JP2010021189A - Photoelectric device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photoelectric device which has high energy conversion efficiency and whose manufacturing processes need not be made complex by combining a quantum well structure with local surface plasmon resonance by metal particulates. <P>SOLUTION: A photoelectric conversion layer 36 having a well layer 42 and a barrier layer 43 laminated alternately is laminated on an upper surface of a third semiconductor layer 35 formed above a substrate 32, and particulates 40 are dispersed on an upper surface of the photoelectric conversion layer 36. Each of the particulates 40 has a composite structure such that an outer peripheral surface of a dielectric core is covered with a shell portion of metal, and provides local surface plasmon resonance with two-wavelength light when receiving incident light. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a photoelectric device, and specifically to a photoelectric device such as a solar cell or a photoelectric sensor having a multiple quantum well structure or a quantum dot structure.

  Known solar cell structures include single-junction solar cells, multi-junction solar cells, and multi-quantum well structure solar cells.

(Single junction solar cell)
The solar cell having the simplest structure is a single junction type, and is generally a pn junction or a pin junction. FIG. 1 is an energy band diagram showing the structure of a general single-junction solar cell. The solar cell of FIG. 1 is configured by joining an n-type semiconductor layer, an i-type semiconductor layer, and a p-type semiconductor layer, and the band gap between the conduction band and the valence band is Eg. When light enters here, light having an energy higher than Eg is absorbed, and electrons 11 are excited from the valence band to the conduction band, holes 12 are generated in the valence band, and an electromotive force is generated in the solar cell. To do.

  However, in such a single-junction solar cell, light with energy smaller than the band gap Eg is transmitted without being absorbed, so that the photoelectric conversion efficiency is low. On the other hand, even if the band gap Eg is reduced, the energy (hν−Eg) larger than the energy at the bottom of the conduction band in the energy (hν: ν is the frequency of light) of the electrons 11 excited to the conduction band. They are immediately released as phonons and are consumed as thermal energy. Therefore, the single junction solar cell has poor photoelectric conversion efficiency.

(Multi-junction solar cell)
FIG. 2 is an energy band diagram showing the structure of the multijunction solar cell. This multi-junction solar cell is formed by stacking pn junctions having different band gaps. In FIG. 2, a pn junction having a band gap of Eg11, a pn junction having a band gap of Eg12, and a pn junction having a band gap of Eg13 are stacked (where Eg11>Eg12> Eg13). In such a multi-junction solar cell, light having an energy larger than Eg11 is absorbed by the pn junction having the band gap Eg11, and then light Eg12 being not absorbed by the pn junction having the band gap Eg11. Light having a large energy is absorbed by the pn junction having the band gap Eg12, and then light having a higher energy than Eg13 is absorbed by the pn junction having the band gap Eg13. Therefore, the multi-junction solar cell can absorb light in a wide wavelength range, reduce thermal energy loss, and realize higher photoelectric conversion efficiency than the single-junction solar cell. An example of the multi-junction solar cell is disclosed in JP-A-8-204215 (Patent Document 1).

  However, in a multi-junction solar cell, it is difficult to achieve both the matching of the lattice constant of each layer and the optimization of the band gap, and an ideal structure cannot be obtained. In addition, the multi-junction solar cell has a complicated layer structure, and process control is difficult. Further, the adjacent layers are connected in series via the tunnel junction 13, and the output current becomes the minimum current value generated in each layer, so that the photoelectric conversion efficiency is easily influenced by the change in the spectrum of sunlight. .

(Multiple quantum well structure)
Therefore, as shown in FIG. 3, a multiple quantum well structure in which semiconductor layers having band gaps Eg1 and Eg2 are repeatedly stacked has been proposed. Here, Eg1 is a band gap in the well layer 14, and Eg2 is a band gap in the barrier layer 15. As such a structure, there is one disclosed in JP-A-11-220150 (Patent Document 2).

  Such a multiple quantum well solar cell basically operates as a solar cell composed of a semiconductor having a bandgap energy of Eg2, but due to the presence of the well layer 14 having a bandgap energy of Eg1 smaller than Eg2. The wavelength range of light that can be absorbed is expanded to the longer wavelength side. Therefore, the current can be increased without reducing the open circuit voltage. In addition, compared to a multi-junction solar cell, there are the advantages that the same material system can be used and that it is easy to manufacture and that the absorption wavelength can be controlled by the size of the multiple quantum well.

  In such a multi-quantum well solar cell, the electrons 11 excited in the well layer 14 must be escaped from the well layer 14. For this purpose, a method using thermal energy and a method using optical energy are used. There is. However, in the method using thermal energy, it is necessary to keep the depth of the quantum well shallow as shown in FIG. 3, so the wavelength range of light that can be absorbed becomes narrow. Therefore, the photoelectric conversion efficiency of the solar cell cannot be increased so much.

  Further, in the method using light energy, light energy is absorbed in order to escape the electrons 11 from the quantum well, so that the electrons 11 are excited from the valence band to the conduction band in the barrier layer 15 as shown in FIG. 3 corresponding to energy Eg2 for excitation, electron Eg1 for exciting the electron 11 from the valence band to the quantum level in the well layer 14, and energy Eg3 for exciting the electron 11 from the quantum level to the conduction band, respectively. Wavelength light can be absorbed. Accordingly, light in a wider range of wavelengths can be absorbed, so that the photoelectric conversion efficiency can be improved and a large current / voltage can be expected.

  As shown in FIG. 5, a quantum dot solar cell in which quantum dots are stacked has also been proposed. In the quantum dot solar cell, quantum dots 17 are arranged in the barrier portion 16, and a three-dimensional quantum well is formed by the quantum dots 17. As a solar cell having such a structure, there is one disclosed in Japanese Patent Application Laid-Open No. 2002-141531 (Patent Document 3). The quantum dot solar cell absorbs light energy based on the same principle as the multiple quantum well solar cell, but the quantum level is made more discrete by using the quantum dot, and the electrons 11 are relaxed to a low energy level in the quantum well. There is a merit that can be difficult.

  However, in a solar cell having a quantum well structure (multiple quantum wells, quantum dots), there are many interfaces where different materials contact each other. For example, in a multiple quantum well solar cell, several tens of well layers and barrier layers are required to sufficiently absorb light. Since the materials and lattice constants are different at the interface between different materials, interface states Ed1 and Ed2 due to dangling bonds (dangling bonds) are likely to occur as shown in FIG. Then, since the electrons 11 and the holes 12 are recombined (interface recombination) through the interface states Ed1 and Ed2, there is a problem that carriers disappear and the photoelectric conversion efficiency of the solar cell is lowered. Further, when a thin multilayer structure such as a multiple quantum well is manufactured or when a large number of quantum dots are manufactured, the manufacturing process becomes complicated as the number of well layers and quantum dots increases.

(Solar cell using plasmon resonance)
A solar cell using plasmon resonance generated in metal fine particles has been proposed. As such a solar cell, there is one disclosed in Japanese Patent Application Laid-Open No. 2006-66550 (Patent Document 4). In this solar cell, as shown in FIG. 7, a transparent electrode 19 is formed on the surface of the photoelectric conversion layer 18, and a back electrode 20 is formed on the back surface, between the back surface of the photoelectric conversion layer 18 and the back electrode 20. Metal fine particles 21 are provided on the surface. When light is incident on the solar cell, the photoelectric conversion layer 18 absorbs the light and converts it into electric energy. Further, when light in a wavelength band having energy larger than the band gap of the photoelectric conversion layer 18 passes through the photoelectric conversion layer 18 and is irradiated onto the metal fine particles 21, the metal fine particles 21 are enhanced around the plasmon resonance. A strong electric field is generated. This electric field excites electrons in the photoelectric conversion layer 18 to the conduction band. Since the electric field formed at this time is enhanced to several tens of times the optical field of incident light, the probability that light is absorbed increases and the conversion efficiency into electric energy increases.

  Moreover, as a solar cell using localized surface plasmon resonance, there is one disclosed in US 2007/0289623 (Patent Document 5). As shown in FIG. 8, this solar cell is provided with a surface plasmon polariton guiding layer 23 made of metal on the surface of a substrate 22, and a quantum dot comprising a semiconductor quantum dot array on the surface. An active layer (quantum dot active layer) 24 and an electrode 25 are provided. Even in a solar cell having such a structure, the absorption of light can be increased by localized surface plasmon resonance, so that the quantum dot active layer 24 can be made thin. Further, this can improve the photoelectric conversion efficiency and reduce the cost.

