JP6474618B2 - Photoelectric conversion element - Google Patents

Description

  The present invention relates to a photoelectric conversion element.

  Patent Document 1 discloses a configuration in which, in a photoelectric conversion element having a quantum structure layer, a light-scattering reflective layer is provided on the main surface of a laminate opposite to the light receiving surface side of the photoelectric conversion element. A structure is described in which, by providing a light-scattering reflective layer, the optical path length of reflected light is increased to improve the absorption of light mainly having a wavelength of 890 to 1100 nm in the sunlight spectrum.

JP 2013-219073 A

  However, in a photoelectric conversion element having quantum dots, it is necessary to efficiently cause two-stage light absorption through an intermediate band in order to improve light absorption of the quantum dots. Two-stage light absorption includes first-stage light absorption (interband transition, which is an optical transition from a valence band to an intermediate band) and second-stage light absorption (subband, which is a transition from an intermediate band to a conduction band). Transition). From the inventors' research, the light absorption at the first stage is not sufficient for improving the efficiency of a solar cell having quantum dots (hereinafter referred to as a quantum dot solar cell). It has become clear that is not enough. When two-stage light absorption is not sufficient, quantum dot solar cells have little additional light absorption compared to conventional single-junction solar cells (for example, Si solar cells and GaAs solar cells) in which no quantum dots are inserted. In addition to improving the conversion efficiency, the quantum dots act as recombination centers and the conversion efficiency is greatly reduced. Therefore, it is very important to efficiently cause two-stage light absorption. Whatever material is used as the material of the quantum dot solar cell, it is essential to efficiently absorb light of a component of 1100 nm or more in the sunlight spectrum by two-stage light absorption.

  In an embodiment of the present invention, it is an object to provide a technique for improving light absorption of a quantum dot in a photoelectric conversion element having quantum dots.

  The photoelectric conversion element in one embodiment of the present invention includes a p-type semiconductor layer, an n-type semiconductor layer, a superlattice semiconductor layer having a structure in which quantum dots are surrounded by a barrier layer, and a conduction band side of the quantum dots. An optical path length increasing element that at least increases an optical path length of light absorbed by an intersubband transition that is a transition from the intermediate band to the conduction band of the barrier layer.

  According to the present disclosure, since the optical path length of the light absorbed by the intersubband transition can be increased at least by the optical path length increasing element, it is possible to improve the absorption of long wavelength light in the quantum dot.

FIG. 1 is a schematic cross-sectional view showing the configuration of the solar cell in the first embodiment. FIG. 2 is a diagram schematically showing an energy band of the solar cell in the present embodiment. FIG. 3 is a diagram in which a part of the schematic diagram of the energy band in FIG. 2 is extracted. FIG. 4 is a partially enlarged view of the texture structure. FIG. 5 is a diagram illustrating a result obtained by calculating the relationship between the texture interval d [μm] and the electric field strength of light inside the solar cell. FIG. 6A shows that in a quantum dot solar cell in which a quantum dot layer is formed of InAs and a barrier layer is formed of GaAs, the theoretical energy limit efficiency increases with the increase in the electric field strength of sunlight corresponding to the intersubband transition. It is a figure which shows how much it improves, and has shown the result under the non-condensing conditions. FIG. 6B shows that in a quantum dot solar cell in which a quantum dot layer is formed of InAs and a barrier layer is formed of GaAs, the theoretical energy limit efficiency increases with the increase in the electric field strength of sunlight corresponding to the intersubband transition. It is a figure which shows how much it improves, and has shown the result on the conditions of 1000 time condensing. FIG. 7A shows that in a quantum dot solar cell in which a quantum dot layer is formed of InAs and a barrier layer is formed of InGaP, the theoretical energy limit efficiency increases as the electric field strength of sunlight corresponding to the intersubband transition increases. It is a figure which shows how much it improves, and has shown the result under the non-condensing conditions. FIG. 7B shows that in a quantum dot solar cell in which a quantum dot layer is formed of InAs and a barrier layer is formed of InGaP, the theoretical energy limit efficiency increases with an increase in the electric field strength of sunlight corresponding to the transition between subbands. It is a figure which shows how much it improves, and has shown the result under the non-condensing conditions. FIG. 8A is a diagram for explaining a method for producing a texture structure of a substrate. FIG. 8B is a diagram for explaining a method for producing a texture structure of a substrate. FIG. 8C is a diagram for explaining a method for producing a texture structure of a substrate. FIG. 8D is a diagram for explaining a method for producing a texture structure of a substrate. FIG. 9 is a diagram showing a configuration of a solar cell formed so that the back surface of the lower electrode is a flat surface. FIG. 10 is a schematic cross-sectional view showing the configuration of the solar cell in the second embodiment. FIG. 11 is a partially enlarged view of the texture structure of the substrate. FIG. 12 is a schematic cross-sectional view showing the configuration of the solar cell in the third embodiment. FIG. 13: is a schematic sectional drawing which shows the structure of the solar cell in 4th Embodiment. FIG. 14 is a schematic cross-sectional view showing the configuration of the solar cell in the fifth embodiment. FIG. 15: is a schematic sectional drawing which shows the structure of the solar cell in 6th Embodiment. FIG. 16 is a schematic cross-sectional view showing the configuration of the solar cell in the seventh embodiment. FIG. 17 is a diagram schematically showing an energy band of the colloidal quantum dot solar cell according to the seventh embodiment. FIG. 18 is a schematic cross-sectional view showing the configuration of the solar cell in the eighth embodiment. FIG. 19 is a schematic cross-sectional view showing the configuration of a solar cell in which two types of wavelength conversion particles, wavelength conversion particles a and wavelength conversion particles b, are applied to a substrate. FIG. 20 is a schematic cross-sectional view showing the configuration of the solar cell in the ninth embodiment. FIG. 21 is a schematic cross-sectional view showing the configuration of the solar cell in the tenth embodiment. FIG. 22 is a schematic cross-sectional view showing the configuration of the solar cell in the eleventh embodiment. FIG. 23 is a schematic cross-sectional view showing the configuration of the solar cell in the twelfth embodiment.

  The photoelectric conversion element in one embodiment of the present invention includes a p-type semiconductor layer, an n-type semiconductor layer, a superlattice semiconductor layer having a structure in which quantum dots are surrounded by a barrier layer, and a conduction band side of the quantum dots. An optical path length increasing element that at least increases an optical path length of light absorbed by an intersubband transition that is a transition from the intermediate band to the conduction band of the barrier layer.

  According to this configuration, the optical path length of the light absorbed by the intersubband transition can be increased at least, so that the absorption of light having a long wavelength can be improved in the quantum dots.

  The optical path length increasing element may have a texture structure formed on the back surface opposite to the light receiving surface of the photoelectric conversion element. According to this configuration, the optical path length of the reflected light can be increased by the texture structure formed on the back surface, so that absorption of long-wavelength light can be improved in the quantum dots.

  The optical path length increasing element may have a texture structure formed on the light receiving surface of the photoelectric conversion element. According to this configuration, since the incident light is refracted and incident in an oblique direction, the optical path length of the incident light can be increased, so that absorption of long-wavelength light can be improved in the quantum dots.

