JP6055619B2 - Solar cell - Google Patents

Description

  The present invention relates to a solar cell having a superlattice structure.

In recent years, photovoltaic devices have attracted attention as a clean energy source that does not emit CO ₂ , and the spread thereof is progressing. The currently most popular photovoltaic element is a single-junction solar cell using silicon. However, conventional silicon solar cells cannot absorb light in the long wavelength region of the sunlight spectrum, and much of the energy of sunlight is not utilized.
In order to make effective use of this unused solar energy, solar cells using a superlattice structure have attracted attention (for example, see Non-Patent Document 1). The superlattice structure has a structure in which quantum layers (quantum dot layers or quantum well layers) of about several nm to 100 nm and barrier layers are alternately stacked. Further, the quantum layer has a feature that the band gap can be freely controlled by size control.
By using a superlattice structure, a new band gap (quantum energy level) can be formed, and a long wavelength region that has been difficult to use in the solar spectrum can be absorbed.

Physical Review B, 82, 1 − 15 (2010)

In a conventional solar cell using a superlattice structure, in addition to absorption in the short wavelength region of the sunlight spectrum, light in the long wavelength region that has been difficult to use is absorbed and photoelectrically converted. However, in conventional superlattice solar cells, the energy corresponding to the band gap of the quantum layer, or the light having energy smaller than the energy corresponding to the band gap of the barrier layer and the band gap of the quantum layer and a certain amount of energy are used. There is a possibility that the light it has is hardly absorbed and cannot be used for photoelectric conversion (Non-Patent Document 1). For this reason, in the solar cell using the conventional superlattice structure, the wavelength range of the light which can be photoelectrically converted becomes narrow, and the increase in photoelectric conversion efficiency is hardly seen. In addition, since the amount of light absorbed by the quantum layer is not sufficient, there is a problem that few carriers are extracted from the quantum layer. In addition, when the number of extracted carriers is small, there is a problem that the voltage of the solar cell decreases.
This invention is made | formed in view of such a situation, and provides the solar cell which the photoelectric conversion efficiency improved by widening the wavelength range of the light which can be photoelectrically converted.

  The present invention includes a photoelectric conversion layer and a wavelength conversion unit including a wavelength conversion material that converts a wavelength of light incident on the photoelectric conversion layer, and the photoelectric conversion layer includes a p-type semiconductor layer and an n-type semiconductor layer. And a superlattice semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer, and the superlattice semiconductor layer has a superlattice structure in which barrier layers and quantum layers are alternately and repeatedly stacked. And a light absorption spectrum having a first light absorption wavelength region and a second light absorption wavelength region on the long wavelength side of the first light absorption wavelength region, wherein the wavelength converter includes the first light absorption wavelength region and the second light absorption wavelength region. Provided is a solar cell characterized in that light having a wavelength within a wavelength region between the light absorption wavelength regions is converted into light having a wavelength within a second light absorption wavelength region.

According to the present invention, the superlattice includes a photoelectric conversion layer having a p-type semiconductor layer, an n-type semiconductor layer, and a superlattice semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer. Since the semiconductor layer has a superlattice structure in which barrier layers and quantum layers are alternately and repeatedly stacked, light incident on the superlattice semiconductor layer can be photoelectrically converted and a photovoltaic force can be generated.
According to the present invention, since the superlattice semiconductor layer has a light absorption spectrum having a first absorption wavelength region and a second absorption wavelength region on the longer wavelength side of the first absorption wavelength region, the first and second absorption wavelengths The incident light in the region can be photoelectrically converted.
According to this invention, the wavelength conversion part containing the wavelength conversion material which converts the wavelength of the light which injects into a photoelectric converting layer is provided, and a wavelength conversion part is a wavelength between the 1st absorption wavelength area | region and a 2nd absorption wavelength area | region. In order to convert light having a wavelength within the region into light having a wavelength within the second absorption wavelength region, light having a wavelength within the wavelength region between the first absorption wavelength region and the second absorption wavelength region is converted to the second absorption wavelength region. The light can be converted into light having an inner wavelength and incident on the photoelectric conversion layer, and photoelectric conversion can be performed. That is, since the light that could not be photoelectrically converted can be converted into light that can be photoelectrically converted and incident on the photoelectric conversion layer, the photoelectric conversion efficiency of the solar cell can be improved. In addition, the amount of light absorbed by the quantum layer increases, and it is possible to solve a problem peculiar to a superlattice structure solar cell that is not seen in a single-junction solar cell in which it is difficult to extract carriers from the quantum layer to the barrier layer. .

It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic perspective view which shows the structure of the solar cell of one Embodiment of this invention. It is a schematic band figure of the photoelectric converting layer contained in the solar cell of one Embodiment of this invention. It is a schematic band figure of the photoelectric converting layer contained in the solar cell of one Embodiment of this invention. It is a schematic band figure of the photoelectric converting layer contained in the solar cell of one Embodiment of this invention. It is explanatory drawing for demonstrating the light absorption spectrum of the superlattice semiconductor layer contained in the solar cell of one Embodiment of this invention, and the light by which the wavelength conversion part contained in the solar cell of one Embodiment of this invention is wavelength-converted. is there. It is a light absorption spectrum of the superlattice semiconductor layer contained in the solar cell of one Embodiment of this invention. It is explanatory drawing for demonstrating the light wavelength-converted by the wavelength conversion part contained in the solar cell of one Embodiment of this invention. It is explanatory drawing explaining the relationship between an absorption peak and a wavelength range. It is explanatory drawing explaining the relationship between an absorption peak and a wavelength range. It is a light absorption spectrum of the superlattice semiconductor layer contained in the solar cell of one Embodiment of this invention. It is a light absorption spectrum of the superlattice semiconductor layer contained in the solar cell of one Embodiment of this invention. It is a light absorption spectrum of the superlattice semiconductor layer contained in the solar cell of one Embodiment of this invention. It is a light absorption spectrum of the superlattice semiconductor layer contained in the solar cell of one Embodiment of this invention. It is a light absorption spectrum of the superlattice semiconductor layer contained in the solar cell of one Embodiment of this invention. It is explanatory drawing for demonstrating the light wavelength-converted by the wavelength conversion part contained in the solar cell of one Embodiment of this invention.

  The solar cell of the present invention includes a photoelectric conversion layer and a wavelength conversion unit including a wavelength conversion material that converts a wavelength of light incident on the photoelectric conversion layer, and the photoelectric conversion layer includes a p-type semiconductor layer, n And a superlattice semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer, and the superlattice semiconductor layer is a superlattice in which barrier layers and quantum layers are alternately and repeatedly stacked. A light absorption spectrum having a structure and having a first absorption wavelength region and a second absorption wavelength region on the longer wavelength side of the first absorption wavelength region, wherein the wavelength conversion unit includes a first absorption wavelength region; The light having a wavelength in the wavelength region between the first absorption wavelength region and the second absorption wavelength region is converted into light having a wavelength in the second absorption wavelength region.

In the present invention, the light absorption wavelength region is a wavelength region in which the superlattice semiconductor layer can absorb most of incident light. The absorption wavelength region can be determined by the light absorption spectrum of the superlattice semiconductor layer, and is a region having an absorption peak. The absorption wavelength region is a region where the absorption rate is 10 or more, where the absorption rate of the baseline of the light absorption spectrum at the absorption peak is 0 and the absorption rate at the top of the absorption peak is 100.
Note that the light absorption spectrum of the superlattice semiconductor layer can be measured by, for example, a spectrophotometer.
In the present invention, the first absorption wavelength region is one of a plurality of absorption wavelength regions of the superlattice semiconductor layer, and the second absorption wavelength region is a plurality of the superlattice semiconductor layer. Is an absorption wavelength region on the long wavelength side of the first absorption wavelength region.
In the present invention, the p-type semiconductor layer, the n-type semiconductor layer, and the superlattice semiconductor layer constitute a photoelectric conversion layer.
In the present invention, the superlattice structure is a structure in which a barrier layer and a quantum layer, both of which are made of a semiconductor and have different band gaps, are repeatedly stacked. The electron wave function of the quantum layer may interact greatly with the electron wave function of the adjacent quantum layer.
In the present invention, the quantum layer is a quantum dot layer or a quantum well layer.
In the present invention, a quantum dot is a semiconductor fine particle having a particle size of 100 nm or less, and is a fine particle surrounded by a semiconductor having a larger band gap than the semiconductor constituting the quantum dot.
In the present invention, the quantum well is a semiconductor thin film having a thickness of 100 nm or less, and is a thin film surrounded by a semiconductor having a larger band gap than the semiconductor constituting the quantum well.

