WO2016127285A1 - 带有表面纳米结构的太阳能电池 - Google Patents

带有表面纳米结构的太阳能电池 Download PDF

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WO2016127285A1
WO2016127285A1 PCT/CN2015/072510 CN2015072510W WO2016127285A1 WO 2016127285 A1 WO2016127285 A1 WO 2016127285A1 CN 2015072510 W CN2015072510 W CN 2015072510W WO 2016127285 A1 WO2016127285 A1 WO 2016127285A1
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solar cell
junction
grating
gallium arsenide
fresnel
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PCT/CN2015/072510
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English (en)
French (fr)
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柳楠
胡昱
何建军
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浙江大学
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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
    • 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 potential barriers
    • H01L31/068—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 potential barriers the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells
    • H01L31/0693—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 potential barriers the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells the devices including, apart from doping material or other impurities, only AIIIBV compounds, e.g. GaAs or InP solar cells
    • 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

Definitions

  • the present invention relates to solar cells, and more particularly to solar cells with surface nanostructures.
  • Photovoltaic power generation has always been a clean energy solution that has received much attention.
  • Solar cells are also one of the most dynamic research areas. Solar cells rely on the photovoltaic effect to achieve effective use of solar energy.
  • solar cells After nearly a century of development, solar cells have been through three generations.
  • the first generation is a crystalline solar cell based on silicon material, and the second generation includes amorphous silicon and poly.
  • Thin film batteries of materials such as polycrystalline silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS).
  • the third generation is a new type of high conversion rate solar cell, including multi-junction or tandem. , intermediate band, carrier-multiplication, and prior up or down energyconversion, of which only multi-junctions are currently being applied to practice.
  • Gallium arsenide is a III-V compound semiconductor material. Unlike silicon, it is a direct bandgap material with better energy conversion efficiency. Moreover, the band gap of gallium arsenide is 1.42 eV, which has good spectral matching with sunlight and has very good absorption characteristics for sunlight. Therefore, gallium arsenide solar cells can be made thinner, up to 5 to 10 microns, compared to silicon solar cells typically having a thickness of 150 microns. In addition, the gallium arsenide solar cell has high temperature resistance. At 300 ° C, the silicon solar cell has stopped operating, and the conversion efficiency of the gallium arsenide battery is still 10%. This feature allows the gallium arsenide battery to be Used in concentrating solar systems.
  • the object of the present invention is to provide a solar cell with a surface nanostructure, which solves the defects of high reflectivity, low photon absorption rate, short circuit current and low open circuit voltage of the conventional solar cell, and is directed to solar energy.
  • the first junction PN junction region and the second junction PN junction region of the battery junction layer are optimized for design.
  • the invention comprises a solar cell, characterized in that a surface nanostructure is formed on the upper surface of the solar cell, the surface nanostructure is a phase grating or a Fresnel lens, and the phase grating or the Fresnel lens is composed of a series of etching grooves. Composition.
  • the surface nanostructure is a phase grating, and the phase grating is composed of a plurality of strip-shaped etching grooves uniformly spaced apart, and the width and spacing of each strip etching groove are designed according to the Huygens-Fresnel principle.
  • the Nerb band method is calculated.
  • the surface nanostructure is a Fresnel lens
  • the Fresnel lens is composed of a plurality of concentric annular etching grooves of the same plane, and the radius of both sides of the concentric annular etching groove is outwardly according to Huygens- Fresnel principle design, calculated by the Fresnel zone method.
  • the depth of the etched trench is one quarter of the center wavelength of the incident light such that the phase difference between the light reflected from the bottom of the etched trench and the light reflected from the upper surface of the surface nanostructure is ⁇ .
  • the refractive index of the solar cell surface nanostructure material is close to 3.
  • the solar cell is a three-junction stacked gallium arsenide solar cell, wherein the first junction material is indium gallium phosphate, the second junction material is gallium arsenide, and the third junction material is indium gallium arsenide solar cell.
  • the upper surface of the triple junction GaAs solar cell has the surface nanostructure for the focus point and the center wavelength of the first junction, and focuses the incident light on the PN of the first junction of the triple junction GaAs solar cell In the knot area.
  • the upper surface of the triple junction GaAs solar cell has the surface nanostructure for the focus point and the center wavelength of the second junction, and the incident light is focused on the PN of the second junction of the triple junction GaAs solar cell In the knot area.
  • the upper surface of the triple junction stacked gallium arsenide solar cell has respective phase gratings for the focus point and the center wavelength of the first junction and the second junction, and the two phase gratings are orthogonal, and the incident light is focused on the three junctions The junction of the first junction and the second junction PN of the stacked gallium arsenide solar cell.
  • the present invention utilizes a new principle mechanism to integrate surface nanostructures (transmissive phase gratings or Fresnel lenses) with solar cells to achieve high performance, small size and high efficiency photovoltaic systems.
  • the surface nanostructure (transmissive phase grating or Fresnel lens) of the present invention can effectively reduce the reflectance of the surface of the solar cell.
  • the surface nanostructure (transmissive phase grating or Fresnel lens) of the present invention can effectively increase the absorption length of photons in the PN junction region of the solar cell.
  • the surface nanostructure (transmissive phase grating or Fresnel lens) of the present invention can effectively increase the intensity of the light field of incident light in the PN junction region of the solar cell.
  • the surface nanostructure (transmissive phase grating or Fresnel lens) of the present invention can effectively improve the effective absorption of photons by the PN junction region of the solar cell.
  • Figure 1 is a schematic illustration of a solar cell of the present invention having a surface phase grating or Fresnel lens nanostructure.
  • FIG. 2 is a schematic view showing the design of a phase grating and a Fresnel lens of the present invention.
  • FIG 3 is a schematic diagram of a Fresnel lens (referred to as a first lens) at the bottom of the absorption layer of the first junction PN junction region of the 500 nm wavelength focal plane designed for the embodiment of the triple junction stacked gallium arsenide cell of the present invention.
  • a Fresnel lens referred to as a first lens
  • Figure 4 is a diagram showing the intensity distribution of incident light at a focal plane of 500 nm wavelength when the focal plane of the Fresnel lens is set at the bottom of the emitter of the PN junction region of the first junction of the cell.
  • Fig. 5 is a graph showing the contrast between the reflectance after the first lens was fabricated on the surface of the triple junction GaAs cell and the reflectance when the first lens was not fabricated.
  • Figure 6 is a graph showing the absorption ratio of the first junction PN junction region after the first lens is fabricated on the surface of the multi-junction stacked gallium arsenide cell and the absorption rate of the first junction PN junction region when the first lens is not fabricated. .
  • FIG. 7 is a schematic diagram of a Fresnel lens (referred to as a second lens) at the bottom of the emitter of the second junction PN junction region of the focal plane of the 750 nm wavelength designed by the present invention.
  • a Fresnel lens referred to as a second lens
  • Figure 8 is a diagram showing the intensity distribution of incident light at a focal plane of 750 nm wavelength when the focal plane of the Fresnel lens is set at the bottom of the emitter of the PN junction region of the second junction of the cell.
  • Fig. 9 is a graph showing the contrast between the reflectance after the second lens is formed on the surface of the multi-junction laminated gallium arsenide cell and the reflectance when the second lens is not formed.
  • Figure 10 is a graph comparing the absorption rate of the second junction PN junction region after the second lens is fabricated on the surface of the multi-junction stacked gallium arsenide cell and the absorption ratio of the second junction PN junction region when the second lens is not fabricated. .
  • FIG. 11 is a schematic diagram of a phase grating (referred to as a first grating) at the bottom of the emitter of a first junction PN junction region of a 500 nm wavelength focal plane designed for an embodiment of a three junction stacked gallium arsenide cell of the present invention.