  The texture interval d of the texture structure is 1.24 / Eci ≦ d ≦ 1.24, where Eci is the energy from the ground level of the conduction band of the quantum dots to the lower level of the conduction band of the barrier layer. / Eci × 3 relationship. According to this configuration, the degree of light absorption due to the optical transition (intersubband transition) from the intermediate energy band to the conduction band of the superlattice semiconductor layer can be effectively improved.

  The texture interval of the texture structure includes a first interval and a second interval, and the first interval and the second interval may be alternately formed. According to this configuration, it is possible to improve not only the light absorption of the intersubband transition but also the degree of light absorption of the optical transition (interband transition) from the valence band to the intermediate energy band in the quantum dot.

  The first interval is d1, the second interval is d2, the energy from the ground level of the conduction band of the quantum dot to the bottom level of the conduction band of the barrier layer is Eci, and the valence band of the quantum dot Assuming that the energy from the ground level to the ground level of the conduction band of the quantum dots is EQD, 1.24 / EQD ≦ d2 ≦ 1.24 / EQD × 3, 1.24 / Eci ≦ d1 ≦ 1.24 / Eci × 3, d1> d2. According to this configuration, it is possible to more effectively improve the degree of light absorption of intersubband transition and light absorption of interband transition.

  You may make it further arrange | position to the recessed part of the unevenness | corrugation of the texture structure formed in the said back surface, and to absorb the incident light and to further provide the wavelength conversion particle | grains which emit at least the light absorbed by the said intersubband transition. According to this configuration, light that could not be reflected by the texture structure formed on the back surface is absorbed by the wavelength conversion particles, and light having a wavelength different from that of the absorbed light is emitted. The degree of absorption can be further improved.

  The optical path length increasing element may be a texture structure formed on a light incident side surface of a layer located on the back side opposite to the light receiving surface of the p-type semiconductor layer and the n-type semiconductor layer. According to this configuration, light can be reflected before the light reaches the back surface of the photoelectric conversion element, for example, the substrate, so that light confinement occurs more efficiently. Thereby, light absorption, especially the light absorption degree of long wavelength light can be improved.

  The substrate further includes a substrate for forming the p-type semiconductor layer, the n-type semiconductor layer, and the superlattice semiconductor layer, and the optical path length increasing element is formed as a texture structure formed on a light incident side surface of the substrate. Also good. According to this configuration, since light can be reflected before reaching the back surface of the substrate, light confinement occurs more efficiently. Thereby, light absorption, especially the light absorption degree of long wavelength light can be improved.

  The optical path length increasing element is a reflective film formed on the back surface opposite to the light receiving surface of the photoelectric conversion element, and the reflective film is conductive from the ground level of the conduction band of the quantum dot to the barrier layer. If the energy up to the lower end level of the band is Eci, it can be configured to reflect at least light having a wavelength of 1.24 / Eci [μm]. According to this configuration, the degree of light absorption of intersubband transition can be effectively improved.

[Embodiment]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated. In addition, in order to make the explanation easy to understand, in the drawings referred to below, the configuration is shown in a simplified or schematic manner, or some components are omitted. Further, the dimensional ratio between the constituent members shown in each drawing does not necessarily indicate an actual dimensional ratio.

  In this specification, the superlattice structure is a structure in which two layers made of a semiconductor and having different band gaps are repeatedly stacked.

  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. The quantum dot layer refers to a layer including a plurality of quantum dots and becomes a well layer having a superlattice structure. The quantum level refers to the discrete energy level of the electrons of the quantum dot. The barrier layer is made of a semiconductor having a larger band gap than the semiconductor constituting the quantum dots, and constitutes a superlattice structure.

  In the following description, an example in which a photoelectric conversion element is applied to a solar cell will be described.

[First embodiment]
FIG. 1 is a schematic cross-sectional view showing the configuration of the solar cell in the first embodiment. The solar cell 100 in the first embodiment includes a substrate 1, a base layer 2, a superlattice semiconductor layer 6, an emitter layer 7, a window layer 8, a contact layer 9, an upper electrode 10, and a lower electrode 11. With. Specifically, the base layer 2 is formed on the substrate 1, and the superlattice semiconductor layer 6 is formed on the base layer 2. An emitter layer 7 is formed on the superlattice semiconductor layer 6, and a window layer 8 is formed on the emitter layer 7. An upper electrode 10 is provided on the window layer 8 via a contact layer 9. The upper electrode 10 is, for example, a grid electrode. A lower electrode 11 is provided on the lower surface (back surface) of the substrate 1.

  In the solar cell 100 shown in FIG. 1, the side on which the upper electrode 10 is provided is the sunlight receiving surface side. Therefore, in this specification, the surface on which the upper electrode 10 is provided is referred to as a light receiving surface, and the surface on which the lower electrode 11 is provided is referred to as a back surface.

  The substrate 1 is a semiconductor containing an n-type impurity and is made of, for example, GaAs, and has a thickness of, for example, 500 μm.

The base layer 2 is a semiconductor containing an n-type impurity, and is made of, for example, GaAs and has a thickness of, for example, 300 nm. Note that a buffer layer may be provided between the substrate 1 and the base layer 2. The buffer layer is formed of a semiconductor containing an n-type impurity, for example, n ⁺ -GaAs, and the thickness thereof can be, for example, 300 nm. This layer can also function as a back surface field (BSF) effect.

  The emitter layer 7 is a semiconductor containing a p-type impurity, and is made of, for example, GaAs and has a thickness of, for example, 250 nm.

The window layer 8 is a semiconductor containing a p-type impurity, and is formed of, for example, Al _0.75 Ga _0.25 As and has a thickness of, for example, 50 nm.

The contact layer 9 is made of a semiconductor containing p-type impurities, for example, p ⁺ -GaAs. In addition, as shown in FIG. 1, you may remove area | regions other than an electrode formation part among the contact layers 9. FIG. In this case, an antireflection film (for example, MgF ₂ / ZnS film) may be provided on the window layer 8.

  The superlattice semiconductor layer 6 has a superlattice structure in which a quantum dot layer 4 composed of a plurality of quantum dots 3 and a barrier layer 5 are alternately and repeatedly stacked, and the valence band and the conduction band of the quantum dots 3 Each forms an intermediate energy band (intermediate energy level). The quantum dot 3 has a structure surrounded by the barrier layer 5.

The quantum dots 3 are made of a semiconductor material having a narrower band gap than the semiconductor material constituting the barrier layer 5 and have quantum levels due to the quantum effect. The quantum dots 3 are made of, for example, InAs, have a width of, for example, 25 nm, and a height of, for example, 8 nm. The quantum dots 3 can be produced, for example, by SK growth. In the SK growth, for example, an n ⁺ -GaAs buffer layer of 300 nm, an n-GaAs of 300 nm, and an undoped GaAs of 300 nm are stacked on an n-GaAs (100) substrate, and an InAs quantum dot is formed thereon. InAs quantum dots are formed by laminating 2.2 ML of an InAs thin film layer on a GaAs film. The reason why the quantum dots 3 are formed in this way is that the quantum dots 3 are formed when the InAs layer exceeds a certain film thickness (critical film thickness), resulting from a difference in lattice constant between the GaAs layer and the InAs layer. This is because the strain energy is reduced. After the InAs quantum dot layer and the GaAs barrier layer 5 are repeatedly formed by an arbitrary number of laminations, 250 nm of p-GaAs, 50 nm of p-Al _0.75 Ga _0.25 As window layer 8 and p ⁺ GaAs contact layer 9. Form.