In the present invention, the quantum dot layer is a layer composed of quantum dots and is a well layer having a superlattice structure.
In the present invention, the quantum well layer is a layer composed of quantum wells and a well layer having a superlattice structure.
In the present invention, the barrier layer is made of a semiconductor material having a wider band gap than the semiconductor material constituting the quantum layer, and forms a potential barrier around the quantum dots or quantum wells.
In the present invention, the intermediate energy level means the upper end of the valence band of the barrier layer or the upper end of the valence band of the quantum layer and the lower end of the energy and the lower end of the conduction band of the barrier layer or the conduction of the quantum layer. An energy level in which electrons photoexcited from the valence band of the quantum layer or the barrier layer can exist for a certain period between the upper end of the band and the lower end of the higher energy, and one or more of the quantum layers Of quantum levels. The intermediate energy level is, for example, an intermediate band or a localized level. Further, all the intermediate energy levels of the superlattice semiconductor layer may be formed from one or a plurality of quantum levels on the conduction band side of the quantum dots. In addition, among the intermediate energy levels of the superlattice semiconductor layer, one intermediate energy level is formed from one or more quantum levels on the valence band side of the quantum dots, and the other intermediate energy levels are The quantum dots may be formed from one or more quantum levels on the conduction band side.

In the present invention, the intermediate band refers to a band connected to one formed in the middle of the forbidden band in the semiconductor constituting the barrier layer. It should be noted that the electron wave function of the quantum dot layer of the superlattice structure interacts with the electron wave function of the adjacent quantum dot layer, resulting in a resonant tunneling effect between the quantum levels of the quantum dots, so that the quantum level becomes one. An intermediate band formed by connecting is also called a mini band.
In the present invention, the localized level is an energy level formed in the middle of the forbidden band in the semiconductor constituting the barrier layer, but is not connected to one. A discrete energy level of electrons formed in a quantum dot surrounded by a potential barrier is also referred to as a quantum level. The quantum level is also referred to as a quantum energy level.

In the solar cell of the present invention, it is preferable that the absorption spectrum has a larger difference between the longest wavelength and the shortest wavelength in the second absorption wavelength region. For example, it is preferable that adjacent quantum layers in the superlattice semiconductor layer are brought close to each other to form a miniband, so that the difference between the longest wavelength and the shortest wavelength in the second absorption wavelength region is increased. From the viewpoint of efficiently absorbing light, the energy difference between the longest wavelength and the shortest wavelength in the second absorption wavelength region is preferably 50 meV or more, and more preferably 100 meV or more. Further, it is preferable to form a miniband from the viewpoint of efficient carrier transportation.
According to such a configuration, the superlattice semiconductor layer can excite valence band electrons to the conduction band through the quantum level of the quantum layer.
In the solar cell of the present invention, the light absorption spectrum has three or more absorption wavelength regions, and the first and second absorption wavelength regions are two regions included in the three or more absorption wavelength regions. It is preferable.
According to such a configuration, the superlattice semiconductor layer can use more incident light for photoelectric conversion, and the photoelectric conversion efficiency can be improved.

In the solar cell of the present invention, the wavelength conversion unit includes two or more types of wavelength conversion materials, and the light absorption spectrum further includes a third absorption wavelength region on the long wavelength side of the second absorption wavelength region, The wavelength conversion unit preferably converts light having a wavelength in a wavelength region between the second absorption wavelength region and the third absorption wavelength region into light having a wavelength in the third absorption wavelength region.
According to such a configuration, the superlattice semiconductor layer can use more incident light for photoelectric conversion, and the photoelectric conversion efficiency can be improved.
In the solar cell of the present invention, in the light absorption spectrum, the energy difference between the longest wavelength and the shortest wavelength in the third absorption wavelength region is preferably 50 meV or more, and more preferably 100 meV or more.
According to such a configuration, the superlattice semiconductor layer can excite valence band electrons to the conduction band through the quantum level of the quantum layer.

In the solar cell of the present invention, the light absorption spectrum has four or more absorption wavelength regions, and the first, second, and third absorption wavelength regions are included in the three or more absorption wavelength regions. A region is preferred.
According to such a configuration, the superlattice semiconductor layer can use more incident light for photoelectric conversion, and the photoelectric conversion efficiency can be improved.
The solar cell of this invention WHEREIN: It is preferable that the said wavelength conversion part contains the wavelength conversion material which converts the wavelength of the light which has a wavelength of 1000 nm or more.
According to such a configuration, the superlattice semiconductor layer can use more incident light for photoelectric conversion, and the photoelectric conversion efficiency can be improved.

In the solar cell of the present invention, the superlattice semiconductor layer is preferably provided so that a miniband is formed.
According to such a configuration, electrons can move in the miniband formed by the quantum levels of the superlattice semiconductor layer, and the probability that the electrons excited in the miniband are excited in the valence band of the barrier layer is increased. And the photoelectric conversion efficiency can be increased.
In the solar cell of the present invention, it is preferable that the wavelength conversion unit is a molded body including a wavelength conversion material, and the photoelectric conversion layer is provided on a surface of the wavelength conversion unit.
According to such a configuration, light whose wavelength is converted inside the molded body can be incident on the photoelectric conversion layer. Moreover, the range in which the photoelectric conversion layer is provided can be narrowed, and the manufacturing cost of the solar cell can be reduced.
In the solar cell of the present invention, the molded body is a rectangular parallelepiped having one light receiving surface, the photoelectric conversion layers are at least four, and are respectively provided on four side surfaces around the light receiving surface of the rectangular parallelepiped. It is preferably provided.
According to such a configuration, light whose wavelength is converted inside the molded body can be incident on the photoelectric conversion layer. Moreover, the range in which the photoelectric conversion layer is provided can be narrowed, and the manufacturing cost of the solar cell can be reduced.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

Configuration of Solar Cell FIGS. 1 to 11 are schematic cross-sectional views or schematic perspective views showing the configuration of the solar cell of this embodiment, and FIGS. 12 to 14 are schematic bands of a photoelectric conversion layer included in the solar cell of this embodiment. FIG. 15, 16, and 20 to 24 are light absorption spectra of a superlattice semiconductor layer included in the solar cell of this embodiment, and FIGS. 17 and 25 are obtained by the wavelength conversion unit included in the solar cell of this embodiment. It is explanatory drawing for demonstrating the light by which wavelength conversion is carried out. 18 and 19 are explanatory diagrams for explaining the relationship between the absorption peak and the absorption wavelength region.

The solar cell 20 of this embodiment includes a photoelectric conversion layer 2 and a wavelength conversion unit 30 that includes a wavelength conversion material that converts the wavelength of light incident on the photoelectric conversion layer 2, and the photoelectric conversion layer 2 is a p-type semiconductor. A layer 12, an n-type semiconductor layer 4, and a superlattice semiconductor layer 10 sandwiched between the p-type semiconductor layer 12 and the n-type semiconductor layer 4, and the superlattice semiconductor layer 10 includes the barrier layer 8 and the quantum layer 5 And a light absorption spectrum having a first absorption wavelength region and a second absorption wavelength region on the long wavelength side of the first absorption wavelength region, and a wavelength conversion The unit 30 is characterized by converting light having a wavelength in the wavelength region between the first absorption wavelength region and the second absorption wavelength region into light having a wavelength in the second absorption wavelength region.
Hereinafter, the solar cell 20 of the present embodiment will be described.

