WO2012069926A2 - Photoelectric conversion device - Google Patents

Photoelectric conversion device Download PDF

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Publication number
WO2012069926A2
WO2012069926A2 PCT/IB2011/003070 IB2011003070W WO2012069926A2 WO 2012069926 A2 WO2012069926 A2 WO 2012069926A2 IB 2011003070 W IB2011003070 W IB 2011003070W WO 2012069926 A2 WO2012069926 A2 WO 2012069926A2
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WO
WIPO (PCT)
Prior art keywords
converter
wavelength
wavelength converter
semiconductor
photoelectric
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Application number
PCT/IB2011/003070
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French (fr)
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WO2012069926A3 (en
Inventor
Nicholas John Ekins-Daukes
Daniel James Farrell
Tomonori Nagashima
Yasuhiko Takeda
Kazutaka Nishikawa
Tomoyoshi Motohiro
Original Assignee
Toyota Jidosha Kabushiki Kaisha
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Publication of WO2012069926A2 publication Critical patent/WO2012069926A2/en
Publication of WO2012069926A3 publication Critical patent/WO2012069926A3/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/055—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means where light is absorbed and re-emitted at a different wavelength by the optical element directly associated or integrated with the PV cell, e.g. by using luminescent material, fluorescent concentrators or up-conversion arrangements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02162—Coatings for devices characterised by at least one potential jump barrier or surface barrier for filtering or shielding light, e.g. multicolour filters for photodetectors
    • H01L31/02165—Coatings for devices characterised by at least one potential jump barrier or surface barrier for filtering or shielding light, e.g. multicolour filters for photodetectors using interference filters, e.g. multilayer dielectric filters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02167—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H01L31/02168—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells the coatings being antireflective or having enhancing optical properties for the solar cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035209—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures
    • H01L31/035218—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures the quantum structure being quantum dots
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/075—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PIN type
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/52—PV systems with concentrators
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/548—Amorphous silicon PV cells

Definitions

  • the invention relates to photoelectric conversion devices, and more particularly to photoelectric conversion devices using wavelength conversion mechanisms.
  • JP-A-2004-296658 As one of the new methods that have been studied so far, multi-junction solar cells using a combination of three different materials have been proposed, as disclosed in Japanese Patent Application Publication No. 2004-296658 (JP-A-2004-296658).
  • the multi-junction solar cells are able to absorb photons of a wide range of wavelengths contained in solar light, and, therefore, provide a high conversion efficiency.
  • solar cells using four to six materials have been studied.
  • the increase in the number of junctions may result in increase of the number of semiconductor interfaces having a high defect density, which may cause recombination loss of caniers and reduction of the conversion efficiency.
  • many types of expensive II I- V compounds need to be used, and the number of manufacturing steps increases, resulting in increased cost, for example.
  • hot-carrier type solar cells to which a photoelectric conversion theory (i.e., hot-carrier theory) different from conventional theories is applied have been proposed, as disclosed in Japanese Patent Application Publication No. 2009-59915 (JP-A-2009-59915).
  • a solar cell of this type is able to absorb photons of a wide range of wavelengths contained in solar light, without increasing the number of junctions (the number of semiconductor materials), and convert the photons into electric power with small energy loss.
  • JP-A-2009-59915 in order to achieve a high conversion efficiency in a solar cell to which the hot-carrier theory is applied, it is necessary to promote energy interaction (exchange) between carriers, and move the carriers while keeping high energy thereof, in a light absorbing layer. While the hot carrier lifetime needs to be at least 1 ns (nanosecond), so as to achieve a high conversion efficiency with which the carriers moved through the light absorbing layer are extracted, the lifetime of existing semiconductor materials is still as short as several ps (picoseconds) to several hundreds of ps. Thus, undeT the present circumstances, the use of the technology as disclosed in JP-A-2009-59P25 is not sufficient to enhance the photoelectric conversion efficiency.
  • a down-conversion type solaT cell thai converts short-wavelength light with high energy into a wavelength that matches the band-gap of the solar cell material, so as to reduce heat (voltage) loss
  • an up-conversion type solar cell that converts long-wavelength light that has small energy and becomes light transmission loss, into a wavelength that matches the band-gap of the solar ceil material.
  • These solar cells use phosphors using (or doped with) rare-earth elements.
  • the known phosphors using rare-earth elements are used, the wavelength range of light that can be absorbed by the solar cells is narrow, and the energy loss that appears during wavelength conversion is large. Thus, it is difficult for the known down-conversion type solar cell and up-conveision type solar cell to achieve sufficiently enhanced photoelectric conversion efficiency.
  • the invention provides a photoelectric conversion device that achieves enhanced photoelectric conversion efficiency.
  • a photoelectric conversion device includes a wavelength converter that includes a semiconductor material that creates electrons and holes when absorbing light, and emits monochromatic light by recombining the electrons and the holes, and a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, and the wavelength converter is disposed upstream of the photoelectric converter as viewed in a travelling direction of the light.
  • the wavelength converter incorporates a hot-carrier mechanism into an optical filter or a photonic crystal, or provides the hot-carrier mechanism with a wavelength selection mechanism, such as surface plasmon resonance of metal nanoparticles, so as to improve the efficiency with which monochromatic light is converted from solar light, while providing the wavelength converter with the function of adjusting the wavelength of the light as desired.
  • a wavelength selection mechanism such as surface plasmon resonance of metal nanoparticles
  • light means multicolor light, such as solar light.
  • the "ypsirearo ... as viewed in a travelling direction of the light” means upstream ... as viewed in a travelling direction of the multicolor light.
  • the wavelength converter is disposed upstream of the photoelectric converter as viewed in a travelling direction of the light means that the wavelength converter and the photoelectric converter are positioned so that the light (multicolor light) that has not reached the photoelectric converter is incident on the wavelength converter, and monochromatic light emitted from the wavelength converter is incident on the photoelectric converter.
  • the wavelength converter is placed on the upstream side as viewed in the traveling direction of the light (multicolor light), and the photoelectric converter is disposed downstream of (he wavelength converter as viewed in the travelling direction of the light (multicolor light).
  • the concept of the "photoelectric conversion device” covers light-detecting elements and others, as well as solar cells. In the following description, multicolor light may be referred simply as "light”.
  • the wavelength converter disposed upstream of the photoelectric converter as viewed in the travelling direction of the light generates electrons and holes (which will be collectively called “carriers" when appropriate) when it receives light, and causes the electrons and holes to recombine and emit monochromatic light, which is then incident on the photoelectric converter. Since the wavelength converter having the function of an optical resonator is arranged to emit monochromatic light, the electrons and holes having high energy through excitation can i>e recombined before losing energy, and otherwise possible energy loss can be reduced.
  • the wavelength converter having the function of the optical resonator is arranged to eventually recombine the electrons and holes, but is not intended to extract the created carriers to the outside as they are. Therefore, there is no need to move the carriers to electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and the amount of energy that would be lost during movement of the earners can be significantly reduced. Furthermore, the use of the semiconductor material in the wavelength converter makes it possible to greatly broaden the wavelength range of light that can be used for generating carriers, as compared with down-conversion type solar cells of the related art using phosphors.
  • the wavelength of the monochromatic light can be adjusted as desired by adjusting (he shape or dimensions of the wavelength converter and suitabl selecting the refractive indices of materials used in the wavelength converter.
  • the monochromatic lighf is incident on the photoelectric converter, so that the energy of the moriochromatic light incident on the photoelectric converter is specified. Therefore, energy loss can be reduced if a semiconductor material having energy gap corresponding to the energy of the monochromatic light incident on the photoelectric converter (more specifically, energy gap that is equal to or smaller by about O.leV than the energy of the monochromatic light) is used for the photoelectric converter. Accordingly, the photoelectric conversion device according to the first aspect of the invention achieves enhanced photoelectric conversion efficiency. According to the first aspect of the invention, the photoelectric conversion efficiency can be easily improved by improving the efficiency of the wavelength converter for converging light into monochromatic light.
  • a photoelectric conversion device includes a wavelength converter that includes a semiconductor material that creates electrons and holes when absorbing light, and emits monochromatic light by recombining the electrons and the holes, and a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, and the wavelength converter is disposed downstream of the photoelectric converter as viewed in a travelling direction of the light.
  • the "downstream ... as viewed in a travefiing direction of the light” means downstream ... as viewed in a travelling direction of the multicolor light.
  • the wavelength converter is disposed downstream of the photoelectric converter as viewed in a travelling direction of the light means that the wavelength converter and the photoelectric converter are positioned so that the light is incident on the photoelectric converter, and the light that has passed through the photoelectric converter is incident on the wavelength converter, which, in turn, emits monochromatic light using the incident light, such that the monochromatic light is incident on the photoelectric converter.
  • the photoelectric converter is placed on the upstream side as viewed in the traveling direction of the light, and the wavelength converter is disposed downstream of the photoelectric converter as viewed in the travelling direction of the light.
  • the wavelength converter generates electrons and holes using light that has passed through the photoelectric converter disposed upstream of the wavelength converter as viewed in the travelling direction of the light, and recombines the electrons and holes thus created.
  • photons with larger energy than the energy gap of the semiconductor material included in the photoelectric converter are absorbed by the p/ioroe/ecfric converter and converted into electric power, and photons that were not converted into electric power in the photoelectric converter are incident on the wavelength converter.
  • the wavelength converter which uses a semiconductor material, absorbs photons with larger energy than the energy gap of the semiconductor material, and generates carriers (electrons and holes).
  • the energy gap of the semiconductor material included in the wavelength converter is smaller than the energy gap of the semiconductor material included in the photoelectric converter.
  • the electrons and holes generated in the wavelength converter interact with the electrons and holes, respectively, generated therein, so as to emit monochromatic light with larger energy than the energy gap of the semiconductor material that forms the photoelectric converter, and the monochromatic light is incident on the photoelectric converter where electric energy is extracted from the incident light,
  • the wavelength range of the light used for conversion into electric energy in the photoelectric converter can be broadened.
  • the wavelength converter is arranged to eventually recombine electrons and holes, but is not intended to extract the generated electrons and holes to the outside as they are.
  • the use of the semiconductor material in the wavelength converter makes it possible to greatly broaden (he wavelength range of light that can be used for generating carriers, as compared with up-conversion type solar cells of the related art using phosphors.
  • the wavelength of the ' monochromatic light can be adjusted as desired by adjusting the shape or dimensions of the wavelength converter and suitably selecting the refractive indices of materials used in the wavelength converter.
  • the photoelectric conversion device achieves enhanced photoelectric conversion efficiency.
  • the photoelectric conversion efficiency can be easily improved by improving the efficiency of the wavelength converter for converging light into monochromatic light.
  • the wavelength converter and the photoelectric converter may be in contact with each other, and an interface between the wavelength converter and the photoelectric converter may have projections, recesses, or a combination of projections and recesses.
  • the wavelength converter and the photoelectric converter may be spaced apart from each other, and at least one surface of the wavelength converter which faces the photoelectric converter may have projections, recesses, or a combination of projections and recesses.
  • protrusions and recesses mean protrusions and recesses whose height and depth are larger than the wavelength of the monochromatic light emitted from the wavelength converter.
  • the wavelength converter may have a semiconductor layer formed of the semiconductor material, and a plurality of transparent material layers having different refractive indices and laminated on each of opposite sides of the semiconductor layer.
  • the plurality of transparent material layers having different refractive indices and laminated on the opposite sides of the semiconductor layer, and the semiconductor layer sandwiched between the transparent material layers cooperate to function as an optical resonator (or optical filter), so that the incident light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
  • the wavelength converter may further include a different-refractive-index materia ⁇ having a refractive index that is different from that of the semiconductor material, and the wavelength converter may have at least one different-refractive-index portion that is formed of the different-refractive-index material and extends through the semiconductor material in a direction from the wavelength converter toward the photoelectric converter.
  • the semiconductor material in which the different-refractive-index portion(s) is/are provided is able to function as an optical resonator (or photonic crystal), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
  • the wavelength converter may further include a different-refractive-index material having a refractive index that is different from thai of the semiconductor material, and the wavelength converter may have a plurality of different-refractive-index portions that are formed of the different-refractive-index material and are three-dimensionaUy dispersed in the semiconductor material.
  • the semiconductor material in which the different-refractive-index portions are three-dimensionally dispersed is able to function as an optical resonator (or photonic crystal), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
  • the wavelength converter may further include metal nanoparticles dispersed in the semiconductor material. This arrangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength.
  • the wavelength converter is able to function as an optical resonator (optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
  • the wavelength converter may have a plurality of semiconductor portions formed of the semiconductor material, and each of the semiconductor portions is interposed between a pair of metal sheets.
  • the metal sheets between which the semiconductor portion is interposed are able to function as a photoconductive antenna. Therefore, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength.
  • the wavelength converter is able to function as an optical resonator (or optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
  • the wavelength converter may have, a plurality of semiconductor portions formed of the semiconductor material, a plurality of metal nanoparticles contained in the semiconductor portions, and a light-permeable insulating material in which the semiconductor portions are dispersed.
  • This arrangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the wavelength converter is able to function as an optical resonator (or optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus copstrucled wavelength converter achieves enhanced photoelectric conversion efficiency,
  • the wavelength converter may have a plurality of semiconductor particles formed of the semiconductor material, a plurality of metal particles in which the semiconductor particles are contained, and a light-permeable insulating material in which the metal particles are dispersed.
  • This arrangement makes it possible to cause surface plasmon resonance to occur around the metal particles. Therefore, the wavelength converter is able to function as an optical resonator (or optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
  • a photoelectric conversion device includes a wavelength converter that includes a semiconductor material that generates electrons and holes when absorbing light, and emits monochromatic light by recorobining the electrons and the holes, and a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, and the wavelength converter consists of a plurality of wavelength converting portions dispersed in the photoelectric converter.
  • the photoelectric conversion device In the photoelectric conversion device according to the third aspect of the invention, electrons and holes are generated in the wavelength converting portions dispersed in the photoelectric converter, using light that has passed through the photoelectric converter. Then, the electrons and holes are recombined in the wavelength converting portions, so that monochromatic light having larger energy than the energy gap of the semiconductor material that forms the photoelectric converter is emitted, and the monochromatic light is incident on the photoelectric converter where electric energy is extracted. With this arrangement, the wavelength range of the light used for conversion into electric energy in the photoelectric converter can be broadened, as in the photoelectric conversion device according to the second aspect of the invention.
  • the wavelength converting portions are arranged to eventually recombine electrons and holes, but not intended to extract the generated electrons and holes to the outside as they are. Therefore, there is no need to move the carriers to electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and the amount of energy that would be lost during movement of the carriers can be significantly reduced. Furthermore, the use of the semiconductor material in the wavelength converting portions makes it possible to greatly broaden the wavelength range of light that can be used for creating carriers, as compared with up-conversion type solar cells of the related art using phosphors. Also, the wavelength of the monochromatic light can be adjusted as desired by adjusting the shape or dimensions of the wavelength converting portions and suitably selecting the refractive indices of materials used in the wavelength converter.
  • the photoelectric conversion device achieves enhanced photoelectric conversion efficiency.
  • the photoelectric conversion efficiency can be easily improved by improving the efficiency with which the wavelength converting portions convert light into monochromatic light.
  • each of the wavelength converting portions may have a semiconductor portion formed of the semiconductor material, and a metal nanoparticie contained in the semiconductor portion, and the semiconductor portions of the wavelength converting portions may be dispensed in the photoelectric converter.
  • This arrangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the wavelength converting portions are able to function as an optical resonator (or optical filter), so that the incident light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength convener achieves enhanced photoelectric conversion efficiency.
  • each of the wavelength converting portions may have a semiconductor portion formed of the semiconductor material, a metal nanoparticle in which the semiconductor portion is contained, and a light-permeable insulting portion in which the metal nanoparticle is contained, and the light-permeable insulating portions of the wavelength converting portions may be dispersed in the photoelectric converter.
  • This anangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the wavelength converting portions are able to function as an optical resonator (or optical filter), so that the incident light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
  • the photoelectric conversion device it is possible to reduce energy loss of the carriers generated therein, while broadening the frequency band of light that can be converted into electric energy. Accordingly, the photoelectric conversion device of the invention achieves enhanced photoelectric conversion efficiency.
  • FIG. 1A is a cross-sectional view sfiowing a solar cell according to a first embodiment of the invention
  • FIG IB is a view illustrating the energy band structure of the solar cell of FIG. 1A
  • FIG. 2A is a cross-sectional view showing a solar cell according lo a second embodiment of the invention
  • FIG. 2B is a view illustrating the energy band structure of the solar cell of FIG 2A;
  • FIG 3A is a cross-sectional view showing a solar cell as a modified example of the first embodiment of the invention.
  • FIG 3B is a cross-sectional view showing a solar cell as another modified example of the first embodiment of the invention.
  • FIG 3C is a cross-sectional view showing a solai cell as a modified example of the second embodiment of the invention.
  • FIG 3D is a cross-sectional view showing a solar cell as another modified example of the second embodiment of the invention.
  • FIG 4 is a cross-sectional view showing one example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention.
  • FIG 5A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention.
  • FIG 5B is a cross-sectional view of the. wavelength converter taken along VB-VB in FIG 5A;
  • FIG 6A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention.
  • FIG. 6B is a cross-sectional view of the wavelength converter taken along VIB-VIB in FIG. 6A;
  • FIG. 7A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention.
  • FIG 7B is a cross-sectional view of the wavelength converter taken along VIIB-VIIB in FIG. 7A;
  • FIG 8 is a cross-sectional view showing another example of wavelength converter that can be used in the solar ceil of the first or second embodiment of the invention.
  • FIG 9A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention.
  • FIG 9B is a cross-sectional view of the wavelength ccmv &r taken along 1XB-1XB in FIG. 9A;
  • FIG. 10 is a view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
  • FIG. 11 is a view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention.
  • FIG. 12 is a cross-sectional view showing a solar cell according to a third embodiment of the invention.
  • FIG 13 is a cross-sectional view showing one example of wavelength converting portion that can be used in the solar cell of the third embodiment of the invention.
  • FIG. 14 is a cross-sectional view showing another example of wavelength converting portion that can be used in the solar cell of the third embodiment of the invention.
  • FIG. 1A is a cross-sectional view showing a solar cell 100 according fo a first embodiment of the invention
  • FIG, IB illustrates the energy band structure of the solar cell 100.
  • the electron energy is greater toward the top of the paper, and the hole energy is greater toward the bottom of the paper.
  • " ⁇ " denotes electrons
  • "o" denotes holes.
  • Egl denotes the energy gap of a semiconductor material contained in a wavelength converter 10 which will be described later
  • Eg2 denotes the energy of monochromaiic light emitted by the wavelength converter 10
  • Eg3 denotes the energy gap of a semiconductor material contained in a photoelectric converter 20 which will be described later.
  • solar light travels from the left-hand side of the paper to the right-hand side.
  • the solar cell 100 includes the wavelength converter 10 having a semiconductor material, and the photoelectric converter 20 having a semiconductor material.
  • the wavelength converter 10 is disposed upstream of the photoelectric converter 20 as viewed in a direction in which soiar fight travels (which will be called "travelling direction of solar light").
  • the solar cell 100 is a down-conversion type solar cell.
  • the semiconductor material contained in the wavelength converter 10 has the energy gap denoted as Egl
  • the wavelength converter 10 has the function of generating monochromatic light having energy denoted as Eg2.
  • the semiconductor material of the photoelectric convener 20 has the energy gap denoted as Eg3.
  • the photoelectric converter 20 has an n layer 21 formed of an n-type semiconductor having the energy gap Eg3. and a p layer 22 formed of a p-type semiconductor having the energy gap Eg3.
  • the n layer 21 and p layer 22 are joined to each other, to form a pn junction 23.
  • a front electrode 24 is connected to the n layer 21, and a rear electrode 25 is connected to the p Ja er 22,
  • solar light with which the solar cell 100 is irradiated is incident on the wavelength converter 10.
  • the solar light contains photons with various levels of energy.
  • the solar light is incident on the semiconductor material contained in the wavelength converter 10
  • only the photons with energy equal to or greater than the energy gap Egl of the semiconductor material are absorbed.
  • the photons are absorbed, electrons having various levels of energy are excited from the valence band of the semiconductor material of the wavelength converter 10 to the conduction band, and holes having various levels of energy are formed in the valence band.
  • an electron energy distribution as shown in FIG IB is formed in the conduction band of the semiconductor material
  • a hole energy distribution as shown in FIG. IB is formed in the valence band of the semiconductor material.
  • the shape of the wavelength converter 10 and the r ' efradive indices of materials used in the converter 10 are suitably selected or adjusted, so that pairs of electrons and holes with specific energy levels, which have an energy difference of Eg2, recombine so as to generate monochromatic light with energy Eg2.