  The barrier layer 5 is made of a semiconductor material having a wider band gap than the semiconductor material constituting the quantum dots 3. The barrier layer 5 forms a potential barrier around the quantum dot layer 4. The barrier layer 5 is made of, for example, GaAs and has a thickness of, for example, 40 nm per layer. If the barrier layer 5 is stacked once after the quantum dot layer 4 / barrier layer 5 is stacked a plurality of times, it is possible to more easily stack the quantum dots. In addition, by inserting a strain compensation layer or the like in the middle of the barrier layer 5, it is possible to more easily multi-layer quantum dots.

In the present embodiment, for example, quantum dots 3 made of InGaAs and barrier layers 5 made of AlGaAs can be used for the superlattice semiconductor layer 6. Moreover, the quantum dot 3 which consists of InAsSb, and the barrier layer 5 which consists of AlAsSb can be used. Other InAs, GaAs, AlAs, InSb, GaSb, AlSb, InP, GaP, material _{AlP, Al x Ga y In 1} -x-y As, Al x Ga y In 1-x-y Sb z As 1-z can also be used for _{Al x Ga y in 1-x} -y P, Al x Ga y in 1-x-y N superlattice semiconductor layer 6 mixed crystal materials such as. Group III-V compound semiconductors, chalcopyrite materials, II-VI group compound semiconductors, group IV compound semiconductors, or mixed crystal materials thereof other than those described above may be used.

  In order to obtain a structure in which light in the vicinity of 1600 nm and 2200 nm in the sunlight spectrum is efficiently absorbed between subbands, a wide band gap material (a material having a larger band gap than GaAs) is used for the superlattice semiconductor layer 6. It is preferable to use it. This is because the InAs / GaAs material often used in conventional quantum dot solar cells has a small band offset of the conduction band, and the absorption wavelength band due to intersubband transition tends to be about 4000 nm or more. Therefore, for example, it is preferable to use InAs / AlAsSb, InGaN / GaN, InAs / InGaP materials or the like as the combination of quantum dots 3 / barrier layers 5.

  In addition, when forming the quantum dot 3 and the barrier layer 5 with the material which consists of mixed crystals, the band gap of a quantum level and the barrier layer 5 can be changed by changing the element ratio of a mixed crystal suitably, or a valence band. The band energy offset (the valence band energy difference between the quantum dots 3 and the barrier layer 5) can be made zero.

  In addition, it is preferable that the quantum dots 3 are directly doped with an n-type dopant or that the barrier layer 5 is modulation-doped with an n-type dopant because the inter-subband light absorption of the quantum dots 3 occurs efficiently. Furthermore, it is more preferable to dope with a dopant concentration similar to the density of the quantum dots 3. This is because if there is no doping in the quantum dots 3 or the barrier layer 5 and no electrons are present in the quantum dots 3, intersubband light absorption occurs only when the electrons are excited by interband absorption. However, if the quantum dots 3 or the barrier layer 5 are doped, subband absorption can occur before interband absorption occurs. That is, when doped, the frequency of intersubband light absorption increases.

  Until now, direct transition type semiconductors such as InAs / GaAs have been mainly used as constituent materials of quantum dot solar cells. Direct transition type semiconductors, unlike indirect transition type semiconductors, have a large light absorption coefficient, and quantum dots 3 using a direct transition type semiconductor material have also been expected to have a large light absorption coefficient. However, the inventors have found that the light absorption coefficient of the quantum dot 3 itself is not so large. In particular, the degree of light absorption due to the intersubband transition which is an optical transition from the ground level (intermediate band) on the conduction band side of the quantum dots 3 to the ground level of the conduction band of the barrier layer 5 is small.

  Therefore, in the solar cell 100 according to the present embodiment, the optical transition from the ground level (intermediate band) on the conduction band side of the quantum dots 3 to the ground level of the conduction band of the barrier layer 5 by increasing the optical path length. An optical path length increasing element is provided in order to increase the optical path length of the light absorbed by the intersubband transition that facilitates light absorption due to a certain intersubband transition. Here, as the optical path length increasing element, a plurality of minute irregularities called texture structures are formed on the back surface side opposite to the light receiving surface of the solar cell 100. In the configuration example shown in FIG. 1, the texture structure 1a is provided on the back surface. In addition, the lower electrode 11 is provided with a texture structure in accordance with the texture structure 1 a of the substrate 1.

  By providing the texture structure 1 a on the substrate 1, light that has not been absorbed by the superlattice semiconductor layer 6 is diffused (scattered) when reflected on the back side of the substrate 1. Reflection in an oblique direction has a longer optical path length than planar reflection, so that light in a long wavelength band absorbed by inter-band transition or inter-subband transition can be efficiently absorbed.

  The texture structure 1 a is designed according to the light absorption wavelength band of the quantum dots 3. The light absorption wavelength band of the quantum dot 3 is a light absorption wavelength due to an optical transition (hereinafter referred to as an interband transition) from the ground level of the valence band of the quantum dot 3 to the ground level of the conduction band of the quantum dot 3. The band and the light absorption wavelength band due to the above-described intersubband transition.

  FIG. 2 is a diagram schematically showing an energy band of the solar cell 100 in the present embodiment. In addition to the interband transition 21 and intersubband transition 22 described above, FIG. 2 shows a bulk transition 23 that is an optical transition from the valence band to the conduction band.

  As described above, the inventors have found that in the quantum dot solar cell, in particular, the intensity of photoexcitation of the intersubband transition 22 is small. Therefore, the texture structure 1 a provided on the substrate 1 is preferably designed according to the light absorption wavelength band of the intersubband transition 22.

  FIG. 3 is a diagram in which a part of the schematic diagram of the energy band in FIG. 2 is extracted. The band gap of the barrier layer 5 is Eg [eV], and the band gap of the quantum dots 3 (the energy between the ground level on the conduction band side and the ground level on the valence band side) of the quantum dots 3 is EQD [eV]. The energy level at the conduction band side of the quantum dot 3 is Eib [eV], the energy level at the lower end of the conduction band of the barrier layer 5 is Ecb [eV], and the barrier is from the ground level of the conduction band of the quantum dot layer 4. The energy up to the energy level at the lower end of the conduction band of the layer 5 is defined as Eci [eV].