JP-A-8-204215 JP-A-11-220150 JP 2002-141531 A JP 2006-66550 A US2007 / 0289623

  However, localized surface plasmon resonance by metal fine particles, which has been performed conventionally, shows an absorption spectrum as shown in FIG. FIG. 9 shows the result of calculating the absorption spectrum of Au particles having a diameter of 100 nm, and an absorption peak appears in the vicinity of the wavelength of 560 nm. Therefore, only light of one wavelength (wavelength corresponding to the maximum peak in FIG. 9) could be absorbed. In the absorption spectrum of FIG. 9, a small and gentle peak is observed at the bottom of the maximum peak (around 950 nm), but this small peak does not contribute to light absorption because the absorption coefficient is small. For this reason, in solar cells such as Patent Documents 4 and 5, even if localized surface plasmon resonance is used, only one excitation can be handled, and improvement in photoelectric conversion efficiency and the number of layers in the quantum well structure can be achieved. The reduction was not effective enough.

  More specifically, when metal fine particles are combined with a multiple quantum well structure as shown in FIG. 4, electrons are excited from the valence band to the quantum level in the well layer using localized surface plasmon resonance of the metal fine particles. Even so, if the number of electrons excited from the quantum level to the conduction band is the same as before, a significant improvement in the overall photoelectric conversion efficiency cannot be expected.

  The present invention has been made in view of such technical problems, and the object of the present invention is to achieve high energy conversion efficiency by combining localized surface plasmon resonance by a metal fine particle and a quantum well structure. And it is providing the photoelectric device which does not complicate a manufacturing process.

  A first photoelectric device according to the present invention includes a multiple quantum well in which a plurality of barrier layers made of a first semiconductor and well layers made of a second semiconductor having a smaller band gap than the first semiconductor are alternately stacked. A photoelectric device having a photoelectric conversion layer having a structure is characterized by having fine particles that perform localized surface plasmon resonance in at least two wavelength bands.

  In the first photoelectric device of the present invention, light having a wavelength that has undergone localized surface plasmon resonance out of incident light generates an enhanced electric field around the fine particles. This enhanced electric field excites electrons in the photoelectric conversion layer from the valence band to the level in the quantum well, from the level in the quantum well to the conduction band, or from the valence band to the conduction band. Electrons can be excited with higher efficiency than directly exciting electrons with incident light. In addition, in the photoelectric device that combines the localized surface plasmon resonance of the fine particles and the multiple quantum well structure, the resonance wavelength band of the localized surface plasmon is at least two wavelength bands, so that light in a wide wavelength range is transmitted in the photoelectric conversion layer. It can be absorbed with high efficiency, and the efficiency of converting light energy into electric energy can be further increased.

  Moreover, according to the 1st photoelectric device of this invention, as a result of making it absorb with high efficiency, the number of layers of a multiple quantum well structure can be reduced. In addition, since the electric field generated around the fine particles is generated in a region about the diameter of the fine particles or several times as much as that, it is reasonable to reduce the thickness of the photoelectric conversion layer to the extent of the electric field. Therefore, the number of barrier layers and well layers can be reduced, the manufacturing process of the photoelectric device can be simplified, and the cost can be reduced. Furthermore, since the number of barrier layers and well layers can be reduced, a decrease in electron collection efficiency due to interface defects can be suppressed, and the photoelectric conversion efficiency of the photoelectric device can be further improved.

  The second photoelectric device of the present invention is a photoelectric device having a quantum dot structure in which at least one quantum dot made of a second semiconductor having a smaller band gap than that of the first semiconductor is included in the barrier portion made of the first semiconductor. A photoelectric device having a conversion layer is characterized by having fine particles that perform localized surface plasmon resonance in at least two wavelength bands.

  In the second photoelectric device of the present invention, light having a wavelength that has undergone localized surface plasmon resonance out of incident light generates an enhanced electric field around the fine particles. This enhanced electric field excites electrons in the photoelectric conversion layer from the valence band to the level in the quantum well, from the level in the quantum well to the conduction band, or from the valence band to the conduction band. Electrons can be excited with higher efficiency than directly exciting electrons with incident light. In addition, in the photoelectric device that combines the localized surface plasmon resonance of fine particles and the quantum dot structure, the resonance wavelength band of the localized surface plasmon is at least two wavelength ranges, so that light in a wide wavelength range is high in the photoelectric conversion layer. It can be absorbed with efficiency, and the efficiency of converting light energy into electrical energy can be further increased.

  Moreover, according to the 2nd photoelectric device of this invention, as a result of making it absorb with high efficiency, the number of lamination | stacking of a quantum dot can be reduced. In addition, since the electric field generated around the fine particles is generated in a region about the diameter of the fine particles or several times as much as that, it is reasonable to reduce the thickness of the photoelectric conversion layer to the extent of the electric field. Therefore, the number of stacked quantum dots and barrier portions can be reduced, the manufacturing process of the photoelectric device can be simplified, and the cost can be reduced. Furthermore, since the number of stacked quantum dots and barrier portions can be reduced, a decrease in electron collection efficiency due to interface defects can be suppressed, and the photoelectric conversion efficiency of the photoelectric device can be further improved.

In an embodiment of the first or second photoelectric device of the present invention, a wavelength corresponding to energy for exciting electrons from a valence band to a level in the quantum well and an electron from the level in the quantum well to the conduction band. When the wavelengths corresponding to the energy to escape are set to λa and λb in order from the short side, and the two resonance wavelengths of the localized surface plasmon resonance generated in the fine particles are set to λ1 and λ2 in order from the short side,
λ1 ≦ λa ≦ λ2 ≦ λb
It is characterized by being.

  According to such an embodiment, electrons can be excited with high efficiency from the valence band to the level in the quantum well by localized surface plasmon resonance at the wavelength λ1 (or λ2), and at the wavelength λ2 (or λ1). Since electrons can be excited from the level in the quantum well to the conduction band by localized surface plasmon resonance, the electrons are efficiently excited from the valence band to the conduction band by two-stage excitation via the quantum level. be able to.

  In this embodiment, it is desirable that the resonance wavelengths λ1 and λ2 are separated from each other by 400 nm or more. If the two resonance wavelengths λ1 and λ2 are separated from each other by about 400 nm, light in a wide wavelength range can be absorbed, and energy conversion efficiency is improved.

  Further, in the absorption spectrum of the fine particles, the peak intensity at the resonance wavelength λ2 on the long wavelength side is preferably 0.7 to 2.0 times the peak intensity at the resonance wavelength λ1 on the short wavelength side. By setting the peak intensity at the resonance wavelength λ2 on the long wavelength side to 0.7 times or more and 2.0 times or less than the peak intensity at the resonance wavelength λ1 on the short wavelength side, the electric field intensity generated by plasmon resonance is the same. Therefore, it is possible to efficiently excite electrons twice.

  Furthermore, the peak intensity at the resonance wavelength λ2 on the long wavelength side is preferably 1.0 to 2.0 times the peak intensity at the resonance wavelength λ1 on the short wavelength side. If the probability of exciting an electron from the valence band to the level in the quantum well is similar to the probability of exciting the level from the level in the quantum well to the conduction band, the electron can be excited twice more efficiently. it can. Compared with the light on the short wavelength side, the light on the long wavelength side is less likely to excite electrons, so the peak intensity at the resonance wavelength λ2 on the long wavelength side is less than the peak intensity at the resonance wavelength λ1 on the short wavelength side. The probability of exciting electrons from the valence band to the level in the quantum well and the probability to excite the level from the level in the quantum well to the conduction band are closer. can do.

  Another embodiment of the first or second photoelectric device of the present invention is characterized in that a distribution representing the number of the fine particles for each size has two or more peaks. According to such an embodiment, localized surface plasmon resonance can be generated at two or more wavelengths by designing the size of the fine particles, and the resonance wavelength can be adjusted by changing the maximum number or size of the fine particle distribution. it can.

  Still another embodiment of the first or second photoelectric device of the present invention is characterized in that the fine particles include two or more kinds of fine particles made of different metals. According to this embodiment, localized surface plasmon resonance can be generated at two or more wavelengths by changing the metal species of the metal fine particles, and the resonance frequency can be adjusted by changing the metal species of the fine particles. it can.

  Still another embodiment of the first or second photoelectric device of the present invention is characterized in that the fine particles include two or more kinds of fine particles composed of a metal and different dielectrics. According to such an embodiment, it is possible to generate localized surface plasmon resonance at two or more wavelengths by changing the types of dielectrics constituting the fine particles, and to change the resonance frequency by changing the types and combinations of the dielectrics. Can be adjusted.