  • electrons in the electron distribution which have different levels of energy from the specific energy level that contributes to generation of light, give and receive energy to and from each other, so that a part of the electrons are provided with the specific energy level that contributes to light generation.
  • holes in the hole distribution which have different levels of energy from the specific energy level that contributes to generation of light, give and receive energy to and from each other, so that a part of the holes are provided with the specific energy level that contributes to light emission. Then, the electrons with the specific energy level that contributes to light emission in the electron distribution, and the* holes having the specific energy level that contributes to light emission in the hole distribution recombine so as to generate monochromatic light with energy Eg2.
  • electrons and holes capable of interacting with each other are limited to electrons and holes having discrete energy levels.
  • the wavelength converter 10 which has the semiconductor material, allows electrons having various levels of energy to interact with each other and allows holes having various levels of energy with each other, so as to create electrons having the specific energy level that contributes to light generation, in the electron distribution, and holes having the specific energy level that contributes to light generation, in the hole distribution.
  • the monochromatic light emitted in the waveleagth converter 10 in this manner travels toward the photoelectric converter 20.
  • the photoelectric converter 20 has the n layer 21 and p layer 22 with the energy gap Eg3.
  • Eg3 is smaller than Eg2 by about O.leV. Therefore, the monochromatic light with energy Eg2 emitted from the wavelength converter 10 can overcome the energy gap of the n layer 21 and p layer 22, so as to be absorbed by the photoelectric converter 20 where electrons and holes are generated.
  • the thus generated electrons and holes are separated by an internal electric field formed by the pn junction 23, almost without losing energy, since the difference between Eg2 and Eg3 is as small as about O.leV.
  • the electrons move to the n layer 21 side, and are collected into the front electrode 24 connected to the n layer 21, while the holes move to the p layer 22 side, and are collected into the rear electrode 25 connected to the p layer 22.
  • the wavelength converter 10 having the function of an optical resonator emits monochromatic light
  • the electrons and holes excited to high levels of energy can be recombined before losing energy, resulting in reduction of energy loss.
  • the wavelength converter 10 is arranged to eventually recombine the electrons and the holes, but not intended to extract the electrons and holes (or carriers) generated therein to the outside as they are. Therefore, in the wavelength converter 10, it is not necessary to move the carriers to electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and it is thus possible to significantly reduce the amount of energy that would be lost during movement of the carriers.
  • the use of the semiconductor material in the wavelength converter 10 makes it possible to significantly broaden the wavelength range of light that can be used for creating the carriers, as compared with the down-conversion type solar cells of the related art using phosphors.
  • the energy of the monochromatic light incident on the photoelectric converter 20 is specified as Eg2. Therefore, energy loss can be reduced by using a semiconductor material having a energy gap corresponding to Eg2, for the photoelectric converter 20.
  • the solar cell 100 according to the invention is able to enhance its photoelectric conversion efficiency.
  • the photoelectric conversion efficiency of the solar eel] 200 can be easily enhanced by enhancing the efficiency of the wavelength converter 10 that converts light into monochromatic light.
  • the energy gap Egl of the semiconductor material contained in the wavelength converter 10 may be, for example, equal to or larger thaa 0.4eV and equal to or smaller than 1.2eV, and the energy Eg2 of the monochromatic light emitted from the wavelength converter 10 may be, for example, equal to or larger than 0.6eV, and equal to or smaller than 1.4eV.
  • the semiconductor material that can be contained in the wavelength converter 10 may be selected from, for example, GalnAs, InAsP, GaSb, Ge, etc.
  • the wavelength converter 10 may be produced by a known method, such as vapor-phase growth or vapor deposition.
  • the energy gap Eg3 of the semiconductor material contained in the photoelectric converter 20 may be, for example, equal to or larger than 0.5eV and equal to or smaller than 1.3eV.
  • the semiconductor material that can be contained in the photoelectric converter 20 may be selected from, for example, GalnAs, InAsP, GaSb, Ge, Si, etc.
  • the ⁇ layer 21 may be produced by adding a known n-type dopant to a selected one of the above-indicated semiconductor materials.
  • the p layer 22 may be produced by adding a known p-type dopant to a selected one of the semiconductor materials.
  • the thickness of the n layer 21 may be, for example, about lOOnm, and the thickness of the p layef 22 may be, for example, about 2 ⁇ .
  • the photoelectric converter 20 may be produced by a known method, such as vapor-phase growth or vapor deposition, and the front electrode 24 and the rear electrode 25 may be produced by a known method, such as a deposition or evaporation method.
  • a known material, such as Au, which can be used for electrodes of solar cells, may be suitably used for forming the front electrode 24, and a known material, such as aluminum or indium tin oxide (ITO), which can be used for electrodes of solar cells, may be suitably used for forming the rear electrode 25.
  • the thickness of the front electrode 24 and the rear electrode 25 may be, for example, about ⁇ .
  • the photoelectric conversion device (down-conversion type photoelectric conversion device) according to the first embodiment of the invention is not limited to the illustrated configuration. Rather, the photoelectric converter included in the photoelectric conversion device (down-conversion type photoelectric conversion device) according to the first embodiment of the invention may have a pin junction.
  • FIG. 2A is a cross-sectional viw showing a solar cell 200 according to a second embodiment of the invention
  • FIG. 2B illustrates the energy band structure of the solar cell 200.
  • the electron energy is greater toward the top of the paper, and the hole energy is greater toward the bottom of the paper.
  • " ⁇ " denotes electrons
  • "o" denotes holes.
  • Eg4 denotes the energy gap of a semiconductor material contained in a wavelength converter 30 which will be described later
  • Eg5 denotes the energy of monochromatic light emitted from the wavelength converter 30, while Eg6 denotes the energy gap of a semiconductor material contained in a photoelectric converter 40 which will be described later.
  • FIG. 2A and FIG. 2B solar light travels from the left-hand side of the paper to the right-hand side.
  • FIG. 2A the same reference numerals as those used in FIG 1A are assigned to the same or corresponding elements as those of the solar cell 100, and further explanation of the elements will not be provided when appropriate.
  • the solar light 200 includes the wavelength converter 30 having a semiconductor material, and the photoelectric converter 40 having a semiconductor materia).
  • the wavelength converter 30 is disposed downstream of the photoelectric converter 40 as viewed in the travelling direction of solar light.
  • the solar cell 200 is an up-conversion type solar cell.
  • the semiconductor material contained in the wavelength converter 30 has the energy gap denoted as Eg4, and the wavelength converter 30 has the function of emitting monochromatic light with energy Eg5.
  • the semiconductor material contained in the photoelectric converter 40 has the energy gap denoted as Eg6.
  • the photoelectric converter 40 has an n layer 41 formed of an n-type semiconductor having the energy gap Eg6, and a p layer 42 formed of a p-type semiconductor having the energy gap Eg6.
  • the n layer 41 and p layer 42 are joined to each other, to form a pn junction 43.
  • a front electrode 24 is connected to the n layer 41, and a rear electrode 44 is connected to the p layer 42.
  • solar light with which the solar cell 200 is irradiated is incident on the photoelectric converter 40.
  • the energy gap Eg6 of the semiconductor material contained in the photoelectric converter 40 is controlled so that the photoelectric converter 40 can absorb only high-energy photons, out of the solar light including photons with various levels of energy. Therefore, when the solar light is incident on the semiconductor material contained in the photoelectric converter 40, only the photons having energy equal (o or greater than the energy gap Eg6 of the semiconductor material are absorbed. Once the photons are absorbed, electrons and holes are generated in the photoelectric converter 40.
  • the thus generated electrons and holes are separated by an internal electric field formed by the n layer 41 and the p layer 42, and the electrons move to the n layer 41 side, to be collected into the front electrode 24 connected into the n layer 41. Also, the holes move to the p layer 42 side, to be collected into the rear electrode 44 connected to the p layer 42.
  • the photoelectric converter 40 absorbs only the photons contained in the solar light and having energy equal to or greater than Eg6. Therefore, the photons contained in the solar light and having energy less than Eg6 pass through the photoelectric converter 40, without being used for photoelectric conversion.
  • the photons that have passed through the photoelectric converter 40 are incident on the wavelength converter 30 disposed downstream of the photoelectric converter 40 as viewed in the travelling direction of the solar light.
  • the energy gap Eg4 of the semiconductor material contained in the wavelength converter 30 is smaller than Eg6, and is controlled so that the wavelength converter 30 can absorb low-energy photons contained in the solar light.
  • the electrons generated in the wavelength converter 30 interact with each other for exchange of energy, while the holes generated in the wavelength converter 30 interact with each other for exchange of energy, so that electrons and holes having an energy difference of Eg5 are generated. Then, the electrons and the holes recombine to generate monochromatic light with energy Eg5.
  • electrons and holes capable of interacting with each other are limited to electrons and holes having discrete energy levels.
  • the wavelength converter 30 which has the semiconductor material, allows electrons having various levels of energy to interact with each other, and allows holes having various levels of energy to interact with each other, so as to emit monochromatic light with energy Eg5,
  • the monochromatic light emitted from the wavelength converter 30 travels toward the photoelectric converter 40.
  • the photoelectric converter 40 has the n layer 41 and p layer 42 having the energy gap Eg6.
  • Eg6 is smaller than Eg5 by about O.leV. Therefore, the monochromatic light with energy Eg5 emitted from the wavelength converter 30 can overcome the band-gap of the n layer 41 and the p layer 42, so as to be absorbed by the photoelectric converter 40, where electrons and holes are generated. The thus generated electrons and holes are separated by an internal electric field formed by the pn junction 43, almost without losing energy, since the difference between Eg5 and Eg6 is as small as about O.leV. Then, the electrons move to the n layer 41 side, and are collected into the front electrode 24 connected to the n layer 41, while the holes move to the p layer 42 side, and are collected into the rear electrode 44 connected to the p layer 42.
  • the photoelectric converter 40 With the solar cell 200 constructed as described above, large-energy photons of the solar light having larger energy than the energy gap of the semiconductor material contained in the photoelectric converter 40 are absorbed by the photoelectric converter 40 to be converted into electric power.
  • the photoelectric converter 40 also receives monochromatic light emitted from the wavelength converter 30 using photons thai passed through the photoelectric converter 40 without being converted into electric power, so as to convert the monochromatic light into electric power.
  • This arrangement makes it possible to broaden the band of light used for conversion into electric energy in the photoelectric converter 40.
  • the wavelength converter 30 is arranged to eventually recombinc the electrons and the holes, but not intended to extract the electrons and holes (carriers) generated therein to the outside as they are.
  • the wavelength converter 30 it is not necessary to move the carriers to the electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and it is thus possible to significantly reduce the amount of energy that would be lost during movement of the carriers. Furthermore, the use of the semiconductor material in the wavelength converter 30 makes it possible to significantly broaden the wavelength range of light that can be used for creating the carriers, as compared with the up-conversion type solar cells of the related art using phosphors.
  • the energy of the monochromatic light incident on the photoelectric converter 40 is specified as Eg5.
  • the solar cell 200 according to the invention is able to enhance its photoelectric conversion efficiency.
  • the photoelectric conversion efficiency of the solar cell 200 can be easily enhanced by enhancing the efficiency of the wavelength converter 30 that converts light i3 ⁇ 4to monochromatic light.
  • the energy gap Eg4 of the semiconductor material contained in the wavelength converter 30 may be, for example, equal to or larger than 0.4eV and equal to or smaller than l. leV, and the energy Eg5 of the monochromatic light emitted from the wavelength converter 30 may be, for example, equal to or larger than 1.4eV and equal to or smaller than 3.3eV.
  • the semiconductor material that can be contained in the wavelength converter 30 may be selected from, for example, GalnAs, IriAsP, GaSb, Ge, etc.
  • the wavelength converter 30 may be produced by a known method, such as vapor-phase growth or vapor deposition.
  • the energy gap Eg6 of the semiconductor material contained in the photoelectric converter 40 may be, for example, equal to or larger than 1.3eV and equal to or smaller than 3.2eV.
  • the semiconductor material that can be contained in the photoelectric converter 40 is selected from; for example, GalnP, AlGaAs, GaAs, etc.
  • the n layer 41 may be produced by adding a known n-type dopant to a selected one of the above-indicated semiconductor materials.
  • the p layer 42 may be produced by adding a known p-type dopant to a selected one of (he semiconductor materials.
  • the thickness of the n layer 41 may be, for example, about lOOnm, and the thickness of the p layer 42 may be, for example, about 2 ⁇ ⁇ ⁇ .
  • the photoelectric converter 40 may be produced by a known method, such as vapor-phase growth or vapor deposition.
  • a known material such as aluminum or indium tin oxide (1TO), which can be used for electrodes of solar cells, may be suitably used for forming the rear electrode 44.
  • the thickness of the rear electrode 44 may be, for example, about ⁇ .
  • the photoelectric conversion device (up-conversion type photoelectric conversion device) according to the second embodiment of the invention is not limited to the illustrated configuration. Rather, the photoelectric converter included in the photoelectric conversion device (up-conversion type photoelectric conversion device) according to the second embodiment of the invention may have a pin junction.
  • wavelength converter is disposed only on one side of the photoelectric converter in the solar cells 100, 200 as described above-, a pair of wavelength converters may be provided such that the photoelectric converter is interposed between the wavelength converters, in the photoelectric conversion device of the invention.
  • the photoelectric conversion devices according to the first and second embodiments of the invention are not limited to these configurations.
  • the wavelength converter and the photoelectric converter may be placed in contact with each other. If the wavelength converter and the photoelectric converter are placed apart from each other (i.e., are not in contact with each other), a substance that permits light to pass therethrough may be placed between the wavelength converter and the photoelectric converter, and the substance may be selected from, for example, air, a transparent resin film, glass, and so forth. If the wavelength converter and the photoelectric converter are placed apart from each other, the wavelength converter is fixed by a fixing means (not shown). As the fixing means, a known fixing means capable of fixing the wavelength converter in position may be used as appropriate.
  • the photoelectric conversion devices according to the first and second embodiments of the invention are not limited to the illustrated configurations.
  • projections and/or recesses are preferably formed at at least one surface of the wavelength converter which faces the photoelectric converter in the case where the wavelength converter and the photoelectric converter are not in contact with each other, or projections and/or recesses are preferaSly formed at the interface between the wavelength converter and the photoelectric converter in the case where the wavelength converter and the photoelectric converter are in contact with each other, in order to make it easier for monochromatic light emitted from the wavelength converter to be incident on the photoelectric converter.
  • FIG 3A through FIG 3D Examples of photoelectric conversion devices provided with the projections and/or recesses are illustrated in FIG 3A through FIG 3D.
  • FIG 3A through FIG 3D the same reference numerals as those used in FIG 1A and FIG 2A are assigned to the same or corresponding constituent elements as those of the solar cells 100, 200, and further explanation of these elements will not be provided when appropriate.
  • FIG 3A is a cross-sectional view showing a solar cell 100a in which projections and/or recesses are formed at the interface between a wavelength converter 10a and a photoelectric converter 20a that are in contact with each other.
  • solar light travels from the left-hand side of the paper to the right-hand side.
  • the wavelength converter 10a is constructed similarly to the wavelength converter 10 of the first embodiment, except that projections and or recesses are formed at one surface that faces the photoelectric converter 20a
  • the photoelectric converter 20a is constructed similarly to the photoelectric converter 20 of the first embodiment, except that the converter 20a has an n layer 21a with projections and/or recesses formed at the interface between the n layer 21a and the wavelength converter 10a.
  • the wavelength converter 10a is disposed upstream of the photoelectric converter 20a as viewed in the travelling direction of the solar light.
  • the solar cell 100a is a down-conversion type solar cell.
  • the solar cell 100a may be produced via a process of forming the n layer 21a on the surface of the wavelength converter 10a provided with the projections and/or recesses, or a process of forming the front electrode 24 and the wavelength converter 10a on the surface of t3 ⁇ 4e n layer 21a provided with the protrusions and recesses.
  • FIG. 3B is a cross-sectional view showing a solar cell 100b having the wavelength converter 10a and the photoelectric converter 20.
  • solar light travels from the left-hand side of the paper to the right-hand side.
  • the wavelength converter 10a and the photoelectric converter 20 are not in contact with each other.
  • the solar cell 100b is constructed similarly to the solar cell 100, except that projections and/or recesses are formed at one surface of the wavelength converter 10a which faces the photoelectric converter 20.
  • the wavelength converter 10a is disposed upstream of the photoelectric converter 20 as viewed in the travelling direction of the solar light; thus, the solar cell 100b is a down-conversion type solar cell.
  • the solar cell lOOb may be produced via a process of forming the wavelength converter 10a on a surface of a substrate that is different from that on which the photoelectric converter 20 is formed.
  • FIG. 3C is a cross-sectional view showing a solar cell 200a in which projections and/or recesses are formed at the interface between a wavelength converter 30a and a photoelectric converter 40a that are in contact with each other.
  • solar light travels from the left-hand side of the paper to the right-hand side.
  • the wavelength converter 30a is constructed similarly to the wavelength converter 30, except that projections and/or recesses are formed at one surface facing the photoelectric converter 40a
  • the photoelectric converter 40a is constructed similarly to the photoelectric converter 40, except that the converter 40a has a p layer 42a with projections and/or recesses formed at the interface between the p layer 42a and the wavelength converter 30a.
  • the wavelength converter 30a is disposed downstream of the photoelectric converter 40a as viewed in the travelling direction of the solar light.
  • the solar cell 200a is an up-conversion type solar cell.
  • the solar cell 200a may be produced via a process of forming the p layer 42a and the rear electrode 44 on the surface of the wavelength converter 30a with the projections and/or recesses, or a process of forming the rear electrode 44 and the wavelength converter 30a on the surface of the p layer 42a with the projections and/or recesses.
  • FIG. 3D is a cross-sectional view showing a solar cell 200b having the wavelength converter 30a and the photoelectric converter 40.
  • solar light travels from the left-hand side of the paper to the right-hand side.
  • the wavelength converter 30a and the photoelectric converter 40 are not in contact with each other.
  • the solar cell 200b is constructed similarly to the solar cell 200, except that projections and/or recesses are formed at one surface of the wavelength converter 30a which faces the photoelectric converter 40.
  • the wavelength converter 30a is disposed downstream of the photoelectric converter 40 as viewed in the travelling direction of the solar light.
  • the solar light 200b is an up-conversion type solar cell.
  • the solar cell 200b may be produced via a process of forming the wavelength converter 30a on a surface of a substrate that is different from that on which the photoelectric converter 40 is formed.
  • the projections and/or recesses are formed in a direction from the wavelength converter 10a, 30a toward the photoelectric converter 20a, 20, 40a, 40.
  • the shapes of the projections and/or recesses are not particularly limited.
  • the projections may be formed in a pyramid shape or columnar shape, and the recesses may be formed by pressing pyramid-shaped projections against the surface concerned, or may be in the form of dimples.
  • the height of the projections/recesses is not particularly limited provided that the height is greater than the wavelength of the monochromatic light emitted from the wavelength converter, and the projections/recesses do not extend through the wavelength converter or photoelectric converter.
  • the height of the projections/recesses may be equal to or greater than 0.5 ⁇ and equal to or less than 20 ⁇ , for example.
  • the interval or pitch of the projections/recesses may be suitably set in accordance with the height of the projections/recesses, and may be, for example, equal to or larger than ⁇ . ⁇ and equal to or smaller than ⁇ .
  • wavelength converters that can be used in the photoelectric conversion devices according to the first and second embodiments of the invention will be described.
  • FIG. 4 is a cross-sectional view showing one example of wavelength converter 11.
  • the vertical direction of the paper is the travelling direction of solar light.
  • the wavelength converter 11 has a semiconductor layer llx formed of a semiconductor material, and four pairs of high-refractive-index transparent material layers 11a and low-refractive-index transparent material layers lib are alternately laminated on each of the opposite sides of the semiconductor layer llx.
  • the high-refractive-index transparent material layer 11a is formed of a transparent material having a larger refractive index than a transparent material of which the low-refractive-index transparent material layer lib is formed
  • the low-refractive-index transparent material layer lib is formed of a transparent material having a smaller refractive index than the transparent material of which the high-refractive-index transparent material layer Ha is formed.
  • the wavelength converter 11 thus constructed is able to function as an optical resonator (or optical filter).
  • the frequency at which electrons and holes generated in the semiconductor layer llx interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converter 11, monochromatic light is emitted through recombination of the electrons and holes generated in the semiconductor layer l lx.
  • the semiconductor material that forms the semiconductor layer llx may be selected»from, for example, InGaAs, InAsP, Ge. and so forth, and the thickness of the semiconductor layer llx (the thickness as measured in the vertical direction of the paper of FIG. 4) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than 300um. Also, where the refractive index of the semiconductor material that forms the semiconductor layer llx is denoted as ns, the thickness of the semiconductor layer llx may be set to l/(2ns) of the wavelength of the monochromatic light to be emitted from the wavelength converter 11.