  • a phase grating referred to as a first grating
  • Figure 12 is a diagram showing the intensity distribution of incident light at a focal plane of 500 nm wavelength when the transmission type grating focal plane is set at the bottom of the PN junction region emitter of the first junction of the cell.
  • Figure 13 is a graph showing the reflectance of the first grating after the first grating was fabricated on the surface of the triple junction GaAs cell and the reflectance when the first grating was not fabricated.
  • Figure 14 is a graph showing the absorption ratio of the first junction PN junction region after the first grating is fabricated on the surface of the multi-junction stacked gallium arsenide cell and the absorption ratio of the first junction PN junction region when the first grating is not fabricated. .
  • FIG. 15 is a schematic diagram of a phase grating (referred to as a second grating) at the bottom of the emitter of the second junction PN junction region of the focal plane of the 750 nm wavelength designed by the present invention.
  • a phase grating referred to as a second grating
  • Figure 16 is a diagram showing the intensity distribution of incident light at a focal plane of 750 nm wavelength when the transmission grating focal plane is set at the bottom of the PN junction region emitter of the second junction of the cell.
  • Figure 17 is a graph comparing the reflectance of a second grating on the surface of a multi-junction stacked gallium arsenide cell with the reflectance of the second grating.
  • Figure 18 is a graph showing the comparison between the absorptance of the second junction PN junction region after the second grating is fabricated on the surface of the multi-junction stacked gallium arsenide cell and the absorption ratio of the second junction PN junction region when the second grating is not fabricated. .
  • FIG. 19 is a schematic diagram of a quadrature phase grating (referred to as a third grating) at the bottom of the first and second junction PN junction regions of the first and second junction PN junction regions, respectively, for the focal planes designed for the 500 nm wavelength and the 750 nm wavelength.
  • a quadrature phase grating referred to as a third grating
  • Figure 20 (a) is a light intensity distribution at the bottom of the emitter of the PN junction region of the first junction of the cell after the light of the wavelength of 500 nm is incident on the surface of the multi-junction stacked gallium arsenide cell on which the third grating is fabricated;
  • the light intensity distribution at the bottom of the emitter of the PN junction region of the second junction of the cell after the light of 750 nm is incident on the surface of the multi-junction GaAs cell on which the third grating is fabricated.
  • Fig. 21 is a graph showing the contrast between the reflectance after the third grating is fabricated on the surface of the multi-junction laminated gallium arsenide cell and the reflectance when the third grating is not fabricated.
  • Figure 22 is a graph showing the absorption ratio of the first junction PN junction region after the third grating is fabricated on the surface of the multi-junction stacked gallium arsenide cell and the absorption ratio of the first junction PN junction region when the third grating is not fabricated. .
  • Figure 23 is a graph showing the absorption ratio of the second junction PN junction region after the third grating is fabricated on the surface of the multi-junction stacked gallium arsenide cell and the absorption ratio of the second junction PN junction region when the third grating is not fabricated. .
  • the present invention includes a solar cell, characterized in that a transmissive phase grating or a Fresnel lens is formed on an upper surface of the solar cell, and the phase grating or the Fresnel lens is composed of a series of etching grooves.
  • the depth of the etched trench is one quarter of the center wavelength of the incident light such that the phase difference between the light reflected from the bottom of the etched trench and the light reflected from the upper surface of the grating or Fresnel lens is ⁇ .
  • the refractive index of the solar cell surface phase grating or Fresnel lens material is close to three.
  • the solar cell is a three-junction stacked gallium arsenide solar cell, wherein the first junction material is indium gallium phosphate, the second junction material is gallium arsenide, and the third junction material is indium gallium arsenide solar cell.
  • the upper surface of the triple junction stacked gallium arsenide solar cell has a phase grating for the focus point and the center wavelength of the first junction, and focuses the incident light on the triple junction stacked gallium arsenide solar cell.
  • a knot in the PN junction area is shown in FIG. 3.
  • the upper surface of the triple junction stacked gallium arsenide solar cell has a phase grating for the focus point and the center wavelength of the second junction, and focuses the incident light on the triple junction stacked gallium arsenide solar cell.
  • the PN junction region of the two junctions In the PN junction region of the two junctions.
  • the upper surface of the triple junction stacked gallium arsenide solar cell has respective phase gratings for the focus point and the center wavelength of the first junction and the second junction, and the two phase gratings are orthogonal, and simultaneously The incident light is focused in the junction of the first junction and the second junction PN of the triple junction stacked gallium arsenide solar cell.
  • the upper surface of the triple junction stacked gallium arsenide solar cell has a Fresnel lens for the focus point and the center wavelength of the first junction, and focuses the incident light on the triple junction stacked gallium arsenide solar cell.
  • the first junction is in the PN junction region.
  • the upper surface of the triple junction stacked gallium arsenide solar cell has a Fresnel lens for the focus point and the center wavelength of the second junction, and focuses the incident light on the triple junction stacked gallium arsenide solar cell.
  • the second junction of the PN junction region is the first junction of the PN junction region.
  • a transmissive phase grating or a Fresnel lens is formed on the upper surface of the solar cell.
  • the phase grating and the Fresnel lens are composed of a series of periodic etching grooves having a period equal to the center wavelength of the junction material and perpendicular to the junction.
  • the incident light in the region is converted into light that propagates in the horizontal direction of the junction region.
  • the depth of the etched trench is one quarter of the center wavelength, such that the phase difference between the light reflected from the bottom of the etched trench and the light reflected from the upper surface of the grating is ⁇ .
  • the solar cell with a transmissive phase grating and a Fresnel lens nanostructure comprises a transmissive phase grating and a Fresnel lens fabricated on the surface of a solar cell.
  • the triple junction GaAs solar cell has a first junction material of indium gallium phosphide, a second junction material of gallium arsenide, and a third junction material of indium gallium arsenide (grown by a double-sided growth method on a gallium arsenide substrate). on).
  • the photoelectric conversion efficiency of a solar cell refers to the ratio of the output power when the optimum load resistance is connected to the external circuit to the total energy incident on the surface of the solar cell.
  • Photoelectric conversion efficiency is a measure of solar cells Important parameters of quality and technical level. In addition to the photo-generated carrier generation capability of the solar cell PN junction itself, there are many external factors that affect the photoelectric conversion efficiency. The common solar cell surface reflectance to sunlight and the solar cell PN junction region absorb photons. Wait.
  • the invention makes the transmission phase grating and the Fresnel lens on the surface of the battery, on the one hand, reduces the surface reflection, on the other hand, the incident light is focused on the PN junction region, or the vertical incident light is deflected by an angle by the grating diffraction effect. , increasing the propagation length of light in the PN junction region, increasing the absorption rate of photons, thereby improving the photoelectric conversion efficiency of the entire solar cell.
  • the transmissive phase grating and the Fresnel lens according to the present invention are a special grating designed based on the Huygens-Fresnel principle for concentrating the parallel light.
  • the Huygens principle describes the wave propagation process.
  • the wavefront of the wave source S is ⁇
  • each point on the ⁇ is a new secondary source that emits a spherical subwave.
  • the envelope surface of all secondary waves forms a new wavefront ⁇ '.
  • the normal direction of the wavefront is the direction of propagation of the wave.
  • This is the Huygens principle.
  • Fresnel introduced the interference phenomenon into the Huygens principle. Since all the secondary waves are from the same source, these secondary waves are coherent, so the light vibration at each point on the new wavefront ⁇ ' is emitted from any wavefront between the source and the point. The superposition result of the secondary wave. This is the Huygens-Fresnel principle.