1. Wavelength Conversion Unit The wavelength conversion unit 30 is a part that converts the wavelength of light incident on the photoelectric conversion layer 2 and includes a wavelength conversion material that converts the wavelength of incident light. The wavelength conversion unit 30 can have, for example, a light receiving surface. The wavelength of light incident from the light receiving surface into the wavelength conversion unit 30 is converted by the wavelength conversion material, and the wavelength converted light is converted from the wavelength conversion unit 30. Released. The light emitted from the wavelength conversion unit 30 enters the photoelectric conversion layer 2 and is used for photoelectric conversion.
The wavelength conversion unit 30 is provided so as to convert light having a wavelength within the wavelength region between the first absorption wavelength region and the second absorption wavelength region into light having a wavelength within the second absorption wavelength region. Here, the first absorption wavelength region is one of a plurality of absorption wavelength regions included in the superlattice semiconductor layer 10, and the second absorption wavelength region is included in the superlattice semiconductor layer 10. It is an absorption wavelength region on the long wavelength side of the first absorption wavelength region among the plurality of absorption wavelength regions.

The wavelength conversion material included in the wavelength conversion unit 30 may be any material as long as it can shift the wavelength of sunlight. For example, ZnO 2, ZnS 3, ZnSe, ZnTe 2, CdS 1, CdSe, CdTe, PbS, PbSe, PbTe, CuInGaS, CuS, InGaZnO, InAs, GaAs, AlAs, InSb, GaSb, AlSb, InP, GaP, AlP, InN, GaN, AlN, Si, Ge, and the like. In addition, inorganic materials, complex materials, glass containing rare earth ions (such as Er ³⁺ , Pr ³⁺ , Tm ³⁺ ) and transition elements, Er-doped garnet crystals (YAG), organic materials, and the like that are mixed crystal materials of these It may be. A plurality of these materials such as a core-shell structure and a core-shell structure may be used. Furthermore, these materials may have a size of several to several tens of nanometers. In that case, the band gap can be freely changed depending on the size.
The type of wavelength conversion material to be used is determined according to the light absorption spectrum of the superlattice semiconductor layer 10, but it is often preferable to use a phosphor made of a semiconductor material. Compared with rare earth and transition metal phosphors, the semiconductor phosphor has a smaller wavelength difference from the light absorption wavelength region to the light emission wavelength region (ie, a smaller Stokes shift) and can perform efficient wavelength conversion. Because.
Further, these materials may be used in a size that causes a quantum effect (approximately Bohr radius as a rough guide). By using a phosphor semiconductor having such a size, the band gap can be freely adjusted, and the absorption wavelength region and the emission wavelength region can be freely controlled. Bohr radii are, for example, 7 nm for InN, 5 nm for CdSe, and 12.5 nm for GaAs.

These wavelength conversion materials may be appropriately selected from materials, mixed crystal ratios, and sizes in accordance with the light absorption spectrum of the superlattice semiconductor layer 10 of the solar cell. Further, it may be preferable to use a plurality of types of wavelength conversion materials according to the light absorption spectrum of the superlattice semiconductor layer 10.
The lower diagram of FIG. 15 is a graph for explaining light that is wavelength-converted by the wavelength conversion unit 30 including two types of wavelength conversion materials. In this graph, the two peaks of light indicated by solid lines are respectively wavelength-converted by the wavelength converter 30 into two peaks of light indicated by dotted lines.
Since the wavelength of light that is wavelength-converted by the wavelength conversion unit 30 is determined according to the light absorption spectrum of the superlattice semiconductor layer 10, it will be described together with the description of the superlattice semiconductor layer 10.

  The wavelength conversion part 30 can be provided in the upper part of the light-receiving surface of the photoelectric converting layer 2, for example like the solar cell 20 shown in FIGS. By providing the wavelength conversion unit 30 in this way, sunlight transmitted through the wavelength conversion unit 30 can be made incident on the photoelectric conversion layer 2, and the wavelength-converted light can be photoelectrically converted. In addition, the wavelength conversion unit 30 may include a first wavelength conversion unit 30a and a second wavelength conversion unit 30b. In this case, the first wavelength conversion unit 30a may include a first wavelength conversion material, and the second wavelength conversion unit 30b may include a second wavelength conversion material. With such a configuration, the wavelength of sunlight can be converted by both the first and second wavelength conversion materials. Moreover, the wavelength conversion part 30 may consist of one wavelength conversion part, and may consist of three or more wavelength conversion parts each containing a different wavelength conversion material.

Moreover, the wavelength conversion part 30 may be formed on the photoelectric conversion layer 2 like the solar cell 20 shown in FIG. 2, for example, and the photoelectric conversion layer 2 of the board | substrate 1 is formed like FIG. The film may be formed on the surface opposite to the surface.
Moreover, the wavelength conversion part 30 may use the wavelength conversion material contained in resin, and may form into a film | membrane the resin containing a wavelength conversion material on the photoelectric converting layer 2, as shown in FIG. You may form into a film on the surface on the opposite side to the surface in which the photoelectric converting layer 2 was formed. Note that FIG. 3 shows a structure in which light that has not been absorbed by the photoelectric conversion layer 2 is wavelength-converted by the wavelength conversion unit 30 and is incident on the photoelectric conversion layer 2 again.
As described above, the structure in which the wavelength conversion unit 30 is provided on the back surface of the solar cell has a structure in which the light absorption region of the wavelength conversion unit 30 is widely present on the short wavelength side (for example, in FIG. This is particularly preferable when E overlaps part or all of the absorption wavelength region A of the photoelectric conversion layer. Moreover, the structure which provides the wavelength conversion part 30 which consists of two or more types of wavelength conversion materials in a solar cell back surface is when the wavelength conversion area E of the wavelength conversion part 30 exists widely in the short wavelength side (for example, in FIG. 15, wavelength conversion). The wavelength conversion region E of the unit 30 is preferably overlapped with a part or all of the light emitting region F of the solar cell), and the solar spectrum having short wavelength light in order from the side closer to the photoelectric conversion layer 2 toward the outside. It is desirable to provide a layer for converting the wavelength of a sunlight spectrum having a long wavelength from the layer for wavelength conversion in a stepwise manner. For example, in the case where the wavelength conversion unit 30 made of two types of wavelength conversion materials is provided on the back surface of the solar cell, a layer that converts the wavelength of a shorter wavelength sunlight spectrum (for example, the wavelength conversion region D in FIG. 15) to 30b in FIG. It is desirable to provide a layer for converting the wavelength of a long-wavelength sunlight spectrum (for example, the wavelength conversion region E in FIG. 15) at 30a.
Such a structure will be described in more detail with reference to FIGS. Here, when the wavelength conversion region D and / or E of the wavelength conversion unit 30 overlaps part or all of the light absorption wavelength region A of the photoelectric conversion layer 2, the wavelength conversion region E is a part of the wavelength conversion region D or Consider the case where everything overlaps. After the sunlight is incident on the photoelectric conversion layer 2, first, most of the sunlight existing in the absorption wavelength regions A, B, and C is absorbed by the solar cell 2, and the sunlight that has not been absorbed is the wavelength conversion unit. Reach 30. Of the unabsorbed sunlight, most of the light existing in the wavelength conversion region D is emitted by the wavelength conversion unit 30b in FIG. 3 at a wavelength suitable for the absorption wavelength region B in FIG. ), And most of the wavelength-converted light is incident on the photoelectric conversion layer 2 again. Further, a part of the light that has not been absorbed by the wavelength converting unit 30b or the wavelength converted by the wavelength converting unit 30b is converted into a wavelength suitable for the absorption wavelength region C in FIG. 15 by the wavelength converting unit 30a (light emitting region). Light having a G spectrum is emitted), and most of the wavelength-converted light is incident on the photoelectric conversion layer 2 again.
By adopting such a structure, there is no possibility that most of the absorption peak A in FIG. 15 is wavelength-converted by the wavelength conversion unit 30a or 30b. In addition, it is preferable because most of the wavelength conversion region D and most of the light emitting region F are not converted by the wavelength conversion unit 30a having the wavelength conversion region E. Furthermore, although the light of the light emission area | region G passes the wavelength conversion part 30b before entering the photoelectric converting layer 2 again, since the possibility that the wavelength conversion part 30b will absorb the light of the light emission area | region G again is lost, it is preferable. In this way, sunlight that has been wasted in the superlattice structure solar cell can be used efficiently, the number of carriers extracted from the quantum layer can be increased, and the voltage of the solar cell can be increased. Photoelectric conversion efficiency can be greatly improved.