  The band gap EQD of the quantum dot layer 4 can be determined from the PL spectrum of the quantum dot layer 4. At this time, the band offset on the valence band side is generally small and can be regarded approximately as Eci≈Eg−EQD. Therefore, this embodiment also regards Eci≈Eg−EQD. In general, quantum dot solar cells have a relationship of Eg> EQD> Eci. If the band offset on the valence band side is very large, the value may be subtracted to calculate Eci (since there is a heavy hole on the valence band side, the quantum dot on the valence band side The energy difference from the ground level to the top of the valence band of the barrier layer is almost equal to the band offset on the valence band side). Each value can also be determined from the quantum structure calculation.

  The texture interval d [μm] of the texture structure 1a is preferably 1.24 / Eci ≦ d ≦ 1.24 / Eci × 3. The texture interval d [μm] is the interval between the centers of the vertices of adjacent convex portions constituting the texture. The center of the vertex of the convex portion is the center of gravity of the surface of the convex portion. Note that 1.24 / Eci [μm] corresponds to a value obtained by converting the energy of the electromagnetic wave of Eci [eV] into a wavelength.

  In a quantum dot solar cell, it is desired that absorption of light having Eci energy corresponding to intersubband transition occurs most efficiently. If absorption of light in this wavelength band does not occur, carrier extraction from the quantum dot layer 4 does not occur efficiently, and an increase in energy conversion efficiency cannot be expected. The absorption of light having Eci energy means that both the intersubband transition in the quantum dot layer 4 and the subsequent carrier extraction can be efficiently generated.

  Further, if Eci is matched with the peak position of the sunlight spectrum (AM1.5G), more efficient light absorption can be expected. For example, it is preferable to use a quantum dot solar cell material close to Eci = 0.56 eV (2200 nm at a wavelength), Eci = 0.78 eV (1600 nm at a wavelength), and a texture structure optimal for it.

  Here, the relationship between the wavelength corresponding to Eci and the distance d [μm] between textures is 1.24 / Eci ≦ d because the effect of increasing light absorption by the texture structure becomes remarkable in this region.

  Further, d ≦ 1.24 / Eci × 3 is set for shortening the solar cell manufacturing process time and reducing the cost. When the texture interval d is large, if the cross-sectional angle of the texture structure (θ in FIG. 4 described later) is the same, an extra semiconductor layer film is required. It is better not to be too big from the viewpoint.

  As will be described later with reference to FIG. 5, the light absorption effect is the highest when the wavelength of light to be absorbed and the texture interval d are approximately the same. Therefore, if the texture interval d is 1.24 / Eci, both the absorption of light between subbands and the extraction of carriers from the quantum dot layer 4 can be efficiently caused.

  From the above viewpoint, the texture interval d is preferably as close as possible to 1.24 / Eci, and more preferably 1.24 / Eci ≦ d ≦ 1.24 / Eci × 1.5.

  FIG. 4 is a partially enlarged view of the texture structure 1a. The texture structure 1a is a pyramid structure, for example. The angle θ shown in FIG. 4 is an angle determined by the crystal plane orientation depending on wet etching conditions during texture formation. For example, if θ is 15 ° <θ <60 °, the effect of increasing the optical path length is large, which is preferable.

  Note that the same applies when wet etching is performed, but the shape of the texture can be made different from the pyramid shape, particularly when dry etching is performed.

  FIG. 5 is a diagram illustrating a result of calculating the relationship between the texture interval d [μm] and the electric field strength inside the solar cell 100. There were three types of light wavelengths: 1.2 [μm], 1.5 [μm], and 2.2 [μm]. The electric field strength shown on the vertical axis indicates the magnification with respect to the case without the texture structure, where 1 is the case where the substrate 1 is not provided with the texture structure 1a. The angle θ of the cross section of the texture structure 1a was 55 °.

  As shown in FIG. 5, the electric field strength is highest when the texture interval d is approximately the same as the wavelength of light. It can also be seen that there is a certain effect of increasing the electric field strength even when the texture interval d is longer than the wavelength of light. For example, for light having a wavelength of 1.2 [μm], the electric field strength is greatest when the texture interval d is 1.2 [μm]. Further, even when the texture interval d is longer than 1.2 [μm], the electric field strength exceeds 1, and there is an effect of increasing the electric field strength.

  FIG. 6A and FIG. 6B show the theory that, in the quantum dot solar cell in which the quantum dot layer 4 is formed of InAs and the barrier layer 5 is formed of GaAs, as the electric field strength of sunlight corresponding to the intersubband transition increases. It is a figure which shows how much a typical energy marginal efficiency improves. FIG. 6A shows the results under non-condensing conditions, and FIG. 6B shows the results under 1000 times condensing conditions. Moreover, in FIG. 6A and 6B, the result of the quantum dot solar cell is shown with the continuous line, and the result of the solar cell (henceforth a bulk solar cell) which is not equipped with the quantum dot layer 4 is shown with the dotted line.

The optical absorption coefficient of the bulk transition 10000 cm ^-1, the optical absorption coefficient between interband transition 5000 cm ^-1, and the optical absorption coefficient of intersubband and 500 cm ^-1. The thickness of the base layer 2 and the emitter layer 7 was 0.5 μm, and the thickness of the quantum dot layer 4 was 3 μm. 6A and 6B show the energy conversion efficiency when the electric field strength is increased by a factor of 1 when the texture structure 1a is not present on the substrate 1 and the electric field strength is increased by providing the texture structure 1a.

  As shown in FIG. 6A, under non-condensing conditions, if the electric field strength is low, the energy conversion efficiency of the quantum dot solar cell is lower than the energy conversion efficiency of the bulk solar cell. On the other hand, if the electric field intensity can be increased to about 3 to 4 times, the energy conversion efficiency of the quantum dot solar cell sufficiently exceeds the energy conversion efficiency of the bulk solar cell.

  On the other hand, as shown in FIG. 6B, the energy conversion efficiency of the quantum dot solar cell does not fall below the energy conversion efficiency of the bulk solar cell under the condition of 1000 times condensing. In particular, in the quantum dot solar cell, the energy conversion efficiency increases as the electric field strength increases until the electric field strength becomes about 3 to 4 times.

7A and 7B show an increase in the electric field strength of sunlight corresponding to the transition between subbands in a quantum dot solar cell in which the quantum dot layer 4 is formed of InAs and the barrier layer 5 is formed of In _0.48 Ga _0.52 P. It is a figure which shows how much theoretical energy limit efficiency improves in connection. FIG. 7A shows the results under non-condensing conditions, and FIG. 7B shows the results under 1000 times condensing conditions. Moreover, in FIG. 7A and FIG. 7B, the result of the quantum dot solar cell is shown with the continuous line, and the result of the bulk solar cell is shown with the dotted line.

As shown in FIGS. 7A and 7B, in the quantum dot solar cell in which the quantum dot layer 4 is formed of InAs and the barrier layer 5 is formed of In _0.48 Ga _0.52 P, the non-condensing condition and the 1000 times condensing condition are used. Under any of the conditions, the energy conversion efficiency of the quantum dot solar cell does not fall below the energy conversion efficiency of the bulk solar cell. In particular, in a quantum dot solar cell, the energy conversion efficiency increases as the electric field strength increases up to about five times the electric field strength under both non-condensing conditions and 1000 times condensing conditions.