  • the Fresnel zone method based on the Huygens-Fresnel principle can be used. As shown in Fig. 2, it is assumed that the incident light is monochromatic parallel light of wavelength ⁇ , and the wavefront at a certain moment is ⁇ , and P is the designed focal point.
  • r 0 + ⁇ /2, r 0 + ⁇ , r 0 +3 ⁇ /2...r 0 +j ⁇ /2 is used as the radius, and a circle is drawn in the wavefront, and the wavefront is divided into a plurality of zones.
  • the optical path difference from the edge of the adjacent ring to the point P is exactly half the wavelength ⁇ /2.
  • These ring bands are Fresnel half-bands, which are etched every other Fresnel half-band to obtain the desired Fresnel lens.
  • n is the refractive index of the phase grating or the Fresnel lens material.
  • the etching groove depth h preferably makes the phase difference between the light reflected from the bottom of the etching groove and the light reflected from the upper surface of the grating is ⁇ , that is,
  • the phase grating material obtained by the above two formulas has an optimum refractive index of 3. Since sunlight has a wide spectral range, the above two equations do not need to be established for a specific wavelength at the same time, and only need to be close.
  • aluminum arsenide is one of the materials that meets the above conditions. It is lattice-matched with gallium arsenide, and the real part of the refractive index is kept at about 3 at a wavelength greater than 670 nm. Moreover, due to its large forbidden band width, the absorption cutoff wavelength is around 440 nm, which is substantially transparent in the visible light wavelength range, which can reduce the loss of incident sunlight in the phase grating.
  • aluminum arsenide is easily oxidized in air, this problem can be solved by incorporating a small amount of gallium into its components.
  • phase grating and the Fresnel lens described above can be fabricated on a single junction solar cell or on a multijunction solar cell.
  • two orthogonal gratings can be designed separately for the focus points on the two different PN junctions and the center wavelengths corresponding to the absorption peaks of the two different PN junctions.
  • the invention is illustrated below by a specific embodiment of two triple junction stacked gallium arsenide solar cells.
  • the first junction of the cell comprises a Fresnel lens layer on the upper surface, a 20 nm thick In 0.51 Al 0.49 P window layer, and a first junction PN junction region comprising a 60 nm layer.
  • the second junction comprises a 30 nm Al 0.4 Ga 0.6 As window layer, the PN junction region comprising an 85 nm GaAs emitter and a 3500 nm GaAs base, wherein the GaAs has an absorption center wavelength of approximately 750 nm.
  • Fresnel lens nanostructures for wavelengths of 500 nm and 750 nm, respectively, such that the diffracted light is focused at the bottom of the emitter of the first junction and the second junction PN junction region.
  • the specific design is as follows:
  • the Fresnel lens material is aluminum arsenide
  • the refractive index n AlAs of aluminum arsenide is 3.36 at a wavelength of 500 nm
  • the etch groove depth of the Fresnel lens is 3.36 at a wavelength of 500 nm
  • In 0.51 Al 0.49 P has a refractive index of 3.18 at a wavelength of 500 nm.
  • In 0.49 Ga 0.51 P has a refractive index of 3.72 at a wavelength of 500 nm.
  • the first lens structure designed in this embodiment is shown in FIG.
  • the absorption center wavelength is 750 nm
  • the wavelength of 750 nm is taken as the center wavelength
  • the half-waveband parameters of the second lens are as follows:
  • the second lens structure designed in this embodiment is shown in FIG.
  • the refractive index and absorptivity of different materials vary with wavelength and affect light. Therefore, in the design process, the real part n and virtual of the complex refractive index of GaAs, In 0.51 Al 0.49 P and In 0.49 Ga 0.51 P materials Department k is within the design considerations.
  • the Fresnel lens of the present invention can be produced in a large area by a nanoimprint method, thereby reducing the manufacturing cost.
  • the Fresnel lens lens is formed on the surface of the gallium arsenide triple junction battery, and has a remarkable anti-reflection effect.
  • the following formula can calculate the weight reflectance (average reflectance) at this time:
  • I( ⁇ ) is the light intensity of the sunlight at the wavelength ⁇
  • R( ⁇ ) is the reflectance of the solar cell at the wavelength ⁇ . It is calculated that the weight reflectance of the gallium arsenide triple junction solar cell sheet with no structure on the surface in the range of 300 nm to 900 nm solar spectrum (AM 1.5) is 35.3%, and the first lens and After the second lens, the weight reflectance decreased to 14.1% and 13.8%, respectively. It can be seen that the Fresnel lens on the surface of the gallium arsenide triple junction solar cell can significantly reduce the surface reflectance. The decrease in surface reflectance means that more photons can be absorbed by the overall solar cell material, which has a very positive effect on increasing the short-circuit current, thereby increasing the photoelectric conversion efficiency of the cell.
  • Figure 6 is a gallium arsenide triple junction solar cell in which the first lens is fabricated in the first junction PN junction region and a gallium arsenide triple junction solar cell without any surface structure.
  • the weight absorption rate (average absorption rate) at this time can be calculated by the following formula:
  • a ( ⁇ ) in the above formula is the absorption rate of the solar cell at the wavelength ⁇ .
  • the weight absorption of the absorption layer of the first junction PN junction region of the first junction of the gallium arsenide triple junction laminate without any surface structure in the solar spectrum (AM1.5) of 300 nm to 900 nm is 31.2%, and The solar cell sheet on which the first lens was fabricated had a weight absorption rate of 35.9% in this layer.
  • the weight absorption rate (average absorption rate) of the solar cell to photons can be calculated:
  • the photon absorption rate of the first junction PN junction region in the wavelength range of 300 nm to 900 nm is 27.8% in the solar cell without the transmission Fresnel lens, and the photon absorption in the region after the first lens is fabricated. The rate was increased to 32.4%.
  • the absorption rate of the second junction PN junction region As shown in Fig. 10, it can be visually seen that the absorption ratio of the junction region is improved after the second lens is fabricated.
  • the light absorption at a central wavelength of 750 nm was increased from 39.3% to 55.0%. It was calculated that the weight absorption of the PN junction absorption layer of the second junction of the gallium arsenide triple junction laminate without any microstructure was in the range of 300 nm to 900 nm (AM 1.5), and the weight absorption rate was 12.9%.
  • the weight absorption rate of the light intensity in the region is increased to 18.8%.
  • the weight absorption rate for photons increased from 16.3% to 24.2%.
  • two orthogonal one-dimensional phase grating nanostructures are designed to focus on the wavelengths of 500 nm and 750 nm, respectively, so that the diffracted light is focused on the PN junction region of the first junction and the second junction.
  • the bottom of the emitter Although such a structure has a lower focusing effect than the Fresnel lens, in practical applications, it can be designed as an orthogonal two-dimensional grating, so that the focus can be simultaneously performed in different junction regions of the battery, and the principle of design Similar to the Fresnel lens, the specific design is as follows:
  • the design of the first lens of Nyer can give the following structural parameters of the grating:
  • the second grating structure designed in this embodiment is as shown in FIG.
  • the focal plane can be obtained in the first junction PN junction region (corresponding wavelength 500 nm) and the second junction PN junction region (corresponding wavelength 750 nm).
  • the two-dimensional transmission type phase grating is as shown in FIG.
  • the refractive index and absorptivity of different materials vary with wavelength and affect light. Therefore, in the design process, the real part n and virtual of the complex refractive index of GaAs, In 0.51 Al 0.49 P and In 0.49 Ga 0.51 P materials Department k is within the design considerations.
  • phase grating nanostructures of the present invention can also be used in single junction solar cells, and two orthogonal gratings can focus on the absorption layer of the same PN junction for two different wavelengths. It is also possible to use only a phase grating in one direction.