  • the thus constructed semiconductor layer llx may be produced by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE).
  • the transparent material that forms the high-refractive-index transparent material layer 11a may be selected from, for example, Ti0 2) Zr0 2 , and so forth, and the thickness of the high-refractive-index transparent material layer 11a (the thickness as measured in the vertical direction of the paper of FIG 4) may be, for example, equal to or larger than 50nm and equal to or smaller than 20Gnm.
  • the refractive index of the transparent material that forms the high-refractive-index transparent material layer 11a is denoted as nh
  • the thickness of the high-refractive-index transparent material layer 11a may be set to l/(4nh) of the wavelength of the monochromatic light to be emitted from the wavelength converter 11.
  • the thus constructed high-refractive-index transparent material layer 11a may be produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering.
  • the transparent material that forms the low-refractive-index transparent material layer lib may be selected from, for example, Si0 2 , MgF 2 , and so forth, and the thickness of the low-refractive-index transparent material layer lib (the thickness as measured in the vertical direction of the paper of FIG 4) may be, for example, equal to or larger than lOOn and equal to or smaller than 300nm.
  • the thickness of the low-refractive-index transparent material layer lib may be set to l/(4nl) of the wavelength of the monochromatic light to be emitted from the wavelength converter 11.
  • the thus constructed low-refractive-index transparent material layer lib may be produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering.
  • the wavelength of the monochromatic light emitted from the wavelength converter 11 can be shortened by reducing "nsxds" and "nhxdh+nlxdl".
  • the wavelength of the monochromatic b ' ght emitted from the wavelength converter 11 can be increased or made longer by increasing "nsxds" and "nhxdh+nlxdl", Also, it is preferable to set the number of pairs of high-refractive-index transparent layers and low-refractive-index transparent layers laminated on each of the opposite sides of the semiconductor layer llx, to four pairs or more, in order to make it easy for the wavelength converter 11 to generate monochromatic light.
  • FIG. 5A is a top view showing another example of wavelength converter 12, and FIG. 5B is a cross-sectional view taken along VB-VB in FIG. 5A.
  • the direction that is perpendicular to the plane of the paper of FIG 5 A, and the vertical direction of the paper of FIG. 5B correspond to the travelling direction of light.
  • the wavelength converter 12 has a plurality of holes. 12a, 12a, . .. formed through a semiconductor portion 12x.
  • the thus constructed wavelength converter 12 is able to function as an optical resonator (or photonic crystal).
  • the frequency at which electrons and holes generated in the semiconductor portion 12x interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converter 12, monochromatic light can be emitted through recombination of the electrons and holes generated in the semiconductor portion 12x.
  • the semiconductor material that forms the semiconductor portion 12x may be selected from, for example, InGaAs, LoAsP, Ge, and so forth, and the thickness of the semiconductor portion 12x (the thickness as measured in the vertical direction of the paper of FIG 5B) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than lOOOnm.
  • the diameter of the holes 12a may be, for example, equal to or larger than 50nm and equal to or smaller than 200nm, and the interval between the centers of adjacent holes 12a, 12a may be, for example, equal to or larger than lOOnm and equal to or smaller than 500nm.
  • the holes 12a, 12a, ... may be hollow, or may be filled with a transparent material having a smaller refractive index than the semiconductor material that forms the semiconductor portion 12x.
  • the transparent material may be selected from Si0 2 , known transparent resins, and so forth.
  • a semiconductor layer having a thickness within the range of about lOOnro to lOOOnm is formed by a known method, such as vapor deposition, more specifically, a metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE), and then, the holes 12a, 12a, ... are formed through the semiconductor layer by a known method, such as photolithography as a combination of dry etching and wet etching, or electron lithography. Then, the holes 12a, 12a, ... are filled as needed with a transparent material having a smaller refractive index than the semiconductor material that forms the semiconductor portion 12x.
  • vapor deposition more specifically, a metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE)
  • MOCVD metal organic chemical vapor deposition
  • MBE molecular beam epitaxy
  • the wavelength of monochromatic light emitted from the wavelength converter 12 can be shortened by reducing "(l-vpl)xnxl+Yplxnyl", or reducing the interval between the centers of adjacent ones of the holes 12a, 12a, ....
  • the wavelength of monochromatic light emitted from the wavelength converter 12 can be increased or made longer by increasing "(l- pl)xnxl+vplxnyl", or increasing the interval between the centers of adjacent ones of the holes 12a, 12a, ....
  • FIG 6A is a top view showing another example of wavelength converter 13, and FIG. 6B is a cross-sectional view taken along VIB-VIB in FIG. 6A.
  • the direction that is perpendicular to the plane of the paper of FIG. 6A, and the vertical direction of the paper of FIG. 6B correspond to the travelling direction of light.
  • the wavelength converter 13 has a semiconductor portion 13x formed of a semiconductor material, and low-refractive- index transparent materia] portions 13a, 13a, ... dispersed in the semiconductor portion 13x.
  • the low-refractive-index transparent material portions 13a are formed of a transparent materia] having a smaller refractive index than the semiconductor material of the semiconductor portion 13 ⁇ .
  • the thus constructed wavelength converter 13 is able to function as an optical resonator (or photonic crystal).
  • the wavelength converter 13 that functions as an optical resonator the frequency at which electrons and holes generated in the semiconductor portion 13x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on ( he wavelength converter 13, monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portion 13x.
  • the semiconductor material that forms the semiconductor portion 13x may be selected from, for example, InGaAs, IjiAsP, Ge, and so forth, and the thickness of the semiconductor portion 13x (the thickness as measured in the vertical direction of the paper of FIG. 6B) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than lOOOnm.
  • the transparent material that forms the low-refractive-index transparent material portions 13a a selected one of known transparent materials, such as Si(1 ⁇ 4 and known transparent resins, which have a smaller refractive index than the semiconductor material that forms the semiconductor portion I3x, may be suitably used.
  • the diameter of the low-refractive-index transparent material portions 13a may be, for example, equal to or larger than 50nm and equal to or smaller than 200nm, and the interval between the centers of adjacent ones of the low-refractive-index transparent material portions 13a, 13a may be, for example, equal to or larger than lOOnm and equal to or smaller than 500nm.
  • the low-refractive-index transparent material portions 13a, 13a, ... are produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering. Then, when the semiconductor portion 13x is formed by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE), a process of forming a part of the semiconductor portion 13x, and a process of dispersing the low-refractive-index transparent material portions 13a, 13a, ... on a surface of the semiconductor portion 13x that is in the middle of formation are repeated, so that the wavelength converter 13 can be fabricated.
  • vapor deposition more specifically, a vacuum evaporation method including ion plating, or sputtering.
  • MOCVD metal organic chemical vapor deposition
  • MBE molecular beam epitaxy
  • the wavelength converter 13 where the refractive index of the semiconductor material that forms the semiconductor portion 13x is denoted as nx2, and the refractive index of the substance that forms the low-refractive-index transparent material portions 13a is denoted as ny2, while the volumetric proportion of the low-refractive-index transparent material portions 13a, 13a, ... in the wavelength converter 33 is denoted as yp2, the wavelength of monochromatic light emitted from the wavelength converter 13 can be shortened by reducing "(l ⁇ Yp2)xnx2+ p2xny2' ⁇ or reducing the interval between the centers of adjacent ones of the low-refractive-index transparent material portions 13a, 13a, ....
  • the wavelength of monochromatic light emitted from the wavelength converter 13 can be increased by increasing "(1- ⁇ 2) ⁇ ⁇ 2+ ⁇ 2 ⁇ 2", or increasing the interval between the centers of adjacent ones of the low-refractive-index transparent material portions 13a, 13a, ....
  • FIG 7A is a top view showing another example of wavelength converter
  • FIG. 7B is a cross-sectional view taken along VIIB-VIIB in FIG 7A.
  • the direction that is perpendicular to the plane of the paper of FIG 7A, and the vertical direction on the paper of FIG. 7B correspond to the travelling direction of light.
  • the wavelength convener 14 has a semiconductor portion 14x formed of a semiconductor material, and metal nanoparticles 14a, 14a, ... dispersed in the semiconductor portion 14x.
  • the wavelength converter 14 is able to function as an optical resonator (or optical filter).
  • the frequency at which electrons and holes generated in the semiconductor portion 14x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converter 14, monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portion I4x.
  • the semiconductor material that forms the semiconductor portion 14x may be selected from, for example, InGaAs, InAsP, Ge, and so forth, and the thickness of the semiconductor portion 4x (the thickness as measured in the vertical direction of the paper of FIG 7B) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than lOOOnm.
  • metal materials that can form the metal nanoparticles 14a include Au, Ag, Al, Pt, etc.
  • the diameter of the metal nanoparticles 14a is preferably controlled to be equal to or larger than lOnm and equal to or smaller than 50Dm, so as to enable the metal nanoparticles 14a to produce surface plasmon resonance.
  • the interval between the centers of adjacent ones of the metal nanoparticles 14a, 14a ... is preferably controlled to be twice or more and five times or less as large as the diameter of the metal nanoparticles 14a, so that the effect of the surface plasmon resonance can be easily obtained over substantially the entire region of the semiconductor portion 14x.
  • the wavelength converter 14 constructed as described above may be fabricated by forming the semiconductor portion 14x by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE), while dispersing the metal nanoparticles 14a, 14a, ... produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering, in the semiconductor portion 14x.
  • vapor deposition more specifically, metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE)
  • MBE molecular beam epitaxy
  • metal nanoparticles 14a, 14a, . .. are dispersed inside the semiconductor portion 14x in the illustrated example of FIG 7A and FIG 7B
  • metal nanoparticles whose surfaces are covered with a transparent insulating material are preferably dispersed in the semiconductor material, so that carriers generated in the semiconductor portion 14x are less likely to be trapped by the metal nanoparticles 14a, 14a, for improvement in the efficiency of the photoelectric conversion device, for example.
  • the preferred example of wavelength converter 15 is illustrated in FIG 8.
  • FIG. 8 is a cross-sectional view showing the wavelength converter 15, and the vertical direction on the paper of FIG 8 is the travelling direction of light.
  • the same reference numerals as used in FIG 7B are assigned to the same or corresponding constituent elements as those of the wavelength converter 14, and further explanation of these elements will not be provided.
  • the wavelength converter 15 has nanoparticles 15a, 15a, ... dispersed in the semiconductor portion 14x, and the nanoparticles 15a, 15a, ... are produced by covering surfaces of the metal nanoparticles 14a, 14a, ... with transparent insulating layers 15b, 15b, ....
  • the surface plasmon resonance can be produced around the nanoparticles 15a, 15a, as in the wavelength converter 14. Since the nanoparticles 15a are formed by covering the surfaces of the metal nonaparticles 14a with the transparent insulating layers 15b, carriers created in the semiconductor portion 14x are prevented from being trapped by the metal nanoparticles 14a. Accordingly, the wavelength converter 15 is able to generate monochromatic light having higher intensity than that emitted from the wavelength converter 14.
  • the transparent insulating layers 15b may be formed of a known insulating material, such as SiOi or a transparent resin.
  • the thickness of the transparent insulating layers 15b is preferably controlled to be equal to or larger than 2nm, so that the transparent insulating layers 15b can effectively prevent the carriers from being trapped by the metal nanoparticles 14a or make it less likely to have the carriers trapped by the metal nanoparticles 14a.
  • the thickness of the transparent insulating layers 15b is preferably controlled to be equal to or smaller than lOnm, so that the effect of the surface plasmon resonance is likely to reach the semiconductor portion 14x.
  • the nanoparticles 15a can be produced by a known method of chemical synthesis, such as a sol-gel method or a solvothermal method.
  • the wavelength converter 14 and the wavelength converter 15 are likely to generate monochromatic light having a longer wavelength as the refractive index of the semiconductor material l4x is increased, and are likely to emit monochromatic light having a shorter, wavelength as the refractive index of the semiconductor material 14x is reduced.
  • FIG. 9A is a top view showing another example of wavelength converter 16, and FIG 9B is a cross-sectional view taken along IXB-IXB of FIG. 9A.
  • the direction perpendicular to the plane of the paper of FIG. 9A and the vertical direction on the paper of FIG. 9B correspond to the travelling direction of light.
  • the wavelength converter 16 has a transparent insulating material portion 16a, and a plurality of photoconductive antenna portions 16b, 16b, ... disposed in the transparent insulating material portion 16a.
  • Each of the photoconductive antenna portions 16b has a semiconductor portion 16x and a pair of metal sheets 16y, 16y, and the semiconductor portion 16x is interposed between the metal sheets 16y, 16y.
  • the metal sheets 16y, 16y function as a photoconductive antenna, at a given frequency that depends on the complex refractive index of the metal sheets 16y, 16y, the semiconductor portion 1 x, and the surrounding transparent insulating material portion 16a, and the size of the metal sheets 16y, 26y. Consequently, the light intensity significantly increases at the semiconductor portions I6x, so that the wavelength converter 16 can function as an optical resonator (or optical filter).
  • the wavelength converter 16 that functions as an optical resonator the frequency at which electrons and holes generated in the semiconductor portions 16x interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converter 16. monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portions 16x.
  • examples of transparent insulating materials that can form the transparent insulating material portion 16a include known transparent insulating materials, such as Si0 2 and transparent resins.
  • examples of semiconductor materials that can form the semiconductor portions 16x include InGaAs, InAsP, Ge, and so forth.
  • the diameter of the semiconductor portions 16x is controlled to be equal to or larger than lOnm, so that surface plasmon resonance can be produced, and is controlled to be equal to or smaller 4 than 200nm, so that the surface plasmon resonance can be produced over the whole semiconductor portion 16x.
  • examples of metal materials that can form the metal sheets 16y include Au, Ag, Al, Pt, and so forth, and the length of the metal sheets 16y (e.g., the length of the metal sheet 16y located at the lower, right-hand side of the paper of FIG. 9A, as measured in the lateral direction on the paper of FIG. 9A) may be controlled to be equal to or larger than lOOnm and equal to or smaller than 400nm, so that the photoconductive antenna portion 16b can function as a photoconductive antenna.
  • the thickness of the metal sheets 16y e.g., the thickness of the metal sheet 16y located at the lower, right-hand side of the paper of FIG. 9A, as measured in the direction perpendicular to the plane of the paper of FIG.
  • the length of the photoconductive antenna portions 16b may be controlled to be equal to or larger than 300nm and equal to or smaller than SOOnm, so that the photoconductive antenna portion 16b can function as a photoconductive antenna.
  • the length of the photoconductive antenna portion 16b may be set to l/(2nt) of the wavelength of monochromatic light to be emitted from the wavelength converter 16, and the interval or pitch of the photoconductive antenna portions 16b as measured in the thickness direction (e.g., the interval of the photoconductive antenna portions 16b located at the lower, right-hand side of the paper of. FIG, 9A, as measured in the vertical direction of the paper of FIG. 9B) may be set to l/(nt) of the wavelength of monochromatic light emitted from the wavelength converter 16.
  • the wavelength converter 16 is likely to emit monochromatic light having a longer wavelength as the length of the photoconductive antenna portion 16b is increased, and is likely to emit monochromatic light having a shorter wavelength as the length of the photoconductive antenna portior? 16b is reduced.
  • the semiconductor portions 16x may be produced by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or a photolithography method as a combination of molecular beam epitaxy ( BE) and dry etching or wet etching, or electron lithography.
  • MOCVD metal organic chemical vapor deposition
  • the metal sheets 16y may be produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or a photolithography method as a combination of sputtering and dry etching or wet etching, or electron lithography.
  • the wavelength converter 16 may be fabricated by producing the transparent insulating material portion 16a by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering, while placing the photoconductive antenna portions 16b in each of which one semiconductor portion 16x is located between projecting parts of a pair of metal sheets I6y, 16y, in the transparent insulating material portion 16a.
  • a known method such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering
  • FIG 10 is a cross-sectional view showing another example of wavelength converter 17.
  • the vertical direction on the paper of FIG 10 is the travelling direction of light.
  • the wavelength converter 17 has a transparent insulating material portion 17a, a plurality of semiconductor portions 17x, 17x, ... dispersed in the transparent insulating material portion 17a, and metal nanoparticles 17b contained in the semiconductor portions 17x.
  • surface plasmon resonance occurs around the metal nanoparticles 17b, 17b, at a particular wavelength that depends on the complex refractive index of the metal nanoparticles 17b and the surrounding semiconductor portions 17x and transparent insulating material portion 17a, and the size of the metal nanoparticles 17b, 17b, ....
  • the wavelength converter 17 is able to function as an optical resonator (or optical filter).
  • the frequency at which electrons and holes created in the semiconductor portions 17x interact wirh electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converter 17, monochromatic light can be emitted through recombination of the electrons and holes generated in the semiconductor portions 17x.
  • examples of transparent insulating materials that can form the transparent insulating material portion 17a include known transparent insulating materials, such as Si0 2 and transparent resins.
  • examples of semiconductor materials that can form the semiconductor portions 17x include InGaAs, InAsP, Ge, and so forth.
  • the diameter of the semiconductor portions 17x may be set to be, for example, equal to or larger than 60nm and equal to or smaller than 250nm (may be larger than the diameter of the metal nanoparticles 17b by an amount equal to or larger than 50nm and equal to or smaller than 200nm).
  • the interval between the centers of adjacent ones of the semiconductor portions 17x, 17x is preferably controlled to be 1.2 times or more as large as the diameter of the semiconductor portions 17x, so that the adjacent semiconductor portions 17x, 17x do not contact each other, and is preferably controlled to be 5 times or less as large as the diameter of each semiconductor portion 17x, so as to make it easy to increase the proportion of the semiconductor portions 17x, 17x, ... in which electrons and holes are generated, in the wavelength converter 17.
  • Examples of metal materials that can form the metal nanoparticles 17b include Au, Ag, Al, Pt, arid so forth.
  • the diameter of the metal nanoparticles 17b is controlled to be equal to or larger than lOnm and equal to or smaller than 50nm, so that surface plasmon resonance can be produced.
  • the wavelength converter 17 is likely to generate monochromatic light having a longer wavelength as the refractive index of the semiconductor material is increased, and is likely to generate monochromatic light having a shorter wavelength as the refractive index of the semiconductor material is reduced.
  • the semiconductor portions 17x that contain the metal nanoparticles 17b may be produced by a known method of chemical Synthesis, such as a sol-gel method, or a solvothermal method.
  • the wavelength converter 17 can be fabricated by producing the transparent insulating material portion 17a by a known method, such as coating or printing, while dispersing the thus produced semiconductor portions 17x, 17x, ... in the transparent insulating material portion 17a.
  • the thickness of the transparent insulating layers is preferably controlled to be equal to or larger than 2nm, so that the transparent insulating layers can effectively prevent the carriers from being trapped by the metal nanoparticles or make it less likely to have the carriers trapped by the metal nanoparticles. Also, the thickness of the transparent insulating layers is preferably controlled to be equal to or smaller than lOnro, so that the effect of the surface plasmon resonance is likely to reach the semiconductor portions.
  • FIG. 11 is a cross-sectional view showing another example of wavelength converter 18.
  • the vertical direction on the paper of FIG 11 is the travelling direction of light.
  • the wavelength converter 18 has a transparent insulating material portion 18a, a plurality of metal particle portions 18b, 18b, ... dispersed in the transparent insulating material portion 18a, and semiconductor particles 18x contained in the metal particle portions 18b.
  • surface plasmon resonance occurs around the metal particle portions 18b, 18b, at a particular wavelength that depends on the complex refractive index of the metal particle portions 18b, 18b, the semiconductor particles 18x, 18x, and the transparent insulating material portion 18a, and the size of the metal particle portions 18b, 18b, ....
  • the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength, and the wavelength converter 18 is able to function as an optical resonator (or optical filter).
  • the wavelength converter 18 that functions as an optical resonator the frequency at which electrons and holes generated in the semiconductor particles 18x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converter 18, monochromatic light can be generated through recombination of the electrons and holes generated in the semiconductor particles 18x,,
  • examples of transparent insulating materials that can form the transparent insulating material portion 18a include known transparent insulating materials, such as Si0 2 and transparent resins.
  • examples of semiconductor materials that can form the semiconductor particles l8x include InGaAs, InAsP, Ge, and so forth. Since the diameter of the metal particle portions 18b that can cause surface plasmon resonance to occur is equal to or larger than lOnxn and equal to or smaller than 50nrn, the diameter of the semiconductor particles 18x may be controlled to be equal to or larger than 5nm and equal to or smaller than 45nm.
  • examples of metal materials that can form the metal particle portions 18b include Au, Ag, Al, Pt, and so forth.