In the quantum dot solar cell in which the quantum dot layer 4 is formed of InAs and the barrier layer 5 is formed of In _0.48 Ga _0.52 P, the barrier layer 5 has a wide band gap, so that the enhancement of the electric field strength is more important. I understand.

  Of the solar cell in the present embodiment, the part other than the texture structure 1a of the substrate 1 can be produced by a known method, for example, a method described in Japanese Patent No. 5509059. For example, the substrate 1 is cleaned with an organic cleaning solution, etched with a sulfuric acid-based etching solution, further washed with running water, and then installed in the MOCVD apparatus. After a GaAs layer to be a base layer 2 and a barrier layer 5 is grown on the substrate 1, a quantum dot layer 4 made of InAs is formed by using a self-organization mechanism. Crystal growth of the barrier layer 5 and the quantum dot layer 4 is repeated to form the superlattice semiconductor layer 6. Subsequently, an emitter layer 7 is formed on the superlattice semiconductor layer 6, a window layer 8 is formed on the emitter layer 7, and a contact layer 9 is formed on the window layer 8. Subsequently, a comb-shaped electrode is formed on the contact layer 9 by photolithography and lift-off technology, and this comb-shaped electrode is used as a mask or a new mask layer is formed, and the contact layer 9 is selectively etched to etch the upper electrode 10. Form. The lower electrode 11 is formed after the texture structure 1a is formed on the back surface of the substrate 1 by a method described later.

  Below, the preparation methods of the texture structure 1a of the board | substrate 1 are demonstrated. 8A to 8D are views for explaining a method for producing the texture structure 1 a of the substrate 1. FIG. 8A shows a quantum dot solar cell made by a known method. In this state, the texture structure 1 a is not formed on the substrate 1.

  In this state, the light-receiving surface of the solar cell is protected with a resist or the like, and after applying the resist to the back surface of the substrate 1, a resist 80 is formed as shown in FIG. 8B by using a photolithography technique.

  Subsequently, as shown in FIG. 8C, a texture structure 1a is formed on the back surface of the substrate 1 by wet etching. For the wet etching, for example, a mixed solution of sulfuric acid and hydrogen peroxide solution or a mixed solution of phosphoric acid and hydrogen peroxide solution can be used. The concentration and temperature of the solution used for wet etching can be appropriately selected according to the size and shape of the texture structure 1a.

  Thereafter, as shown in FIG. 8D, a lower electrode 11 is formed on the back surface of the substrate 1 on which the texture structure 1a is formed. The lower electrode 11 may be any material that can take ohmic contact, but a metal such as AuGe / Ni / Au is better than a transparent electrode such as ITO. This is because the metal has a higher reflectivity for long-wavelength light than the transparent electrode. The lower electrode 11 is produced using electron beam evaporation or sputtering. The lower electrode 11 may be formed directly on the base layer 2 by recess etching a part of the region.

  The lower electrode 11 may be formed (laminated) so that the back surface of the electrode (the surface not in contact with the substrate 1) is a flat surface. FIG. 9 is a diagram showing a configuration of the solar cell 100 formed so that the back surface of the lower electrode 11 is a flat surface.

  From the viewpoint of the ease of the production process of the texture structure 1a, the following production method may be employed. That is, the contact layer 9, the window layer 8, the emitter layer 7, the superlattice semiconductor layer 6, and the base layer 2 are formed in this order on the substrate 1, and then another supporting substrate (for example, a silicon substrate or plastic) is formed by a fusion technique or the like. Transfer to the substrate. Then, after peeling off the original substrate 1, a texture structure is formed on the substrate 1. You may transfer a quantum dot solar cell element to the board | substrate 1 from which the texture structure was formed from the beginning. As another advantage of using such a method, since a thin substrate is used, light absorption (free carrier absorption) by the substrate can be suppressed.

[Second Embodiment]
FIG. 10 is a schematic cross-sectional view showing the configuration of the solar cell in the second embodiment. In the solar cell in the second embodiment, the texture structure 1b of the substrate 1 is formed by a quadrangular frustum.

  The texture interval d [μm] of the texture structure 1b is preferably 1.24 / Eci ≦ d ≦ 1.24 / Eci × 3, as in the solar cell in the first embodiment. In particular, as described in the first embodiment, it is better that the texture interval d is closer to 1.24 / Eci, and further when 1.24 / Eci ≦ d ≦ 1.24 / Eci × 1.5. preferable.

  FIG. 11 is a partially enlarged view of the texture structure 1 b of the substrate 1. For example, if the angle θ of the cross section of the texture structure 1b shown in FIG. 11 is 15 ° <θ <50 °, the effect of increasing the optical path length is great. Further, the effect of increasing the optical path length is greater when the side e [μm] of the lower surface of the quadrangular pyramid shown in FIG. 11 is not so large. Therefore, e [μm] is preferably 0 <e <1.24 / Eci / 2.

  Of the method for manufacturing a solar cell in the present embodiment, a part different from the method for manufacturing a solar cell in the first embodiment, that is, a method for manufacturing the texture structure 1b of the substrate 1 will be described. For example, if a resist having a wide horizontal width is used as the resist 80 to be formed on the back surface of the substrate 1 as described with reference to FIG. 8B, the texture structure 1b having the structure of a quadrangular pyramid with a flat projection is formed. can do.

[Third Embodiment]
FIG. 12 is a schematic cross-sectional view showing the configuration of the solar cell in the third embodiment. In the solar cell 100 according to the third embodiment, the texture structure 120a is provided on the light receiving surface instead of the back surface of the solar cell 100.

  In the solar cell 100 in the third embodiment, the texture forming semiconductor layer 120 is formed on the window layer 8 in order to provide the texture structure 120 a on the light receiving surface. That is, the texture structure 120 a is formed on the light receiving surface side of the semiconductor layer 120. The semiconductor layer 120 can be formed using, for example, the same material as the emitter layer 7 and the window layer 8.

  Even when the texture structure 120a is provided on the light receiving surface of the solar cell 100, the incident light is refracted and incident in an oblique direction, so that the optical path length of the incident light is increased. Thereby, the absorption degree of the long wavelength light by intersubband transition can be improved.

  Of the solar cell manufacturing method according to the present embodiment, parts different from the solar cell manufacturing method according to the first embodiment will be described. In the solar cell in the present embodiment, no texture structure is formed on the back surface of the substrate 1. Further, after forming the semiconductor layer 120 on the window layer 8, the contact layer 9 is formed on the semiconductor layer 120. Thereafter, after forming the upper electrode 10 as described in the first embodiment, the texture structure 120a is formed on the semiconductor layer 120 by the same method as the method for forming the texture structure 1a of the substrate 1 described in the first embodiment. Form.

[Fourth Embodiment]
FIG. 13: is a schematic sectional drawing which shows the structure of the solar cell in 4th Embodiment. In the solar cell 100 according to the fourth embodiment, not only the back surface of the solar cell 100 but also the light receiving surface is provided with a texture structure. That is, the texture structure 1 a is formed on the back surface of the substrate 1, and the texture structure 120 a is also formed on the semiconductor layer 120 formed on the window layer 8.