  • the phase grating of the present invention may also adopt a periodic structure whose period is close to the center wavelength (ie, ⁇ /n) in the material of the first junction region, and does not focus the incident light, but converts the normally incident light to substantially along the junction.
  • the area spreads light horizontally, thereby increasing the absorption length and absorption rate.
  • Fig. 12 it can be seen from Fig. 12 that after the incident light of the wavelength of 500 nm passes through the first grating, a plurality of diffraction orders appear at the bottom of the absorption layer of the first junction PN junction region, and the intensity of the main diffraction order is high and the range is very high. Small (width is 100nm, much smaller than the 2 micron width of the entire unit), the focusing function is basically achieved. As shown in Fig. 16, the incident light of 750 nm wavelength passes through the second grating and has a very good focusing effect on the absorption layer of the second junction PN junction region. As can be seen from FIG.
  • the weight reflectance of the gallium arsenide triple junction solar cell sheet with no structure on the surface in the range of 300 nm to 900 nm solar spectrum (AM 1.5) is 35.3%, and the first grating is fabricated.
  • the weight reflectance dropped to 10.1%.
  • the weight reflectances of the second grating and the third grating cell were calculated to be 12.6% and 10.2%, respectively. Comparing the anti-reflection effect of the Fresnel lens, the anti-reflection effect after the surface phase grating is made is slightly better than the Fresnel lens.
  • the absorbance at the center wavelength of 500 nm increased from 51.6% to 82.7% (after making the first grating) and 67.5% (after making the third grating).
  • the weight absorption of the absorption layer of the first junction PN junction region of the first junction of the gallium arsenide multi-junction cell without any surface structure in the range of 300 nm to 900 nm solar spectrum (AM 1.5) is 31.2%.
  • the weight absorption rates of the cells of the first grating and the third grating in the layer reached 50.9% and 40.1%, respectively.
  • the photon absorption rate was increased from 27.8% to 44.0% (after the first grating produced) and 36.2% (after the third grating was fabricated).
  • the solar cell of the grating has an influence on the absorption rate of the first junction PN junction region, which is slightly lower than that of the solar cell in which the first grating is fabricated.
  • the absorption ratio of the junction region is improved.
  • the light absorption at a central wavelength of 750 nm was increased from 39.3% to 56.1% (after making the second grating) and 60.4% (after making the third grating).
  • the weight of the PN junction absorption layer of the second junction GaAs junction region of the second junction of the gallium arsenide triple junction cell without any microstructure is in the range of 300 nm to 900 nm solar spectrum (AM 1.5)
  • the absorption rate was 12.9%, and the weight absorption rate of the region was increased to 19.7% and 20.1%, respectively, after the second grating and the third grating were fabricated.
  • the weight absorption rate for photons increased from 16.3% to 25.2% and 25.7%, respectively.
  • the transmissive phase grating and the Fresnel lens nanostructure of the present invention have many advantages and have outstanding technical effects.
  • the phase grating and the Fresnel lens can be fabricated in a large area by nanoimprinting, thereby reducing the manufacturing cost;