  • the interval between the centers of adjacent ones of the metal particle portions 18b, 18b is preferably 1.2 times or more as large as the diameter of the metal particle portions 18b, so that the adjacent metal particle portions 18b, 18b do not contact each other, and is preferably 5 times or less as large as the diameter of the metal particle portions 18b, so that the proportion of the metal particle portions 18b, 18b, ... in the wavelength converter 18, can be readily increased.
  • the wavelength converter 18 is likely to emit monochromatic light having a longer wavelength as the refractive index of the semiconductor particles I8x is increased, and is likely to emit monochromatic light having a shorter wavelength as the refractive index of the semiconductor particles 18x is reduced.
  • the metal particle portions 18b that contain the semiconductor particles. 18x may be produced by a known method of chemical synthesis, such as a sol-gel method, or a solvothermal method.
  • the wavelength converter 18 can be produced by producing the transparent insulating material portion 18a by a known method, such as coating or printing, while dispersing the thus produced metal particle portions 18b, 18b, ... in the transparent insulating material portion 18a.
  • FIG. 12 is a cross-sectional view showing a solar cell 300 according to a third embodiment of the invention.
  • the same reference numerals as those used in FIG. 1A are used for identifying the same or corresponding constituent elements as those of the solar cell 100, and further explanation of these elements will not be provided.
  • the solar cell 300 has an n layer 51, an i layer 52, and a p layer 53, and the n layer 51, i layer 52, and p layer 53 form a pin junction 54.
  • the i layer 52 mainly functions as a photoelectric converter, and wavelength converting portions 55, 55, ... (which may be simply referred to as "wavelength converter 55" when appropriate) each having a semiconductor material are dispersed in the i layer 52.
  • wavelength converter 55 which may be simply referred to as "wavelength converter 55" when appropriate
  • the solar cell 300 is an up-con version type solar cell.
  • the wavelength converter 55 has a semiconductor material with energy gap Eg4, and has the function of emitting monochromatic light with energy Eg5.
  • the i layer 52 that functions as the photoelectric converter is formed of a semiconductor material with energy gap Eg6
  • the n layer 51 to which a front electrode 24 is connected is formed of an n-type semiconductor with the energy gap Eg6, and a p layer 53 to which the rear electrode 25 is connected is formed of a p type semiconductor with the energy gap Eg6.
  • solar light incident on the solar cell 300 passes through the n layer 51, and is incident on the semiconductor material (which may be referred to as "photoelectric converter 52" when appropriate) of the i layer 52 disposed around the wavelength converting portions 55.
  • the energy gap Eg6 of the photoelectric converter 52 is controlled so that the photoelectric converter 52 can absorb only high-energy photons, out of the solar light containing photons with various levels of energy. Therefore, when the solar light is incident on the photoelectric converter 52, the photoelectric converter 52 absorbs only the photons with energy equal to or greater than the energy gap Eg6 of the photoelectric converter 52. Once the photons are absorbed, electrons and holes are created in the photoelectric converter 52.
  • the thus created electrons and holes are separated by an internal electric field formed by the n layer 51 and the p layer 53, and the electrons move to the n layer 51 side, and are collected into the front electrode 24 connected to the n layer 51. Also, the holes move to the p layer 53 side, and are collected into the rear electrode 25 connected to the p layer 53.
  • the photoelectric converter 52 absorbs only the photons contained in the solar light and having energy equal to or greater than Eg6. Therefore, photons with energy less than Eg6, out of the photons contained in the solar light, pass through the photoelectric converter 52 without being used for photoelectric conversion, and reach the wavelength converter 55.
  • the energy gap Eg4 of the semiconductor material contained in the wavelength converter 55 is smaller than Eg6, and is controlled so that the wavelength converter 55 can absorb low-energy photons contained in the solar light. Therefore, when the light is incident on the semiconductor material contained in the wavelength converter 55, only the photons with energy equal to or greater than the energy gap Eg4 of the semiconductor material are absorbed.
  • the electrons generated in the wavele-Dgth converter 55 interact with each other and give and receive energy to and from each other, while the holes generated in the wavelength converter 55 interact with each other and give and receive energy to and from each other, so that pairs of electrons and holes with specific energy levels, which have an energy difference of Eg5, are generated. Then, the electrons and the holes recorabine to emit monochromatic light with energy Eg5.
  • electrons and boles capable of interacting with each other are limited to electrons and holes having discrete energy levels.
  • the wavelength converter 55 which has the semiconductor material, allows electrons with various levels of energy to interact with each other, and allows holes with various levels of energy to interact with each other, so as to emit monochromatic light having energy Eg5.
  • the monochromatic light thus emitted from the wavelength converter 55 travels toward the photoelectric converter 52.
  • the energy gap of the photoelectric converter 52 is Eg6.
  • Eg6 is smaller than Eg5 by about O. leV. Therefore, the monochromatic light with energy Eg5 emitted from the wavelength converter 55 can overcome the band-gap of the photoelectric converter 52, so as to be absorbed by the photoelectric converter 52 where electrons and holes are generated. The thus generated electrons and holes are separated by an internal electric field formed by the pin junction 54, almost without losing energy, since the difference between Eg5 and Eg6 is as small as about 0.1 eV. Then, the electrons move to the n layer 51 side, and are collected into the front electrode 24 connected to the n layer 51. Also, the holes move to the p layer 53 side, and are collected into the rear electrode 25 connected to the p layer 53.
  • the solar cell 300 constructed as described above, large-energy photons of solar light having larger energy than the energy gap of the photoelectric converter 52 are absorbed by the photoelectric converter 52 and converted into electric power.
  • the photoelectric converter 52 also receives monochromatic light emitted from the wavelength converter 55 using photons that passed through the photoelectric converter 52 without being converted into electric power, so as to convert the monochromatic light into electric power.
  • This arrangement makes it possible to broaden the band of light used for conversion into electric energy in the photoelectric converter 52.
  • the wavelength converter 55 is arranged to eventually recombine the electrons and the holes, but not intended to extract the created electrons and holes (carriers) to the outside as they are.
  • the wavelength converter 55 it is not necessary to move the carriers to the electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and it is thus possible to significantly reduce the amount of energy that would be lost during movement of the carriers. Furthermore, the use of the semiconductor material in the wavelength converter 55 makes it possible to significantly broaden the wavelength range of light that can be used for creating the carriers, as compared with up-conversion type solar cells of the related art using phosphors.
  • the energy of the monochromatic light incident on the photoelectric converter 52 is specified as Eg5, Therefore, energy loss can be reduced by using a semiconductor material with energy gap corresponding to Eg5, for the photoelectric converter 52.
  • the solar cell 300 according to the invention is able to enhance its photoelectric conversion efficiency.
  • the photoelectric conversion efficiency of the solar cell 300 can be easily enhanced by enhancing the efficiency of the wavelength converter 55 that converts light into monochromatic light.
  • the energy gap Eg4 of the semiconductor material contained in the wavelength converter 55 may be, for example, equal to or higher than 0.4eV and equal to or lower than l.leV, and the energy Eg5 of the monochromatic light emitted from the wavelength converter 55 may be, for example, equal to or higher than l,4eV, and equal to or lower than 3.3eV.
  • semiconductor materials that can be contained in the wavelength converting portions 55 include GalnAs, InAsP, GaSb, Ge, and so forth.
  • the wavelength converting portions 55 may be produced by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), or a known method of chemical synthesis, such as a sol-gel method, or a solvotherma] method.
  • the wavelength converting portions 55, 55, ... can be dispersed in the ⁇ layer 52, by repeating a process of producing a part of the photoelectric converter 52, dispersing the wavelength converting portions 55, 55, ... on a surface of the photoelectric converter 52, and producing another part of the photoelectric converter 52 on the surface of the photoelectric converter 52 on which the wavelength converting portions 55, 55, ... are dispersed.
  • the energy gap Eg6 of the n layer, photoelectric converter (i layer) 52, and the p layer 53 may be, for example, equal to or higher than 1.3eV and equal to or lower than 3.2e V.
  • semiconductor materials that can be used for forming the n layer 51, photoelectric converter 52, and p layer 53 include GalnP, AlGaAs, GaAs, and so forth.
  • the n layer 51 may be produced by adding a known n-type dopant to a selected one of the above-indicated semiconductor materials.
  • the p layer 53 may be produced by adding a known p-type dopant to a selected one of the semiconductor materials.
  • the thickness of the n layer 51 and the p layer 53 may be, for example, about lOOnm, and the thickness of the i layer 52 may be, for example, about 200nm.
  • the n layer 51, photoelectric converter 52, and the p layer 53 may be produced by a known method, such as vapor deposition or vapor-phase growth.
  • the photoelectric conversion device (up-conversion type photoelectric conversion device) according to the third embodiment of the invention is not limited to this configuration.
  • wavelength converting portions may be dispersed in the n layer and/or the p layer, in addition to the ⁇ layer.
  • wavelength converters which can be used in the photoelectric conversion device according to the third embodiment of the invention will be described.
  • FIG. 13 is a cross-sectional view showing one example of wavelength converter 56.
  • wavelength converting portions 56, 56, ... that constitute the wavelength converter 56 are used in place of the wavelength converting portions 55, 55, ... as shown in FIG 12.
  • each of the wavelength converting portions 56 has a metal nanoparticle 56a covered with a semiconductor portion 56x.
  • surface piasmon resonance occurs around the metal nanoparticle 56, at a particular wavelength that depends on the complex refractive index of the metal nanoparticle 56a and the surrounding semiconductor portion 56x, and the size of the metal nanoparticle 56a.
  • the wavelength converting portion 56 is able to function as an optical resonator (or optical filter).
  • the frequency at which electrons and holes created in the semiconductor portion 56x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converting portion 56, for example, monochromatic light can be emitted through recombination of the electrons and holes generated in the semiconductor portion 56x.
  • examples of semiconductor materials that can form the semiconductor portion 56x include InGaAs, InAsP, Ge, and so forth, and the diameter of the semiconductor portion 56x may be, for example, equal to or larger than 60nm and equal to or smaller than 250nm (or may be larger than the diameter of the metal nanoparticles 56a by an amount equal to or larger than 50nm and equal to or smaller than 200nm).
  • the interval between the centers of adjacent ones of the wavelength converting portions 56, 56 is preferably 1.2 times or more as large as the diameter of the wavelength converting portion 56, so that the adjacent wavelength converting portions 56, 55 do not contact each other, and is preferably 5 times or less as iarge as the diameter of the wavelength converting portion 56, so that the proportion of the wavelength converting portions 56, 56, ... in which electrons and holes are generated, in the photoelectric converter, can be readily increased.
  • Examples of metal materials that can form the metal nanoparticle 56a include Au, Ag, Al, Pt, and so forth.
  • the diameter of the metal nanoparticle 56a is controlled to be equal to or larger than lOnm and equal to or smaller than 50nm, so as to enable the metal nanoparticle 56a to produce surface plasmon resonance.
  • the wavelength converting portion 56 is likely to emit monochromatic light having a longer wavelength as the refractive index of the semiconductor portion 56x is increased, and is likely to generate monochromatic light having a shorter wavelength as the refractive index of the semiconductor portion 56x is reduced.
  • the wavelength converting portion 56 may be produced by successively growing the metal nanoparticle 56a and the semiconductor portion 56x, by a known method of chemical synthesis, such as a sol-gel method, or a solvothermal method.
  • a known method of chemical synthesis such as a sol-gel method, or a solvothermal method.
  • the thickness of the transparent insulating layer is preferably controlled to be equal to or larger than 2nm, so that the transparent insulating layer can effectively prevent carriers from being trapped by the metal nanoparticle or make it less likely to have the carriers trapped by the metal nanoparticle. Also, the thickness of the transparent insulating layer is preferably controlled to be equal to or smaller than lOnm, so that the effect of the surface plasmon resonance is likely to reach the semiconductor portion.
  • FIG. 14 is a cross-sectional view showing another example of wavelength converter 57.
  • wavelength converting portions 57, 57, ... that constitute the wavelength converter 57 are used in place of the wavelength converting portions 55, 55, ... as shown in FIG 12.
  • each of the wavelength converting portions 57 has a metal nanoparticle 57a containing a semiconductor portion 57x, and a transparent material layer 57b with which the metal nanoparticle 57a containing the semiconductor portion 57x is covered.
  • the wavelength converting portion 57 constructed as described above, surface plasmon resonance occurs around the metal nanoparticle, at a particular wavelength that depends on the complex refractive index of the metal nanoparticle 57a, a semiconductor material of the semiconductor portion 57x, and a transparent material of the transparent material layer 57b, and the size of the metal nanoparticle 57a.
  • the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength.
  • the wavelength converting portion 57 is able to function as an optical resonator (or optica! filter).
  • the frequency at which electrons and holes generated in the semiconductor portion S7x interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converting portion 57, for example, monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portion 57x.
  • the transparent material that forms the transparent material layer 57b may be selected from known transparent insulating materials, such as Si0 2 and transparent resins.
  • the thickness of the transparent material layer 57b is preferably controlled to be equal to or larger than 2nm and equal to or smaller than lOnm, so that the carriers created in the photoelectric converter are effectively prevented from being trapped by the metal nanoparticles 57a.
  • the semiconductor material that forms the semiconductor portion 57x may be selected from, for example, InGaAs, InAsP, Ge, and so forth.
  • the diameter of the metal nanoparticle 57a which enables the metal nanoparticle 57a to produce surface plasmon resonance is equal to or larger than lOnm and equal to or smaller than 50nm
  • the diameter of the semiconductor portion 57x may be controlled to be equal to or larger than 5nm and equal to or smaller than 45am.
  • metal materials that can form the metal nanoparticle 57a include Au, Ag, Al, Pt, and so forth.
  • the interval between the centers of adjacent ones of the wavelength converting portions 57, 57 is preferably 1.2 limes or more as large as the diameter of the wavelength converting portion 57, so that the adjacent wavelength converting portions 57, 57 do not contact each other, and is preferably 5 times or less as large as the diameter of the wavelength converting portion 57, so that the proportion of the wavelength converting portions 57, 57, ... in the photoelectric converter can be readily increased.
  • the wavelength converting portion 57 is liSely to generate monochromatic light having a longer wavelength as the size of the semiconductor portion 57x is increased, and is likely to emit monochromatic light having a shorter wavelength as the size of the semiconductor portion 57x is reduced.
  • the wavelength converting.portion 57 may be produced by successively growing the semiconductor portion 57x, metal nanoparticle 57a, and the transparent material layer 57b, by a known method of chemicaJ synthesis, such as a sol-gel method, or a solvoihe mal method.

Abstract

A photoelectric conversion device includes a wavelength converter that includes a semiconductor material that generates electrons and holes when absorbing light, and emits monochromatic light by recombining the electrons and the holes, and a photoelectric converter that generates electrons and holes when receiving' the monochromatic light emitted from the wavelength converter, and has a pin junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter.

Description

PHOTOELECTRIC CONVERSION DEVICE
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The invention relates to photoelectric conversion devices, and more particularly to photoelectric conversion devices using wavelength conversion mechanisms.
2. Description of Related Art
[0002] Solar cells have the advantages that the amount of emission of carbon dioxide (CO2) per unit of electricity produced is small, and that no fuel is required to generate electric power. Therefore, various types of solar cells have been actively studied. At present, among solar cells that are in practical use, single-junction solar cells each having one pn junction and using single-crystal silicon or multi-crystalline silicon are predominantly used. However, the theoretical limit of the photoelectric conversion efficiency (which will be called "theoretical limit efficiency") of the single-junction solar cell is still as small a£ about 30%; therefore, new methods for improving the photoelectric conversion efficiency have been studied.
[0003] As one of the new methods that have been studied so far, multi-junction solar cells using a combination of three different materials have been proposed, as disclosed in Japanese Patent Application Publication No. 2004-296658 (JP-A-2004-296658). The multi-junction solar cells are able to absorb photons of a wide range of wavelengths contained in solar light, and, therefore, provide a high conversion efficiency. To further improve the efficiency, solar cells using four to six materials have been studied. However, the increase in the number of junctions may result in increase of the number of semiconductor interfaces having a high defect density, which may cause recombination loss of caniers and reduction of the conversion efficiency. Also, many types of expensive II I- V compounds need to be used, and the number of manufacturing steps increases, resulting in increased cost, for example.
[0004] In the meantime, as a means for improving the conversion efficiency, hot-carrier type solar cells to which a photoelectric conversion theory (i.e., hot-carrier theory) different from conventional theories is applied have been proposed, as disclosed in Japanese Patent Application Publication No. 2009-59915 (JP-A-2009-59915). A solar cell of this type is able to absorb photons of a wide range of wavelengths contained in solar light, without increasing the number of junctions (the number of semiconductor materials), and convert the photons into electric power with small energy loss.
[0005] According to the technology disclosed in JP-A-2009-59915, in order to achieve a high conversion efficiency in a solar cell to which the hot-carrier theory is applied, it is necessary to promote energy interaction (exchange) between carriers, and move the carriers while keeping high energy thereof, in a light absorbing layer. While the hot carrier lifetime needs to be at least 1 ns (nanosecond), so as to achieve a high conversion efficiency with which the carriers moved through the light absorbing layer are extracted, the lifetime of existing semiconductor materials is still as short as several ps (picoseconds) to several hundreds of ps. Thus, undeT the present circumstances, the use of the technology as disclosed in JP-A-2009-59P25 is not sufficient to enhance the photoelectric conversion efficiency.
[0006] Other than the above-mentioned types of solar cells, a down-conversion type solaT cell thai converts short-wavelength light with high energy into a wavelength that matches the band-gap of the solar cell material, so as to reduce heat (voltage) loss, and an up-conversion type solar cell that converts long-wavelength light that has small energy and becomes light transmission loss, into a wavelength that matches the band-gap of the solar ceil material, have been developed. These solar cells use phosphors using (or doped with) rare-earth elements. However, if the known phosphors using rare-earth elements are used, the wavelength range of light that can be absorbed by the solar cells is narrow, and the energy loss that appears during wavelength conversion is large. Thus, it is difficult for the known down-conversion type solar cell and up-conveision type solar cell to achieve sufficiently enhanced photoelectric conversion efficiency.
SUMMARY OF THE INVENTION
[0007] The invention provides a photoelectric conversion device that achieves enhanced photoelectric conversion efficiency.
[0008] A photoelectric conversion device according to a first aspect of the invention includes a wavelength converter that includes a semiconductor material that creates electrons and holes when absorbing light, and emits monochromatic light by recombining the electrons and the holes, and a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, and the wavelength converter is disposed upstream of the photoelectric converter as viewed in a travelling direction of the light. In the present invention, the wavelength converter incorporates a hot-carrier mechanism into an optical filter or a photonic crystal, or provides the hot-carrier mechanism with a wavelength selection mechanism, such as surface plasmon resonance of metal nanoparticles, so as to improve the efficiency with which monochromatic light is converted from solar light, while providing the wavelength converter with the function of adjusting the wavelength of the light as desired.
[0009] In the present invention as described above, "light" means multicolor light, such as solar light. The "ypsirearo ... as viewed in a travelling direction of the light" means upstream ... as viewed in a travelling direction of the multicolor light. Namely, "the wavelength converter is disposed upstream of the photoelectric converter as viewed in a travelling direction of the light" means that the wavelength converter and the photoelectric converter are positioned so that the light (multicolor light) that has not reached the photoelectric converter is incident on the wavelength converter, and monochromatic light emitted from the wavelength converter is incident on the photoelectric converter. More specifically, the wavelength converter is placed on the upstream side as viewed in the traveling direction of the light (multicolor light), and the photoelectric converter is disposed downstream of (he wavelength converter as viewed in the travelling direction of the light (multicolor light). Also, in the present invention, the concept of the "photoelectric conversion device" covers light-detecting elements and others, as well as solar cells. In the following description, multicolor light may be referred simply as "light".
[0010] In the photoelectric conversion device according to the first aspect of the invention, the wavelength converter disposed upstream of the photoelectric converter as viewed in the travelling direction of the light generates electrons and holes (which will be collectively called "carriers" when appropriate) when it receives light, and causes the electrons and holes to recombine and emit monochromatic light, which is then incident on the photoelectric converter. Since the wavelength converter having the function of an optical resonator is arranged to emit monochromatic light, the electrons and holes having high energy through excitation can i>e recombined before losing energy, and otherwise possible energy loss can be reduced. Also, the wavelength converter having the function of the optical resonator is arranged to eventually recombine the electrons and holes, but is not intended to extract the created carriers to the outside as they are. Therefore, there is no need to move the carriers to electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and the amount of energy that would be lost during movement of the earners can be significantly reduced. Furthermore, the use of the semiconductor material in the wavelength converter makes it possible to greatly broaden the wavelength range of light that can be used for generating carriers, as compared with down-conversion type solar cells of the related art using phosphors. Also, the wavelength of the monochromatic light can be adjusted as desired by adjusting (he shape or dimensions of the wavelength converter and suitabl selecting the refractive indices of materials used in the wavelength converter. In addition, the monochromatic lighf is incident on the photoelectric converter, so that the energy of the moriochromatic light incident on the photoelectric converter is specified. Therefore, energy loss can be reduced if a semiconductor material having energy gap corresponding to the energy of the monochromatic light incident on the photoelectric converter (more specifically, energy gap that is equal to or smaller by about O.leV than the energy of the monochromatic light) is used for the photoelectric converter. Accordingly, the photoelectric conversion device according to the first aspect of the invention achieves enhanced photoelectric conversion efficiency. According to the first aspect of the invention, the photoelectric conversion efficiency can be easily improved by improving the efficiency of the wavelength converter for converging light into monochromatic light.