  The texture structure 1a formed on the substrate 1 can be the same structure as the texture structure described in the first embodiment. Moreover, the texture structure 120a formed on the light receiving surface of the solar cell 100 can be the same as the texture structure described in the third embodiment. As described in the third embodiment, the texture-forming semiconductor layer 120 can be formed using, for example, the same material as the emitter layer 7 and the window layer 8.

  According to the solar cell 100 in the fourth embodiment, since the texture structure is provided not only on the back surface of the solar cell 100 but also on the light receiving surface, the substructure is compared with the configuration in which the texture structure is provided only on the back surface or only on the light receiving surface. The degree of absorption of long-wavelength light due to interband transition can be further improved.

  In the solar cell in the fourth embodiment, the texture structure 1a of the substrate 1 can be formed by the method described in the first embodiment, and the texture structure 120a of the semiconductor layer 120 has been described in the third embodiment. It can be formed by a method.

[Fifth Embodiment]
FIG. 14 is a schematic cross-sectional view showing the configuration of the solar cell in the fifth embodiment. In the solar cell according to the fifth embodiment, the lower electrode 11 is a grid electrode like the upper electrode 10. The structure other than the lower electrode 11 is the same as the configuration of the solar cell in the fourth embodiment.

  Of the solar cell in this embodiment, a part different from the solar cell manufacturing method in the fourth embodiment will be described. The lower electrode 11 forms a grid electrode on the back surface of the substrate 1 by the same method as the method for forming the upper electrode 10. Thereafter, the texture structure 1a is formed on the substrate 1 by the same method as that for forming the texture structure 1a described in the first embodiment.

[Sixth Embodiment]
FIG. 15: is a schematic sectional drawing which shows the structure of the solar cell in 6th Embodiment. In the solar cell 100 in the sixth embodiment, the texture interval d of the texture structure 1 c provided on the substrate 1 is not uniform. Specifically, the texture structure 1c has two types of intervals, d1 [μm] and d2 [μm], as the texture interval d [μm].

The distances d1 [μm] and d2 [μm] between the two types of textures satisfy the relationships of the following expressions (1) to (3). However, EQD [eV] in the equation (1) is a band gap of the quantum dot layer 4 as described with reference to FIG.
1.24 / EQD ≦ d2 ≦ 1.24 / EQD × 3 (1)
1.24 / Eci ≦ d1 ≦ 1.24 / Eci × 3 (2)
d1> d2 (3)

  The distances d1 [μm] and d2 [μm] between the two types of textures are alternately arranged. However, the distances d1 [μm] and d2 [μm] between the two types of textures may be randomly arranged.

  As described above, the degree of absorption of light having Eci energy corresponding to the intersubband transition can be improved by providing the texture of the interval d1. Further, by providing the texture with the interval d2, it is possible to improve the degree of absorption of light having EQD energy corresponding to the interband transition. Therefore, by forming the texture structure 1c having two types of texture intervals d1 [μm] and d2 [μm] on the substrate 1, light absorption of inter-band transition and light of inter-subband transition in the quantum dot layer 4 are obtained. Both absorptions can occur efficiently.

  The texture structure having two kinds of texture intervals d1 [μm] and d2 [μm] may be formed on the light receiving surface of the solar cell 100 as described in the third embodiment, or the fourth As described in the embodiment, the solar cell 100 may be formed on both the light receiving surface and the back surface.

  In the solar cell according to the present embodiment, the resist 80 formed on the back surface of the substrate 1 described with reference to FIG. 8B has two types of texture intervals d by using two types of resists having horizontal widths. A texture structure can be formed.

[Seventh Embodiment]
In the first to sixth embodiments, quantum dot solar cells using SK growth have been described. However, the quantum dot solar cell may be a colloidal quantum dot solar cell.

  FIG. 16 is a schematic cross-sectional view showing the configuration of the solar cell in the seventh embodiment. This quantum dot solar cell 100 is a colloidal quantum dot solar cell, and FIG. 16 shows an example of the configuration of a core / shell type quantum dot solar cell. That is, the quantum dot 3A is a core / shell colloidal quantum dot.

In the core / shell type quantum dot solar cell 100 shown in FIG. 16, a layer 160 of tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO) is formed on the substrate 1, and TiO ₂ is further formed thereon. A layer 161 is formed. Further, a quantum dot layer 162 including the quantum dots 3A is formed on the TiO ₂ layer 161, and the upper electrode 10 is further provided thereon. The texture structure 1a is provided on the back surface of the substrate 1, and the lower electrode 11 is provided on the back surface, as in the solar cell 100 shown in FIG.

  The colloidal quantum dots are not limited to the core / shell type. Examples of the material of the quantum dot include ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, PbS, PbSe, PbTe, CuInGaS, CuS, InGaZnO, InAs, GaAs, AlAs, InSb, GaSb, AlSb, InP, GaP, AlP, InN, GaN, AlN, Si, Ge, or the like can be used.

  As materials for the core / shell type quantum dots, the above-mentioned materials can be used in combination. For example, CdS / ZnSe, PbS / CdS, PbSe / CdSe, PbSe / PbS, PbTe / CdTe, InAs / InGaAs, InN / InGaN, InSb / GaSb, InGaP / GaP may be used as the core / shell material combination. it can.

  For example, a PbSe / CdTe core-shell quantum dot can be produced as follows. PbSe quantum dots are mixed with PbO, oleic acid, and 1-octadecene, added with trioctylphosphine selenide, and heated at 180 degrees. Next, a solution prepared by heating cadmium cyclohexabutyrate in oleamine and a solution prepared by heating selenium powder in octadecene are alternately poured into a PbSe quantum dot solution to form a CdTe shell on a PbSe quantum dot. Layers can be formed. By using colloidal quantum dots as the quantum dots 3A, a simple manufacturing process such as spin coating or ink jet printing applied on the substrate 1 can be used.

As the substrate 1, a flexible substrate such as plastic or glass can be used. A layer 160 of tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO) is formed on the substrate 1 by sputtering or the like, and a TiO ₂ layer 161 is formed thereon by sputtering or the like. Further thereon, a solution containing the colloidal quantum dots 3A is laminated by spin coating or the like. If a thick film is desired, spin coating is performed multiple times. Thereafter, if an Au / Ag electrode or the like is formed by vapor deposition, it functions as a solar cell. For example, a PbSe / CdTe-QD octane solution may be used as the solution. In such a solar cell, holes flow to the quantum dot layer 162 side and electrons flow to the TiO ₂ layer 161 side. That is, the quantum dot layer 162 functions as a p-type layer, and the TiO ₂ layer 161 functions as an n-type layer. The use of the core-shell type quantum dot material is more preferable because the quantum effect is easily obtained.

  FIG. 17 is a diagram schematically showing an energy band of the colloidal quantum dot solar cell in the present embodiment. FIG. 17 illustrates an interband transition 21A, an intersubband transition 22A, and a bulk transition 23A.

[Eighth Embodiment]
FIG. 18 is a schematic cross-sectional view showing the configuration of the solar cell in the eighth embodiment. The solar cell 100 in the eighth embodiment further includes wavelength conversion particles 170 in addition to the configuration of the solar cell in the fifth embodiment. The wavelength converting particle 170 has a function of absorbing light and emitting light absorbed by intersubband transition.