  • the phase grating and the Fresnel lens are fabricated in a triple junction laminated gallium arsenide solar energy
  • the surface of the battery, so these nanostructures and triple junction GaAs solar cells have a high degree of integration, greatly reducing the cost of solar cells.
  • the reflectivity of the surface of the solar cell can be significantly reduced, which can effectively improve the utilization of sunlight by the solar cell.
  • phase grating and the Fresnel lens have a converging effect on the light, the absorption length of the photon in the PN junction region is greatly increased. More importantly, the specially designed phase grating increases the light field intensity of the target PN junction absorption region at the focal plane, which is beneficial to the generation of photogenerated carriers, can greatly improve the short-circuit current of the battery, and can effectively improve the open circuit of the battery. Voltage.

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Abstract

一种带有表面纳米结构的太阳能电池。在太阳能电池的上表面制作有表面纳米结构，表面纳米结构为相位光栅或者菲涅尔透镜，相位光栅或者菲涅尔透镜均由一系列刻蚀槽构成，刻蚀槽的深度由公式计算得出，使得入射光从光栅上表面传播到下表面的相位差为π；刻蚀槽的宽度和间隔根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得到。利用表面纳米结构与太阳能电池集成，实现高性能、小尺寸高效率光伏系统；可以有效降低太阳能电池表面的反射率；有效增加光子在太阳能电池PN结区的吸收长度；有效增加入射光在太阳能电池的PN结区的光场强度；有效提高太阳能电池的PN结区对光子的有效吸收。

Description

带有表面纳米结构的太阳能电池 技术领域
本发明涉及了太阳能电池，尤其涉及了带有表面纳米结构的太阳能电池。
背景技术
光伏发电一直是备受关注的清洁能源解决方案，太阳能电池也是目前最具活力的研究领域之一。太阳能电池依靠光伏效应来实现对太阳能的有效利用，在经近一个世纪的发展之后，太阳能电池已历经三代，第一代是基于硅材料的晶体太阳能电池，第二代是包括非晶硅、聚晶硅(polycrystallinesilicon)、碲化镉(CdTe)、硒化铜铟镓(CIGS)等材料的薄膜型电池，第三代是新型高转换率太阳能电池，包括多结型(multi-junction or tandem)、中间能带性(intermediate band)、载流子倍增型(carrier-multiplication)、光子能量转变型(prior up or down energyconversion)，其中只有多结型目前被应用到了实际当中。
目前市场上主流的太阳能电池大多为硅太阳能电池。但是硅太阳能电池的光电转换效率极限只有24％。为了达到更高的转换效率，人们开始研制基于砷化镓材料的太阳能电池。单结的砷化镓太阳能电池的理论极限约为27％。
砷化镓属于III-V族化合物半导体材料，与硅不同，它是直接带隙材料，有着更好的能量转化效率。而且，砷化镓的带隙为1.42eV，与太阳光的光谱匹配能力好，对太阳光有非常好的吸收特性。因此，相比于硅太阳能电池通常150微米的厚度，砷化镓太阳能电池可以做得更薄，达到5～10微米。此外，砷化镓太阳能电池具有耐高温的特性，在300℃的条件下，硅太阳能电池已经停止运作，而砷化镓电池的转换效率仍然有10％，这一特性使得砷化镓电池可以被用于聚光太阳能系统。
为突破单结太阳能电池的转换效率极限，人们开始研究基于不同禁带宽度材料的多结叠层太阳能电池结构。1994年，美国的清洁能源实验室(NREL)发布GaInP/GaAs聚光多结太阳能电池，效率超过30％。如今，多结叠层太阳能电池的已成为转换效率最高的太阳能电池结构，并不断刷新记录。根据NREL最新的统计数据，截至本发明撰写之时，转换效率最高已达到44.7％。
在此基础上，很多人希望通过制作表面纳米结构的方式获得进一步改良，其中包括纳米线、纳米锥、减反层镀膜等技术，通过降低表面反射率的方法提高太阳能电池材料整体对入射光的吸收率，从而提高光电转换效率。
发明内容
针对现有技术的不足，本发明的目的是提出一种带有表面纳米结构的太阳能电池，解决传统太阳能电池反射率高、光子吸收率低、短路电流和开路电压较低等缺点，并针对太阳能电池结层的第一结PN结区和第二结PN结区进行设计优化。
本发明的目的是通过以下技术方案来实现的：
本发明包括太阳能电池，其特征在于：在所述太阳能电池的上表面制作有表面纳米结构，表面纳米结构为相位光栅或者菲涅尔透镜，相位光栅或者菲涅尔透镜均由一系列刻蚀槽构成。
所述的表面纳米结构的刻蚀槽的深度由公式h＝λ/[2(n-1)]计算得出，使得入射光从光栅上表面传播到下表面的相位差为π，其中λ为入射光的中心波长，n是太阳能电池表面纳米结构材料的折射率；刻蚀槽的宽度和间隔根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得到。
所述的表面纳米结构为相位光栅，相位光栅由多条平行间隔均布的条形刻蚀槽构成，各个条形刻蚀槽的宽度和间隔根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得到。
所述的表面纳米结构为菲涅尔透镜，菲涅尔透镜由同一平面的多个同心环状刻蚀槽构成，所有同心圆环刻蚀槽两侧边缘的半径向外依次根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得到。
所述刻蚀槽的深度为入射光的中心波长的四分之一，使从刻蚀槽底部反射的光与从表面纳米结构上表面反射的光的相位差为π。
所述太阳能电池表面纳米结构材料的折射率接近3。
所述太阳能电池为三结叠层砷化镓太阳能电池，其中第一结材料为铟镓磷，第二结材料为砷化镓，第三结材料为铟镓砷的太阳能电池。
所述三结叠层砷化镓太阳能电池上表面，针对第一结的聚焦点和中心波长有该表面纳米结构，把入射光聚焦在三结叠层砷化镓太阳能电池的第一结的PN结区中。
所述三结叠层砷化镓太阳能电池上表面，针对第二结的聚焦点和中心波长有该表面纳米结构，把入射光聚焦在三结叠层砷化镓太阳能电池的第二结的PN结区中。
所述三结叠层砷化镓太阳能电池上表面，分别针对第一结和第二结的聚焦点和中心波长有各自的相位光栅，两个相位光栅正交，同时把入射光聚焦在三结叠层砷化镓太阳能电池的第一结和第二结PN的结区中。
本发明具有的有益效果是：
1.本发明利用一个新的原理机制将表面纳米结构(透射型相位光栅或菲涅尔透镜)与太阳能电池集成，实现高性能、小尺寸高效率光伏系统。
2.本发明的表面纳米结构(透射型相位光栅或菲涅尔透镜)可有效降低太阳能电池表面的反射率。
3.本发明的表面纳米结构(透射型相位光栅或菲涅尔透镜)可有效增加光子在太阳能电池PN结区的吸收长度。
4.本发明的表面纳米结构(透射型相位光栅或菲涅尔透镜)可有效增加入射光在太阳能电池的PN结区的光场强度。
5.本发明的表面纳米结构(透射型相位光栅或菲涅尔透镜)可有效提高太阳能电池的PN结区对光子的有效吸收。
附图说明
图1是本发明带有表面相位光栅或菲涅尔透镜纳米结构的太阳能电池的示意图。
图2是本发明的相位光栅和菲涅尔透镜设计示意图。
图3是本发明的针对三结叠层砷化镓电池的实施例设计的500纳米波长焦平面在第一结PN结区吸收层底部的菲涅尔透镜(简称第一种透镜)的示意图。