[0011] A photoelectric conversion device according to a second aspect of the invention includes a wavelength converter that includes a semiconductor material that creates electrons and holes when absorbing light, and emits monochromatic light by recombining the electrons and the holes, and a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, and the wavelength converter is disposed downstream of the photoelectric converter as viewed in a travelling direction of the light.
[0012] Here, the "downstream ... as viewed in a travefiing direction of the light" means downstream ... as viewed in a travelling direction of the multicolor light. Naraeiy, "the wavelength converter is disposed downstream of the photoelectric converter as viewed in a travelling direction of the light" means that the wavelength converter and the photoelectric converter are positioned so that the light is incident on the photoelectric converter, and the light that has passed through the photoelectric converter is incident on the wavelength converter, which, in turn, emits monochromatic light using the incident light, such that the monochromatic light is incident on the photoelectric converter. More specifically, the photoelectric converter is placed on the upstream side as viewed in the traveling direction of the light, and the wavelength converter is disposed downstream of the photoelectric converter as viewed in the travelling direction of the light.
[0013] In the photoelectric conversion device according to the second aspect of the invention, the wavelength converter generates electrons and holes using light that has passed through the photoelectric converter disposed upstream of the wavelength converter as viewed in the travelling direction of the light, and recombines the electrons and holes thus created. Namely, according to the second aspect of the invention, photons with larger energy than the energy gap of the semiconductor material included in the photoelectric converter are absorbed by the p/ioroe/ecfric converter and converted into electric power, and photons that were not converted into electric power in the photoelectric converter are incident on the wavelength converter. The wavelength converter, which uses a semiconductor material, absorbs photons with larger energy than the energy gap of the semiconductor material, and generates carriers (electrons and holes). In this connection, the energy gap of the semiconductor material included in the wavelength converter is smaller than the energy gap of the semiconductor material included in the photoelectric converter. Then, the electrons and holes generated in the wavelength converter interact with the electrons and holes, respectively, generated therein, so as to emit monochromatic light with larger energy than the energy gap of the semiconductor material that forms the photoelectric converter, and the monochromatic light is incident on the photoelectric converter where electric energy is extracted from the incident light, With this arrangement, the wavelength range of the light used for conversion into electric energy in the photoelectric converter can be broadened. Also, the wavelength converter is arranged to eventually recombine electrons and holes, but is not intended to extract the generated electrons and holes to the outside as they are. Therefore, there is no need to move the carriers to electrodes, as in hot-carrier type solar cells of the related art using (he quantum structure, and the amount of energy that would be lost during movement of the carriers can be significantly reduced. Furthermore, the use of the semiconductor material in the wavelength converter makes it possible to greatly broaden (he wavelength range of light that can be used for generating carriers, as compared with up-conversion type solar cells of the related art using phosphors. Also, the wavelength of the' monochromatic light can be adjusted as desired by adjusting the shape or dimensions of the wavelength converter and suitably selecting the refractive indices of materials used in the wavelength converter. In addition, energy loss can be reduced if a semiconductor material with energy gap corresponding to the energy of the monochromatic light incident on the photoelectric converter (more specifically, energy gap that is equal to or smaller by about O.leV than the energy of the monochromatic light) is used for the photoelectric converter. Accordingly, the photoelectric conversion device according to the second aspect of the invention achieves enhanced photoelectric conversion efficiency. According to the second aspect of the invention, the photoelectric conversion efficiency can be easily improved by improving the efficiency of the wavelength converter for converging light into monochromatic light.
[0014] In the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter and the photoelectric converter may be in contact with each other, and an interface between the wavelength converter and the photoelectric converter may have projections, recesses, or a combination of projections and recesses. In another form of the invention, the wavelength converter and the photoelectric converter may be spaced apart from each other, and at least one surface of the wavelength converter which faces the photoelectric converter may have projections, recesses, or a combination of projections and recesses. With the projections and/or recesses thus formed, the proportion of monochromatic light that reaches the photoelectric converter, in the monochromatic Light emitted from the wavelength converter, is likely to be increased; therefore, the photoelectric conversion efficiency can be easily enhanced.
[0015] The above-indicated "protrusions and recesses" mean protrusions and recesses whose height and depth are larger than the wavelength of the monochromatic light emitted from the wavelength converter.
[0016] In the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter may have a semiconductor layer formed of the semiconductor material, and a plurality of transparent material layers having different refractive indices and laminated on each of opposite sides of the semiconductor layer. With this arrangement, the plurality of transparent material layers having different refractive indices and laminated on the opposite sides of the semiconductor layer, and the semiconductor layer sandwiched between the transparent material layers cooperate to function as an optical resonator (or optical filter), so that the incident light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
[0017] In the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter may further include a different-refractive-index materia} having a refractive index that is different from that of the semiconductor material, and the wavelength converter may have at least one different-refractive-index portion that is formed of the different-refractive-index material and extends through the semiconductor material in a direction from the wavelength converter toward the photoelectric converter. With this arrangement, the semiconductor material in which the different-refractive-index portion(s) is/are provided is able to function as an optical resonator (or photonic crystal), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
[0018] In the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter may further include a different-refractive-index material having a refractive index that is different from thai of the semiconductor material, and the wavelength converter may have a plurality of different-refractive-index portions that are formed of the different-refractive-index material and are three-dimensionaUy dispersed in the semiconductor material. With this anangement, the semiconductor material in which the different-refractive-index portions are three-dimensionally dispersed is able to function as an optical resonator (or photonic crystal), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
[0019] In the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter may further include metal nanoparticles dispersed in the semiconductor material. This arrangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength. Thus, the wavelength converter is able to function as an optical resonator (optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
[0020] In the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter may have a plurality of semiconductor portions formed of the semiconductor material, and each of the semiconductor portions is interposed between a pair of metal sheets. In this arrangement, the metal sheets between which the semiconductor portion is interposed are able to function as a photoconductive antenna. Therefore, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength. Thus, the wavelength converter is able to function as an optical resonator (or optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
[0021] In the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter may have, a plurality of semiconductor portions formed of the semiconductor material, a plurality of metal nanoparticles contained in the semiconductor portions, and a light-permeable insulating material in which the semiconductor portions are dispersed. This arrangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the wavelength converter is able to function as an optical resonator (or optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus copstrucled wavelength converter achieves enhanced photoelectric conversion efficiency,
[0022] Γη the photoelectric conversion device according to the first or second aspect of the invention, the wavelength converter may have a plurality of semiconductor particles formed of the semiconductor material, a plurality of metal particles in which the semiconductor particles are contained, and a light-permeable insulating material in which the metal particles are dispersed. This arrangement makes it possible to cause surface plasmon resonance to occur around the metal particles. Therefore, the wavelength converter is able to function as an optical resonator (or optical filter), so that the light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
[0023] A photoelectric conversion device according to a third aspect of the invention includes a wavelength converter that includes a semiconductor material that generates electrons and holes when absorbing light, and emits monochromatic light by recorobining the electrons and the holes, and a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, and the wavelength converter consists of a plurality of wavelength converting portions dispersed in the photoelectric converter.
[0024] In the photoelectric conversion device according to the third aspect of the invention, electrons and holes are generated in the wavelength converting portions dispersed in the photoelectric converter, using light that has passed through the photoelectric converter. Then, the electrons and holes are recombined in the wavelength converting portions, so that monochromatic light having larger energy than the energy gap of the semiconductor material that forms the photoelectric converter is emitted, and the monochromatic light is incident on the photoelectric converter where electric energy is extracted. With this arrangement, the wavelength range of the light used for conversion into electric energy in the photoelectric converter can be broadened, as in the photoelectric conversion device according to the second aspect of the invention. Also, the wavelength converting portions are arranged to eventually recombine electrons and holes, but not intended to extract the generated electrons and holes to the outside as they are. Therefore, there is no need to move the carriers to electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and the amount of energy that would be lost during movement of the carriers can be significantly reduced. Furthermore, the use of the semiconductor material in the wavelength converting portions makes it possible to greatly broaden the wavelength range of light that can be used for creating carriers, as compared with up-conversion type solar cells of the related art using phosphors. Also, the wavelength of the monochromatic light can be adjusted as desired by adjusting the shape or dimensions of the wavelength converting portions and suitably selecting the refractive indices of materials used in the wavelength converter. In addition, energy Joss can be reduced if a semiconductor material with energy gap corresponding to the energy of the monochromatic light incident on the photoelectric converter (more specifically, energy gap that is equal to or smaller by about O.leV than the energy of the monochromatic light) is used for the photoelectric converter. Accordingly, the photoelectric conversion device according to the third aspect of the invention achieves enhanced photoelectric conversion efficiency. According to the third aspect of the invention, too, the photoelectric conversion efficiency can be easily improved by improving the efficiency with which the wavelength converting portions convert light into monochromatic light.
[0025] In the photoelectric conversion device according to the third aspect of the invention, each of the wavelength converting portions may have a semiconductor portion formed of the semiconductor material, and a metal nanoparticie contained in the semiconductor portion, and the semiconductor portions of the wavelength converting portions may be dispensed in the photoelectric converter. This arrangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the wavelength converting portions are able to function as an optical resonator (or optical filter), so that the incident light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength convener achieves enhanced photoelectric conversion efficiency.
[0026] In the photoelectric conversion device according to the third aspect of the invention, each of the wavelength converting portions may have a semiconductor portion formed of the semiconductor material, a metal nanoparticle in which the semiconductor portion is contained, and a light-permeable insulting portion in which the metal nanoparticle is contained, and the light-permeable insulating portions of the wavelength converting portions may be dispersed in the photoelectric converter. This anangement makes it possible to cause surface plasmon resonance to occur around the metal nanoparticles. Therefore, the wavelength converting portions are able to function as an optical resonator (or optical filter), so that the incident light can be converted into monochromatic light. Accordingly, the photoelectric conversion device including the thus constructed wavelength converter achieves enhanced photoelectric conversion efficiency.
[0027] In the photoelectric conversion device according to the present invention, it is possible to reduce energy loss of the carriers generated therein, while broadening the frequency band of light that can be converted into electric energy. Accordingly, the photoelectric conversion device of the invention achieves enhanced photoelectric conversion efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
FIG. 1A is a cross-sectional view sfiowing a solar cell according to a first embodiment of the invention;
FIG IB is a view illustrating the energy band structure of the solar cell of FIG. 1A; FIG. 2A is a cross-sectional view showing a solar cell according lo a second embodiment of the invention; FIG. 2B is a view illustrating the energy band structure of the solar cell of FIG 2A;
FIG 3A is a cross-sectional view showing a solar cell as a modified example of the first embodiment of the invention;
FIG 3B is a cross-sectional view showing a solar cell as another modified example of the first embodiment of the invention;
FIG 3C is a cross-sectional view showing a solai cell as a modified example of the second embodiment of the invention;
FIG 3D is a cross-sectional view showing a solar cell as another modified example of the second embodiment of the invention;
FIG 4 is a cross-sectional view showing one example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
FIG 5A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
FIG 5B is a cross-sectional view of the. wavelength converter taken along VB-VB in FIG 5A;
FIG 6A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
FIG. 6B is a cross-sectional view of the wavelength converter taken along VIB-VIB in FIG. 6A;
FIG. 7A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
FIG 7B is a cross-sectional view of the wavelength converter taken along VIIB-VIIB in FIG. 7A;
FIG 8 is a cross-sectional view showing another example of wavelength converter that can be used in the solar ceil of the first or second embodiment of the invention;
FIG 9A is a top view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
FIG 9B is a cross-sectional view of the wavelength ccmv &r taken along 1XB-1XB in FIG. 9A; FIG. 10 is a view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
FIG. 11 is a view showing another example of wavelength converter that can be used in the solar cell of the first or second embodiment of the invention;
FIG. 12 is a cross-sectional view showing a solar cell according to a third embodiment of the invention;
FIG 13 is a cross-sectional view showing one example of wavelength converting portion that can be used in the solar cell of the third embodiment of the invention; and
FIG. 14 is a cross-sectional view showing another example of wavelength converting portion that can be used in the solar cell of the third embodiment of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0029] Solar cells according to some embodiments of the invention will be described with reference to the drawings. It is to be understood that the embodiments as described below are illustrative only, and the invention is not limited to these embodiments. In the drawings, reference numerals are not assigned to some illustrations.
[0030] FIG. 1A is a cross-sectional view showing a solar cell 100 according fo a first embodiment of the invention, and FIG, IB illustrates the energy band structure of the solar cell 100. In FIG IB, the electron energy is greater toward the top of the paper, and the hole energy is greater toward the bottom of the paper. Also, in FIG IB, "·" denotes electrons, and "o" denotes holes. Egl denotes the energy gap of a semiconductor material contained in a wavelength converter 10 which will be described later, and Eg2 denotes the energy of monochromaiic light emitted by the wavelength converter 10, while Eg3 denotes the energy gap of a semiconductor material contained in a photoelectric converter 20 which will be described later. In FIG 1A and FIG. IB, solar light travels from the left-hand side of the paper to the right-hand side.
[0031] As shown in FIG. 1A and FIG. B, the solar cell 100 includes the wavelength converter 10 having a semiconductor material, and the photoelectric converter 20 having a semiconductor material. The wavelength converter 10 is disposed upstream of the photoelectric converter 20 as viewed in a direction in which soiar fight travels (which will be called "travelling direction of solar light"). Namely, the solar cell 100 is a down-conversion type solar cell. The semiconductor material contained in the wavelength converter 10 has the energy gap denoted as Egl, and the wavelength converter 10 has the function of generating monochromatic light having energy denoted as Eg2. The semiconductor material of the photoelectric convener 20 has the energy gap denoted as Eg3. The photoelectric converter 20 has an n layer 21 formed of an n-type semiconductor having the energy gap Eg3. and a p layer 22 formed of a p-type semiconductor having the energy gap Eg3. The n layer 21 and p layer 22 are joined to each other, to form a pn junction 23. A front electrode 24 is connected to the n layer 21, and a rear electrode 25 is connected to the p Ja er 22,
[0032] In operation, solar light with which the solar cell 100 is irradiated is incident on the wavelength converter 10. The solar light contains photons with various levels of energy. When the solar light is incident on the semiconductor material contained in the wavelength converter 10, only the photons with energy equal to or greater than the energy gap Egl of the semiconductor material are absorbed. Once the photons are absorbed, electrons having various levels of energy are excited from the valence band of the semiconductor material of the wavelength converter 10 to the conduction band, and holes having various levels of energy are formed in the valence band. Namely, when light is incident on the semiconductor material of the wavelength converter 10, an electron energy distribution as shown in FIG IB is formed in the conduction band of the semiconductor material, and a hole energy distribution as shown in FIG. IB is formed in the valence band of the semiconductor material.
[0033] As shown in FIG. IB, the shape of the wavelength converter 10 and the r'efradive indices of materials used in the converter 10 are suitably selected or adjusted, so that pairs of electrons and holes with specific energy levels, which have an energy difference of Eg2, recombine so as to generate monochromatic light with energy Eg2. At the same time, electrons in the electron distribution, which have different levels of energy from the specific energy level that contributes to generation of light, give and receive energy to and from each other, so that a part of the electrons are provided with the specific energy level that contributes to light generation. Similarly, holes in the hole distribution, which have different levels of energy from the specific energy level that contributes to generation of light, give and receive energy to and from each other, so that a part of the holes are provided with the specific energy level that contributes to light emission. Then, the electrons with the specific energy level that contributes to light emission in the electron distribution, and the* holes having the specific energy level that contributes to light emission in the hole distribution recombine so as to generate monochromatic light with energy Eg2. In down-conversion type solar cells of the related art using phosphors, electrons and holes capable of interacting with each other are limited to electrons and holes having discrete energy levels. On the other hand, the wavelength converter 10, which has the semiconductor material, allows electrons having various levels of energy to interact with each other and allows holes having various levels of energy with each other, so as to create electrons having the specific energy level that contributes to light generation, in the electron distribution, and holes having the specific energy level that contributes to light generation, in the hole distribution. The monochromatic light emitted in the waveleagth converter 10 in this manner travels toward the photoelectric converter 20.
[0034] The photoelectric converter 20 has the n layer 21 and p layer 22 with the energy gap Eg3. In this embodiment, Eg3 is smaller than Eg2 by about O.leV. Therefore, the monochromatic light with energy Eg2 emitted from the wavelength converter 10 can overcome the energy gap of the n layer 21 and p layer 22, so as to be absorbed by the photoelectric converter 20 where electrons and holes are generated. The thus generated electrons and holes are separated by an internal electric field formed by the pn junction 23, almost without losing energy, since the difference between Eg2 and Eg3 is as small as about O.leV. Then, the electrons move to the n layer 21 side, and are collected into the front electrode 24 connected to the n layer 21, while the holes move to the p layer 22 side, and are collected into the rear electrode 25 connected to the p layer 22.
[0035] In the solar cell 100 in which the wavelength converter 10 having the function of an optical resonator emits monochromatic light, the electrons and holes excited to high levels of energy can be recombined before losing energy, resulting in reduction of energy loss. Also, the wavelength converter 10 is arranged to eventually recombine the electrons and the holes, but not intended to extract the electrons and holes (or carriers) generated therein to the outside as they are. Therefore, in the wavelength converter 10, it is not necessary to move the carriers to electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and it is thus possible to significantly reduce the amount of energy that would be lost during movement of the carriers. Furthermore, the use of the semiconductor material in the wavelength converter 10 makes it possible to significantly broaden the wavelength range of light that can be used for creating the carriers, as compared with the down-conversion type solar cells of the related art using phosphors. In addition, in the solar cell 100 in which the monochromatic light emitted from the wavelength converter 10 is incident on the photoelectric converter 20, the energy of the monochromatic light incident on the photoelectric converter 20 is specified as Eg2. Therefore, energy loss can be reduced by using a semiconductor material having a energy gap corresponding to Eg2, for the photoelectric converter 20. Thus, the solar cell 100 according to the invention is able to enhance its photoelectric conversion efficiency. The photoelectric conversion efficiency of the solar eel] 200 can be easily enhanced by enhancing the efficiency of the wavelength converter 10 that converts light into monochromatic light.
[0036] In the solar cell 100, the energy gap Egl of the semiconductor material contained in the wavelength converter 10 may be, for example, equal to or larger thaa 0.4eV and equal to or smaller than 1.2eV, and the energy Eg2 of the monochromatic light emitted from the wavelength converter 10 may be, for example, equal to or larger than 0.6eV, and equal to or smaller than 1.4eV. The semiconductor material that can be contained in the wavelength converter 10 may be selected from, for example, GalnAs, InAsP, GaSb, Ge, etc. The wavelength converter 10 may be produced by a known method, such as vapor-phase growth or vapor deposition.
[0037J The energy gap Eg3 of the semiconductor material contained in the photoelectric converter 20 may be, for example, equal to or larger than 0.5eV and equal to or smaller than 1.3eV. The semiconductor material that can be contained in the photoelectric converter 20 may be selected from, for example, GalnAs, InAsP, GaSb, Ge, Si, etc. In the photoelectric converter 20, the υ layer 21 may be produced by adding a known n-type dopant to a selected one of the above-indicated semiconductor materials. The p layer 22 may be produced by adding a known p-type dopant to a selected one of the semiconductor materials. The thickness of the n layer 21 may be, for example, about lOOnm, and the thickness of the p layef 22 may be, for example, about 2μηι. The photoelectric converter 20 may be produced by a known method, such as vapor-phase growth or vapor deposition, and the front electrode 24 and the rear electrode 25 may be produced by a known method, such as a deposition or evaporation method. A known material, such as Au, which can be used for electrodes of solar cells, may be suitably used for forming the front electrode 24, and a known material, such as aluminum or indium tin oxide (ITO), which can be used for electrodes of solar cells, may be suitably used for forming the rear electrode 25. The thickness of the front electrode 24 and the rear electrode 25 may be, for example, about Ιμιη.