Furthermore, the wavelength conversion part 30 is provided by providing the photoelectric converting layer 2 on the side surface 51 of the luminescence converter including the wavelength conversion part 30 made of one or a plurality of wavelength conversion materials as in the solar cell shown in FIG. The sunlight incident on the light enters the photoelectric conversion layer 2 after being condensed and wavelength-converted. Thereby, the improvement of the conversion efficiency of the solar cell 20 can be expected. Here, the luminescence converter refers to a configuration including a wavelength conversion material, and is formed by mixing glass, resin, or the like in order to fix the wavelength conversion material. Further, the luminescence converter receives sunlight from the surface 50, repeats wavelength conversion and light emission inside the luminescence converter, and light is totally reflected on the surface 50 and its back surface, and finally from the four edge surfaces 51 of the rectangular parallelepiped. A device that emits sunlight that has been condensed and wavelength-converted. The photoelectric conversion efficiency of the solar cell 20 can be improved by providing the photoelectric conversion layers 2 on the four edge surfaces 51 of the luminescence converter.
Moreover, in addition to being able to wavelength-convert sunlight in a spectral region that could not be absorbed conventionally by using a luminescent converter, there is an effect that sunlight is condensed and applied to the superlattice structure solar cell. When sunlight is collected and applied to the superlattice solar cell, the two-stage light absorption through the quantum levels and the intermediate band in the superlattice solar cell is greatly improved, and carriers can be efficiently extracted. it can. Thus, due to the two effects of the wavelength conversion effect and the light condensing effect, the rate of increase in the conversion efficiency of the superlattice solar cell with or without the luminescent converter is the conversion of the single junction solar cell with or without the luminescent converter. Compared to the rate of increase in efficiency, the structure in which a superlattice solar cell is installed in a luminescent converter is a conversion efficiency that was not seen in the conventional structure in which a single-junction solar cell was installed in a luminescent converter. A significant increase is seen.
Furthermore, with such a structure, the superlattice structure solar cell 20 is configured with a usage amount of about the edge area, so that the usage amount and cost of the material are reduced.
Moreover, since the solar cell 20 is reduced in weight, the solar cell 20 can be attached to a window or a building material, mounted on a roof, or used regardless of location.

2. n-type semiconductor layer (base layer) and p-type semiconductor layer (emitter layer)
The n-type semiconductor layer (base layer) 4 is made of a semiconductor containing n-type impurities, and the p-type semiconductor layer (emitter layer) 12 is made of a semiconductor containing p-type impurities.
The n-type semiconductor layer 4 and the p-type semiconductor layer 12 constitute the photoelectric conversion layer 2 with the superlattice semiconductor layer 10 sandwiched therebetween, and light is incident on these layers to generate photovoltaic power. The light incident on the photoelectric conversion layer 2 is light that has been wavelength-converted by the wavelength conversion unit 30.
The n-type semiconductor layer 4 and the p-type semiconductor layer 12 can be formed by, for example, the MOCVD method.

  The n-type semiconductor layer 4 can be electrically connected to the n-type electrode 18, and the p-type semiconductor layer 12 can be electrically connected to the p-type electrode 17. As a result, the photovoltaic force generated between the n-type semiconductor layer 4 and the p-type semiconductor layer 12 can be output to an external circuit via the n-type electrode 18 and the p-type electrode 17. Further, the contact layer 15 may be provided between the n-type semiconductor layer 4 and the n-type electrode 18 or between the p-type semiconductor layer 12 and the p-type electrode 17.

3. Superlattice Semiconductor Layer The superlattice semiconductor layer 10 is sandwiched between an n-type semiconductor layer (base layer) 4 and a p-type semiconductor layer (emitter layer) 12 to constitute the photoelectric conversion layer 2. The superlattice semiconductor layer 10 has a superlattice structure in which the quantum layers 5 and the barrier layers 8 are alternately and repeatedly stacked.

  The superlattice semiconductor layer 10 has a band structure as shown in the band diagrams of FIGS. In these band diagrams, the horizontal axis is the distance in the z direction shown in FIGS. 1 to 10, and the vertical axis is the energy of electrons. In addition, the lower end of the conduction band, the upper end of the valence band, and the quantum level of the quantum layer are indicated by solid lines. Further, the superlattice semiconductor layer 10 having a band structure as shown in the band diagram of FIG. 12 has a light absorption spectrum as shown in the upper diagram of FIG. In this case, the wavelength converter 30 is provided so as to convert the wavelength of incident light as shown in the lower diagram of FIG.

  The superlattice semiconductor layer 10 having a band structure as shown in FIG. 12 has an energy corresponding to the energy difference between the upper end of the valence band of the barrier layer 8 and the lower end of the conduction band of the barrier layer 8 (the band gap of the barrier layer 8). Light having (wavelength) is absorbed, and electrons in the valence band of the barrier layer 8 are excited to the conduction band of the barrier layer 8 as indicated by an arrow 200 in FIG. Such light absorption due to the band gap of the barrier layer 8 appears as an absorption peak A in the light absorption spectrum shown in the upper diagram of FIG. It is absorption of light up to energy that is somewhat larger than energy (light within the absorption wavelength region A). Therefore, the superlattice semiconductor layer 10 can absorb light in the absorption wavelength region A due to the band gap of the barrier layer 8.

  In addition, the superlattice semiconductor layer 10 having the band structure as shown in FIG. 12 has a quantum level of the valence band of the quantum layer 5 (or the valence band upper end of the barrier layer 8) and the conduction band of the quantum layer 5. Light having an energy (wavelength) corresponding to an energy difference from the quantum level (band gap of the quantum layer) is absorbed, and electrons in the quantum level of the valence band of the quantum layer 5 (arrow 201 in FIG. 12) Or electrons in the valence band of the barrier layer 8) are excited to the quantum level of the conduction band of the quantum layer 5. Such light absorption due to the band gap of the quantum layer 5 appears as an absorption peak B in the light absorption spectrum shown in the upper diagram of FIG. 15, and from the energy corresponding to the band gap of the quantum layer 5 from this energy. Absorption of light up to a certain large energy (light in the absorption wavelength region B) is obtained. Therefore, the superlattice semiconductor layer 10 can absorb light in the absorption wavelength region B due to the band gap of the quantum layer 5. Here, the energy difference between the longest wavelength and the shortest wavelength in the absorption wavelength region B is preferably 50 meV or more, and more preferably 100 meV or more.

  Furthermore, the superlattice semiconductor layer 10 having the band structure as shown in FIG. 12 has energy (wavelength) corresponding to the energy difference between the quantum level of the conduction band of the quantum layer 5 and the lower end of the conduction band of the barrier layer 8. The electrons having the quantum level in the conduction band of the quantum layer 5 are excited to the conduction band of the barrier layer 8 as indicated by an arrow 202 in FIG. The light absorption due to the energy difference between the quantum level of the conduction band of the quantum layer 5 and the bottom of the conduction band of the barrier layer 8 is as shown by the absorption peak C in the light absorption spectrum shown in the upper diagram of FIG. Appearance is absorption of light (light in the light absorption wavelength region C) from energy corresponding to this energy difference to energy somewhat larger than this energy. Therefore, the superlattice semiconductor layer 10 can absorb light in the absorption wavelength region C due to the energy difference between the quantum level of the conduction band of the quantum layer 5 and the lower end of the conduction band of the barrier layer 8. Here, the energy difference between the longest wavelength and the shortest wavelength in the absorption wavelength region C is preferably 50 meV or more, and more preferably 100 meV or more.