图4是当透菲涅尔透镜聚焦面设定在电池第一结的PN结结区发射极底部时，500纳米波长入射光在聚焦面处的光强分布。
图5是在三结叠层砷化镓电池表面制作了第一种透镜后的反射率与未制作第一种透镜时反射率的对比曲线图。
图6是在多结叠层砷化镓电池表面制作了第一种透镜后的第一结PN结区的吸收率与未制作第一种透镜时第一结PN结区吸收率的对比曲线图。
图7是本发明提出的针对750纳米波长设计的焦平面在第二结PN结区发射极底部的菲涅尔透镜(简称第二种透镜)的示意图。
图8是当菲涅尔透镜聚焦面设定在电池第二结的PN结结区发射极底部时，750纳米波长入射光在聚焦面处的光强分布。
图9是在多结叠层砷化镓电池表面制作了第二种透镜后的反射率与未制作第二种透镜时反射率的对比曲线图。
图10是在多结叠层砷化镓电池表面制作了第二种透镜后的第二结PN结区的吸收率与未制作第二种透镜时第二结PN结区吸收率的对比曲线图。
图11是本发明的针对三结叠层砷化镓电池的实施例设计的500纳米波长焦平面在第一结PN结区发射极底部的相位光栅(简称第一种光栅)的示意图。
图12是当透射型光栅聚焦面设定在电池第一结的PN结结区发射极底部时，500纳米波长入射光在聚焦面处的光强分布。
图13是在三结叠层砷化镓电池表面制作了第一种光栅后的反射率与未制作第一种光栅时反射率的对比曲线图。
图14是在多结叠层砷化镓电池表面制作了第一种光栅后的第一结PN结区的吸收率与未制作第一种光栅时第一结PN结区吸收率的对比曲线图。
图15是本发明提出的针对750纳米波长设计的焦平面在第二结PN结区发射极底部的相位光栅(简称第二种光栅)的示意图。
图16是当透射型光栅聚焦面设定在电池第二结的PN结结区发射极底部时，750纳米波长入射光在聚焦面处的光强分布。
图17是在多结叠层砷化镓电池表面制作了第二种光栅后的反射率与未制作第二种光栅时反射率的对比曲线图。
图18是在多结叠层砷化镓电池表面制作了第二种光栅后的第二结PN结区的吸收率与未制作第二种光栅时第二结PN结区吸收率的对比曲线图。
图19是本发明提出的同时针对500纳米波长和750纳米波长设计的焦平面分别在第一、二结PN结区发射极底部的正交相位光栅(简称第三种光栅)的示意图。
图20(a)是500纳米波长的光入射到制作了第三种光栅的多结叠层砷化镓电池表面后，电池第一结的PN结结区发射极底部的光强分布；(b)是750纳米波长的光入射到制作了第三种光栅的多结叠层砷化镓电池表面后，电池第二结的PN结结区发射极底部的光强分布。
图21是在多结叠层砷化镓电池表面制作了第三种光栅后的反射率与未制作第三种光栅时反射率的对比曲线图。
图22是在多结叠层砷化镓电池表面制作了第三种光栅后的第一结PN结区的吸收率与未制作第三种光栅时第一结PN结区吸收率的对比曲线图。
图23是在多结叠层砷化镓电池表面制作了第三种光栅后的第二结PN结区的吸收率与未制作第三种光栅时第二结PN结区吸收率的对比曲线图。
具体实施方式
下面结合附图和实施例对本发明作进一步说明。
如图1所示，本发明包括太阳能电池；其特征在于：在所述太阳能电池的上表面制作透射型相位光栅或菲涅尔透镜，相位光栅或菲涅尔透镜由一系列刻蚀槽构成，所述刻蚀槽的深度由公式h＝λ/[2(n-1)]计算得出，使得入射光从光栅或菲涅尔透镜上表面传播到下表面的相位差为π，其中λ为入射光的中心波 长，n是太阳能电池表面相位光栅或菲涅尔透镜材料的折射率；所述刻蚀槽的宽度和间隔根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得出。
所述刻蚀槽的深度为入射光的中心波长的四分之一，使从刻蚀槽底部反射的光与从光栅或菲涅尔透镜上表面反射的光的相位差为π。
所述太阳能电池表面相位光栅或菲涅尔透镜材料的折射率接近3。
所述太阳能电池为三结叠层砷化镓太阳能电池，其中第一结材料为铟镓磷，第二结材料为砷化镓，第三结材料为铟镓砷的太阳能电池。
如图3所示，所述三结叠层砷化镓太阳能电池上表面，针对第一结的聚焦点和中心波长有相位光栅，把入射光聚焦在三结叠层砷化镓太阳能电池的第一结的PN结区中。
如图7所示，所述三结叠层砷化镓太阳能电池上表面，针对第二结的聚焦点和中心波长有相位光栅，把入射光聚焦在三结叠层砷化镓太阳能电池的第二结的PN结区中。
如图11所示，所述三结叠层砷化镓太阳能电池上表面，分别针对第一结和第二结的聚焦点和中心波长有各自的相位光栅，两个相位光栅正交，同时把入射光聚焦在三结叠层砷化镓太阳能电池的第一结和第二结PN的结区中。
如图16所示，所述三结叠层砷化镓太阳能电池上表面，针对第一结的聚焦点和中心波长有菲涅尔透镜，把入射光聚焦在三结叠层砷化镓太阳能电池的第一结的PN结区中。
如图17所示，所述三结叠层砷化镓太阳能电池上表面，针对第二结的聚焦点和中心波长有菲涅尔透镜，把入射光聚焦在三结叠层砷化镓太阳能电池的第二结的PN结区中。
在太阳能电池的上表面制作透射型相位光栅或菲涅尔透镜，相位光栅和菲涅尔透镜由一系列具有的周期性刻蚀槽构成，其周期等于结区材料的中心波长，将垂直于结区入射的光转为沿着结区水平方向传播的光。
所述刻蚀槽深度为中心波长的四分之一，使从刻蚀槽底部反射的光与从光栅上表面反射的光的相位差为π。
如图1所示，本发明所述的带有透射型相位光栅和菲涅尔透镜纳米结构的太阳能电池包含一种制作在太阳能电池表面的透射型相位光栅和菲涅尔透镜，该实施例为三结叠层砷化镓太阳能电池，其第一结材料为铟镓磷，第二结材料为砷化镓，第三结材料为铟镓砷(通过双面生长法生长在砷化镓衬底上)。
太阳能电池的光电转换效率是指在外部回路上连接最佳负载电阻时的输出功率与入射到太阳能电池表面的总能量之比。光电转换效率是衡量太阳能电池 的质量和技术水平的重要参数。除太阳能电池PN结本身的光生载流子产生能力之外，影响光电转换效率的外部因素有很多，常见的有太阳能电池表面对于太阳光的反射率，太阳能电池PN结结区对光子的吸收能力等。
对于太阳能电池，不仅需要通过降低电池表面的反射率来提升整体的光子吸收，更要增加光子在PN结结区的吸收，从而增加光生载流子密度，进一步增大太阳能电池的短路电流和开路电压。
本发明通过在电池表面制作透射型相位光栅和菲涅尔透镜，一方面减少表面反射，另一方面使得入射光聚焦在PN结结区，或者通过光栅衍射效应使垂直入射的光偏折一个角度，增加光在PN结结区的传播长度，提升光子的吸收率，进而提高整个太阳能电池的光电转换效率。
下面将详述透射型相位光栅和菲涅尔透镜的设计细节。
本发明涉及的透射型相位光栅和菲涅尔透镜是基于惠更斯—菲涅尔原理设计的一种特殊光栅，用来对平行光起到汇聚作用。
惠更斯原理描述了波的传播过程。假设在某一时刻，波源S的波阵面为∑，∑上的每一点都是一个发出球面次波新的次波源。在一段时间后，所有次波的包络面形成了一个新的波阵面∑’。波面的法线方向就是波的传播方向。这个就是惠更斯原理。菲涅尔将干涉现象引入了惠更斯原理。由于所有的次波都来自同一个光源，因此这些次波是相干的，所以新的波阵面∑’上的每一点的光振动都是光源和该点之间任意一个波阵面上发出的次波的叠加结果。这个就是惠更斯—菲涅尔原理。
本发明对于透射型相位光栅和菲涅尔透镜的设计，可以采用基于惠更斯—菲涅尔原理的菲涅尔波带法。如图2所示，假设入射光是波长为λ的单色平行光，其在某一时刻的波面是∑，P是设计的聚焦点。假设P点到波面∑的距离是r0，以r0+λ/2，r0+λ，r0+3λ/2…r0+jλ/2为半径在法平面内画圆，这些圆与波面∑相交并将其分割成很多区段，相邻区段对应边缘到P点的光程差正好是半波长λ/2(相位差为π)。将这些区段沿法平面正交方向延伸，就形成一系列条形波带，这些条形半波带就是所需要的透射型相位聚焦光栅。如果以P为圆心，以r0+λ/2，r0+λ，r0+3λ/2…r0+jλ/2为半径，在波面∑内画圆，将波面∑分成很多的环带。这些相邻环带对应边缘到P点的光程差正好是半波长λ/2。这些环带就是菲涅尔半波带，每隔一个菲涅尔半波带进行刻蚀，就可以得到所需要的菲涅尔透镜。
为了实现入射光在P点处相位增强，需要补偿相邻菲涅尔半波带对应边缘到P点的λ/2光程差(π相位差)，因此需要制作透射型相位光栅或菲涅尔透镜， 将每隔一个的条形波带刻蚀一定深度，使得入射光从光栅或菲涅尔透镜上表面传播到下表面的相位差为π(或π的奇数倍)，因此刻蚀槽深度h应满足：
Figure PCTCN2015072510-appb-000001
上式中n是相位光栅或菲涅尔透镜材料的折射率。
同时，为了减小表面反射，刻蚀槽深度h最好能使从刻蚀槽底部反射的光与从光栅上表面反射的光的相位差为π，即满足
Figure PCTCN2015072510-appb-000002