[0038] While the solar cell 100 as illustrated above includes the photoelectric converter 20 having the pn junction, the photoelectric conversion device (down-conversion type photoelectric conversion device) according to the first embodiment of the invention is not limited to the illustrated configuration. Rather, the photoelectric converter included in the photoelectric conversion device (down-conversion type photoelectric conversion device) according to the first embodiment of the invention may have a pin junction.
[0039] FIG. 2A is a cross-sectional viw showing a solar cell 200 according to a second embodiment of the invention, and FIG. 2B illustrates the energy band structure of the solar cell 200. In FIG. 2B, the electron energy is greater toward the top of the paper, and the hole energy is greater toward the bottom of the paper. Also, in FIG 2B, "·" denotes electrons, and "o" denotes holes. In FIG. 2B, Eg4 denotes the energy gap of a semiconductor material contained in a wavelength converter 30 which will be described later, and Eg5 denotes the energy of monochromatic light emitted from the wavelength converter 30, while Eg6 denotes the energy gap of a semiconductor material contained in a photoelectric converter 40 which will be described later. In FIG 2A and FIG. 2B, solar light travels from the left-hand side of the paper to the right-hand side. In FIG. 2A, the same reference numerals as those used in FIG 1A are assigned to the same or corresponding elements as those of the solar cell 100, and further explanation of the elements will not be provided when appropriate.
[0040] As shown in FIG 2A and FIG. 2B, the solar light 200 includes the wavelength converter 30 having a semiconductor material, and the photoelectric converter 40 having a semiconductor materia). The wavelength converter 30 is disposed downstream of the photoelectric converter 40 as viewed in the travelling direction of solar light. Namely, the solar cell 200 is an up-conversion type solar cell. The semiconductor material contained in the wavelength converter 30 has the energy gap denoted as Eg4, and the wavelength converter 30 has the function of emitting monochromatic light with energy Eg5. The semiconductor material contained in the photoelectric converter 40 has the energy gap denoted as Eg6. The photoelectric converter 40 has an n layer 41 formed of an n-type semiconductor having the energy gap Eg6, and a p layer 42 formed of a p-type semiconductor having the energy gap Eg6. The n layer 41 and p layer 42 are joined to each other, to form a pn junction 43. A front electrode 24 is connected to the n layer 41, and a rear electrode 44 is connected to the p layer 42.
[0041] In operation, solar light with which the solar cell 200 is irradiated is incident on the photoelectric converter 40. The energy gap Eg6 of the semiconductor material contained in the photoelectric converter 40 is controlled so that the photoelectric converter 40 can absorb only high-energy photons, out of the solar light including photons with various levels of energy. Therefore, when the solar light is incident on the semiconductor material contained in the photoelectric converter 40, only the photons having energy equal (o or greater than the energy gap Eg6 of the semiconductor material are absorbed. Once the photons are absorbed, electrons and holes are generated in the photoelectric converter 40. The thus generated electrons and holes are separated by an internal electric field formed by the n layer 41 and the p layer 42, and the electrons move to the n layer 41 side, to be collected into the front electrode 24 connected into the n layer 41. Also, the holes move to the p layer 42 side, to be collected into the rear electrode 44 connected to the p layer 42.
[00421 As described above, the photoelectric converter 40 absorbs only the photons contained in the solar light and having energy equal to or greater than Eg6. Therefore, the photons contained in the solar light and having energy less than Eg6 pass through the photoelectric converter 40, without being used for photoelectric conversion. The photons that have passed through the photoelectric converter 40 are incident on the wavelength converter 30 disposed downstream of the photoelectric converter 40 as viewed in the travelling direction of the solar light. The energy gap Eg4 of the semiconductor material contained in the wavelength converter 30 is smaller than Eg6, and is controlled so that the wavelength converter 30 can absorb low-energy photons contained in the solar light. Therefore, when light is incident on the semiconductor material contained in the wavelength converter 30, only the photons with energy equal to or greater than the energy gap Eg4 of the semiconductor material are absorbed. Once the photons are absorbed, electrons with various levels of energy are excited from the valence band to the conduction band, and holes with various levels of energy are formed in the valence band. Namely, when light is incident on the semiconductor material of the wavelength converter 30, an electron energy distribution as shown in FIG 2B is formed in the conduction band of the semiconductor material, and a hole energy distribution as shown in FIG. 2B is formed in the valence band of the semiconductor material.
[0043] The electrons generated in the wavelength converter 30 interact with each other for exchange of energy, while the holes generated in the wavelength converter 30 interact with each other for exchange of energy, so that electrons and holes having an energy difference of Eg5 are generated. Then, the electrons and the holes recombine to generate monochromatic light with energy Eg5. In up-conversion type solar cells of the related art using phosphors, electrons and holes capable of interacting with each other are limited to electrons and holes having discrete energy levels. On the other hand, the wavelength converter 30, which has the semiconductor material, allows electrons having various levels of energy to interact with each other, and allows holes having various levels of energy to interact with each other, so as to emit monochromatic light with energy Eg5, The monochromatic light emitted from the wavelength converter 30 travels toward the photoelectric converter 40.
[0044] The photoelectric converter 40 has the n layer 41 and p layer 42 having the energy gap Eg6. In this embodiment, Eg6 is smaller than Eg5 by about O.leV. Therefore, the monochromatic light with energy Eg5 emitted from the wavelength converter 30 can overcome the band-gap of the n layer 41 and the p layer 42, so as to be absorbed by the photoelectric converter 40, where electrons and holes are generated. The thus generated electrons and holes are separated by an internal electric field formed by the pn junction 43, almost without losing energy, since the difference between Eg5 and Eg6 is as small as about O.leV. Then, the electrons move to the n layer 41 side, and are collected into the front electrode 24 connected to the n layer 41, while the holes move to the p layer 42 side, and are collected into the rear electrode 44 connected to the p layer 42.
[0045] With the solar cell 200 constructed as described above, large-energy photons of the solar light having larger energy than the energy gap of the semiconductor material contained in the photoelectric converter 40 are absorbed by the photoelectric converter 40 to be converted into electric power. The photoelectric converter 40 also receives monochromatic light emitted from the wavelength converter 30 using photons thai passed through the photoelectric converter 40 without being converted into electric power, so as to convert the monochromatic light into electric power. This arrangement makes it possible to broaden the band of light used for conversion into electric energy in the photoelectric converter 40. Also, the wavelength converter 30 is arranged to eventually recombinc the electrons and the holes, but not intended to extract the electrons and holes (carriers) generated therein to the outside as they are. Therefore, in the wavelength converter 30, it is not necessary to move the carriers to the electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and it is thus possible to significantly reduce the amount of energy that would be lost during movement of the carriers. Furthermore, the use of the semiconductor material in the wavelength converter 30 makes it possible to significantly broaden the wavelength range of light that can be used for creating the carriers, as compared with the up-conversion type solar cells of the related art using phosphors. In addition, in the solar cell 200 in which the monochromatic light emitted from the wavelength converter 30 is incident on the photoelectric converter 40, the energy of the monochromatic light incident on the photoelectric converter 40 is specified as Eg5. Therefore, energy loss can be reduced by using a semiconductor material having a energy gap corresponding to Eg5, for the photoelectric converter 40. Thus, the solar cell 200 according to the invention is able to enhance its photoelectric conversion efficiency. The photoelectric conversion efficiency of the solar cell 200 can be easily enhanced by enhancing the efficiency of the wavelength converter 30 that converts light i¾to monochromatic light.
[0046] In the solar cell 200, the energy gap Eg4 of the semiconductor material contained in the wavelength converter 30 may be, for example, equal to or larger than 0.4eV and equal to or smaller than l. leV, and the energy Eg5 of the monochromatic light emitted from the wavelength converter 30 may be, for example, equal to or larger than 1.4eV and equal to or smaller than 3.3eV. The semiconductor material that can be contained in the wavelength converter 30 may be selected from, for example, GalnAs, IriAsP, GaSb, Ge, etc. The wavelength converter 30 may be produced by a known method, such as vapor-phase growth or vapor deposition.
[0047] The energy gap Eg6 of the semiconductor material contained in the photoelectric converter 40 may be, for example, equal to or larger than 1.3eV and equal to or smaller than 3.2eV. The semiconductor material that can be contained in the photoelectric converter 40 is selected from; for example, GalnP, AlGaAs, GaAs, etc. In the photoelectric converter 40, the n layer 41 may be produced by adding a known n-type dopant to a selected one of the above-indicated semiconductor materials. The p layer 42 may be produced by adding a known p-type dopant to a selected one of (he semiconductor materials. The thickness of the n layer 41 may be, for example, about lOOnm, and the thickness of the p layer 42 may be, for example, about 2μηι. The photoelectric converter 40 may be produced by a known method, such as vapor-phase growth or vapor deposition. A known material, such as aluminum or indium tin oxide (1TO), which can be used for electrodes of solar cells, may be suitably used for forming the rear electrode 44. The thickness of the rear electrode 44 may be, for example, about Ιμτη.
[0048] While the solar cell 200 as illustrated above includes the photoelectric converter 40 having ihe pn junction, the photoelectric conversion device (up-conversion type photoelectric conversion device) according to the second embodiment of the invention is not limited to the illustrated configuration. Rather, the photoelectric converter included in the photoelectric conversion device (up-conversion type photoelectric conversion device) according to the second embodiment of the invention may have a pin junction.
[0049] While the wavelength converter is disposed only on one side of the photoelectric converter in the solar cells 100, 200 as described above-, a pair of wavelength converters may be provided such that the photoelectric converter is interposed between the wavelength converters, in the photoelectric conversion device of the invention.
[0050] While the solar cell 100 in which the wavelength converter 10 is placed apart from the photoelectric converter 20, and the solar cell 200 in which the wavelength converter 30 is placed apart from the photoelectric converter 40 are illustrated in FIG. 1A and FIG 2A, respectively, the photoelectric conversion devices according to the first and second embodiments of the invention are not limited to these configurations. In the photoelectric conversion devices according to the first embodiment and the second embodiment, the wavelength converter and the photoelectric converter may be placed in contact with each other. If the wavelength converter and the photoelectric converter are placed apart from each other (i.e., are not in contact with each other), a substance that permits light to pass therethrough may be placed between the wavelength converter and the photoelectric converter, and the substance may be selected from, for example, air, a transparent resin film, glass, and so forth. If the wavelength converter and the photoelectric converter are placed apart from each other, the wavelength converter is fixed by a fixing means (not shown). As the fixing means, a known fixing means capable of fixing the wavelength converter in position may be used as appropriate.
f0051] While the solar cell 100 in which one surface of the wavelength converter 10 which faces the photoelectric converter 20 is a smooth surface, and the solar cell 200 in which one surface of the wavelength converter 30 which faces the photoelectric converter 40 is a smooth surface are illustrated in FIG. 1A and FIG. 2A, respectively, the photoelectric conversion devices according to the first and second embodiments of the invention are not limited to the illustrated configurations. In the photoelectric conversion devices according to the first and second embodiments of the invention, projections and/or recesses are preferably formed at at least one surface of the wavelength converter which faces the photoelectric converter in the case where the wavelength converter and the photoelectric converter are not in contact with each other, or projections and/or recesses are preferaSly formed at the interface between the wavelength converter and the photoelectric converter in the case where the wavelength converter and the photoelectric converter are in contact with each other, in order to make it easier for monochromatic light emitted from the wavelength converter to be incident on the photoelectric converter. With the projections and/or recesses thus provided, the proportion of monochromatic light emitted from the wavelength converter and reflected by the surface of the wavelength converter which faces the photoelectric converter can be reduced. Examples of photoelectric conversion devices provided with the projections and/or recesses are illustrated in FIG 3A through FIG 3D. In FIG 3A through FIG 3D, the same reference numerals as those used in FIG 1A and FIG 2A are assigned to the same or corresponding constituent elements as those of the solar cells 100, 200, and further explanation of these elements will not be provided when appropriate.
[0052] FIG 3A is a cross-sectional view showing a solar cell 100a in which projections and/or recesses are formed at the interface between a wavelength converter 10a and a photoelectric converter 20a that are in contact with each other. In FIG 3A, solar light travels from the left-hand side of the paper to the right-hand side. The wavelength converter 10a is constructed similarly to the wavelength converter 10 of the first embodiment, except that projections and or recesses are formed at one surface that faces the photoelectric converter 20a, and the photoelectric converter 20a is constructed similarly to the photoelectric converter 20 of the first embodiment, except that the converter 20a has an n layer 21a with projections and/or recesses formed at the interface between the n layer 21a and the wavelength converter 10a. In the solar cell 100a, the wavelength converter 10a is disposed upstream of the photoelectric converter 20a as viewed in the travelling direction of the solar light. Thus, the solar cell 100a is a down-conversion type solar cell. The solar cell 100a may be produced via a process of forming the n layer 21a on the surface of the wavelength converter 10a provided with the projections and/or recesses, or a process of forming the front electrode 24 and the wavelength converter 10a on the surface of t¾e n layer 21a provided with the protrusions and recesses.
[0053] FIG. 3B is a cross-sectional view showing a solar cell 100b having the wavelength converter 10a and the photoelectric converter 20. In FIG. 3B, solar light travels from the left-hand side of the paper to the right-hand side. As shown in FIG 3B, the wavelength converter 10a and the photoelectric converter 20 are not in contact with each other. The solar cell 100b is constructed similarly to the solar cell 100, except that projections and/or recesses are formed at one surface of the wavelength converter 10a which faces the photoelectric converter 20. The wavelength converter 10a is disposed upstream of the photoelectric converter 20 as viewed in the travelling direction of the solar light; thus, the solar cell 100b is a down-conversion type solar cell. The solar cell lOOb may be produced via a process of forming the wavelength converter 10a on a surface of a substrate that is different from that on which the photoelectric converter 20 is formed.
[0054] FIG. 3C is a cross-sectional view showing a solar cell 200a in which projections and/or recesses are formed at the interface between a wavelength converter 30a and a photoelectric converter 40a that are in contact with each other. In FIG. 3C, solar light travels from the left-hand side of the paper to the right-hand side. The wavelength converter 30a is constructed similarly to the wavelength converter 30, except that projections and/or recesses are formed at one surface facing the photoelectric converter 40a, and the photoelectric converter 40a is constructed similarly to the photoelectric converter 40, except that the converter 40a has a p layer 42a with projections and/or recesses formed at the interface between the p layer 42a and the wavelength converter 30a. In the solar cell 200a, the wavelength converter 30a is disposed downstream of the photoelectric converter 40a as viewed in the travelling direction of the solar light. Thus, the solar cell 200a is an up-conversion type solar cell. The solar cell 200a may be produced via a process of forming the p layer 42a and the rear electrode 44 on the surface of the wavelength converter 30a with the projections and/or recesses, or a process of forming the rear electrode 44 and the wavelength converter 30a on the surface of the p layer 42a with the projections and/or recesses.
[0055] FIG. 3D is a cross-sectional view showing a solar cell 200b having the wavelength converter 30a and the photoelectric converter 40. In FIG. 3D, solar light travels from the left-hand side of the paper to the right-hand side. As shown in FIG. 3D, the wavelength converter 30a and the photoelectric converter 40 are not in contact with each other. The solar cell 200b is constructed similarly to the solar cell 200, except that projections and/or recesses are formed at one surface of the wavelength converter 30a which faces the photoelectric converter 40. The wavelength converter 30a is disposed downstream of the photoelectric converter 40 as viewed in the travelling direction of the solar light. Thus, the solar light 200b is an up-conversion type solar cell. The solar cell 200b may be produced via a process of forming the wavelength converter 30a on a surface of a substrate that is different from that on which the photoelectric converter 40 is formed.
[0056] In FIG. 3A through FIG. 3D, the projections and/or recesses are formed in a direction from the wavelength converter 10a, 30a toward the photoelectric converter 20a, 20, 40a, 40. In the photoelectric conversion device of the invention, the shapes of the projections and/or recesses are not particularly limited. For example, the projections may be formed in a pyramid shape or columnar shape, and the recesses may be formed by pressing pyramid-shaped projections against the surface concerned, or may be in the form of dimples. Also, in the photoelectric conversion device of the invention, the height of the projections/recesses is not particularly limited provided that the height is greater than the wavelength of the monochromatic light emitted from the wavelength converter, and the projections/recesses do not extend through the wavelength converter or photoelectric converter. For example, the height of the projections/recesses may be equal to or greater than 0.5μιη and equal to or less than 20μηι, for example. The interval or pitch of the projections/recesses may be suitably set in accordance with the height of the projections/recesses, and may be, for example, equal to or larger than Ο.ΐμιτι and equal to or smaller than ΙΟμτη.
[0057] In the following, some examples of wavelength converters that can be used in the photoelectric conversion devices according to the first and second embodiments of the invention will be described.
[0058] FIG. 4 is a cross-sectional view showing one example of wavelength converter 11. In FIG. 4, the vertical direction of the paper is the travelling direction of solar light. As shown in FIG 4, the wavelength converter 11 has a semiconductor layer llx formed of a semiconductor material, and four pairs of high-refractive-index transparent material layers 11a and low-refractive-index transparent material layers lib are alternately laminated on each of the opposite sides of the semiconductor layer llx. Here, the high-refractive-index transparent material layer 11a is formed of a transparent material having a larger refractive index than a transparent material of which the low-refractive-index transparent material layer lib is formed, and the low-refractive-index transparent material layer lib is formed of a transparent material having a smaller refractive index than the transparent material of which the high-refractive-index transparent material layer Ha is formed. The wavelength converter 11 thus constructed is able to function as an optical resonator (or optical filter). In the wavelength converter 11 that functions as an optical resonator, the frequency at which electrons and holes generated in the semiconductor layer llx interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converter 11, monochromatic light is emitted through recombination of the electrons and holes generated in the semiconductor layer l lx.
[00S9] In the wavelength converter 11, the semiconductor material that forms the semiconductor layer llx may be selected»from, for example, InGaAs, InAsP, Ge. and so forth, and the thickness of the semiconductor layer llx (the thickness as measured in the vertical direction of the paper of FIG. 4) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than 300um. Also, where the refractive index of the semiconductor material that forms the semiconductor layer llx is denoted as ns, the thickness of the semiconductor layer llx may be set to l/(2ns) of the wavelength of the monochromatic light to be emitted from the wavelength converter 11. The thus constructed semiconductor layer llx may be produced by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE).
[0060] The transparent material that forms the high-refractive-index transparent material layer 11a may be selected from, for example, Ti02) Zr02, and so forth, and the thickness of the high-refractive-index transparent material layer 11a (the thickness as measured in the vertical direction of the paper of FIG 4) may be, for example, equal to or larger than 50nm and equal to or smaller than 20Gnm. Where the refractive index of the transparent material that forms the high-refractive-index transparent material layer 11a is denoted as nh, the thickness of the high-refractive-index transparent material layer 11a may be set to l/(4nh) of the wavelength of the monochromatic light to be emitted from the wavelength converter 11. The thus constructed high-refractive-index transparent material layer 11a may be produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering.
[0061] The transparent material that forms the low-refractive-index transparent material layer lib may be selected from, for example, Si02, MgF2, and so forth, and the thickness of the low-refractive-index transparent material layer lib (the thickness as measured in the vertical direction of the paper of FIG 4) may be, for example, equal to or larger than lOOn and equal to or smaller than 300nm. Where the refractive index of the transparent material that forms the lo -refractive-index transparent material layer lib is denoted as ni, the thickness of the low-refractive-index transparent material layer lib may be set to l/(4nl) of the wavelength of the monochromatic light to be emitted from the wavelength converter 11. The thus constructed low-refractive-index transparent material layer lib may be produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering.
[0062] In the wavelength converter 11, where the thickness of the semiconductor layer llx is denoted as ds, and the thickness of the high-refractive-index transparent material layer 11a is denoted as dh, while the thickness of the low-refractive-index transparent material layer lib is denoted as dl, and the refractive index of the semiconductor material that forms the semiconductor layer llx is denoted as ns, the wavelength of the monochromatic light emitted from the wavelength converter 11 can be shortened by reducing "nsxds" and "nhxdh+nlxdl". On the other hand, the wavelength of the monochromatic b'ght emitted from the wavelength converter 11 can be increased or made longer by increasing "nsxds" and "nhxdh+nlxdl", Also, it is preferable to set the number of pairs of high-refractive-index transparent layers and low-refractive-index transparent layers laminated on each of the opposite sides of the semiconductor layer llx, to four pairs or more, in order to make it easy for the wavelength converter 11 to generate monochromatic light.