The absorption wavelength region is a wavelength region in which the superlattice semiconductor layer 10 can absorb most of incident light. The absorption wavelength region is a region where the absorption rate is 10 or more, where the absorption rate of the baseline of the light absorption spectrum at the absorption peak is 0 and the absorption rate at the top of the absorption peak is 100. When the baseline of the light absorption spectrum is inclined, the average can be set to zero.
For example, when the superlattice semiconductor layer 10 has an absorption peak as shown in FIG. 18, the absorption wavelength region of this absorption peak is a region between two alternate long and short dash lines in FIG. When two absorption peaks overlap outside the absorption wavelength region, the peak is divided and a baseline is drawn. When two or more absorption peaks overlap in the absorption wavelength region as shown in FIG. 19, one absorption wavelength region is defined by the overlapping absorption peaks. In this case, the peak of the absorption peak is the peak of the highest absorption peak among the overlapping absorption peaks.

  When the superlattice semiconductor layer 10 has a light absorption spectrum as shown in the upper diagram of FIG. 15, among the light incident on the superlattice semiconductor layer 10, within the absorption wavelength region A, within the absorption wavelength region B, and within the absorption wavelength region C Is absorbed by the superlattice semiconductor layer 10 and used for photoelectric conversion. However, the light having a wavelength in the wavelength region between the absorption wavelength region A and the absorption wavelength region B and the light having a wavelength in the wavelength region between the absorption wavelength region B and the absorption wavelength region C are superlattice semiconductor layers. 10 hardly absorbs and is hardly used for photoelectric conversion.

  In such a case, a wavelength conversion unit 30 including first and second wavelength conversion materials capable of converting the wavelength of light as shown in the lower diagram of FIG. 15 is provided. In this case, the wavelength at which the first wavelength conversion material converts light having a wavelength in the wavelength region between the absorption wavelength region A and the absorption wavelength region B indicated by the solid line into light in the absorption wavelength region B indicated by the dotted line. As the conversion material, the second wavelength conversion material converts light having a wavelength in the wavelength region between the absorption wavelength region B and the absorption wavelength region C indicated by the solid line into light in the absorption wavelength region C indicated by the dotted line. A wavelength conversion material is used. By providing such a wavelength conversion unit 30, the amount of light absorbed in the superlattice semiconductor layer 10 and used for photoelectric conversion can be increased, and the photoelectric conversion efficiency of the solar cell 20 can be improved.

In such a case, the first absorption wavelength region is the absorption wavelength region A or B. When the first absorption wavelength region is the absorption wavelength region A, the second absorption wavelength region is the absorption wavelength region B or C. When the first absorption wavelength region is the absorption wavelength region B, the second absorption wavelength region is the absorption wavelength region C. Therefore, the wavelength conversion unit 30 may convert the wavelength of light in the wavelength region between the absorption wavelength region A and the absorption wavelength region B into light of the wavelength in the absorption wavelength region C.
Further, when the first absorption wavelength region is the absorption wavelength region A and the second absorption wavelength region is the absorption wavelength region B, the absorption wavelength region C may be the third absorption wavelength region.

FIG. 13 shows the band of the superlattice semiconductor layer 10 when a miniband is formed by the quantum level of the valence band of the quantum layer 5 and the miniband is formed by the quantum level of the conduction band of the quantum layer 5. In this case, the optical excitation indicated by the arrow 300 in FIG. 13 corresponds to the optical excitation indicated by the arrow 200 in FIG. 12, the optical excitation indicated by the arrow 301 in FIG. 13 corresponds to the optical excitation indicated by the arrow 301 in FIG. Corresponds to the optical excitation indicated by the arrow 202 in FIG. In such a case, since the carrier can move through the miniband, it is possible to increase the probability of the two-stage photoexcitation of electrons and to increase the photoelectric conversion efficiency.
In addition, it is preferable to form a miniband because an absorption wavelength region caused by the quantum layer is widened. From the viewpoint of efficient light absorption, the energy difference between the longest wavelength and the shortest wavelength in the absorption wavelength region due to the quantum layer is preferably 50 meV or more, and more preferably 100 meV or more.

The quantum layer 5 may be a quantum dot layer 6 composed of a plurality of quantum dots 7 like the solar cell 20 shown in FIGS. 1 to 3, 6, 7, 9, and 10. It may be a quantum dot 7 formed in a nanowire as in the solar cell 20, or may be a quantum well layer 9 as in the solar cell 20 shown in FIG.
The quantum dot layer 6 is a layer including a plurality of quantum dots 7, and the quantum dots 7 are made of a semiconductor material having a narrower band gap than the semiconductor material constituting the barrier layer 8, and the quantum level is increased by the quantum effect. Have.
The plurality of quantum dots 7 included in the superlattice semiconductor layer 10 may be composed of the same material or may include quantum dots 7 composed of different materials. Further, when the plurality of quantum dots 7 included in the superlattice semiconductor layer 10 are composed of mixed crystals, the plurality of quantum dots 7 may include a plurality of types of quantum dots 7 having different mixed crystal ratios. The number of quantum energy levels of the superlattice solar cell can be changed by including the quantum dots 7 composed of a plurality of types of materials and mixed crystal ratios. Such quantum dots 7 can be provided, for example, like the quantum dots 7a and 7b included in the solar cell 20 illustrated in FIG. 7, and the quantum dots 7a to 7 included in the solar cell 20 illustrated in FIG. 7d can be provided.
According to theoretical calculations, the conversion efficiency of a solar cell having a plurality of types of quantum dots is 63% when one type of quantum dot is used, but when two types of quantum dots are used, the maximum efficiency is 70%, 73% when 3 types of quantum dots are used, and 75% when 4 types of quantum dots are used. On the other hand, when five or more types of quantum dots are used, the increase in efficiency is remarkably slowed down, so that a high potential is obtained with one to four types of quantum dots, and the highest potential is obtained with four types of quantum dots.

The plurality of quantum dots 7 included in the superlattice semiconductor layer 10 may be composed of substantially the same size in the x direction, the size in the y direction, and the size in the z direction. A plurality of types of dots 7 may be included. By including quantum dots 7 having a plurality of sizes, the number of quantum energy levels of the superlattice solar cell can be changed. Such quantum dots 7 can be provided, for example, like the quantum dots 7a and 7b included in the solar cell 20 shown in FIG. 6, and the quantum dots 7a to 7a included in the solar cell 20 shown in FIG. 7d can be provided.
The superlattice semiconductor layer 10 including the quantum dots 7 can be produced, for example, by SK growth.

The barrier layer 8 is made of a semiconductor material having a wider band gap than the semiconductor material constituting the quantum dots 7, and forms a potential barrier around the quantum dots 7.
In this embodiment, the solar cell 20, the superlattice semiconductor layer 10, may be used, for example In _x Ga _1-x quantum dot layer 6 made of _{As, Al x Ga 1-x} As consisting barrier layer 8 . Further, it is possible to use a barrier layer 8 composed of In _x As _1-x quantum dot layer 6 made of _{Sb, Al x As 1-x} Sb. In addition, materials of InAs, GaAs, AlAs, InSb, GaSb, AlSb, InP, GaP, and AlP and mixed crystal materials thereof may be used for the superlattice semiconductor layer 10. Moreover, a barrier layer 8 constituting the superlattice semiconductor layer 10, as the material constituting the quantum dot layer _{6, Al x Ga y In 1} -xy As, Al x Ga y In 1-xy Sb z As 1 - z, Al _{x Ga y In 1-xy P} , or the like can be used _{Al x Ga y In 1-xy} N. Other than those described above, III-V group compound semiconductors, chalcopyrite materials, II-VI group compound semiconductors, group IV semiconductors, or mixed crystal materials thereof may be used.

  The quantum dot layer 6 and the barrier layer 8 made of a mixed crystal change the element ratio of the mixed crystal appropriately to change the lattice constant according to a desired value or the substrate, or to change the valence band energy offset (quantum dot layer 6 and the barrier layer 8 can be made zero.