由以上二式可得相位光栅材料的最佳折射率为3。由于太阳光具有很宽的光谱范围，所以以上二式不需要对特定的波长同时成立，只需要接近即可。例如：砷化铝是比较符合上述条件的材料之一。它与砷化镓晶格匹配，折射率实部在波长大于670nm时基本保持在3左右。而且由于其禁带宽度较大，吸收截止波长在440nm左右，在可见光波长范围内基本透明，这可以减小入射的太阳光在相位光栅内的损耗。虽然砷化铝在空气中容易被氧化，但这个问题可通过在其组分中掺入少量镓解决。
上述相位光栅和菲涅尔透镜可以制作在单结太阳能电池上，也可制作在多结太阳能电池上。在多结太阳能电池的情况下，可以针对两个不同PN结上的聚焦点和对应这两个不同PN结吸收峰的中心波长分别设计制作两个正交的光栅。
下面通过两个三结叠层砷化镓太阳能电池的具体实施例说明本发明。
实施例1
下面通过一个三结叠层砷化镓太阳能电池的具体实施例说明本发明。参照图1的层状结构和参数，该电池的第一结包含一个位于上表面的菲涅尔透镜层，一个20nm厚的In0.51Al0.49P窗口层，第一结PN结区包含一个60nm的In0.49Ga0.51P发射极和1200nm的In0.49Ga0.51P基极，In0.49Ga0.51P的吸收中心波长为500纳米。第二结包含一个30nm的Al0.4Ga0.6As窗口层，其PN结区包含一个85nm的GaAs发射极和3500nm的GaAs基极，其中GaAs的吸收中心波长大约为750纳米。
我们设计的菲涅尔透镜纳米结构，分别针对500纳米和750纳米的波长，使得的衍射光聚焦在第一结和第二结PN结区的发射极底部。具体设计如下：
对于三结叠层砷化镓太阳能电池的第一结，其吸收中心波长为500纳米，因此取λ＝500nm进行设计。假设菲涅尔透镜材料为砷化铝，砷化铝的折射率nAlAs在500纳米波长处为3.36，则菲涅尔透镜的刻蚀槽深度：
Figure PCTCN2015072510-appb-000003
菲涅尔透镜的焦距：f＝20nm+60nm＝80nm
In0.51Al0.49P在500纳米波长处的折射率为3.18。
In0.49Ga0.51P在500纳米波长处的折射率为3.72。
等效折射率：
Figure PCTCN2015072510-appb-000004
各个菲涅尔半波带半径Rm满足以下公式：
Figure PCTCN2015072510-appb-000005
其中m是第m个圆环。由此计算可得第一种透镜的半波带参数如下表：
  中心波带 波带1 波带2 波带3 波带4 波带5 波带6
内半径(nm) 0 195 333 466 599 731 863
外半径(nm) 122 265 400 533 665 797 929
本实施例设计的第一种透镜结构如图3所示。
类似地对于三结叠层砷化镓太阳能电池第二结，其吸收中心波长为750纳米，因此取750纳米波长为中心波长进行设计，根据图1的结构参数计算得第二种透镜齿高h＝183nm，第二种透镜的半波带参数如下表：
  中心波带 波带1 波带2 波带3 波带4 波带5 波带6
内半径(nm) 0 875 1276 1609 1910 2191 2461
外半径(nm) 609 1088 1448 1762 2052 2327 2592
本实施例设计的第二种透镜结构如图7所示。
不同的材料的折射率与吸收率都随波长变化，并对光产生影响，因此在设计的过程中，GaAs、In0.51Al0.49P和In0.49Ga0.51P材料复折射率的实部n与虚部k都在设计的考虑范围内。
本发明的菲涅尔透镜可以通过纳米压印的方法进行大面积制作，从而降低制作成本。
效果分析
1，聚焦效果分析：
由图4可以看出，500纳米波长的入射光经过第一种透镜之后在第一结PN结区的发射极底部出现了很多衍射级次，而其中主衍射级次的强度很高且范围很小(宽度大约为100nm，远小于整个单元的宽度)，实现了非常好的聚焦功能。如图8所示，750纳米波长的入射光经过第二种透镜之后在第二结PN结区的发射极底部也呈现出非常好的聚焦效果。
2，减反射效果分析：
如图5和图9所示，将菲涅尔透镜透镜制作在砷化镓三结叠层电池表面后，起到了很明显的减反射效果。以下公式可以计算出此时的权重反射率(平均反射率)：
Figure PCTCN2015072510-appb-000006
上式中I(λ)是太阳光在波长λ处的光强，R(λ)是太阳能电池在波长λ处的反射率。经计算得出，表面未制作任何结构的砷化镓三结叠层太阳能电池片在300nm～900nm太阳能光谱(AM1.5)范围内的权重反射率为35.3％，而制作了第一种透镜和第二种透镜后，权重反射率分别降到14.1％和13.8％。由此可见，在砷化镓三结叠层太阳能电池表面制作菲涅尔透镜可以显著降低其表面反射率。而表面反射率的降低则意味着有更多的光子可以被整体太阳能电池材料吸收，这对增大短路电流，从而增大提高电池的光电转换效率有着非常积极的作用。
3，PN结区光子吸收率分析：
尽管在太阳能电池表面制作菲涅尔透镜可以降低表面反射率，从而增大整体吸收，但并不是所有被整体太阳能电池材料吸收掉的光子都可以被转换为载流子。在砷化镓三结叠层太阳能电池结构中，在特定波长范围内的光子只有被每一结的PN结区吸收掉，才能产生有效的光生载流子。因此，我们需要进一步分析该透射型菲涅尔透镜对目标结PN结区吸收率的影响。
如图6是制作了第一种透镜的砷化镓三结叠层太阳能电池在第一结PN结区的吸收率与未作任何表面结构的砷化镓三结叠层太阳能电池在该层结区吸收率的对比曲线。图中吸收率曲线在700nm附近的陡降与In0.49Ga0.51P在室温下1.85eV的禁带宽度相吻合。由这两幅图可以直观地看出，制作了菲涅尔透镜纳米结构后，目标结PN结区(焦平面)的吸收率获得了显著的提升。在中心波长500纳米处的吸收率从51.5％被提高到59.3％。由于作为菲涅尔透镜材料的砷化铝在440nm以下有较强吸收，因此制作了菲涅尔透镜后，第一结PN结区对440nm以下的光子吸收受到了影响。
通过以下公式可以计算出此时的权重吸收率(平均吸收率)：
Figure PCTCN2015072510-appb-000007
上式中A(λ)是太阳能电池在波长λ处的吸收率。经过该公式计算，未制作任何表面结构的砷化镓三结叠层电池片第一结PN结区吸收层在300nm～900nm太阳能光谱(AM1.5)范围内的权重吸收率为31.2％，而制作了第一种透镜的太阳能电池片在该层的权重吸收率达到了35.9％。根据下式可以计算出太阳能电池对光子的权重吸收率(平均吸收率)：
Figure PCTCN2015072510-appb-000008
经计算，未制作透射型菲涅尔透镜的太阳能电池电池在300nm-900nm波长范围内第一结PN结区的光子吸收率为27.8％，而制作了第一种透镜后，该区域的光子吸收率被提高到32.4％。
同样地，对于第二结PN结区的吸收率，如图10所示，可以直观地看出制作了第二种透镜后，该结区吸收率获得了提高。在中心波长750纳米处的光强吸收率从39.3％被提高到55.0％。经计算，未制作任何微结构的砷化镓三结叠层电池片第二结PN结区吸收层在300nm～900nm太阳能光谱(AM1.5)范围内的权重吸收率为12.9％，而制作了第二种透镜后，该区域光强的权重吸收率被提高到18.8％。而对光子的权重吸收率则从16.3％提高到24.2％。
实施例2
参照图1的层状结构和参数，设计两个正交的一维相位光栅纳米结构，分别针对500纳米和750纳米的波长，使得的衍射光聚焦在第一结和第二结的PN结区发射极底部。这样的结构与菲涅尔透镜相比虽然聚焦作用有所下降，但在实际应用中由于可以设计成正交的二维光栅，从而使得同时在电池的不同结区都可以进行聚焦，设计的原理与菲涅尔透镜相似，具体设计如下：
对于三结叠层砷化镓太阳能电池的第一结，其吸收中心波长为500纳米，因此取λ＝500nm进行设计。假设光栅材料为砷化铝，砷化铝的折射率nAlAs在500纳米波长处为3.36，根据公式h＝λ/[2(n-1)]计算得出刻蚀槽深度为106nm，类比菲涅尔第一种透镜的设计可得出第一种光栅的结构参数如下表：
Figure PCTCN2015072510-appb-000009
Figure PCTCN2015072510-appb-000010
实施例设计的第一种光栅结构如图11所示。类似地对于三结叠层砷化镓太阳能电池第二结，其吸收中心波长为750纳米，因此取750纳米波长为中心波长进行设计，根据图1的结构参数计算得第二种光栅齿高h＝183nm，波带参数如下表：
Figure PCTCN2015072510-appb-000011
本实施例设计的第二种光栅结构如图15所示。
通过将以上两种光栅结构正交，齿高取中间值h＝145nm，就可得到焦平面分别在第一结PN结区(对应波长500纳米)和第二结PN结区(对应波长750纳米)的二维透射型相位光栅，如图19所示。