  Still another embodiment of the first or second photoelectric device of the present invention is characterized in that the fine particles include two or more kinds of fine particles having different shapes. According to this embodiment, localized surface plasmon resonance can be generated at two or more wavelengths by fine particle shape design, and the resonance frequency can be adjusted by changing the shape of each fine particle.

  Further, in this embodiment, one kind of fine particles among the fine particles is a rod-shaped fine particle made of a metal, and the length in the longitudinal direction is twice or more the size in the short direction, The dimension in the short direction is 10 nm or more and 100 nm or less. If the rod-shaped metal fine particles having such dimensions are included, localized surface plasmon resonance in a long wavelength region can be generated in two or more types of fine particles.

  Further, in this embodiment, one kind of fine particles is composed of a cylindrical core portion made of a dielectric and a ring portion made of a metal surrounding the core portion in a ring shape. The diameter of the circular cross section of the core part is 40 nm or more and 300 nm or less, the thickness of the ring part is 1 nm or more and 20 nm or less, the height of the ring part is 5 nm or more, and the outer diameter of the ring part is 0 It is characterized by being less than 5 times. If composite fine particles having such a structure and size are included, localized surface plasmon resonance in a long wavelength region can be generated in a plurality of types of fine particles.

  Further, in this embodiment, one kind of fine particles is a fine particle composed of a core portion made of a dielectric and a shell portion made of a metal covering the outer surface of the core portion in a shell shape, The outer dimension of the core part is 40 nm or more and 200 nm or less, and the thickness of the shell part is 1 nm or more and 20 nm or less. If composite fine particles having such a structure and size are included, localized surface plasmon resonance in a long wavelength region can be generated in a plurality of types of fine particles.

  Still another embodiment of the first or second photoelectric device of the present invention is configured such that the fine particles include a core portion made of a dielectric and a shell portion made of metal with an outer surface of the core portion covered in a shell shape. The outer dimension of the core part is 150 nm or more and 600 nm or less, and the thickness of the shell part is 1 nm or more and 20 nm or less. This is a condition for generating localized surface plasmon resonance at two wavelengths by fine particles having a shell structure. That is, if the outer diameter of the core is smaller than 150 nm, the peak of the absorption coefficient cannot be obtained on the long wavelength side, and if the outer diameter of the core is larger than 600 nm, a part of the incident light is scattered. This is because it becomes difficult for light to enter the photoelectric conversion layer. Further, when the thickness of the shell portion is larger than 20 nm, the absorption coefficient of the long wavelength side peak is decreased, and when the thickness of the shell portion is smaller than 1 nm, plasmon resonance is difficult to occur and sufficient electric field strength can be obtained. Because it disappears.

  Still another embodiment of the first or second photoelectric device of the present invention is such that the fine particles are distributed on a surface, and the coverage of the fine particles covering the surface is 1% or more and 30% or less. It is a feature. If the coverage of fine particles is less than 1%, light in a specific wavelength region cannot be plasmon-resonated with the fine particles, and if the coverage of fine particles is greater than 30%, light in a wavelength region that does not resonate is emitted from the particle layer. It is because it becomes difficult to permeate | transmit.

  Still another embodiment of the first or second photoelectric device of the present invention is such that the fine particles are distributed on a certain surface, and the fine particle distribution surface and the second semiconductor most distant from the fine particle distribution surface. The distance between them is not more than twice the average value of the external dimensions of the fine particles. The electric field generated around the fine particles gradually attenuates as the distance from the fine particles increases, and the spread is estimated to be approximately the diameter of the fine particles. Therefore, if the distance from the fine particle to the farthest second semiconductor is more than twice the external dimension of the fine particle (for example, the diameter in the case of a spherical fine particle), an electric field with sufficient strength reaches the entire photoelectric conversion layer. Therefore, it is desirable that the distance to the farthest second semiconductor is not more than twice the outer diameter diameter of the fine particles.

  According to still another embodiment of the first photoelectric device of the present invention, there is provided a third semiconductor layer having a larger band gap than the well layer and a thickness larger than the thickness of one layer of the barrier layer. And the third semiconductor layer is provided at an adjacent position on the opposite side to the light incident side of the photoelectric conversion layer. According to such an embodiment, light that has not been absorbed by the quantum well structure can be absorbed by the third semiconductor layer, so that the conversion efficiency can be further improved.

  Still another embodiment of the second photoelectric device of the present invention is a third semiconductor layer having a band gap larger than that of the quantum dots and a thickness larger than a distance between the quantum dots in the light transmission direction. The third semiconductor layer is provided at an adjacent position on the opposite side to the light incident side of the photoelectric conversion layer. According to such an embodiment, light that has not been absorbed by the quantum dots or the barrier portion can be absorbed by the third semiconductor layer, so that the conversion efficiency can be further improved.

  In these embodiments, it is desirable that the third semiconductor layer is formed of the same material as the first semiconductor. If the third semiconductor layer is formed using the same material as the first semiconductor, the manufacturing process of the solar cell can be simplified.

  Still another embodiment of the first photoelectric device of the present invention is characterized in that the well layer has a width of 10 nm or less and the barrier layer has a width of 10 nm or less. According to this embodiment, the level of the quantum well can be discretized, so that carriers can be made difficult to relax. Further, since electrons are easily tunneled between well layers, carrier recombination can be suppressed.

  According to still another embodiment of the second photoelectric device of the present invention, one of the height, width, and depth of the quantum dots has a length of 10 nm or less, and the remaining two sides have a length of 30 nm or less. It is characterized by being. According to this embodiment, the level of the quantum well can be discretized, so that carriers can be made difficult to relax.

  The means for solving the above-described problems in the present invention has a feature in which the above-described constituent elements are appropriately combined, and the present invention enables many variations by combining such constituent elements. .

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

(First embodiment)
FIG. 10 is a schematic cross-sectional view showing the structure of the solar cell according to Embodiment 1 of the present invention. In this solar cell 31, an n-GaAs buffer layer 33 is laminated on the upper surface of an n ⁺ -GaAs substrate 32, and an n-AlGaAs layer 34 is laminated thereon. Further, a third semiconductor layer 35 made of i-AlGaAs is stacked on the n-AlGaAs layer 34, and a photoelectric conversion layer 36 having a multiple quantum well structure is stacked on the upper surface thereof. A p-AlGaAs emitter layer 37 for moving electrons in the plane direction is provided on the upper surface of the photoelectric conversion layer 36, and a p-type electrode 39 and a p-type electrode are formed on part of the upper surface of the p-AlGaAs emitter layer 37. A p ⁺ -GaAs contact layer 38 which is in ohmic contact with 39 is stacked. Of the upper surface of the p-AlGaAs emitter layer 37, fine particles 40 are dispersed and attached to a region exposed from the p-type electrode 39. Further, on the lower surface of the ^{n +} -GaAs substrate 32 is prepared n-type electrode 41.

FIG. 11 is an enlarged schematic perspective view showing the structure of the photoelectric conversion layer 36. The photoelectric conversion layer 36 having a multiple quantum well structure is formed by laminating a plurality of well layers 42 made of i-InGaAs layers and barrier layers 43 made of i-AlGaAs layers. For example, in this embodiment, the well layer 42 formed of an i-In _0.9 Ga _0.1 As layer having a thickness of 5 nm is formed so that the well layer 42 is placed on the third semiconductor layer 35 having a thickness of 200 nm. The barrier layers 43 made of i-Al _0.4 Ga _0.6 As layers having a thickness of 5 nm are alternately laminated by five layers.

FIG. 12 shows the band structure of the solar cell 31 having the above structure obtained by calculation. According to the calculation, the band gap between the valence band (Ev) and the quantum level (Eq) of the well layer 42 is Eg1, and the band between the quantum level (Eq) of the well layer 42 and the conduction band (Ec). If the gap is Eg3,
Eg1 = 1.32 eV (converted wavelength 943 nm)
Eg3 = 0.61 eV (converted wavelength: 2042 nm)
It is. In addition, the band gap Eg2 at the time of direct transition from the valence band (Ev) to the conduction band (Ec) without going through the quantum level is
Eg2 = 1.92 eV (converted wavelength: 645 nm)
It is. Since the third semiconductor layer 35 is made of a material having the same composition as that of the barrier layer 43, the band gap Eg4 between the valence band (Ev) and the conduction band (Ec) in the third semiconductor layer 35 is also Eg2. Similarly,
Eg4 = 1.92 eV (converted wavelength: 645 nm)
It became.