As shown in FIG. 4, the superlattice semiconductor layer 10 may have a structure (quantum dot nanowire structure) in which quantum dots 7 are stacked in a nanowire having a diameter of 100 nm or less and a plurality of nanowires are arranged. In the structure shown in FIG. 4, the size of the quantum dots 7 included in one nanowire is the same.
Such a structure is characterized in that the quantum dots 7 can be stacked with the strain reduced as much as possible, and as a result, uniform quantum dots 7 can be stacked.
Moreover, although it can be filled with the resin 40 like BCB in the space | gap of several nanowire, it has the characteristic that a wavelength conversion material can also be buried instead. In this case, the gaps of the plurality of nanowires become the wavelength conversion unit 30. In this case, the wavelength-converted light can be reflected a plurality of times inside the solar cell (the effective optical path length increases), and the light can be used more effectively.
Similarly to FIGS. 1 to 3, the wavelength conversion unit 30 can be formed immediately above the superlattice structure solar cell 20 or on the front side or the back side of the superlattice structure solar cell.

  Although the superlattice structure solar cell 20 using the quantum dots 7 has been described above, the present invention can also be applied to the superlattice structure solar cell 20 having the quantum well layer 9 as shown in FIG. That is, even in a superlattice structure solar cell using a quantum well, there is a problem that an absorption spectrum region caused by the quantum layer 5 is narrow.

  Also in the superlattice structure solar cell 20 using the quantum well, the wavelength conversion unit 30 is formed immediately above the superlattice structure solar cell 20 or on the front surface side or the back surface side of the superlattice structure solar cell 20 as in FIGS. can do.

  FIG. 6 shows a structure in which quantum dots 7a and 7b having the same material / mixed crystal ratio but different sizes are alternately stacked. With such a structure, two different energy levels can be formed at the intermediate level. Such a structure is not a technique in which quantum dots 7a and 7b of different sizes are arranged alternately, but the superlattice structure is divided into two parts, the same size quantum dots 7 are stacked at the lower part of the superlattice structure, and the size is arranged at the upper part Different quantum dots 7 may be stacked.

  FIG. 7 shows a structure in which quantum dots 7a and 7b having the same size but different materials and mixed crystal ratios are alternately stacked. With such a structure, two different energy levels can be formed at the intermediate level. Such a structure is not a technique in which quantum dots 7a and 7b having different materials and mixed crystal ratios are arranged alternately, but quantum dots 7a and 7b having different materials and mixed crystal ratios are stacked on the lower and upper parts of the superlattice structure, respectively. May be.

FIG. 8 shows a structure (quantum dot nanowire structure) in which quantum dots 7 are stacked in nanowires 43 having a diameter of 100 nm or less. In FIG. 8, the sizes of the quantum dots 7 included in one nanowire 43 are the same.
Forming nanowires 43 including quantum dots 7 of two different sizes in a high density, about half each within a square area of 300 nm or less per side (about the wavelength of the ultraviolet region), leaving excess sunlight It can absorb without. It is preferable that the quantum dot nanowire structures having two different sizes are alternately arranged because sunlight can be efficiently absorbed.
In addition, when such a structure is used, distortion is relaxed in the x direction, so that quantum dots having different sizes (including sizes in both the x direction and the z direction) can be easily formed. It is also easy to form quantum dot nanowire structures having two or more different sizes.
Wavelength conversion can be efficiently performed by providing a wavelength conversion material above the gap between the nanowires 43 of the quantum dot nanowires or the superlattice structure solar cell.

  FIG. 9 shows a structure in which quantum dots 7a, 7b, 7c, and 7d having the same material / mixed crystal ratio but different sizes are periodically stacked. With such a structure, four different energy levels can be formed in the intermediate level. Such a structure is not a technique in which quantum dots 7a, 7b, 7c, and 7d having different sizes are periodically arranged, but the superlattice structure is divided into four parts, and different quantum dots 7a, 7b, 7c, and 7d are provided for the respective parts. May be laminated.

  FIG. 10 shows a structure in which quantum dots 7a, 7b, 7c, and 7d having the same size but different materials and mixed crystal ratios are periodically stacked. With such a structure, four different energy levels can be formed in the intermediate level. Such a structure is not a technique of periodically arranging quantum dots 7a, 7b, 7c, 7d of different materials / mixed crystal ratios, but the superlattice structure is divided into four parts, and different quantum dots 7a, 7b are divided into respective parts. , 7c, 7d may be laminated.

  For example, the superlattice solar cell 20 using GaAs for the barrier layer 8 and InGaAs for the quantum layer 5 can freely change the quantum energy level of the quantum layer 5 by appropriately adjusting the composition and size of InGaAs. Energy conversion efficiency can be increased when the superlattice semiconductor layer 10 has a light absorption spectrum as shown in FIG. In this case, the superlattice semiconductor layer 10 has a band structure as shown in FIG. An absorption peak A in FIG. 16 indicates light absorption due to the band gap of the barrier layer material, an absorption peak B in FIG. 16 indicates light absorption corresponding to photoexcitation indicated by an arrow 201 in FIG. 12, and an absorption peak C in FIG. The light absorption corresponding to the optical excitation of the arrow 202 of FIG. 12 is shown. 16 may correspond to the photoexcitation indicated by the arrow 202 in FIG. 12, and the absorption peak C in FIG. 16 may correspond to the photoexcitation indicated by the arrow 201 in FIG. The peak width and absorption wavelength of the absorption peak B and absorption peak C due to the quantum layer 5 in FIG. 16 are examples, and the absorption peak A of the barrier layer material is not necessarily the peak width or absorption wavelength described in this figure. And the absorption peaks B and C resulting from the quantum layer 5 can increase the energy conversion efficiency.

When the superlattice semiconductor layer 10 has a light absorption spectrum as shown in FIG. 16, the energy conversion efficiency of the solar cell 20 increases when incident light is wavelength-converted by the wavelength conversion unit 30 as shown in FIG. 17. That is, a wavelength conversion material that converts the wavelength of sunlight from a wavelength of about 890 nm to about 1300 nm (spectral region before 403 is wavelength-converted) into sunlight of about 1300 nm (spectral region after 404 is wavelength-converted); A wavelength conversion unit that converts the wavelength of sunlight from about 1400 nm to about 2700 nm (spectral region before 405 is wavelength-converted) into sunlight of about 2700 nm (spectral region after 406 is wavelength-converted) Should be prepared.
The wavelength conversions 407 and 408 do not necessarily require wavelength conversion of all the spectral regions 403 and 405. If at least a part of one of the spectral regions 403 and 405 can be converted, energy conversion efficiency can be improved. Can be expensive.

  Examples of the wavelength conversion material that converts the wavelength into light of about 1300 nm include PbS, PbSe, PbTe, InAs, InGaAs, InN, InGaN, InSb, and InGaSb. When the band gap of the bulk material for wavelength conversion does not match the desired wavelength of 1300 nm after wavelength conversion, the size can be adjusted to the desired wavelength using the quantum effect by setting the size to the order of nm. These may be a core-shell structure or a mixed crystal material. Examples of the core-shell structure include PbS / CdS, PbSe / CdSe, PbTe / CdTe, InAs / InGaAs, InN / InGaN, and InSb / InGaSb.

  Examples of the wavelength conversion material that converts the wavelength of light to about 2700 nm include PbS, PbSe, PbTe, InAs, InGaAs, InSb, and InGaSb. When the band gap of the bulk material for wavelength conversion does not match the desired wavelength of 2700 nm after wavelength conversion, the size can be adjusted to the desired wavelength using the quantum effect by setting the size to the order of nm. These may be a core-shell structure or a mixed crystal material. Examples of the core-shell structure include PbS / CdS, PbSe / CdSe, PbTe / CdTe, InAs / InGaAs, InSb / InGaSb, and the like.

The above configuration is preferably made of two types of wavelength conversion materials (even if the same material has different sizes and mixed crystal ratios, but different types), but either one of the wavelength conversions 407 or 408, or both of them are plural. The wavelength conversion material may be used, and absorption and emission may be repeated a plurality of times and then converted to a desired wavelength.
For wavelength conversion, it is preferable to perform quantum efficiency as high as possible from the viewpoint of higher energy conversion efficiency. As such a material, a wavelength conversion material of an organic material that can use the Forster mechanism can also be used.