[0063] FIG. 5A is a top view showing another example of wavelength converter 12, and FIG. 5B is a cross-sectional view taken along VB-VB in FIG. 5A. The direction that is perpendicular to the plane of the paper of FIG 5 A, and the vertical direction of the paper of FIG. 5B correspond to the travelling direction of light. As shown in FIG. 5 A and FIG. 5B, the wavelength converter 12 has a plurality of holes. 12a, 12a, . .. formed through a semiconductor portion 12x. The thus constructed wavelength converter 12 is able to function as an optical resonator (or photonic crystal). In the wavelength converter 12 that functions as an optical resonator, the frequency at which electrons and holes generated in the semiconductor portion 12x interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converter 12, monochromatic light can be emitted through recombination of the electrons and holes generated in the semiconductor portion 12x.
[0064] In the wavelength converter 12, the semiconductor material that forms the semiconductor portion 12x may be selected from, for example, InGaAs, LoAsP, Ge, and so forth, and the thickness of the semiconductor portion 12x (the thickness as measured in the vertical direction of the paper of FIG 5B) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than lOOOnm. Also, the diameter of the holes 12a may be, for example, equal to or larger than 50nm and equal to or smaller than 200nm, and the interval between the centers of adjacent holes 12a, 12a may be, for example, equal to or larger than lOOnm and equal to or smaller than 500nm.
[0065] In the wavelength converter 12, the holes 12a, 12a, ... may be hollow, or may be filled with a transparent material having a smaller refractive index than the semiconductor material that forms the semiconductor portion 12x. The transparent material may be selected from Si02, known transparent resins, and so forth.
[0066] To fabricate the wavelength converter 12 constructed as described above, a semiconductor layer having a thickness within the range of about lOOnro to lOOOnm is formed by a known method, such as vapor deposition, more specifically, a metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE), and then, the holes 12a, 12a, ... are formed through the semiconductor layer by a known method, such as photolithography as a combination of dry etching and wet etching, or electron lithography. Then, the holes 12a, 12a, ... are filled as needed with a transparent material having a smaller refractive index than the semiconductor material that forms the semiconductor portion 12x. [0067] In the wavelength converter 12, where the refractive index of the semiconductor material that forms the semiconductor portion 12x is denoted as nxl, and the refractive index of the substance present in the holes 12a is denoted as nyl, while the volumetric proportion of the holes 12a, 12a, ... in the wavelength converter 12 is denoted as γρΐ, the wavelength of monochromatic light emitted from the wavelength converter 12 can be shortened by reducing "(l-vpl)xnxl+Yplxnyl", or reducing the interval between the centers of adjacent ones of the holes 12a, 12a, .... On the other hand, the wavelength of monochromatic light emitted from the wavelength converter 12 can be increased or made longer by increasing "(l- pl)xnxl+vplxnyl", or increasing the interval between the centers of adjacent ones of the holes 12a, 12a, ....
[0068] FIG 6A is a top view showing another example of wavelength converter 13, and FIG. 6B is a cross-sectional view taken along VIB-VIB in FIG. 6A. The direction that is perpendicular to the plane of the paper of FIG. 6A, and the vertical direction of the paper of FIG. 6B correspond to the travelling direction of light. As shown in FIG. 6A and FIG. 6B, the wavelength converter 13 has a semiconductor portion 13x formed of a semiconductor material, and low-refractive- index transparent materia] portions 13a, 13a, ... dispersed in the semiconductor portion 13x. The low-refractive-index transparent material portions 13a are formed of a transparent materia] having a smaller refractive index than the semiconductor material of the semiconductor portion 13χ. The thus constructed wavelength converter 13 is able to function as an optical resonator (or photonic crystal). In the wavelength converter 13 that functions as an optical resonator, the frequency at which electrons and holes generated in the semiconductor portion 13x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on (he wavelength converter 13, monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portion 13x.
[0069] In the wavelength converter 13, the semiconductor material that forms the semiconductor portion 13x may be selected from, for example, InGaAs, IjiAsP, Ge, and so forth, and the thickness of the semiconductor portion 13x (the thickness as measured in the vertical direction of the paper of FIG. 6B) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than lOOOnm. As the transparent material that forms the low-refractive-index transparent material portions 13a, a selected one of known transparent materials, such as Si(¼ and known transparent resins, which have a smaller refractive index than the semiconductor material that forms the semiconductor portion I3x, may be suitably used. In the wavelength converter 13, the diameter of the low-refractive-index transparent material portions 13a may be, for example, equal to or larger than 50nm and equal to or smaller than 200nm, and the interval between the centers of adjacent ones of the low-refractive-index transparent material portions 13a, 13a may be, for example, equal to or larger than lOOnm and equal to or smaller than 500nm.
[0070] To fabricate the wavelength converter 13 constructed as described above, the low-refractive-index transparent material portions 13a, 13a, ... are produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering. Then, when the semiconductor portion 13x is formed by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE), a process of forming a part of the semiconductor portion 13x, and a process of dispersing the low-refractive-index transparent material portions 13a, 13a, ... on a surface of the semiconductor portion 13x that is in the middle of formation are repeated, so that the wavelength converter 13 can be fabricated.
[0071] In the wavelength converter 13, where the refractive index of the semiconductor material that forms the semiconductor portion 13x is denoted as nx2, and the refractive index of the substance that forms the low-refractive-index transparent material portions 13a is denoted as ny2, while the volumetric proportion of the low-refractive-index transparent material portions 13a, 13a, ... in the wavelength converter 33 is denoted as yp2, the wavelength of monochromatic light emitted from the wavelength converter 13 can be shortened by reducing "(l~Yp2)xnx2+ p2xny2'\ or reducing the interval between the centers of adjacent ones of the low-refractive-index transparent material portions 13a, 13a, .... On the other hand, the wavelength of monochromatic light emitted from the wavelength converter 13 can be increased by increasing "(1-γρ2)χηχ2+γρ2χην2", or increasing the interval between the centers of adjacent ones of the low-refractive-index transparent material portions 13a, 13a, ....
[0072] FIG 7A is a top view showing another example of wavelength converter
14, and FIG. 7B is a cross-sectional view taken along VIIB-VIIB in FIG 7A. The direction that is perpendicular to the plane of the paper of FIG 7A, and the vertical direction on the paper of FIG. 7B correspond to the travelling direction of light. As shown in FIG 7A and FIG 7B, the wavelength convener 14 has a semiconductor portion 14x formed of a semiconductor material, and metal nanoparticles 14a, 14a, ... dispersed in the semiconductor portion 14x. In the wavelength converter 14 constructed as described above, surface plasmon resonance Occurs around the metal nanoparticles 14a, 14a, at a particular frequency that depends on the complex refractive index of the metal nanoparticles 14a, 14a, ... and the surrounding semiconductor portion 14x, and the size of the metal nanoparticles 14a, 14a, .... As a result, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength. Thus, the wavelength converter 14 is able to function as an optical resonator (or optical filter). In the wavelength converter 14 that functions as an optical resonator, the frequency at which electrons and holes generated in the semiconductor portion 14x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converter 14, monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portion I4x.
[0073] In the wavelength converter 14, the semiconductor material that forms the semiconductor portion 14x may be selected from, for example, InGaAs, InAsP, Ge, and so forth, and the thickness of the semiconductor portion 4x (the thickness as measured in the vertical direction of the paper of FIG 7B) may be controlled to be, for example, equal to or larger than lOOnm and equal to or smaller than lOOOnm. Examples of metal materials that can form the metal nanoparticles 14a include Au, Ag, Al, Pt, etc. In the wavelength converter 14, the diameter of the metal nanoparticles 14a is preferably controlled to be equal to or larger than lOnm and equal to or smaller than 50Dm, so as to enable the metal nanoparticles 14a to produce surface plasmon resonance. The interval between the centers of adjacent ones of the metal nanoparticles 14a, 14a ... is preferably controlled to be twice or more and five times or less as large as the diameter of the metal nanoparticles 14a, so that the effect of the surface plasmon resonance can be easily obtained over substantially the entire region of the semiconductor portion 14x.
[0074] The wavelength converter 14 constructed as described above may be fabricated by forming the semiconductor portion 14x by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE), while dispersing the metal nanoparticles 14a, 14a, ... produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering, in the semiconductor portion 14x.
[00751 While the metal nanoparticles 14a, 14a, . .. are dispersed inside the semiconductor portion 14x in the illustrated example of FIG 7A and FIG 7B, metal nanoparticles whose surfaces are covered with a transparent insulating material are preferably dispersed in the semiconductor material, so that carriers generated in the semiconductor portion 14x are less likely to be trapped by the metal nanoparticles 14a, 14a, for improvement in the efficiency of the photoelectric conversion device, for example. The preferred example of wavelength converter 15 is illustrated in FIG 8.
[0076] FIG. 8 is a cross-sectional view showing the wavelength converter 15, and the vertical direction on the paper of FIG 8 is the travelling direction of light. In FIG. 8, the same reference numerals as used in FIG 7B are assigned to the same or corresponding constituent elements as those of the wavelength converter 14, and further explanation of these elements will not be provided. As shown in FIG. 8, the wavelength converter 15 has nanoparticles 15a, 15a, ... dispersed in the semiconductor portion 14x, and the nanoparticles 15a, 15a, ... are produced by covering surfaces of the metal nanoparticles 14a, 14a, ... with transparent insulating layers 15b, 15b, .... In the wavelength converter 15 constructed as described above, too, the surface plasmon resonance can be produced around the nanoparticles 15a, 15a, as in the wavelength converter 14. Since the nanoparticles 15a are formed by covering the surfaces of the metal nonaparticles 14a with the transparent insulating layers 15b, carriers created in the semiconductor portion 14x are prevented from being trapped by the metal nanoparticles 14a. Accordingly, the wavelength converter 15 is able to generate monochromatic light having higher intensity than that emitted from the wavelength converter 14.
[0077] In the wavelength converter 15, the transparent insulating layers 15b may be formed of a known insulating material, such as SiOi or a transparent resin. The thickness of the transparent insulating layers 15b is preferably controlled to be equal to or larger than 2nm, so that the transparent insulating layers 15b can effectively prevent the carriers from being trapped by the metal nanoparticles 14a or make it less likely to have the carriers trapped by the metal nanoparticles 14a. Also, the thickness of the transparent insulating layers 15b is preferably controlled to be equal to or smaller than lOnm, so that the effect of the surface plasmon resonance is likely to reach the semiconductor portion 14x. In the wavelength converter 15, the nanoparticles 15a can be produced by a known method of chemical synthesis, such as a sol-gel method or a solvothermal method.
[0078] The wavelength converter 14 and the wavelength converter 15 are likely to generate monochromatic light having a longer wavelength as the refractive index of the semiconductor material l4x is increased, and are likely to emit monochromatic light having a shorter, wavelength as the refractive index of the semiconductor material 14x is reduced.
[0079] FIG. 9A is a top view showing another example of wavelength converter 16, and FIG 9B is a cross-sectional view taken along IXB-IXB of FIG. 9A. The direction perpendicular to the plane of the paper of FIG. 9A and the vertical direction on the paper of FIG. 9B correspond to the travelling direction of light. As shown in FIG. 9A and FIG 9B, the wavelength converter 16 has a transparent insulating material portion 16a, and a plurality of photoconductive antenna portions 16b, 16b, ... disposed in the transparent insulating material portion 16a. Each of the photoconductive antenna portions 16b has a semiconductor portion 16x and a pair of metal sheets 16y, 16y, and the semiconductor portion 16x is interposed between the metal sheets 16y, 16y. In the wavelength converter 16 constructed as described above, the metal sheets 16y, 16y function as a photoconductive antenna, at a given frequency that depends on the complex refractive index of the metal sheets 16y, 16y, the semiconductor portion 1 x, and the surrounding transparent insulating material portion 16a, and the size of the metal sheets 16y, 26y. Consequently, the light intensity significantly increases at the semiconductor portions I6x, so that the wavelength converter 16 can function as an optical resonator (or optical filter). In the wavelength converter 16 that functions as an optical resonator, the frequency at which electrons and holes generated in the semiconductor portions 16x interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converter 16. monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portions 16x.
[0080] In the wavelength converter 16, examples of transparent insulating materials that can form the transparent insulating material portion 16a include known transparent insulating materials, such as Si02 and transparent resins. Also, examples of semiconductor materials that can form the semiconductor portions 16x include InGaAs, InAsP, Ge, and so forth. The diameter of the semiconductor portions 16x is controlled to be equal to or larger than lOnm, so that surface plasmon resonance can be produced, and is controlled to be equal to or smaller4 than 200nm, so that the surface plasmon resonance can be produced over the whole semiconductor portion 16x. Also, examples of metal materials that can form the metal sheets 16y include Au, Ag, Al, Pt, and so forth, and the length of the metal sheets 16y (e.g., the length of the metal sheet 16y located at the lower, right-hand side of the paper of FIG. 9A, as measured in the lateral direction on the paper of FIG. 9A) may be controlled to be equal to or larger than lOOnm and equal to or smaller than 400nm, so that the photoconductive antenna portion 16b can function as a photoconductive antenna. To the same end, the thickness of the metal sheets 16y (e.g., the thickness of the metal sheet 16y located at the lower, right-hand side of the paper of FIG. 9A, as measured in the direction perpendicular to the plane of the paper of FIG. 9A) may be controlled to be equal to or larger than 5nm and equal to or smaller than 20nm. Also, the length of the photoconductive antenna portions 16b (e.g., the length of the photoconductive antenna portion 16b located at the lower, right-hand side of the paper of FIG. 9A, as measured in the lateral direction on the paper of FIG. 9A) may be controlled to be equal to or larger than 300nm and equal to or smaller than SOOnm, so that the photoconductive antenna portion 16b can function as a photoconductive antenna. Where the refractive index of the transparent insulating material that forms the transparent insulating material portion 16a is denoted as nt, the length of the photoconductive antenna portion 16b may be set to l/(2nt) of the wavelength of monochromatic light to be emitted from the wavelength converter 16, and the interval or pitch of the photoconductive antenna portions 16b as measured in the thickness direction (e.g., the interval of the photoconductive antenna portions 16b located at the lower, right-hand side of the paper of. FIG, 9A, as measured in the vertical direction of the paper of FIG. 9B) may be set to l/(nt) of the wavelength of monochromatic light emitted from the wavelength converter 16.
[0081] The wavelength converter 16 is likely to emit monochromatic light having a longer wavelength as the length of the photoconductive antenna portion 16b is increased, and is likely to emit monochromatic light having a shorter wavelength as the length of the photoconductive antenna portior? 16b is reduced.
[0082] In the wavelength converter 16 constructed as described above, the semiconductor portions 16x may be produced by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD), or a photolithography method as a combination of molecular beam epitaxy ( BE) and dry etching or wet etching, or electron lithography. The metal sheets 16y may be produced by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or a photolithography method as a combination of sputtering and dry etching or wet etching, or electron lithography. Then, the wavelength converter 16 may be fabricated by producing the transparent insulating material portion 16a by a known method, such as vapor deposition, more specifically, a vacuum evaporation method including ion plating, or sputtering, while placing the photoconductive antenna portions 16b in each of which one semiconductor portion 16x is located between projecting parts of a pair of metal sheets I6y, 16y, in the transparent insulating material portion 16a.
[0083] FIG 10 is a cross-sectional view showing another example of wavelength converter 17. The vertical direction on the paper of FIG 10 is the travelling direction of light. As shown in FIG 10, the wavelength converter 17 has a transparent insulating material portion 17a, a plurality of semiconductor portions 17x, 17x, ... dispersed in the transparent insulating material portion 17a, and metal nanoparticles 17b contained in the semiconductor portions 17x. In the wavelength converter 17 constructed as described above, surface plasmon resonance occurs around the metal nanoparticles 17b, 17b, at a particular wavelength that depends on the complex refractive index of the metal nanoparticles 17b and the surrounding semiconductor portions 17x and transparent insulating material portion 17a, and the size of the metal nanoparticles 17b, 17b, .... As a result, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength. Thus, the wavelength converter 17 is able to function as an optical resonator (or optical filter). In the wavelength converter 17 that functions as an optical resonator, the frequency at which electrons and holes created in the semiconductor portions 17x interact wirh electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converter 17, monochromatic light can be emitted through recombination of the electrons and holes generated in the semiconductor portions 17x.
[0084] In the wavelength converter 17, examples of transparent insulating materials that can form the transparent insulating material portion 17a include known transparent insulating materials, such as Si02 and transparent resins. Also, examples of semiconductor materials that can form the semiconductor portions 17x include InGaAs, InAsP, Ge, and so forth. The diameter of the semiconductor portions 17x may be set to be, for example, equal to or larger than 60nm and equal to or smaller than 250nm (may be larger than the diameter of the metal nanoparticles 17b by an amount equal to or larger than 50nm and equal to or smaller than 200nm). Also, the interval between the centers of adjacent ones of the semiconductor portions 17x, 17x is preferably controlled to be 1.2 times or more as large as the diameter of the semiconductor portions 17x, so that the adjacent semiconductor portions 17x, 17x do not contact each other, and is preferably controlled to be 5 times or less as large as the diameter of each semiconductor portion 17x, so as to make it easy to increase the proportion of the semiconductor portions 17x, 17x, ... in which electrons and holes are generated, in the wavelength converter 17.
[0085] Examples of metal materials that can form the metal nanoparticles 17b include Au, Ag, Al, Pt, arid so forth. In the wavelength converter 17, the diameter of the metal nanoparticles 17b is controlled to be equal to or larger than lOnm and equal to or smaller than 50nm, so that surface plasmon resonance can be produced. The wavelength converter 17 is likely to generate monochromatic light having a longer wavelength as the refractive index of the semiconductor material is increased, and is likely to generate monochromatic light having a shorter wavelength as the refractive index of the semiconductor material is reduced.
[0086] In the wavelength converter 17, the semiconductor portions 17x that contain the metal nanoparticles 17b may be produced by a known method of chemical Synthesis, such as a sol-gel method, or a solvothermal method. The wavelength converter 17 can be fabricated by producing the transparent insulating material portion 17a by a known method, such as coating or printing, while dispersing the thus produced semiconductor portions 17x, 17x, ... in the transparent insulating material portion 17a. In the wavelength converter 17, it is preferable that metai nanoparticles that are covered with transparent insulating layers formed of a known insulating material, such as Si<½ or a transparent resin, are contained in semiconductor portions, so that carriers created in the semiconductor portions 17x are less likely or unlikely to be trapped by the metal nanoparticles 17b. In the case where the metal nanoparticles are covered with the transparent insulating layers, the thickness of the transparent insulating layers is preferably controlled to be equal to or larger than 2nm, so that the transparent insulating layers can effectively prevent the carriers from being trapped by the metal nanoparticles or make it less likely to have the carriers trapped by the metal nanoparticles. Also, the thickness of the transparent insulating layers is preferably controlled to be equal to or smaller than lOnro, so that the effect of the surface plasmon resonance is likely to reach the semiconductor portions.
[0087] FIG. 11 is a cross-sectional view showing another example of wavelength converter 18. The vertical direction on the paper of FIG 11 is the travelling direction of light. As shown in FIG. 11, the wavelength converter 18 has a transparent insulating material portion 18a, a plurality of metal particle portions 18b, 18b, ... dispersed in the transparent insulating material portion 18a, and semiconductor particles 18x contained in the metal particle portions 18b. In the wavelength converter 18 thus constructed, surface plasmon resonance occurs around the metal particle portions 18b, 18b, at a particular wavelength that depends on the complex refractive index of the metal particle portions 18b, 18b, the semiconductor particles 18x, 18x, and the transparent insulating material portion 18a, and the size of the metal particle portions 18b, 18b, .... As a result, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength, and the wavelength converter 18 is able to function as an optical resonator (or optical filter). In the wavelength converter 18 that functions as an optical resonator, the frequency at which electrons and holes generated in the semiconductor particles 18x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converter 18, monochromatic light can be generated through recombination of the electrons and holes generated in the semiconductor particles 18x,,
[0088] In the wavelength converter 18, examples of transparent insulating materials that can form the transparent insulating material portion 18a include known transparent insulating materials, such as Si02 and transparent resins. Also, examples of semiconductor materials that can form the semiconductor particles l8x include InGaAs, InAsP, Ge, and so forth. Since the diameter of the metal particle portions 18b that can cause surface plasmon resonance to occur is equal to or larger than lOnxn and equal to or smaller than 50nrn, the diameter of the semiconductor particles 18x may be controlled to be equal to or larger than 5nm and equal to or smaller than 45nm. Examples of metal materials that can form the metal particle portions 18b include Au, Ag, Al, Pt, and so forth. The interval between the centers of adjacent ones of the metal particle portions 18b, 18b is preferably 1.2 times or more as large as the diameter of the metal particle portions 18b, so that the adjacent metal particle portions 18b, 18b do not contact each other, and is preferably 5 times or less as large as the diameter of the metal particle portions 18b, so that the proportion of the metal particle portions 18b, 18b, ... in the wavelength converter 18, can be readily increased. The wavelength converter 18 is likely to emit monochromatic light having a longer wavelength as the refractive index of the semiconductor particles I8x is increased, and is likely to emit monochromatic light having a shorter wavelength as the refractive index of the semiconductor particles 18x is reduced.