The fine particles 40 have a shell structure, and the dielectric core 45 is covered with a metal shell 46 as shown in FIG. Specifically, the fine particles 40 are obtained by covering the surface of a substantially spherical silica (SiO ₂ ) having a diameter of 340 nm with a metal shell 46 made of Au having a thickness of 13 nm. The fine particles 40 are attached to the upper surface of the photoelectric conversion layer 36 so as to have a number density of about 2 × 10 ⁸ cm ⁻² . The fine particles 40 do not need to be regularly arranged, are randomly distributed to such an extent that they can be regarded as being almost uniform when viewed macroscopically, and the coverage of the fine particle distribution is approximately 20%. The coverage is the area of the region in which the fine particles 40 are distributed as viewed from the direction perpendicular to the surface on which the fine particles 40 are distributed (for example, a region surrounded by a two-dot chain line in FIG. 13B). Is the ratio of the total area of the fine particles 40 included in the region (for example, the sum of the orthogonal projection areas of the fine particles hatched in FIG. 13B).

  FIG. 14 shows the result of calculating the absorption spectrum of the silica-Au fine particles 40 having the shell structure as described above. According to the absorption spectrum of the fine particles 40, absorption peaks are observed at λ 1 = 900 nm and λ 2 = 1520 nm, and localized surface plasmon resonance occurs in a wide wavelength range including this resonance wavelength.

  In addition, as the spherical dielectric core 45 constituting the fine particles 40, polystyrene, PMMA (polymethyl methacrylate), or the like can be used in addition to silica. In addition to Au, Ag, Pt, Cu, Al, Pd, or the like can also be used as the metal shell 46 that covers the dielectric core 45 in a spherical shell shape.

In order to generate two or more peaks in the absorption spectrum by one kind of fine particles 40 in which the dielectric core 45 is covered with the metal shell 46,
Diameter of dielectric core 45: 150 nm or more and 600 nm or less Film thickness of metal shell 46: It is desirable that the thickness be 1 nm or more and 20 nm or less.

  In such fine particles 40, the resonance wavelength shifts to the longer wavelength side as the diameter of the dielectric core 45 is larger, and the resonance wavelength shifts to the shorter wavelength side as the diameter of the dielectric core 45 is smaller. When the diameter is smaller than 150 nm, an absorption coefficient peak cannot be obtained on the long wavelength side. On the other hand, when the diameter of the dielectric core 45 is larger than 600 nm, a part of the light is scattered by diffraction scattering, and it becomes difficult for the light to enter the photoelectric conversion layer 36. Therefore, the diameter of the dielectric core 45 is preferably 150 nm or more and 600 nm or less. In particular, if the diameter of the dielectric core 45 is 260 nm or more and 400 nm or less, the intensity ratio between the short wavelength side peak and the long wavelength side peak can be optimized.

  Further, the resonance wavelength shifts to the longer wavelength side as the metal shell 46 is thinner, and the resonance wavelength shifts to the shorter wavelength side as the metal shell 46 is thicker. The thickness of the metal shell 46 is more than 20 nm. As the value increases, the absorption coefficient of the long wavelength peak decreases. On the other hand, if the film thickness of the metal shell 46 is smaller than 1 nm, plasmon resonance is difficult to occur, and sufficient electric field strength cannot be obtained. Therefore, the film thickness of the metal shell 46 is preferably 1 nm or more and 20 nm or less. Especially, if the film thickness of the metal shell 46 is 5 nm or more and 15 nm or less, the intensity ratio between the short wavelength side peak and the long wavelength side peak can be optimized.

  In order to efficiently excite electrons in the photoelectric conversion layer 36 having a multiple quantum well structure, the probability of exciting electrons from the valence band to the quantum level Eq of the well layer 42 and the electrons from the quantum level Eq to the conduction band. It is preferable that the probability of escape is balanced. Therefore, it is sufficient that the electric field intensity generated by the light on the short wavelength side and the electric field intensity generated by the light on the long wavelength side are approximately the same, and the ratio of the absorption coefficient due to localized surface plasmon resonance is the peak on the short wavelength side. The peak value on the long wavelength side with respect to the value is preferably in the range of 0.7 to 2.0. In the absorption spectrum of FIG. 14, the peak value on the long wavelength side of the two absorption peaks is 0.85 times the peak value on the short wavelength side, so that electrons can be excited efficiently. In the multiple quantum well structure, the light on the long wavelength side is less likely to excite electrons than the light on the short wavelength side. In order to make the probability of exciting electrons from the valence band to the quantum level Eq of the well layer 42 and the probability of escaping electrons from the quantum level Eq to the conduction band, The peak value on the long wavelength side is preferably in the range of 1.0 to 2.0 times.

  According to the solar cell 31 having such a structure, light having a specific wavelength among the light incident on the solar cell 31 generates a strong electric field in the vicinity of the fine particles 40 by performing local surface plasmon resonance. As shown in FIG. 15, the intensity of the electric field decreases exponentially as the distance from the surface of the fine particles increases. The distance at which the electric field intensity is 1 / e is called a see-through distance and is an index indicating the magnitude of the electric field spread. Since the seepage distance of the fine particles 40 is substantially equal to the fine particle diameter D, it is considered that the electric field extends around the fine particles 40 to an area of the distance D to 2D. When this electric field acts on the photoelectric conversion layer 36, electrons can be excited, so that light energy can be finally converted into electron energy. Moreover, since the electric field strength generated by local surface plasmon resonance is enhanced several tens of times that of incident light, light energy is converted into electrical energy with high efficiency in a very thin region near the fine particles 40. It becomes possible.

Speaking of the first embodiment, as shown in FIG. 14, the fine particles 40 have absorption peaks at λ1 = 900 nm and λ2 = 1520 nm. Further, in the well layer 42, a conversion wavelength corresponding to energy E1 [eV] for exciting electrons from the valence band to the level Eq in the quantum well and energy E3 [eV] for escaping electrons from the quantum level Eq to the conduction band. Λa = hc / (E1 × 1.602 × 10 ⁻¹⁹ ) = 943 nm and λb = hc / (E3 × 1.602 × 10 ⁻¹⁹ ) = 2042 nm (h is Planck's constant, c is the speed of light). Therefore,
λ1 ≦ λa ≦ λ2 ≦ λb
Is satisfied. Therefore, it is possible to excite electrons to the quantum well with an electric field with light of wavelength λ1, and to escape electrons from the quantum well with an electric field with light of wavelength λ2. As a result, electrons can be excited from the valence band to the conduction band by high-efficiency by twice excitation using localized surface plasmon resonance, and the photoelectric conversion efficiency of the solar cell 31 can be improved. In addition, since the resonance wavelengths λ1 and λ2 are separated by 400 nm or more, light having a wide range of wavelengths can be absorbed. Furthermore, since the resonance wavelength can be changed by changing the material and dimensions of the fine particles, or changing the shape, structure, combination, etc. of the fine particles as will be described later, the resonance wavelength can be changed according to the band structure of the photoelectric conversion layer 36. The microparticles can be designed to obtain a resonance wavelength.

  On the other hand, light in the vicinity of a wavelength of 645 nm corresponding to the excitation energy Eg3 in the barrier layer 43 has a small absorption coefficient due to localized surface plasmons (see FIG. 14), and therefore easily reaches the photoelectric conversion layer 36 without plasmon resonance. For this reason, the electric field generated by localized surface plasmon resonance can hardly be utilized for excitation in the barrier layer 43, but as in a general multiple quantum well structure, electrons are conducted from the valence band by light absorption in the barrier layer 43. It can be excited to the band.

When the fine particles 40 are densely distributed and the coverage of the fine particle distribution is larger than 30%, light that does not plasmon resonate with the fine particles 40 is transmitted through the surface where the fine particles 40 are distributed and enters the photoelectric conversion layer 36. It becomes difficult to do. On the other hand, when the coverage of the fine particle distribution is smaller than 1%, a large amount of light that does not interact with the fine particles 40 increases, so that sufficient electric field strength cannot be obtained by plasmon resonance. In particular, if the coverage of the fine particles 40 is 15% or more and 25% or less, sufficient electric field strength can be obtained by plasmon resonance, and light having a wavelength that does not cause plasmon resonance can be incident on the photoelectric conversion layer 36. Accordingly, the coverage of the fine particle distribution is preferably 1% or more and 30% or less, and particularly preferably 15% or more and 25% or less. In Embodiment 1, since the density of the fine particles 40 is 2 × 10 ⁸ cm ⁻² and the coverage of the fine particles 40 is about 20%, it is within the optimum range.