FIG. 20 shows a superlattice semiconductor layer formed using, for example, any one of AlGaInAs, AlGaInP, and AlSb for the barrier layer 8 and using any one of InGaAs, InPSb, InAsN, and InAsSb for the quantum layer 5. 10 is a light absorption spectrum. In this case, PbS, PbSe, PbTe, InAs, InGaAs are wavelength conversion materials for converting the wavelength of light in the wavelength region between the absorption wavelength region A and the absorption wavelength region B into light in the absorption wavelength region B. InN, InGaN, InSb, InGaSb, CuInGaSe ₂ , CuInTe _2, or the like can be used.
Further, as a wavelength conversion material for converting the wavelength of light in the wavelength region between the absorption wavelength region B and the absorption wavelength region C into light in the absorption wavelength region C, PbS, PbSe, PbTe, InAs, InGaAs, InSb, InGaSb, or the like can be used.
In any wavelength conversion, when the band gap of the bulk material for wavelength conversion does not match the desired wavelength after wavelength conversion, the size can be adjusted to the desired wavelength using the quantum effect by setting the size to the order of nm.

Figure 21 uses a CuIn (GaAl) Se ₂ on the barrier layer 8, which is a light absorption spectrum of the superlattice semiconductor layer 10 fabricated using a CuInGaSe ₂ the quantum layer 5. In this case, PbS, PbSe, PbTe, InAs, InGaAs are wavelength conversion materials for converting the wavelength of light in the wavelength region between the absorption wavelength region A and the absorption wavelength region B into light in the absorption wavelength region B. InN, InGaN, InSb, InGaSb, CuInGaSe ₂ , CuInTe _2, or the like can be used. When the band gap of the bulk material for wavelength conversion does not match the desired wavelength after wavelength conversion, the size can be adjusted to the desired wavelength using the quantum effect by setting the size to the order of nm.
Further, as a wavelength conversion material for converting the wavelength of light in the wavelength region between the absorption wavelength region B and the absorption wavelength region C into light in the absorption wavelength region C, PbS, PbSe, PbTe, InAs, InGaAs, InN, InGaN, InSb, InGaSb, CuInGaSe ₂ , CuInTe ₂ or the like can be used. When the band gap of the bulk material for wavelength conversion does not match the desired wavelength after wavelength conversion, the size can be adjusted to the desired wavelength using the quantum effect by setting the size to the order of nm.
By providing the wavelength conversion material as described above, it is possible not only to absorb a spectrum that has not been sufficiently absorbed, but also to efficiently extract carriers from the quantum layer 5 to the barrier layer 8.

  The above-described configuration has been mainly described for the superlattice structure solar cell in which the light absorption spectrum of the superlattice semiconductor layer 10 has three absorption wavelength regions as shown in FIGS. The present invention can be similarly applied to a superlattice structure solar cell having a light absorption spectrum of 4 or more.

For a superlattice structure solar cell in which the light absorption spectrum of the superlattice semiconductor layer 10 has four or more absorption wavelength regions, the number of preferred spectral regions increases when the wavelength is converted. However, as will be described later, when the light absorption spectrum is dense, it is sometimes preferable that the number of preferable spectral regions is smaller when wavelength conversion is performed.
A superlattice structure solar cell in which the light absorption spectrum of the superlattice semiconductor layer 10 has five absorption wavelength regions will be described below.

A superlattice structure solar cell in which the light absorption spectrum of the superlattice semiconductor layer 10 has five absorption wavelength regions has, for example, a structure having two types of quantum dots as shown in FIGS. It is like that.
In FIG. 22, for example, one of AlGaInAs, AlGaInP, and AlSb is used for the barrier layer, and at least one of the size, material, and mixed crystal ratio is different from at least one of InGaAs, InPSb, InAsN, and InAsSb for the quantum layer. It is the optical absorption spectrum of the superlattice semiconductor layer 10 produced using.

  In the superlattice solar cell having such a superlattice semiconductor layer 10, for example, light in a wavelength region from 900 nm to near 1200 nm is wavelength-converted into light in a wavelength region near 1200 nm by the wavelength conversion unit 30, for example, from 1200 nm to near 1800 nm. The light in the wavelength region from 1800 nm to wavelength light in the vicinity of 1800 nm may be converted into light, and the light in the wavelength region from 1800 nm to near 2800 nm may be converted into light in the wavelength region near 2800 nm.

Examples of wavelength conversion materials for wavelength conversion to light in the wavelength region near 1200 nm include PbS, PbSe, PbTe, InAs, InGaAs, InN, InGaN, InSb, InGaSb, CuInGaSe ₂ , and CuInTe ₂ . Examples of the wavelength conversion material that converts the wavelength into light include PbS, PbSe, PbTe, InAs, InGaAs, InSb, and InGaSb. Examples of the wavelength conversion material that converts the wavelength into light in the wavelength region near 2800 nm include PbS, PbSe, PbTe, InAs, InGaAs, InSb, and InGaSb. In any wavelength conversion, when the band gap of the bulk material for wavelength conversion does not match the desired wavelength after wavelength conversion, the size can be adjusted to the desired wavelength using the quantum effect by setting the size to the order of nm.
As the configuration of the superlattice structure solar cell having an absorption spectrum as shown in FIG. 22, for example, solar cells as shown in FIGS.

Hereinafter, a superlattice structure solar cell in which the light absorption spectrum of the superlattice semiconductor layer 10 has nine wavelength absorption regions will be described.
The superlattice structure solar cell in which the light absorption spectrum of the superlattice semiconductor layer 10 has nine wavelength absorption regions has, for example, a structure having four different quantum dots as shown in FIGS.
In FIG. 23, for example, one type of AlGaInAs, AlGaInP, and AlSb is used for the barrier layer 8, and any one of the size, material, and mixed crystal ratio is different for the quantum layer 5 from, for example, InGaAs, InPSb, InAsN, and InAsSb. It is the light absorption spectrum of the superlattice semiconductor layer 10 produced using the kind.
In this case, since the light absorption spectrum becomes dense, the number of wavelength conversion materials may be small. For example, since the absorption wavelength regions A to E in FIG. 23 are dense, it is not necessary to convert the wavelength of the light between them by the wavelength conversion unit 30, for example, in the wavelength region between the absorption wavelength regions E and F The wavelength conversion unit 30 may convert the wavelength of light and light in the wavelength region between the absorption wavelength regions F and G. As described above, when a plurality of types of quantum layers 5 are used, the types of wavelength conversion materials can be reduced.
As the configuration of the superlattice structure solar cell having the absorption spectrum as shown in FIG. 23, for example, solar cells as shown in FIGS.

4). Manufacturing method of solar cell having superlattice structure Quantum dot layer 6 is formed by a method called Stranski-Krastanov (SK) growth using molecular beam epitaxy (MBE) method or metal organic chemical vapor deposition (MOCVD) method. Quantum dots can be produced by using an electronic lithography technique, a droplet epitaxy method, or the like. In the SK growth method, the mixed crystal ratio of the quantum dots can be adjusted by changing the composition ratio of the raw materials in the above method, and the size of the quantum dots can be adjusted by changing the growth temperature, pressure, deposition time, etc. Can do.

  In the production of the solar cell of the present embodiment, for example, a solar cell having a superlattice structure is produced by using a molecular beam epitaxy (MBE) method or a metal organic chemical vapor deposition method (MOCVD) excellent in film thickness control. can do. Here, a manufacturing method of one embodiment of a solar cell having a superlattice structure will be described.

  For example, the n-GaAs substrate (n-type semiconductor substrate) 1 is cleaned with an organic cleaning solution, then cleaned with a sulfuric acid cleaning solution, further washed with running water, and then placed in an MOCVD apparatus. A buffer layer 3 is formed on this substrate. The buffer layer 3 is a layer for improving the crystallinity of the light absorption layer to be formed thereon, and for example, a GaAs layer is formed. Subsequently, a 300 nm-thick n-type GaAs base layer (n-type semiconductor layer) 4 and a GaAs layer serving as a barrier layer 8 are crystal-grown on the buffer layer 3, and then a quantum dot made of InAs is used using a self-organization mechanism. Layer 6 is formed. At this time, the size and composition ratio of the quantum dots can be adjusted to desired values by appropriately changing the deposition time, temperature, pressure, raw material supply amount, raw material composition ratio, and the like.