  Although the texture structure 1a of the substrate 1 is a structure that effectively reflects the light that could not be absorbed, it does not ideally reflect all the light. Therefore, by providing the wavelength conversion particle 170, the wavelength conversion particle 170 absorbs the light that could not be reflected by the substrate 1, and emits the light that is absorbed by the intersubband transition, whereby the long wavelength light due to the intersubband transition Further improve the degree of absorption.

  The wavelength converting particles 170 are formed on the back surface of the substrate 1 so as to fill the concave and convex portions of the texture structure 1 a of the substrate 1. For example, when wavelength conversion particles contained in a solvent such as an octane solution are applied by spin coating or the like and then heated at, for example, 200 ° C., the solvent is vaporized and the wavelength conversion particles adhere to each other to form a film. The wavelength conversion particle 170 is made of, for example, a nanoparticle fluorescent material or semiconductor nanoparticle of about 1 to 1000 nm. As the particle size is smaller, the wavelength conversion particles can be applied to the concave and convex portions of the texture structure 1a without gaps, and particles of about 1 to 50 nm are more preferable.

  When the photon energy is Eph [eV], the wavelength conversion particle 170 preferably absorbs sunlight energy in a band of Eci <Eph <EQD and converts it into light having Eci energy. This is because the light absorption spectrum band of the quantum dots 3 is not wide. The wavelength conversion particle 170 emits light having a wavelength capable of intersubband transition, so that the sunlight component of the long wavelength light is efficiently absorbed by the intersubband transition, and the two-stage light absorption is efficiently performed. Occur. Thereby, carriers are efficiently extracted from the quantum dots 3 and the current increases.

  The material of the wavelength conversion particle 170 may be any material as long as it can shift the wavelength of sunlight. For example, PbS, PbSe, PbTe, CuInGaS, InAs, GaAs, AlAs, InSb, GaSb, AlSb, InP, GaP, AlP, InN, GaN, AlN, Si, Ge, and mixed crystal materials thereof can be used. Further, complex materials, rare earth ions (Er3 +, Pr3 +, Tm3 +, etc.), glass containing transition elements, Er-doped garnet crystals (YAG), organic materials, and the like may be used.

  A plurality of the above materials may be used for the core-shell structure, the core-shell structure, and the like. Furthermore, these materials may have a size of several nanometers to several tens of nanometers. In that case, the band gap can be freely changed depending on the size.

  The wavelength converting particles 170 applied to the substrate 1 are preferably not only one type but also two types. This is because the quantum dot 3 does not have a wide light absorption spectrum band not only in intersubband transition but also in interband transition.

  FIG. 19 is a schematic cross-sectional view showing the configuration of the solar cell 100 in which two types of wavelength conversion particles 170, wavelength conversion particles 170a and wavelength conversion particles 170b, are applied to the substrate 1. The wavelength converting particle 170a converts the sunlight spectrum in the band Eci <Eph <EQD into light having Eci energy, and the wavelength converting particle b converts the sunlight spectrum in the band EQD <Eph <Eg into the energy of EQD. It is preferable to convert it into the light which has. Thereby, the sunlight component of long wavelength light can be efficiently absorbed not only by the transition between subbands but by the interband transition.

  Here, the wavelength conversion particle 170a is a photon emitted by the wavelength conversion particle 170b after wavelength conversion (for example, a photon having EQD energy) or a photon having an energy larger than the EQD that the wavelength conversion particle 170b should originally absorb. There is a possibility of absorption. Therefore, it is preferable that the wavelength conversion particle 170b is first applied to the back surface of the substrate 1 and then the wavelength conversion particle 170a is applied thereafter so that sunlight enters the wavelength conversion particle 170b first. In addition, when the ratio of the wavelength conversion particle 170a is lower than the wavelength conversion particle 170b, the wavelength conversion particles 170a and 170b may be mixed and applied.

  In the solar cell in this embodiment, the structure other than the wavelength conversion particles is manufactured by the same method as that in the solar cell in the fifth embodiment, and then a resist is disposed on the back surface of the lower electrode 11 to form the texture structure 1a of the substrate 1. It can manufacture by apply | coating a wavelength conversion particle | grain so that the recessed part of an unevenness | corrugation may be filled.

[Ninth Embodiment]
FIG. 20 is a schematic cross-sectional view showing the configuration of the solar cell in the ninth embodiment. In the solar cell 100 according to the ninth embodiment, the buffer layer 190 is provided on the base layer 2, and the texture structure 2 a is provided on the surface of the base layer 2 that is in contact with the buffer layer 190. Superlattice semiconductor layer 6 is formed on buffer layer 190.

  According to the solar cell 100 in the ninth embodiment, since sunlight is reflected by the texture structure 2a of the buffer layer 190 before reaching the substrate 1, light confinement occurs more efficiently. This is because the substrate itself may cause light absorption (free carrier absorption). Thereby, light absorption, especially the light absorption degree of long wavelength light can be improved.

  The texture structure 2a formed on the base layer 2 can be formed by the same method as the formation method of the texture structure 1a of the substrate 1 described in the first embodiment. When the part which is not demonstrated in 1st Embodiment among the manufacturing methods of the solar cell in this embodiment is demonstrated, as demonstrated in 1st Embodiment, it is a base layer using the resist and etching which were patterned. After the texture structure 2a is formed on 2, the buffer layer 190 is formed on the base layer 2, and the superlattice semiconductor layer 6 is formed on the buffer layer 190 by the same method as described in the first embodiment. To do.

[Tenth embodiment]
FIG. 21 is a schematic cross-sectional view showing the configuration of the solar cell in the tenth embodiment. In the solar cell 100 according to the tenth embodiment, the buffer layer 200 is provided between the substrate 1 and the base layer 2, and the texture structure 1 d is provided on the surface of the substrate 1 in contact with the buffer layer 200. Also in this structure, since sunlight is reflected by the texture structure 1d provided on the upper surface of the substrate 1, light confinement occurs more efficiently, and the light absorption, particularly the light absorption degree of long wavelength light, can be improved. it can.

  The texture structure 1d formed on the substrate 1 can be formed by the same method as the formation method of the texture structure 1a of the substrate 1 described in the first embodiment. When the part which is not demonstrated in 1st Embodiment among the manufacturing methods of the solar cell in this embodiment is demonstrated, after forming the texture structure 1d in the board | substrate 1, the buffer layer 200 is formed on the board | substrate 1, Base layer 2 is formed on buffer layer 200.

[Eleventh embodiment]
In the solar cell in the sixth embodiment, there are two kinds of texture intervals d of the texture structure provided on the substrate 1. In the solar cell in the eleventh embodiment, there are three or more kinds of texture intervals d of the texture structure provided on the substrate 1.