  Further, light having a wavelength of about 645 nm that has not been absorbed by the barrier layer 43 reaches the third semiconductor layer 35. Light in the vicinity of a wavelength of 645 nm that is not absorbed by the barrier layer 43 is further absorbed by the third semiconductor layer 35, and is converted from light energy to electrical energy in the third semiconductor layer 35. For this reason, the third semiconductor layer 35 is preferably different from the energy of the wavelength at which the band gap E4 resonates with the localized surface plasmon resonance. Furthermore, in order to collect the carriers generated in the photoelectric conversion layer 36 without loss, it is preferable that the band gap is the same as that of the barrier layer 43. If these conditions are satisfied, the third semiconductor layer 35 may be made of a material different from that of the barrier layer 43. However, if the same material is used for the third semiconductor layer 35 and the barrier layer 43 as in the first embodiment, the manufacturing process of the solar cell 31 can be simplified.

  Here, considering the width (thickness) of the well layer 42 and the barrier layer 43, 10 nm or less is preferable and 5 nm or less is more preferable for the following reasons. If the width of the well layer 42 is too large, many quantum levels are generated in the well layer 42. When there are a large number of quantum levels, electrons flowing through the conduction band of the barrier layer 43 tend to fall into the quantum levels in the well layer 42 and lose energy as heat. Therefore, considering the level in the well layer 42, the width of the well layer 42 is preferably 10 nm or less. Further, when the width of the barrier layer 43 decreases, carriers can move from the well layer 42 to the well layer 42 by tunneling through the barrier layer 43. When electrons excited to the quantum level Eq in the well layer 42 can move by tunneling between the adjacent well layers 42, the mobility of the electrons increases. When the electrons move at high speed, the holes and electrons Since these are spatially separated, it becomes difficult to recombine, and the carrier collection efficiency increases. In order to enable such effective tunneling, the width of the barrier layer 43 is preferably 10 nm or less.

  Further, if the well layer 42 and the barrier layer 43 are thinned to facilitate the tunneling of electrons, a miniband 44 is formed between the quantum levels Eq of the well layer 42. When the miniband 44 is formed, the probability of electron transition increases and it becomes easier to excite electrons. In order to form such a miniband 44, it is preferable that the widths of the well layer 42 and the barrier layer 43 are both 5 nm or less.

  In Embodiment 1, since the widths of the well layer 42 and the barrier layer 43 are each as small as 5 nm, as shown in FIG. 14, a miniband is generated in the vicinity of the quantum level Eq, and electrons are tunneled in the miniband. You can move. For this reason, recombination of an electron and a hole can be suppressed and the efficiency of the solar cell 31 can be improved more.

  Note that the third semiconductor layer 35 may be omitted. In the case of eliminating the third semiconductor layer 35, for example, fine particles that resonate with light having a wavelength corresponding to the band gap E3 of the barrier layer 43 may be mixed to increase light absorption in the barrier layer 43.

(Production process)
Next, a manufacturing method of the solar cell 31 will be described with reference to FIGS. In order to produce the solar cell 31, it may be produced as follows using a molecular beam epitaxy method (MBE). First, an n ⁺ -GaAs substrate 32 containing Si as an impurity is prepared, and the substrate 32 is organically cleaned using a solvent such as acetone or methanol. After the surface of the substrate 32 is etched with a sulfuric acid-based etchant to remove oxide films and the like, the substrate 32 is carried into the MBE apparatus and set. Next, in the MBE apparatus, each layer is epitaxially grown to the following thickness sequentially under a reduced pressure of a growth temperature of 580 ° C., a growth rate of 1.0 μm / h, and an As4 pressure of 2 × 10 ⁻⁵ Torr (FIGS. 10 and 11). reference).
N-GaAs buffer layer 33 1 μm
N-Al _0.4 Ga _0.6 As layer 34 600 nm
Third semiconductor layer 35 200 nm
(I-Al _0.4 Ga _0.6 As layer)
・ Photoelectric conversion layer 36 (5 layers each of well layer and barrier layer)
Well layer 42 (i-In _0.9 Ga _0.1 As) 5 nm
Barrier layer 43 (i-Al _0.4 Ga _0.6 As) 5 nm
P-Al _0.4 Ga _0.6 As emitter layer 37 50 nm
・ P ⁺ -GaAs contact layer 38 50 nm
Si may be used for the n-type dopant and Be for the p-type dopant.

Finally, the p-type p ⁺ -GaAs contact layer 38 is partially removed by etching, a p-type electrode 39 is provided on the p ⁺ -GaAs contact layer 38, and the n ⁺ -GaAs substrate 32 has n on the lower surface. A mold electrode 41 is provided. Next, the fine particles 40 are provided on the upper surface of the photoelectric conversion layer 36 as follows.

  Note that the method of stacking the crystal layers is not limited to the MBE method, and a metal organic chemical vapor deposition method (MOCVD) or the like may be used.

FIG. 16 is a schematic view showing a method for producing the fine particles 40. First, as shown in FIG. 16A, nanoparticles having an average particle diameter of about 340 nm are made of silica to form a dielectric core 45. Next, as shown in FIG. 16B, a silane coupling agent terminated with an amine 47 is attached to the dielectric core 45. When the dielectric core 45 thus treated is placed in the HAuCl ₄ solution, the Au ions 48 are reduced by the amine 47 and attracted to the surface of the dielectric core 45 as shown in FIG. As shown in (d), an Au thin film (metal shell 46) grows on the surface of the dielectric core 45.

  The solution containing the fine particles 40 is applied onto the surface of the photoelectric conversion layer 36 by spin coating, and the solvent is dried and removed, whereby a large number of fine particles 40 are substantially uniform on the surface of the photoelectric conversion layer 36 as shown in FIG. Can be attached to.

(Combination of other materials)
In the above embodiment, AlGaAs is used for the material of the barrier layer and InGaAs is used for the material of the well layer for the GaAs substrate, but other combinations are possible. For example, the following combinations may be used. The composition ratio of each material is not particularly limited as long as it is optimally adjusted in consideration of the band gap and lattice strain.
(1) Substrate material: GaAs
Barrier layer material: GaAs
Well layer material: InGaAs
According to this combination of materials, the band gap in the quantum well structure can be optimized and matched to the sunlight spectrum (this feature also applies to the first embodiment).
(2) Substrate material: GaAs
Barrier layer material: AlGaAs
Well layer material: GaAs
According to this combination of materials, the lattice constants of AlGaAs and GaAs are almost the same, and since there is no lattice distortion, the crystallinity can be increased and the loss due to recombination can be reduced.
(3) Substrate material: GaAs
Barrier layer material: GaAsP
Well layer material: InGaAs
According to this combination of materials, distortion can be compensated by combining GaAsP having a lattice constant smaller than that of the GaAs substrate and InGaAs having a large lattice constant, and the occurrence of transition can be suppressed, so that the crystallinity can be increased.
(4) Substrate material: InP
Barrier layer material: _In x _{Ga 1-x} As
Well layer _{material: In z Ga 1-z As} (x <z)
According to this combination of materials, distortion is compensated by combining In _x Ga _1-x As, which has a smaller lattice constant than InP substrate, and In _z Ga _1-z As, which has a larger lattice constant, thereby suppressing the occurrence of transition. Therefore, crystallinity can be increased.
(5) Substrate material: Si
Barrier layer material: SiO ₂
Well layer material: Si
(6) Substrate material: Si
Barrier layer material: Si ₃ N ₄
Well layer material: Si
(7) Substrate material: Si
Barrier layer material: SiC
Well layer material: Si
Since Si-based materials are abundant, according to the combination of materials (5) to (7), the raw material costs can be reduced.

  FIG. 17 shows a modification of the first embodiment. In the modification of FIG. 17, the emitter layer is not provided, and the fine particles 40 are distributed on the upper surface of the photoelectric conversion layer 36 having a multiple quantum well structure. By providing the fine particles 40 directly on the upper surface of the photoelectric conversion layer 36, the electric field generated by the fine particles 40 can easily reach the photoelectric conversion layer 36, so that the conversion efficiency can be increased. In this case, it is preferable to reduce the period of the p-type electrode 39 and the contact layer 38 in order to increase the carrier collection efficiency.