The crystal growth of the barrier layer 8 and the quantum dot layer 6 is repeated from the quantum dot layer 6 closest to the n-type semiconductor layer 4 to the quantum dot layer closest to the p-type semiconductor layer 12. At this time, when the quantum dot layer is n-type, for example, the quantum dot layer 6 performs crystal growth while introducing silane (SiH ₄ ), and introduces Si into the barrier layer 8. Si may be directly introduced into the quantum dots 7.
Subsequently, a p-type GaAs layer (p-type semiconductor layer) 12 having a thickness of 250 nm is crystal-grown, and then an AlAs layer is formed as the window layer 14.

Subsequently, a p-type electrode 17 is formed on the contact layer 15 by a photolithography technique, a lift-off technique, and an etching technique, whereby a solar cell having a superlattice structure can be formed. The contact layer 15 is preferably removed by etching except under the p-type electrode 17.
For example, Si can be used as the n-type dopant, and Zn can be used as the p-type dopant. Other n-type dopants include, for example, S, Se, Sn, Te, C, and other p-type dopants include Be, Mg, and the like. Examples of the electrode material include AuGeNi and AgSn for the n-type GaAs layer, and AuZn and AgInZn for the p-type GaAs layer. These can be formed by resistance heating vapor deposition or EB vapor deposition.
It can be similarly produced using InAsSb quantum dots and AlSb barrier layers. In the case of these materials, if the substrate is GaSb, the lattice mismatch is reduced, which is more preferable.
The wavelength conversion unit 30 may be provided on the light receiving surface side of the photoelectric conversion layer 2 as shown in FIG. 1 or may be provided on the light receiving surface of the photoelectric conversion layer 2 as shown in FIG. Further, the wavelength conversion unit 30 may be provided on the substrate 1 as shown in FIG. 3, or may already be provided inside the luminescence converter as shown in FIG.
Moreover, the wavelength conversion part 30 may use wavelength conversion material as it is, and may seal and use it for resin or glass.

  When the light absorption region of the wavelength conversion unit 30 is widely present on the short wavelength side (for example, in FIG. 15, the wavelength conversion region D and / or E of the wavelength conversion unit 30 is a part of the light absorption wavelength region A of the photoelectric conversion layer 2 or A structure in which the wavelength conversion unit 30 is provided on the back surface of the solar cell is preferable. Moreover, when providing the wavelength conversion part 30 which consists of two or more types of wavelength conversion materials in a solar cell back surface, from the layer which wavelength-converts the solar spectrum which has short wavelength light in order toward the outer side from the side close | similar to a solar cell. It is desirable to provide a layer for wavelength-converting the sunlight spectrum having a long wavelength (for example, in FIG. 15, the wavelength conversion region E of the wavelength conversion unit 30 overlaps part or all of the absorption wavelength region B of the solar cell. Preferred if). For example, in the case where a wavelength conversion unit made of two kinds of wavelength conversion materials is provided on the back surface of the solar cell, a layer 30a for converting the wavelength of a shorter wavelength sunlight spectrum (for example, wavelength conversion region D in FIG. 15) is added to 30b in FIG. It is desirable to provide a layer for converting the wavelength of the long-wavelength sunlight spectrum (for example, the wavelength conversion region E in FIG. 15).

  By adopting such a structure, there is no possibility that most of the absorption peak A in FIG. 15 is wavelength-converted by the wavelength conversion unit 30a or 30b. In addition, it is preferable because the wavelength conversion unit 30a (having the wavelength conversion region E) most of the wavelength conversion region D and most of the light emission region F does not have a risk of wavelength conversion. In this way, sunlight that has been wasted in the superlattice structure solar cell can be used efficiently, the number of carriers extracted from the quantum layer can be increased, and the voltage of the solar cell can be increased. Photoelectric conversion efficiency can be greatly improved.

1: n-type semiconductor substrate 2: photoelectric conversion layer 3: buffer layer 4: base layer (n-type semiconductor layer) 5: quantum layer 6: quantum dot layer 7: quantum dot 8: barrier layer 9: quantum well layer 10: super Lattice semiconductor layer 12: Emitter layer (p-type semiconductor layer) 14: Window layer 15: Contact layer 17: P-type electrode 18: N-type electrode 19: Antireflection film 20: Solar cell 30a, 30b: Wavelength conversion unit 40: Resin 43: Nanowire 47: SiO ₂ film 50: Sunlight incident surface 51: Side surface

Claims (14)

A photoelectric conversion layer, and a wavelength conversion unit including a wavelength conversion material that converts the wavelength of light incident on the photoelectric conversion layer,
The photoelectric conversion layer includes a p-type semiconductor layer, an n-type semiconductor layer, and a superlattice semiconductor layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer,
The superlattice semiconductor layer has a superlattice structure in which barrier layers and quantum layers are alternately and repeatedly stacked, and has a first absorption wavelength region and a second absorption wavelength region on the long wavelength side of the first absorption wavelength region Having a light absorption spectrum having
The wavelength conversion unit converts light having a wavelength in the wavelength region between the first absorption wavelength region and the second absorption wavelength region into light having a wavelength in the second absorption wavelength region ,
The solar cell according to claim 1, wherein the wavelength conversion unit is provided on a side of the photoelectric conversion layer opposite to a side on which sunlight is incident .   The solar cell according to claim 1, wherein the quantum layer is a quantum dot layer. The light absorption spectrum has three or more absorption wavelength regions,
The solar cell according to claim 1 or 2, wherein the first and second absorption wavelength regions are two regions included in the three or more absorption wavelength regions. The wavelength conversion unit includes two or more types of wavelength conversion materials,
The light absorption spectrum further has a third absorption wavelength region on the long wavelength side of the second absorption wavelength region,
The wavelength conversion unit according to any one of claims 1 to 3, wherein the wavelength conversion unit converts light having a wavelength in a wavelength region between a second absorption wavelength region and a third absorption wavelength region into light having a wavelength in the third absorption wavelength region. The solar cell according to one. The wavelength conversion unit has a plurality of wavelength conversion layers made of different types of wavelength conversion materials,
The solar cell according to any one of claims 1 to 4, wherein the plurality of wavelength conversion layers are made of a wavelength conversion material capable of wavelength-converting longer wavelength light as the distance from the photoelectric conversion layer increases. The light absorption spectrum has four or more absorption wavelength regions,
The solar cell according to claim 5 , wherein the first, second, and third absorption wavelength regions are three regions included in the four or more absorption wavelength regions. Wherein the wavelength conversion material, a solar cell according to any one of claims 1 to 6 which is a semiconductor material. The superlattice structure is a solar cell according to any one of claims 1 to 7 barrier layer and the quantum dot layer is a quantum dot nanowire structure are alternately laminated. The said quantum dot nanowire structure is a solar cell of Claim 8 provided with the said wavelength conversion part between quantum dot nanowires. The solar cell according to any one of claims 1 to 9 , wherein the wavelength conversion unit includes a wavelength conversion material that converts a wavelength of light having a wavelength of 1000 nm or more. Said superlattice semiconductor layer, a solar cell according to any one of claims 1-10 which is provided so as to form a miniband. The light absorption spectrum, the solar cell according to any one of claims 1 to 11 energy difference shortest wavelength and the longest wavelength of the second light absorption wavelength region is not less than 50 meV. The wavelength conversion part is a molded body containing a wavelength conversion material,
The photoelectric conversion layer, the solar cell according to any one of claims 1 to 12 provided on the surface of the wavelength converting part. The molded body is a rectangular parallelepiped having one light receiving surface,
14. The solar cell according to claim 13 , wherein the number of the photoelectric conversion layers is at least four and provided on four side surfaces around the light receiving surface of the rectangular parallelepiped.