  FIG. 22 is a schematic cross-sectional view showing the configuration of the solar cell in the eleventh embodiment. In the configuration shown in FIG. 22, there are three types of texture intervals d [μm] of the texture structure 1 e provided on the substrate 1, d1 [μm], d2 [μm], and d3 [μm]. However, the texture spacing d [μm] may be four or more. Further, the intervals d [μm] between the three or more types of textures may be regularly arranged or irregularly arranged.

  If the shortest interval among the three or more types of intervals d [μm] is dmin [μm], dmin [μm] is set to satisfy 1.24 / EQD ≦ dmin.

  According to the solar cell in the eleventh embodiment, both the inter-band transition light absorption and the inter-subband transition light absorption in the quantum dot layer 4 efficiently occur over a wide spectral region of sunlight.

  The texture structure having three types of texture intervals d1 [μm], d2 [μm], and d3 [μm] may be formed on the light receiving surface of the solar cell 100 as described in the third embodiment. However, as described in the fourth embodiment, the solar cell 100 may be formed on both the light receiving surface and the back surface.

  In the solar cell according to the present embodiment, as the resist 80 formed on the back surface of the substrate 1 described with reference to FIG. 8B, three or more types of texture intervals d are used by using a resist having three or more types of horizontal widths. A texture structure having can be formed.

[Twelfth embodiment]
In the solar cell in the twelfth embodiment, a reflection film for reflecting light that could not be absorbed by the superlattice semiconductor layer 6 was used as an optical path length increasing element for at least increasing light absorbed by the intersubband transition. Provide.

  FIG. 23 is a schematic cross-sectional view showing the configuration of the solar cell in the twelfth embodiment. The solar cell 100 in the twelfth embodiment has a reflective film 220. The reflective film 220 is provided on the back surface of the substrate 1 and reflects at least light having a wavelength of 1.24 / Eci [μm].

  In order to improve the degree of light absorption of intersubband transition, the reflective film 220 preferably reflects light with a wavelength of 1.24 / Eci [μm] most efficiently, and 1.24 / Eci [μm]. It is preferable to reflect 90% or more of light having a wavelength of. The reflection film 220 may be a multi-layer film or a metal film using silver, for example, as long as it can efficiently reflect light having a wavelength of 1.24 / Eci [μm]. The reflective film 220 can be formed using, for example, a vacuum deposition method or a sputtering method.

  A diffusion plate (scattering member) may be provided between the substrate 1 and the reflective film 220. By providing the diffusion plate, light can be scattered in an oblique direction, so that the optical path length of the reflected light can be further increased.

  As described above, according to the solar cell in the first to twelfth embodiments, at least the optical path length of the light absorbed by the intersubband transition is increased by the optical path length increasing element such as the texture structure and the reflective film described in each embodiment. Can be increased. Therefore, the photoelectric conversion element according to the embodiment of the present invention has a structure in which a quantum dot is surrounded by a barrier layer, sandwiched between a p-type semiconductor layer, an n-type semiconductor layer, a p-type semiconductor layer, and an n-type semiconductor layer. A superlattice semiconductor layer, and an optical path length increasing element that at least increases an optical path length of light absorbed by an intersubband transition that is a transition from the intermediate band on the conduction band side of the quantum dot to the conduction band of the barrier layer. Just do it.

  The present invention is not limited to the embodiment described above. For example, in the above-described embodiment, an example in which the photoelectric conversion element is applied to a solar cell has been described. However, the photoelectric conversion element can also be used as a photodetector. When used as a photodetector, it may be designed according to the wavelength to be detected. For example, ECi = 0.2 eV (wavelength 6 μm) can be used as a 6 μm wavelength sensing device.

  Further, a quantum dot solar cell manufactured by the SK growth method may be transferred to a flexible substrate or a plastic substrate. In that case, it may be transferred to a substrate provided with a texture structure in advance, and the quantum dot solar cell itself is not exposed to the wet etching process or the dry etching process used in producing the texture structure.

  In the above-described embodiment, the base layer 2 is described as an n-type semiconductor layer and the emitter layer 7 as a p-type semiconductor layer. However, the base layer 2 may be a p-type semiconductor layer and the emitter layer 7 may be an n-type semiconductor layer.

  The features described in the above-described embodiments can be used in appropriate combination.

DESCRIPTION OF SYMBOLS 1 Substrate 1a, 1b, 1c, 1d, 1e, 120a Texture structure 2 Base layer 3, 3A Quantum dot 4 Quantum dot layer 5 Barrier layer 6 Superlattice semiconductor layer 7 Emitter layer 100 Solar cell 110 Semiconductor layer 220 Reflective film

Claims (10)

a p-type semiconductor layer;
an n-type semiconductor layer;
A superlattice semiconductor layer having a structure in which a quantum dot is surrounded by a barrier layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer;
An optical path length increasing element that at least increases the optical path length of the light absorbed by the intersubband transition that is a transition from the intermediate band on the conduction band side of the quantum dot to the conduction band of the barrier layer;
A photoelectric conversion element comprising:   The photoelectric conversion element according to claim 1, wherein the optical path length increasing element is a texture structure formed on a back surface opposite to a light receiving surface of the photoelectric conversion element.   The photoelectric conversion element according to claim 1, wherein the optical path length increasing element is a texture structure formed on a light receiving surface of the photoelectric conversion element.   The texture interval d of the texture structure is 1.24 / Eci ≦ d ≦ 1.24, where Eci is the energy from the ground level of the conduction band of the quantum dots to the lower level of the conduction band of the barrier layer. The photoelectric conversion element of Claim 2 or 3 which has a relationship of / Ecix3.   The photoelectric conversion element according to any one of claims 2 to 4, wherein the texture interval of the texture structure includes a first interval and a second interval.   The first interval is d1, the second interval is d2, the energy from the ground level of the conduction band of the quantum dot to the bottom level of the conduction band of the barrier layer is Eci, and the valence band of the quantum dot Assuming that the energy from the ground level to the ground level of the conduction band of the quantum dots is EQD, 1.24 / EQD ≦ d2 ≦ 1.24 / EQD × 3, 1.24 / Eci ≦ d1 ≦ 1.24 The photoelectric conversion element according to claim 5, having a relationship of / Eci × 3 and d1> d2.   The wavelength conversion particles according to claim 2, further comprising wavelength conversion particles disposed in concave and convex portions of the texture structure formed on the back surface, which absorb incident light and emit at least light absorbed by the intersubband transition. Photoelectric conversion element.   The optical path length increasing element is a texture structure formed on a light incident side surface of a layer located on a back surface opposite to a light receiving surface of the p-type semiconductor layer and the n-type semiconductor layer. 1. The photoelectric conversion element according to 1. A substrate for forming the p-type semiconductor layer, the n-type semiconductor layer, and the superlattice semiconductor layer;
The photoelectric conversion element according to claim 1, wherein the optical path length increasing element is a texture structure formed on a light incident side surface of the substrate. The optical path length increasing element is a reflective film formed on the back surface opposite to the light receiving surface of the photoelectric conversion element,
The reflection film reflects at least light having a wavelength of 1.24 / Eci [μm], where Eci is energy from the ground level of the conduction band of the quantum dots to the lower level of the conduction band of the barrier layer. The photoelectric conversion element according to claim 1.

Images