  However, it is desirable that an electric field generated in the vicinity of the fine particles 40 covers the entire photoelectric conversion layer 36 regardless of whether or not another semiconductor layer is interposed between the photoelectric conversion layer 36 and the fine particles 40. That is, if the distance between the surface of the fine particle 40 and the photoelectric conversion layer 36 farthest from the fine particle 40 is too large with respect to the diameter of the fine particle 40, a quantum well is formed outside the region where the electric field is enhanced. Therefore, it becomes more susceptible to defects due to quantum wells, although the amount of light absorption does not increase. On the other hand, if the distance between the surface of the fine particle 40 and the quantum well layer farthest from the fine particle 40 is too small with respect to the diameter of the fine particle 40, the energy of the strong electric field generated by the localized surface plasmon resonance can be sufficiently absorbed. become unable. For this reason, the distance from the surface on which the fine particles 40 are distributed to the lower surface of the photoelectric conversion layer 36 is set to be not more than twice the diameter of the fine particles 40 (particularly a value that is less than twice and close to twice the diameter). It is preferable.

  In consideration of such points, when other semiconductor layers are interposed between the photoelectric conversion layer 36 and the fine particles 40, the number of the well layers 42 and the barrier layers 43 that can be formed within the range covered by the electric field is determined. Therefore, the fine particles 40 are desirably provided on the upper surface of the photoelectric conversion layer 36.

(Second Embodiment)
In the first embodiment, the case where two peaks are generated in the absorption spectrum of local surface plasmon resonance by one type of fine particle 40 has been described. However, by mixing a plurality of types of fine particles having different resonance wavelengths, in a plurality of wavelength regions. Plasmon resonance is also possible.

  Examples of the fine particles that can be used for such applications include spherical fine particles 40a, disk-shaped fine particles 40b, rod-shaped fine particles 40c, and ring-shaped fine particles 40d as shown in FIGS. 18 (a) to 18 (e). And shell-like fine particles 40e. Of these, spherical fine particles 40a and disk-shaped fine particles 40b are likely to resonate on the short wavelength side, and rod-shaped fine particles 40c, ring-shaped fine particles 40d, and shell-shaped fine particles 40e are likely to resonate on the long wavelength side. FIG. 20 is described in Patent Document 4, but shows a difference in resonance wavelength between spherical fine particles and rod-shaped fine particles.

  Furthermore, the wavelength of plasmon resonance can be adjusted by changing the material and size of these fine particles. Further, the present invention is not limited to the above shape, and cubic fine particles, quadrangular prism-shaped fine particles, triangular prism-shaped fine particles, triangular pyramid-shaped fine particles, quadrangular pyramid-shaped fine particles, and the like can also be used. In the following, the terms long wavelength (range) and short wavelength (range) are used, but this is a relative one that changes depending on the relationship with the wavelength to be compared.

For example, in order to resonate in the short wavelength region, spherical metal particles 40a having the following size (the whole being made of metal) may be used (FIG. 18A).
(Average) diameter D1 of metal sphere: 10 nm or more and 200 nm or less In particular, in the case of spherical Au fine particles having a diameter of 100 nm, an absorption spectrum as shown in FIG. 9 is obtained.

Further, in order to resonate in the short wavelength region, a disk-shaped metal fine particle 40b having the following size (the whole being made of metal) may be used (FIG. 18B).
Bottom diameter D2: 10 nm or more and 300 nm or less Disc thickness H2: 5 nm or more and 0.5 times or less of bottom surface diameter D2
(5nm ≦ H2 ≦ D2 / 2)

For example, in order to resonate in the long wavelength region, rod-shaped metal fine particles 40c (all of which are made of metal) such as a cylinder or square column having the following size may be used (FIG. 18 (c)). .
Length in the short direction D3: 10 nm or more and 100 nm or less Longitudinal length H3: 2 to 10 times the length D3 in the short direction Here, the length D3 in the short direction means the bottom diameter or one side The length H3 in the longitudinal direction indicates the height. When the length D3 in the short direction of the fine particle 40c is shorter than 10 nm, plasmon resonance is difficult to occur, and sufficient electric field strength cannot be obtained in the vicinity of the fine particle 40c. On the other hand, when the length D3 in the short direction is 100 nm or more, the light is scattered by diffraction or the like by the fine particles 40c, and a part of the light hardly reaches the photoelectric conversion layer 36. Accordingly, the length in the short direction is preferably 10 nm ≦ D 3 ≦ 100 nm. Further, when the ratio of the length H3 in the longitudinal direction to the length D3 in the short direction of the fine particles 40c is smaller than twice, the resonance wavelength does not shift to the long wavelength side. On the other hand, when this ratio is larger than 10 times, it is difficult to produce the fine particles 40c. Therefore, the ratio of the length H3 in the longitudinal direction to the length D3 in the short direction is preferably 2 ≦ (H3 / D3) ≦ 10.

  Furthermore, if the length of the rod-shaped metal fine particles 40c in the longitudinal direction is 4 to 10 times the length in the short direction, a strong peak of the absorption spectrum can be obtained on the long wavelength side.

As another form for resonating at a long wavelength, fine particles 40d in which a ring portion 52 made of a metal film is formed in an annular shape around a cylindrical dielectric core 51 may be used (FIG. 18D). In order to resonate the fine particles 40d in the long wavelength region, the size of the dielectric core 51 is d4: 40 nm or more and 300 nm or less. The thickness T4 of the ring part 52: 1 nm or more and 20 nm or less. Thus, it is desirable that the outer diameter is 0.5 times or less. If the diameter of the dielectric core 51 is smaller than 40 nm, the resonance wavelength does not shift to a long wavelength. On the other hand, when the thickness is larger than 300 nm, the light is scattered by the fine particles 40d, so that a part of the light hardly reaches the photoelectric conversion layer 36. Therefore, the diameter of the dielectric core 51 is preferably 40 nm ≦ d 4 ≦ 300 nm. In addition, when the thickness T4 of the ring portion 52 is smaller than 1 nm, plasmon resonance hardly occurs, and sufficient electric field strength cannot be obtained in the vicinity of the fine particles 40d. On the other hand, when the thickness T4 is greater than 20 nm, the absorption coefficient on the long wavelength side becomes small. Therefore, the thickness of the ring portion 52 is preferably 1 nm ≦ T4 ≦ 20 nm. Further, when the height H4 of the fine particle 40d is smaller than 5 nm, plasmon resonance hardly occurs and sufficient electric field strength cannot be obtained in the vicinity of the fine particle 40d. On the other hand, when the height H4 is larger than ½ of the outer peripheral diameter (d4 + 2 × T4) of the fine particles 40d, part of the light hardly reaches the photoelectric conversion layer 36 due to scattering. Accordingly, the height of the fine particles 40d is preferably 5 nm ≦ H4 ≦ (d4 + 2 × T4) / 2.

As another form for resonating at a long wavelength, fine particles 40e in which a metal shell 46 made of a metal thin film is formed around a shell-like dielectric core 45 may be used (FIG. 18 (e)). . In order to resonate the fine particles 40e in the long wavelength region, the size is
The diameter D5 of the dielectric core 45: 40 nm or more and 200 nm or less The film thickness T5 of the metal shell 46: 1 nm or more and 20 nm or less is desirable. When the diameter D5 of the dielectric core 45 is smaller than 40 nm, the resonance wavelength does not shift to the long wavelength side. On the other hand, when the diameter D5 is larger than 200 nm, resonance occurs in a plurality of wavelength regions. Therefore, the diameter of the dielectric core 45 is preferably 40 nm ≦ D5 ≦ 200 nm. Further, when the film thickness T5 of the metal shell 46 is smaller than 1 nm, plasmon resonance hardly occurs in the fine particles 40e, and sufficient electric field strength cannot be obtained. On the other hand, when the film thickness T5 is larger than 20 nm, the light absorption coefficient is decreased. Therefore, the thickness of the metal shell 46 is preferably 1 nm ≦ T5 ≦ 20 nm.

  In addition, as a manufacturing method of the fine particles 40a to 40c that do not include the dielectric as shown in FIGS. 18A to 18C, the metal film formed on the surface of the photoelectric conversion layer 36 is patterned as shown in FIG. Alternative methods are possible. In this method, a resist 49 is formed on the surface of the photoelectric conversion layer 36 (FIG. 19A), and the resist 49 is patterned by an electron beam exposure method (FIG. 19B), thereby forming fine particles. A minute opening is formed in the resist 49 at the position to expose the photoelectric conversion layer 36 from the resist 49. Thereafter, a metal is evaporated from above the resist 49 to deposit a metal film 50 on the resist 49 and the upper surface of the photoelectric conversion layer 36 (FIG. 19C). Next, when the resist 49 is dissolved with a solvent and the metal film 50 on the resist 49 is lifted off, the fine particles 40 are formed by the metal film 50 remaining on the upper surface of the photoelectric conversion layer 36 (FIG. 19D). According to such a method, the shape and size of the fine particles, the distance between the fine particles 40, and the like can be controlled, and fine particles 40a to 40c having different sizes and shapes can be mixed.

