Thin film solar cell and method for manufacturing thin film solar cell

Abstract

The present invention provides: a thin film solar cell which is capable of repairing damage caused by radiation; and a method for manufacturing a thin film solar cell. A p-type light absorption layer (103) of this thin film solar cell (100) has a first region (R1) and a second region (R2), which are obtained by dividing the p-type light absorption layer (103) into two equal parts. The p-type light absorption layer (103) contains Cu as a group I element, and contains Ga and In as group III elements. The average value of the ratio C1 between the number of atoms of Cu and the number of atoms of the group III elements in the first region (R1) is lower than the average value of the ratio C1 in the second region (R2). The average value of the ratio G1 between the number of atoms of Ga and the number of atoms of the group III element in the first region (R1) is lower than the average value of the ratio G1 in the second region (R2), and the average value of the ratio G1 in the p-type light absorption layer (103) is 0.2 to 0.4 inclusive. The absolute value of the rate of change of the ratio G1 from the first electrode layer (102) side to the second electrode layer (105) side becomes the largest in the first region (R1), and the ratio G1 in the p-type light absorption layer (103) is on a downward trend from the first electrode layer (102) side to the second electrode layer (105) side in the first region (R1).

Description

Thin-film solar cell and method for manufacturing thin-film solar cell

[0001] The present invention relates to a thin-film solar cell and a method for manufacturing a thin-film solar cell.

[0002] Thin-film solar cells are one type of solar cell. Thin-film solar cells have a chalcopyrite structure containing Cu, In, Ga, Se, S, etc. as a p-type light absorbing layer. ２ There are CIS-based thin-film solar cells that use aromatic compounds. As shown in Non-Patent Document 1, academic research into CIS-based thin-film solar cells is being actively conducted. Patent Document 1 also describes the atomic profile of a p-type light absorption layer aimed at improving the photoelectric conversion efficiency of CIS-based thin-film solar cells. Thin-film solar cells are sometimes used in outer space, and Non-Patent Document 2 describes thin-film solar cells for use in outer space.

[0003] Patent No. 6297038

[0004] Physica Status Solidi. Rapid Research Letters, 2019, Vol.13, pp.1900415Advanced Energy Materials, 2022, Vol.12, Issue 29, pp.2200125

[0005] Strong radiation exists in space, and thin-film solar cells placed in space are exposed to this strong radiation, which causes defects in the light-absorbing layer within the thin-film solar cells and degrades their performance.

[0006] Therefore, an object of the present invention is to provide a thin-film solar cell that can withstand radiation and has a long life, and a method for manufacturing the thin-film solar cell.

[0007] A thin-film solar cell according to one aspect of the present disclosure includes a substrate, a first electrode layer provided on the substrate, and a I-III-VI ２The present invention relates to a semiconductor device comprising a p-type light absorbing layer containing a Group I compound and an n-type second electrode layer provided on the p-type light absorbing layer, wherein the p-type light absorbing layer has a first region on the second electrode layer side when the p-type light absorbing layer is divided into two equal parts in a depth direction of the p-type light absorbing layer, and a second region other than the first region. The p-type light absorbing layer contains Cu as a Group I element and Ga and In as Group III elements, and the average atomic ratio of Cu to the Group III elements in the first region is lower than the average atomic ratio of Cu to the Group III elements in the second region. The average atomic ratio of Ga to the Group III elements in the first region is lower than the average atomic ratio of Ga to the Group III elements in the second region, and the average atomic ratio of Ga to the Group III elements in the p-type light absorbing layer is 0.2 or more and 0.4 or less. In the p-type light absorbing layer, the absolute value of the rate of change of the ratio of the number of atoms of Ga to the group III element from the first electrode layer side to the second electrode layer side is greatest in the first region, and the ratio of the number of atoms of Ga to the group III element in the p-type light absorbing layer tends to decrease from the first electrode layer side to the second electrode layer side in the first region.

[0008] A method for manufacturing a thin-film solar cell according to one embodiment of the present disclosure includes a first step of providing a first electrode layer on a substrate, and a second step of providing a first electrode layer on the first electrode layer. ２and a third step of providing an n-type second electrode layer on the p-type light absorbing layer, wherein the p-type light absorbing layer has a first region on the second electrode layer side when the p-type light absorbing layer is divided into two equal parts in the depth direction of the p-type light absorbing layer, and a second region other than the first region, the p-type light absorbing layer containing Cu as a Group I element and Ga and In as Group III elements, and the second step is performed by dividing the first region into two equal parts such that an average value of the atomic ratio of Cu to the Group III element in the first region is lower than an average value of the atomic ratio of Cu to the Group III element in the second region, and the Ga and In in the first region are lower than an average value of the atomic ratio of Cu to the Group III element in the second region. The p-type light absorbing layer is provided so that the average value of the atomic ratio of the Group III elements is lower than the average value of the atomic ratio of Ga to the Group III elements in the second region, the average value of the atomic ratio of Ga to the Group III elements in the p-type light absorbing layer is 0.2 or more and 0.4 or less, the absolute value of the rate of change of the atomic ratio of Ga to the Group III elements from the first electrode layer side to the second electrode layer side in the p-type light absorbing layer is greatest in the first region, and the atomic ratio of Ga to the Group III elements in the p-type light absorbing layer tends to decrease from the first electrode layer side to the second electrode layer side in the first region.

[0009] According to the present invention, it is possible to provide a thin-film solar cell that can withstand radiation and has a long life, and a method for manufacturing the thin-film solar cell.