[0089] In the wavelengt converter 18, the metal particle portions 18b that contain the semiconductor particles. 18x may be produced by a known method of chemical synthesis, such as a sol-gel method, or a solvothermal method. The wavelength converter 18 can be produced by producing the transparent insulating material portion 18a by a known method, such as coating or printing, while dispersing the thus produced metal particle portions 18b, 18b, ... in the transparent insulating material portion 18a.
[0090] FIG. 12 is a cross-sectional view showing a solar cell 300 according to a third embodiment of the invention. In FIGL 12, the same reference numerals as those used in FIG. 1A are used for identifying the same or corresponding constituent elements as those of the solar cell 100, and further explanation of these elements will not be provided.
[0091] As shown in FIG. 12, the solar cell 300 has an n layer 51, an i layer 52, and a p layer 53, and the n layer 51, i layer 52, and p layer 53 form a pin junction 54. In the solar cell 300, the i layer 52 mainly functions as a photoelectric converter, and wavelength converting portions 55, 55, ... (which may be simply referred to as "wavelength converter 55" when appropriate) each having a semiconductor material are dispersed in the i layer 52. In the solar cell 300, light that has passed through a part of the i layer 52 is incident on the wavelength converter 55. Thus, the solar cell 300 is an up-con version type solar cell. The wavelength converter 55 has a semiconductor material with energy gap Eg4, and has the function of emitting monochromatic light with energy Eg5. The i layer 52 that functions as the photoelectric converter is formed of a semiconductor material with energy gap Eg6 The n layer 51 to which a front electrode 24 is connected is formed of an n-type semiconductor with the energy gap Eg6, and a p layer 53 to which the rear electrode 25 is connected is formed of a p type semiconductor with the energy gap Eg6.
[0092] In operation, solar light incident on the solar cell 300 passes through the n layer 51, and is incident on the semiconductor material (which may be referred to as "photoelectric converter 52" when appropriate) of the i layer 52 disposed around the wavelength converting portions 55. The energy gap Eg6 of the photoelectric converter 52 is controlled so that the photoelectric converter 52 can absorb only high-energy photons, out of the solar light containing photons with various levels of energy. Therefore, when the solar light is incident on the photoelectric converter 52, the photoelectric converter 52 absorbs only the photons with energy equal to or greater than the energy gap Eg6 of the photoelectric converter 52. Once the photons are absorbed, electrons and holes are created in the photoelectric converter 52. The thus created electrons and holes are separated by an internal electric field formed by the n layer 51 and the p layer 53, and the electrons move to the n layer 51 side, and are collected into the front electrode 24 connected to the n layer 51. Also, the holes move to the p layer 53 side, and are collected into the rear electrode 25 connected to the p layer 53.
[0093] As described above, the photoelectric converter 52 absorbs only the photons contained in the solar light and having energy equal to or greater than Eg6. Therefore, photons with energy less than Eg6, out of the photons contained in the solar light, pass through the photoelectric converter 52 without being used for photoelectric conversion, and reach the wavelength converter 55. The energy gap Eg4 of the semiconductor material contained in the wavelength converter 55 is smaller than Eg6, and is controlled so that the wavelength converter 55 can absorb low-energy photons contained in the solar light. Therefore, when the light is incident on the semiconductor material contained in the wavelength converter 55, only the photons with energy equal to or greater than the energy gap Eg4 of the semiconductor material are absorbed. Once the photons are absorbed, electrons with various levels of energy are excited from the valence band to the conduction band, and holes with various levels of energy are formed in the valence band. Namely, when light is incident on the semiconductor material of the wavelength converter 55, the electron energy distribution as shown in FIG, 2B is formed in the conduction band of the semiconductor material, and the hole energy distribution as shown in FIG. 2B is formed in the valence band of the semiconductor material, as in the wavelength converter 30 of the solar cell 200.
[0094] The electrons generated in the wavele-Dgth converter 55 interact with each other and give and receive energy to and from each other, while the holes generated in the wavelength converter 55 interact with each other and give and receive energy to and from each other, so that pairs of electrons and holes with specific energy levels, which have an energy difference of Eg5, are generated. Then, the electrons and the holes recorabine to emit monochromatic light with energy Eg5. In up-conversion type solar cells of the related art using phosphors, electrons and boles capable of interacting with each other are limited to electrons and holes having discrete energy levels. On the other hand, the wavelength converter 55, which has the semiconductor material, allows electrons with various levels of energy to interact with each other, and allows holes with various levels of energy to interact with each other, so as to emit monochromatic light having energy Eg5. The monochromatic light thus emitted from the wavelength converter 55 travels toward the photoelectric converter 52.
[0095] The energy gap of the photoelectric converter 52 is Eg6. In this embodiment, Eg6 is smaller than Eg5 by about O. leV. Therefore, the monochromatic light with energy Eg5 emitted from the wavelength converter 55 can overcome the band-gap of the photoelectric converter 52, so as to be absorbed by the photoelectric converter 52 where electrons and holes are generated. The thus generated electrons and holes are separated by an internal electric field formed by the pin junction 54, almost without losing energy, since the difference between Eg5 and Eg6 is as small as about 0.1 eV. Then, the electrons move to the n layer 51 side, and are collected into the front electrode 24 connected to the n layer 51. Also, the holes move to the p layer 53 side, and are collected into the rear electrode 25 connected to the p layer 53.
[0096] With the solar cell 300 constructed as described above, large-energy photons of solar light having larger energy than the energy gap of the photoelectric converter 52 are absorbed by the photoelectric converter 52 and converted into electric power. The photoelectric converter 52 also receives monochromatic light emitted from the wavelength converter 55 using photons that passed through the photoelectric converter 52 without being converted into electric power, so as to convert the monochromatic light into electric power. This arrangement makes it possible to broaden the band of light used for conversion into electric energy in the photoelectric converter 52. Also, the wavelength converter 55 is arranged to eventually recombine the electrons and the holes, but not intended to extract the created electrons and holes (carriers) to the outside as they are. Therefore, in the wavelength converter 55, it is not necessary to move the carriers to the electrodes, as in hot-carrier type solar cells of the related art using the quantum structure, and it is thus possible to significantly reduce the amount of energy that would be lost during movement of the carriers. Furthermore, the use of the semiconductor material in the wavelength converter 55 makes it possible to significantly broaden the wavelength range of light that can be used for creating the carriers, as compared with up-conversion type solar cells of the related art using phosphors. In addition, in the solar cell 300 in which the monochromatic light emitted from the wavelength converter 55 is incident on the photoelectric converter 52, the energy of the monochromatic light incident on the photoelectric converter 52 is specified as Eg5, Therefore, energy loss can be reduced by using a semiconductor material with energy gap corresponding to Eg5, for the photoelectric converter 52. Thus, the solar cell 300 according to the invention is able to enhance its photoelectric conversion efficiency. The photoelectric conversion efficiency of the solar cell 300 can be easily enhanced by enhancing the efficiency of the wavelength converter 55 that converts light into monochromatic light.
[0097] In the solar cell 300, the energy gap Eg4 of the semiconductor material contained in the wavelength converter 55 may be, for example, equal to or higher than 0.4eV and equal to or lower than l.leV, and the energy Eg5 of the monochromatic light emitted from the wavelength converter 55 may be, for example, equal to or higher than l,4eV, and equal to or lower than 3.3eV. Examples of semiconductor materials that can be contained in the wavelength converting portions 55 include GalnAs, InAsP, GaSb, Ge, and so forth. The wavelength converting portions 55 may be produced by a known method, such as vapor deposition, more specifically, metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), or a known method of chemical synthesis, such as a sol-gel method, or a solvotherma] method. The wavelength converting portions 55, 55, ... can be dispersed in the ί layer 52, by repeating a process of producing a part of the photoelectric converter 52, dispersing the wavelength converting portions 55, 55, ... on a surface of the photoelectric converter 52, and producing another part of the photoelectric converter 52 on the surface of the photoelectric converter 52 on which the wavelength converting portions 55, 55, ... are dispersed.
[0098] Also, the energy gap Eg6 of the n layer, photoelectric converter (i layer) 52, and the p layer 53 may be, for example, equal to or higher than 1.3eV and equal to or lower than 3.2e V. Examples of semiconductor materials that can be used for forming the n layer 51, photoelectric converter 52, and p layer 53 include GalnP, AlGaAs, GaAs, and so forth. In the solar cell 300, the n layer 51 may be produced by adding a known n-type dopant to a selected one of the above-indicated semiconductor materials. The p layer 53 may be produced by adding a known p-type dopant to a selected one of the semiconductor materials. The thickness of the n layer 51 and the p layer 53 may be, for example, about lOOnm, and the thickness of the i layer 52 may be, for example, about 200nm. The n layer 51, photoelectric converter 52, and the p layer 53 may be produced by a known method, such as vapor deposition or vapor-phase growth.
[0099] While the solar eel] 300 as described above has the wavelength converting portions 55 dispersed only in the i layer 52 in the illustrated embodiment, the photoelectric conversion device (up-conversion type photoelectric conversion device) according to the third embodiment of the invention is not limited to this configuration. In the photoelectric conversion device (uprconversion type photoelectric conversion device) according to the third embodiment of the invention, wavelength converting portions may be dispersed in the n layer and/or the p layer, in addition to the Ί layer.
[0100] In the following, some examples of wavelength converters which can be used in the photoelectric conversion device according to the third embodiment of the invention will be described.
[0101] FIG. 13 is a cross-sectional view showing one example of wavelength converter 56. When the photoelectric conversion device according to the third embodiment of the invention is provided with the wavelengih converter 56, wavelength converting portions 56, 56, ... that constitute the wavelength converter 56 are used in place of the wavelength converting portions 55, 55, ... as shown in FIG 12. As shown in FIG. 13, each of the wavelength converting portions 56 has a metal nanoparticle 56a covered with a semiconductor portion 56x. In the thus constructed wavelength converting portion 56, surface piasmon resonance occurs around the metal nanoparticle 56, at a particular wavelength that depends on the complex refractive index of the metal nanoparticle 56a and the surrounding semiconductor portion 56x, and the size of the metal nanoparticle 56a. As a result, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength. Thus, the wavelength converting portion 56 is able to function as an optical resonator (or optical filter). In the wavelength converting portion 56 that functions as an optical resonator, the frequency at which electrons and holes created in the semiconductor portion 56x interact with electrons and holes, respectively, created therein can be increased. Therefore, when light is incident on the wavelength converting portion 56, for example, monochromatic light can be emitted through recombination of the electrons and holes generated in the semiconductor portion 56x.
[0102] In the wavelength converting portion 56, examples of semiconductor materials that can form the semiconductor portion 56x include InGaAs, InAsP, Ge, and so forth, and the diameter of the semiconductor portion 56x may be, for example, equal to or larger than 60nm and equal to or smaller than 250nm (or may be larger than the diameter of the metal nanoparticles 56a by an amount equal to or larger than 50nm and equal to or smaller than 200nm). In the photoelectric conversion device according to the third embodiment of the invention, the interval between the centers of adjacent ones of the wavelength converting portions 56, 56 is preferably 1.2 times or more as large as the diameter of the wavelength converting portion 56, so that the adjacent wavelength converting portions 56, 55 do not contact each other, and is preferably 5 times or less as iarge as the diameter of the wavelength converting portion 56, so that the proportion of the wavelength converting portions 56, 56, ... in which electrons and holes are generated, in the photoelectric converter, can be readily increased.
[0103] Examples of metal materials that can form the metal nanoparticle 56a include Au, Ag, Al, Pt, and so forth. In the wavelength converting portion 56, the diameter of the metal nanoparticle 56a is controlled to be equal to or larger than lOnm and equal to or smaller than 50nm, so as to enable the metal nanoparticle 56a to produce surface plasmon resonance. The wavelength converting portion 56 is likely to emit monochromatic light having a longer wavelength as the refractive index of the semiconductor portion 56x is increased, and is likely to generate monochromatic light having a shorter wavelength as the refractive index of the semiconductor portion 56x is reduced.
[0104] The wavelength converting portion 56 may be produced by successively growing the metal nanoparticle 56a and the semiconductor portion 56x, by a known method of chemical synthesis, such as a sol-gel method, or a solvothermal method. In the wavelength converting portion 5ό, it is preferable to cover the metal nanoparticle with a transparent insulating layer formed of a known insulating material, such as Si02 or a transparent resin, and containing the metal nanoparticle covered with the transparent insulating layer in the semiconductor portion, so that the carriers created in the semiconductor portion 56x aie less likely or unlikely to be trapped by the metal nanoparticle 56a. In the case where the metal nanoparticle is covered with the transparent insulating layer, the thickness of the transparent insulating layer is preferably controlled to be equal to or larger than 2nm, so that the transparent insulating layer can effectively prevent carriers from being trapped by the metal nanoparticle or make it less likely to have the carriers trapped by the metal nanoparticle. Also, the thickness of the transparent insulating layer is preferably controlled to be equal to or smaller than lOnm, so that the effect of the surface plasmon resonance is likely to reach the semiconductor portion.
[0105] FIG. 14 is a cross-sectional view showing another example of wavelength converter 57. When the photoelectric conversion device according to the third embodiment is provided with the wavelength converter 57, wavelength converting portions 57, 57, ... that constitute the wavelength converter 57 are used in place of the wavelength converting portions 55, 55, ... as shown in FIG 12. As shown in FIG. 14, each of the wavelength converting portions 57 has a metal nanoparticle 57a containing a semiconductor portion 57x, and a transparent material layer 57b with which the metal nanoparticle 57a containing the semiconductor portion 57x is covered. In the wavelength converting portion 57 constructed as described above, surface plasmon resonance occurs around the metal nanoparticle, at a particular wavelength that depends on the complex refractive index of the metal nanoparticle 57a, a semiconductor material of the semiconductor portion 57x, and a transparent material of the transparent material layer 57b, and the size of the metal nanoparticle 57a. As a result, the density of states of photons having a given wavelength can be increased to be significantly larger than those of photons having wavelengths around the given wavelength. Thus, the wavelength converting portion 57 is able to function as an optical resonator (or optica! filter). In the wavelength converting portion 57 that functions as an optical resonator, the frequency at which electrons and holes generated in the semiconductor portion S7x interact with electrons and holes, respectively, generated therein can be increased. Therefore, when light is incident on the wavelength converting portion 57, for example, monochromatic light can be emitted through recombination of the electrons and holes created in the semiconductor portion 57x.
[0106] In the wavelength converting portion 57, the transparent material that forms the transparent material layer 57b may be selected from known transparent insulating materials, such as Si02 and transparent resins. The thickness of the transparent material layer 57b is preferably controlled to be equal to or larger than 2nm and equal to or smaller than lOnm, so that the carriers created in the photoelectric converter are effectively prevented from being trapped by the metal nanoparticles 57a. The semiconductor material that forms the semiconductor portion 57x may be selected from, for example, InGaAs, InAsP, Ge, and so forth. Since the diameter of the metal nanoparticle 57a which enables the metal nanoparticle 57a to produce surface plasmon resonance is equal to or larger than lOnm and equal to or smaller than 50nm, the diameter of the semiconductor portion 57x may be controlled to be equal to or larger than 5nm and equal to or smaller than 45am. Examples of metal materials that can form the metal nanoparticle 57a include Au, Ag, Al, Pt, and so forth. The interval between the centers of adjacent ones of the wavelength converting portions 57, 57 is preferably 1.2 limes or more as large as the diameter of the wavelength converting portion 57, so that the adjacent wavelength converting portions 57, 57 do not contact each other, and is preferably 5 times or less as large as the diameter of the wavelength converting portion 57, so that the proportion of the wavelength converting portions 57, 57, ... in the photoelectric converter can be readily increased. The wavelength converting portion 57 is liSely to generate monochromatic light having a longer wavelength as the size of the semiconductor portion 57x is increased, and is likely to emit monochromatic light having a shorter wavelength as the size of the semiconductor portion 57x is reduced. [0107] The wavelength converting.portion 57 may be produced by successively growing the semiconductor portion 57x, metal nanoparticle 57a, and the transparent material layer 57b, by a known method of chemicaJ synthesis, such as a sol-gel method, or a solvoihe mal method.

Claims

CLAIMS:
1. A photoelectric conversion device comprising:
a wavelength converter that includes a semiconductor material that generates electrons and holes when absorbing light, and emits monochromatic light by recombining the generated electrons and the created holes; and
a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, wherein
the wavelength converter is disposed upstream of the photoelectric converter as viewed in a travelling direction of the light.
2. A photoelectric conversion device comprising:
a wavelength converter that includes a semiconductor material that generates electrons and holes when absorbing light, and emits monochromatic light by recombining the generated electrons and the created holes; and
a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, wherein
the wavelength converter is disposed downstream of the photoelectric converter as viewed in a travelling direction of the light.
3. The photoelectric conversion device according to claim 1 or claim 2, wherein the wavelength converter and the photoelectric converter are in contact with each other, and an interface between the wavelength converter and the photoelectric converter has projections, recesses, or a combination of projections and recesses.
4. The photoelectric conversion device according to claim 1 or claim 2, wherein the wavelength converter and the photoelectric converter are spaced apart from each other, and at least one surface of the wavelength converter which faces the photoelectric converter has projections, recesses, or a combination of projections and recesses.
5. The photoelectric conversion device according to any one of claims 1 - 4, wherein the wavelength converter has a semiconductor layer formed of the semiconductor material, and a plurality of transparent material layers having different refractive indices and laminated on each of opposite sides of the semiconductor layer.
6. The photoelectric conversion device according to any one of claims 1 - 4, wherein:
the wavelength converter further includes a different-refractive-index material having a refractive index that is different from that of the semiconductor material; and the wavelength converter has at least one different-refractive-index portion that is formed of the different-refractive-index material and extends through the semiconductor material in a direction from the wavelength converter toward the photoelectric converter.
7. The photoelectric conversion device according to claim 6, wherein said at least one different-refractive-index portion comprises a plurality of holes.
8. The photoelectric conversion device according to any one of claims 1 - 4, wherein:
the wavelength converter further includes a different-refractive-index material having a refractive index that is different from that of the semiconductor material; and the wavelength converter has a plurality of different-refractive-index portions that are formed of the different-refractive-index material and are three-dimensionally dispersed in the semiconductor material.
9. The photoelectric conversion device according to claim 8, wherein the different-refractive-index material has a smaller refractive index than the semiconductor material.
10. The photoelectric conversion device according to any one of claims 1 - 4, wherein the wavelength converter further includes a plurality of metal nanoparticles dispersed in the semiconductor material.
11. The photoelectric conversion device according to claim 10, wherein the metal nanoparticles are covered with light-permeable insulating layers.
12. The photoelectric conversion device according to any one of claims 1 - 4, wherein the wavelength converter has a plurality of semiconductor portions formed of the semiconductor material, and each of the semiconductor portions is interposed between a pair of metal sheets.
13. The photoelectric conversion device according to any one of claims 1 - 4, wherein the wavelength converter has a plurality of semiconductor portions formed of the semiconductor material, a plurality of metal nanoparticles contained in the semiconductor portions, and a light-permeable insulating material in which the semiconductor portions are dispersed.
14. The photoelectric conversion device according to any one of claims 1 - 4, wherein the wavelength converter has a plurality of semiconductor particles formed of the semiconductor material, a plurality of metal particles in which the semiconductor particles are contained, and a light-permeable insulating material in which the metal particles are dispersed.
15. A photoelectric conversion device comprising:
a wavelength converter that includes a semiconductor material that generates electrons and holes when absorbing light, and emits monochromatic light by recornbining the generated electrons and the created holes; and
a photoelectric converter that generates electrons and holes when receiving the monochromatic light emitted from the wavelength converter, and has a pn junction or a pin junction that separates and moves the electrons and the holes generated in the photoelectric converter when the monochromatic light is incident on the photoelectric converter, wherein
the wavelength converter comprises a plurality of wavelength converting portions dispersed in the photoelectric converter.
16. The photoelectric conversion device according to claim 15, wherein each of the wavelength converting portions has a semiconductor portion formed of the semiconductor material, and a metal nanoparticle contained in the semiconductor portion, and the semiconductor portions of the wavelength converting portions are dispersed in the photoelectric converter.
17. The photoelectric conversion device according to claim 15, wherein each of the wavelength converting portions has a semiconductor portion formed of the semiconductor material, a metal nanoparticle in which the semiconductor portion is contained, and a light-permeable insulting portion in which the metal nanoparticle is contained, and the light-permeable insulating portions of the wavelength converting portions are dispersed in the photoelectric converter.
PCT/IB2011/003070 2010-11-26 2011-11-25 Photoelectric conversion device WO2012069926A2 (en)

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