(Third embodiment)
In the first embodiment, a case where two peaks are generated in the absorption spectrum of local surface plasmon resonance by one kind of fine particle 40 will be described. In the second embodiment, fine particles 40a having different shapes or structures are used. Although the case where plasmon resonance is performed in a plurality of wavelength regions by combining ~ 40e and the like has been described, there are other methods of performing plasmon resonance in a plurality of wavelength regions.

One of them is a method of causing plasmon resonance in a plurality of wavelength regions by arranging fine particles 53 close to each other as shown in FIG. This is because when the metal fine particles 53 are brought close to each other, the electric fields interact with each other, so that two peaks are generated in the absorption spectrum. In order to have two or more peaks in the absorption spectrum by bringing them close to each other, it is preferable to set the size of the fine particles 53 and the distance between the particles as follows.
Diameter D6 of the fine particles 53: 10 nm to 300 nm or less Distance W6 between the fine particles 53: Less than twice the diameter D6 If the diameter D6 of the fine particles 53 is smaller than 10 nm, the resonance wavelength on the short wavelength side is too short and larger than 300 nm This is because part of the light is less likely to enter the photoelectric conversion layer 36 due to scattering. Further, if the distance W6 between the fine particles 53 is larger than twice the diameter D6, the exuding distance of the electric field (substantially equal to the diameter D6) is not overlapped and it becomes difficult to interact with each other. Is preferably not more than twice the diameter D6, more preferably not more than one time the diameter D6.

  Another method is a method of mixing fine particles 54a and 54b having different sizes as shown in FIG. The wavelength of local surface plasmon resonance shifts to the longer wavelength side as the size of the fine particles increases. Therefore, the diameter Dl of the large particle 54a is determined so as to obtain the target resonance wavelength on the long wavelength side, and the diameter Ds of the small particle 54b is determined so as to obtain the target resonance wavelength on the short wavelength side. In addition, the number Nl of the fine particles 54a having a large size is determined so that a desired absorption coefficient can be obtained at the resonance wavelength on the long wavelength side, and the size of the fine particle 54a can be obtained at the resonance wavelength on the short wavelength side. If the number Ns of small particles 54b is determined, the particle distribution as shown in FIG. 22 can be designed. Then, if the fine particles adjusted according to the distribution are dispersed on the surface of the photoelectric conversion layer 36, a target absorption spectrum can be obtained.

  As another method, there is a method of preparing fine particles by mixing two or more different metal materials. Since the plasmon resonance wavelength can be changed by changing the dielectric constant of the metal, a plurality of resonance wavelengths can be provided by mixing metals having different dielectric constants. For example, since the fine particle of Au has a longer resonance wavelength than the fine particle of Ag, plasmon resonance can be performed on the short wavelength side and the long wavelength side by mixing Ag and Au to produce the fine particle.

  As another method, there is a method of producing fine particles by mixing two or more different dielectric materials. When the dielectric constant inside the fine particles and the dielectric constant of the surrounding medium are increased, the resonance wavelength of plasmon resonance shifts to the longer wavelength side, so it is possible to resonate at multiple wavelengths by mixing dielectric materials with different dielectric constants. . For example, two or more kinds of dielectric materials are mixed to produce a spherical dielectric core 45 and the surface thereof is covered with a metal shell 46 to form shell-type fine particles.

(Fourth embodiment)
FIG. 23 is a perspective view schematically showing a solar cell 61 according to Embodiment 4 of the present invention. In the solar cell 61, the third semiconductor layer 63 formed above the substrate 62 is formed, and the photoelectric conversion layer 64 having a quantum dot structure is formed on the third semiconductor layer 63. An electrode 68 is formed on a part of the upper surface. Further, fine particles 67 are distributed in a region exposed from the electrode 68 on the upper surface of the photoelectric conversion layer 64. In the photoelectric conversion layer 64, a plurality of quantum dots 66 are discretely distributed in the barrier portion 65. The quantum dots 66 are formed of a semiconductor material having an energy gap smaller than that of the barrier portion 65, and the quantum dots 66 are aligned in the light transmission direction (vertical direction). An electrode 69 is provided on the lower surface of the substrate 62. The barrier portion 65 corresponds to the barrier layer 43 of the first embodiment, and the quantum dot 66 corresponds to the well layer 42 of the first embodiment.

  While the well layer 42 is provided in a layer form in the first embodiment, the solar cell 61 is also provided in the second embodiment except that the quantum dots 66 are formed in a dot form (discrete minute regions). The same material is used as in the first embodiment.

  When light is incident from the light receiving surface side, light having a plurality of resonance wavelengths is plasmon-resonated by the fine particles 67 to generate a strong electric field in the vicinity of the fine particles 67. The quantum dot 66 absorbs light having a resonance wavelength and excites electrons from the valence band to the level in the quantum dot and excites electrons from the level in the quantum dot to the conduction band. In the barrier section 65, light other than the resonance wavelength is absorbed, and electrons are excited from the valence band to the conduction band. Further, light that is larger than the band gap of the third semiconductor layer 63 among the light transmitted through the photoelectric conversion layer 64 is absorbed by the third semiconductor layer 63. In addition, since the solar cell 61 has a quantum dot structure, the quantum levels in the quantum dots 66 (three-dimensional quantum wells) are further discretized, and the electrons that have flowed through the conduction band of the barrier portion 65 are quantum. It becomes difficult to relax to the quantum level in the well.

  In the case of the quantum dot 66, in order to discretize the quantum level, one side of the height, depth, and width of the quantum dot 66 is 10 nm or less, and the length of the remaining two sides is 30 nm or less. It is desirable. In particular, if the thickness (height) is 5 nm and the depth and width are 20 nm, the quantum levels can be sufficiently discretized.

  The quantum dots 66 are preferably aligned in the height direction, and the distance between the quantum dots 66 in the height direction (the shortest distance between the surfaces of the quantum dots 66) is preferably 10 nm or less. This facilitates electrons to tunnel in the height direction and flow. At this time, the quantum dots 66 do not have to be aligned in a horizontal plane (width direction and depth direction), and may be regularly arranged or randomly arranged. Further, if the distance between the quantum dots 66 in the height direction is set to 5 nm or less, a miniband is formed and the probability of excitation of electrons can be increased.

(Fifth embodiment)
FIG. 24 is a perspective view showing the structure of the solar cell 71 according to the fifth embodiment. In this solar cell 71, the barrier layer 75 and the well layer 76 are alternately stacked on the upper surface of the third semiconductor layer 73 located above the substrate 72 by self-organization using lattice strain, thereby having a multiple quantum well structure. A photoelectric conversion layer 74 is manufactured. When the photoelectric conversion layer 74 is produced by self-organization using lattice strain, the wetting layer 77 and the island-shaped portion 78 are formed. Even with such a structure, the same effect as that of the quantum dot can be obtained. Also at this time, it is desirable that the height of the island-shaped portion 78 is 10 nm or less and the diameter (width and depth) of the island-shaped portion 78 in the horizontal plane is 30 nm or less.

  Further, fine particles 79 that are localized surface plasmon resonance with two or more wavelengths are dispersed on the upper surface of the photoelectric conversion layer 74, and an electrode 80 is provided on a part of the upper surface of the photoelectric conversion layer 74. The electrode 81 is also provided.

(Sixth embodiment)
The fine particles may be provided on the back surface side with respect to the light incident direction. In general, it is desirable to arrange fine particles on the light incident surface side in a substrate type solar cell, and it is desirable to arrange fine particles on the back side in a super straight type solar cell.

  In the solar cell 91 of Embodiment 6 shown in FIG. 25, the transparent electrode 93 is provided on the back surface of the transparent substrate 92, and the third semiconductor layer 94 is formed on the back surface of the transparent electrode 93. Further, a photoelectric conversion layer 95 having a multiple quantum well structure including a barrier layer and a well layer is laminated on the back surface of the transparent electrode 93. As in the first embodiment, a binary or ternary III-V group compound semiconductor such as GaAs can be used for the photoelectric conversion layer 95, but other materials may be used. For example, a thin film system such as a-Si, CIS, CIGS, or CdTe may be used in addition to single crystal Si or polycrystalline Si.

  On the back surface of the photoelectric conversion layer 95, a dielectric 96 including fine particles 97 having a plurality of resonance wavelengths is applied, and an electrode 98 is provided. The dielectric 96 including the fine particles 97 is preferably applied after the photoelectric conversion layer 95 is formed in order not to affect the film formation process of the photoelectric conversion layer 95.