不同的材料的折射率与吸收率都随波长变化，并对光产生影响，因此在设计的过程中，GaAs、In0.51Al0.49P和In0.49Ga0.51P材料复折射率的实部n与虚部k都在设计的考虑范围内。
当然，本发明的相位光栅纳米结构也可用于单结的太阳能电池，两个正交的光栅可以针对两个不同的波长聚焦在同一PN结的吸收层。也可以仅采用一个方向的相位光栅。
本发明的相位光栅也可采用周期性结构，其周期接近第一结区材料中的中心波长(即λ/n)，不对入射光进行聚焦，而是将垂直入射的光转为基本沿着结区水平传播的光，从而增大吸收长度和吸收率。
效果分析
1，聚焦效果分析：
由图12可以看出，500纳米波长的入射光经过第一种光栅之后在第一结PN结区的吸收层底部出现了很多衍射级次，而其中主衍射级次的强度很高且范围很小(宽度在100nm，远小于整个单元的2微米宽度)，基本上实现了聚焦功能。如图16所示，750纳米波长的入射光经过第二种光栅之后在第二结PN结区的吸收层也有非常好的聚焦效果。由图20可以看出，当将上述第一种光栅和第二种光栅两个透射型相位光栅正交获得第三种光栅后，出现了正交的衍射级 次。由于短波长对齿高的变化更为敏感，所以500纳米波长的入射光在第一结区吸收层的聚焦效果受到了一定影响，但仍能看到一定的聚焦作用，如图20(a)；而750纳米的入射光则在第二结结区区发射极底部仍然具有非常明显的聚焦效果，如图20(b)。
2，减反射效果分析：
如图13所示，将第一种光栅制作在砷化镓多结叠层电池表面后，起到了很明显的减反射效果。经计算得出，表面未制作任何结构的砷化镓三结叠层太阳能电池片在300nm～900nm太阳能光谱(AM1.5)范围内的权重反射率为35.3％，而制作了第一种光栅后，权重反射率降到10.1％。而根据图17和图21所示，计算得出制作了第二种光栅和第三种光栅电池片的权重反射率分别为12.6％和10.2％。对比菲涅尔透镜的减反射效果，制作表面相位光栅后的减反射效果略优于菲涅尔透镜。
3，吸收层光子吸收率分析：
如图14和图22所示，分别是制作了第一种光栅和第三种光栅的砷化镓三结叠层太阳能电池在第一结PN结区的吸收率与未作任何表面结构的砷化镓三结叠层太阳能电池在该层结区吸收率的对比曲线。图中吸收率曲线在700nm附近的陡降与In0.49Ga0.51P在室温下1.85eV的禁带宽度相吻合。由这两幅图可以直观地看出，制作了透射型相位光栅结构后，目标结PN结区(焦平面)的吸收率获得了显著的提升。在中心波长500纳米处的吸收率从51.6％分别提高到82.7％(制作第一种光栅后)和67.5％(制作第三种光栅后)。
经过公式计算，未制作任何表面结构的砷化镓多结叠层电池片第一结PN结区吸收层在300nm～900nm太阳能光谱(AM1.5)范围内的权重吸收率为31.2％，而制作了第一种光栅和第三种光栅的电池片在该层的权重吸收率分别达到了50.9％和40.1％。而光子吸收率则从27.8％，分别被提高到44.0％(制作了的第一种光栅后)和36.2％(制作了第三种光栅后)。由于作为光栅材料的砷化铝在440nm以下有较强吸收，而第三种光栅的高度比第一种光栅多了40nm，这增加了短波在第三种光栅内的损耗，因此制作了第三种光栅的太阳能电池在第一结PN结区的吸收率受到影响，略低于制作了第一种光栅的太阳能电池。
同样地，对于第二结PN结区的吸收率，如图18和图23所示，可以直观地看出制作了第二种光栅和第三种光栅后，该结区吸收率获得了提高。在中心波长750纳米处的光强吸收率从39.3％分别被提高到56.1％(制作第二种光栅后)和60.4％(制作第三种光栅后)。经计算，未制作任何微结构的砷化镓三结叠层电池片第二结PN结区吸收层在300nm～900nm太阳能光谱(AM1.5)范围内的权重 吸收率为12.9％，而制作了第二种光栅和第三种光栅后，该区域的权重吸收率分别被提高到19.7％和20.1％。而对光子的权重吸收率则从16.3％分别提高到25.2％和25.7％。
对比制作了菲涅尔透镜的三结叠层砷化镓太阳能电池，制作表面相位光栅对所述太阳能电池第一结和第二结PN结区光子的有效吸收率的贡献更为显著。
由上述实施例可见，本发明的透射型相位光栅和菲涅尔透镜纳米结构有很多优点，具有突出显著的技术效果。一方面，相位光栅和菲涅尔透镜可以通过纳米压印的方法进行大面积制作，从而降低制作成本；另一方面，因为该相位光栅和菲涅尔透镜制作在三结叠层砷化镓太阳能电池表面，所以这些纳米结构和三结叠层砷化镓太阳能电池具有高度的集成性，大大的降低了太阳能电池的成本。同时，太阳能电池表面的反射率也能显著降低，能够有效提高太阳能电池对于太阳光的利用。由于该相位光栅和菲涅尔透镜对光线具有汇聚作用，导致光子在PN结区的吸收长度大幅增加。更重要的是，经过特殊设计的相位光栅增大了焦平面处目标PN结吸收区的光场强度，有益于光生载流子的产生，能大幅提高电池的短路电流，能有效提高电池的开路电压。
本发明的实施例只是用来解释说明本发明，而不是对本发明进行限制，在本发明的精神和权利要求的保护范围内，对本发明作出的任何修改和改变，都落入本发明的保护范围。

Claims (10)

  1. 一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：在所述太阳能电池的上表面制作有表面纳米结构，表面纳米结构为相位光栅或者菲涅尔透镜，相位光栅或者菲涅尔透镜均由一系列刻蚀槽构成。
  2. 根据权利要求1所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述的表面纳米结构的刻蚀槽的深度由公式h＝λ/[2(n-1)]计算得出，使得入射光从光栅上表面传播到下表面的相位差为π，其中λ为入射光的中心波长，n是太阳能电池表面纳米结构材料的折射率；刻蚀槽的宽度和间隔根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得到。
  3. 根据权利要求1或2所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述的表面纳米结构为相位光栅，相位光栅由多条平行间隔均布的条形刻蚀槽构成，各个条形刻蚀槽的宽度和间隔根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得到。
  4. 根据权利要求1或2所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述的表面纳米结构为菲涅尔透镜，菲涅尔透镜由同一平面的多个同心环状刻蚀槽构成，所有同心圆环刻蚀槽两侧边缘的半径向外依次根据惠更斯-菲涅尔原理设计，由菲涅尔波带法计算得到。
  5. 根据权利要求1～4任一所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述刻蚀槽的深度为入射光的中心波长的四分之一，使从刻蚀槽底部反射的光与从表面纳米结构上表面反射的光的相位差为π。
  6. 根据权利要求1～4任一所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述太阳能电池表面纳米结构材料的折射率接近3。
  7. 根据权利要求1～4任一所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述太阳能电池为三结叠层砷化镓太阳能电池，其中第一结材料为铟镓磷，第二结材料为砷化镓，第三结材料为铟镓砷的太阳能电池。
  8. 根据权利要求1～4任一所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述三结叠层砷化镓太阳能电池上表面，针对第一结的聚焦点和中心波长有该表面纳米结构，把入射光聚焦在三结叠层砷化镓太阳能电池的第一结的PN结区中。
  9. 根据权利要求1～4任一所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述三结叠层砷化镓太阳能电池上表面，针对第二结的聚焦点和中心波长有该表面纳米结构，把入射光聚焦在三结叠层砷化镓太阳能电池的第二结的PN结区中。
  10. 根据权利要求1或3任一所述的一种带有表面纳米结构的太阳能电池，包括太阳能电池，其特征在于：所述三结叠层砷化镓太阳能电池上表面，分别针对第一结和第二结的聚焦点和中心波长有各自的相位光栅，两个相位光栅正交，同时把入射光聚焦在三结叠层砷化镓太阳能电池的第一结和第二结PN的结区中。
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