  In the structure in which the fine particles 97 are arranged on the back surface, when light enters from the transparent substrate 92 side, the light is absorbed by the third semiconductor layer 94 and the fine particles 97 are transmitted by the light transmitted through the third semiconductor layer 94 and the photoelectric conversion layer 95. Is localized surface plasmon resonance. Then, electrons in the photoelectric conversion layer 95 are excited by a strong electric field generated in the vicinity of the fine particles 97, and light having a resonance wavelength is absorbed by the photoelectric conversion layer 95.

  In each of the above embodiments, the case where the present invention is applied to a solar cell has been described. However, the present invention can also be applied to a photoelectric sensor or the like. The present invention can also be applied to dye-sensitized solar cells and organic solar cells.

FIG. 1 is an energy band diagram showing the structure of a general single-junction solar cell. FIG. 2 is an energy band diagram showing the structure of a multi-junction solar cell. FIG. 3 is an energy band diagram showing the structure of a solar cell having a multiple quantum well structure. FIG. 4 is an energy band diagram showing the structure of a solar cell having a multiple quantum well structure having deep quantum wells. FIG. 5 is a schematic perspective view showing a solar cell having a quantum dot structure. FIG. 6 is an energy band diagram showing a state in which interface states Ed1 and Ed2 due to dangling bonds are generated in the well layer. FIG. 7 is a schematic cross-sectional view of a solar cell using metal fine particles. FIG. 8 is a schematic cross-sectional view of another solar cell using metal fine particles. FIG. 9 is a diagram showing an absorption spectrum of metal fine particles. FIG. 10 is a schematic cross-sectional view showing the structure of the solar cell according to Embodiment 1 of the present invention. FIG. 11 is a schematic perspective view showing an enlarged photoelectric conversion layer of the above solar cell. FIG. 12 is an energy band diagram of the solar cell of the first embodiment. FIG. 13A is a cross-sectional view showing the structure of fine particles having a shell structure. FIG. 13B is a diagram for explaining the coverage of fine particles. FIG. 14 is a diagram showing an absorption spectrum of the fine particles having the shell structure shown in FIG. FIG. 15 is a diagram showing the intensity of the electric field generated in the vicinity of the fine particles by the localized surface plasmon resonance. FIGS. 16A to 16D are diagrams showing a method for producing shell-structured fine particles. FIG. 17 is a schematic cross-sectional view showing the structure of a solar cell according to a modification of the first embodiment. FIGS. 18A to 18E are perspective views showing shapes or structures of various fine particles. FIGS. 19A to 19D are diagrams showing a method for producing fine particles not containing a dielectric. FIG. 20 is a diagram showing the difference in resonance wavelength between spherical fine particles and rod-shaped fine particles. FIGS. 21A and 21B are schematic diagrams for explaining a method for resonating at two or more wavelengths. FIG. 22 is a diagram for explaining the design of the distribution of fine particles. FIG. 23 is a perspective view schematically showing the structure of the solar cell according to Embodiment 4 of the present invention. FIG. 24 is a perspective view schematically showing the structure of the solar cell according to embodiment 5 of the present invention. FIG. 25 is a cross-sectional view schematically showing the structure of the solar cell according to Embodiment 6 of the present invention.

Explanation of symbols

31, 61, 71 Solar cell 32, 62 Substrate 35, 63 Third semiconductor layer 36, 64 Photoelectric conversion layer 39 P-type electrode 40, 40a, 40b, 40c, 40d, 40e Fine particles 41 N-type electrode 42 Well layer 43 Barrier Layer 45 Dielectric core 46 Metal shell 51 Dielectric core 52 Ring part 53, 54a, 54b, 67, 69 Fine particle 65 Barrier part 66 Quantum dot

Claims (21)

In a photoelectric device having a photoelectric conversion layer having a multiple quantum well structure in which a plurality of barrier layers made of a first semiconductor and well layers made of a second semiconductor having a smaller band gap than the first semiconductor are alternately stacked. ,
A photoelectric device comprising fine particles that perform localized surface plasmon resonance in at least two wavelength bands. In a photoelectric device having a photoelectric conversion layer having a quantum dot structure in which at least one quantum dot made of a second semiconductor having a smaller band gap than the first semiconductor is included in the barrier portion made of the first semiconductor.
A photoelectric device comprising fine particles that perform localized surface plasmon resonance in at least two wavelength bands. The wavelength corresponding to the energy that excites electrons from the valence band to the level in the quantum well and the wavelength that corresponds to the energy that escapes electrons from the level in the quantum well to the conduction band are set to λa and λb in order from the short side, When the two resonance wavelengths of localized surface plasmon resonance generated in the fine particles are λ1 and λ2 in order from the short side,
λ1 ≦ λa ≦ λ2 ≦ λb
The photoelectric device according to claim 1, wherein the photoelectric device is a device.   The photoelectric device according to claim 3, wherein the resonance wavelengths λ1 and λ2 are separated from each other by 400 nm or more.   In the absorption spectrum of the fine particles, the peak intensity at the resonance wavelength λ2 on the long wavelength side is 0.7 to 2.0 times the peak intensity at the resonance wavelength λ1 on the short wavelength side, The photoelectric device according to claim 4.   The peak intensity at the resonance wavelength λ2 on the long wavelength side is 1.0 to 2.0 times the peak intensity at the resonance wavelength λ1 on the short wavelength side. Photoelectric device.   The photoelectric device according to claim 1, wherein the distribution representing the number of the fine particles for each size has two or more peaks.   The photoelectric device according to claim 1, wherein the fine particles include two or more kinds of fine particles made of different metals.   3. The photoelectric device according to claim 1, wherein the fine particles include two or more kinds of fine particles made of a metal and different dielectrics.   The photoelectric device according to claim 1, wherein the fine particles include two or more kinds of fine particles having different shapes.   One kind of the fine particles is a rod-shaped fine particle made of a metal, and has a length in the longitudinal direction more than twice that in the short direction, and a size in the short direction of 10 nm or more. The photoelectric device according to claim 10, wherein the photoelectric device is 100 nm or less.   One kind of the fine particles is a fine particle composed of a cylindrical core portion made of a dielectric and a ring portion made of a metal surrounding the core portion in a ring shape, and the core portion has a circular shape. The diameter of the cross section is 40 nm or more and 300 nm or less, the thickness of the ring part is 1 nm or more and 20 nm or less, the height of the ring part is 5 nm or more and 0.5 times or less of the outer diameter of the ring part. The photoelectric device according to claim 10, wherein the photoelectric device is characterized.   One kind of the fine particles is a fine particle composed of a core portion made of a dielectric and a shell portion made of a metal covering the outer surface of the core portion in a shell shape, and the outer dimension of the core portion is 40 nm. The photoelectric device according to claim 10, wherein the thickness is 200 nm or less and the thickness of the shell portion is 1 nm or more and 20 nm or less.   The fine particles are composed of a core portion made of a dielectric and a shell portion made of a metal covering the outer surface of the core portion in a shell shape, and the outer dimensions of the core portion are 150 nm or more and 600 nm or less. The photoelectric device according to claim 1, wherein the thickness is 1 nm or more and 20 nm or less.   3. The photoelectric device according to claim 1, wherein the fine particles are distributed on a certain surface, and a covering ratio of the fine particles covering the surface is 1% or more and 30% or less.   The fine particles are distributed on a certain surface, and the distance between the fine particle distribution surface and the second semiconductor farthest from the fine particle distribution surface is not more than twice the average value of the external dimensions of the fine particles. The photoelectric device according to claim 1, wherein the photoelectric device is characterized in that   A third semiconductor layer having a larger band gap than the well layer and having a thickness larger than the thickness of one layer of the barrier layer, the third semiconductor layer being light incident on the photoelectric conversion layer The photoelectric device according to claim 1, wherein the photoelectric device is provided at an adjacent position opposite to the side.   A third semiconductor layer having a larger band gap than the quantum dots and having a thickness larger than a distance in the light transmission direction of the quantum dots, the third semiconductor layer being a light of the photoelectric conversion layer; The photoelectric device according to claim 2, wherein the photoelectric device is provided at an adjacent position opposite to the incident side.   The photoelectric device according to claim 17, wherein the third semiconductor layer is formed of the same material as the first semiconductor.   The photoelectric device according to claim 1, wherein the well layer has a width of 10 nm or less, and the barrier layer has a width of 10 nm or less.   3. The photoelectric device according to claim 2, wherein one of the height, width, and depth of the quantum dots has a length of 10 nm or less and the remaining two sides have a length of 30 nm or less. 4. .

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