[0010] FIG. 1 is a cross-sectional view of a thin-film solar cell according to an embodiment. FIG. 2 is a view showing a Cu profile in a p-type light absorbing layer according to an embodiment. FIG. 3 is a view showing a Ga profile in a p-type light absorbing layer according to an embodiment. FIG. 4 is a view showing another example of a Ga profile in a p-type light absorbing layer according to an embodiment. FIG. 5 is a view showing another example of a Ga profile in a p-type light absorbing layer according to an embodiment. FIG. 6 is a cross-sectional view illustrating an example of a method for manufacturing a thin-film solar cell according to an embodiment. FIG. 7 is a cross-sectional view illustrating an example of a method for manufacturing a thin-film solar cell according to an embodiment. FIG. 8 is a cross-sectional view illustrating an example of a method for manufacturing a thin-film solar cell according to an embodiment. FIG. 9 is a cross-sectional view illustrating an example of a method for manufacturing a thin-film solar cell according to an embodiment. FIG. 10 is a cross-sectional view illustrating an example of a method for manufacturing a thin-film solar cell according to an embodiment. FIG. 11 is a view illustrating the recovery effect of a thin-film solar cell according to an embodiment. FIG. 12 is a view illustrating the recovery effect of a thin-film solar cell according to an embodiment. FIG. 13 is a view illustrating the recovery effect of a thin-film solar cell according to an embodiment. FIG. 14 is a view comparing the recovery effects of the thin-film solar cell according to an embodiment with other thin-film solar cells.

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the drawings, components with the same reference numerals have the same or similar configurations.

[0012] 1 shows a cross-sectional view of a thin-film solar cell 100 according to this embodiment. The thin-film solar cell 100 has a substrate 101, a first electrode layer 102, a p-type light absorbing layer 103, an electron transport layer 104, a second electrode layer 105, and a grid electrode 106. The thin-film solar cell 100 receives light from the grid electrode 106 side and generates electricity.

[0013] The substrate 101 may be, for example, a glass substrate such as soda lime glass or low-alkali glass, a metal substrate such as a stainless steel plate, or a polyimide resin substrate. The first electrode layer 102 may be, for example, a metal conductive layer disposed on the substrate 101 and made of a metal such as Mo, Cr, or Ti. The film thickness of the first electrode layer 102 may be, for example, 200 to 800 nm.

[0014] The p-type light absorbing layer 103 is disposed on the first electrode layer 102, for example, and２ The p-type light absorbing layer 103 is formed of a group compound. The p-type light absorbing layer 103 contains Cu as a group I element, Ga and In as group III elements, and Se and S as group VI elements. The thickness of the p-type light absorbing layer 103 can be, for example, 1 to 3 μm.

[0015] The p-type light absorbing layer 103 has, in the depth direction of the p-type light absorbing layer 103, a first region R1 on the second electrode layer 105 side when the p-type light absorbing layer 103 is divided into two equal parts, and a second region R2, different from the first region, on the first electrode layer 102 side. Furthermore, the first region R1 has, in the depth direction of the p-type light absorbing layer 103, a third region R3 on the second electrode layer 105 side when the first region R1 is divided into two equal parts, and a fourth region R4 on the second region R2 or first electrode layer 102 side when the first region R1 is divided into two equal parts.

[0016] The electron transport layer 104 is disposed on the p-type light absorbing layer 103 and is made of, for example, Zn(O,S,OH) 2 , which has n-type conductivity, is transparent, and has high resistance. ｘ The electron transport layer 104 can be formed to a thickness of, for example, 20 to 150 nm. The electron transport layer 104 can be formed using a thin film of a II-VI group compound semiconductor such as CdS, ZnS, or ZnO, or a mixed crystal thereof such as Zn(O,S). ｘ Specific examples include In ２ O ３ , In ２ S ３ , In(O, S, OH) ｘ The insulating layer 10 may also be formed from an In-based compound semiconductor thin film such as

[0017] The second electrode layer 105 is disposed on the electron transport layer 104 and may be made of, for example, ZnO:B, which has n-type conductivity, a wide band gap, transparency, and low resistance. The second electrode layer 105 may be formed as a transparent conductive film having a thickness of, for example, 0.1 to 1.5 μm. Alternatively, the second electrode layer 105 may be formed as a transparent conductive film made of ZnO:Al, ZnO:Ga, and ITO.

[0018] The grid electrode 106 is, for example, an electrode disposed on the second electrode layer 105 to extract electricity from the second electrode layer 105. The grid electrode 106 is made of a conductive material, and for example, a metal material, an alloy material, or a transparent conductive material can be used.

[0019] <Examples> Next, the atomic ratio (C1) between Cu and group III elements and the atomic ratio (G1) between Ga and group III elements in the p-type light absorbing layer 103 of the thin-film solar cell 100 according to this embodiment will be described. The inventors manufactured three thin-film solar cells 100 according to this embodiment (A to C), and will describe the ratios C1 and G1 measured for the three examples with reference to Figures 2 to 5. The atomic numbers are measured, for example, by measuring a cross section of the light absorbing layer by energy dispersive X-ray spectroscopy using a scanning electron microscope.

[0020] Fig. 2 shows an example of the atomic ratio C1 of Cu to group III elements in the p-type light absorbing layer 103 in Example A of the thin-film solar cell 100. The horizontal axis in Fig. 2 represents the depth from the surface of the p-type light absorbing layer 103 on the second electrode layer 105 side. The horizontal axis is also used in Figs. 3 to 5. The atomic ratio C1 of Cu to group III elements is calculated as the ratio of (number of Cu atoms) / (number of group III element atoms). The group III elements are, for example, In and Ga.

[0021] The ratio C1 of the entire p-type light absorbing layer 103 is lower than 1.0. In the example of FIG. 2, the average value of the ratio C1 of the entire layer is 0.91. If the ratio C1 is 1.0 or more, the photoelectric conversion efficiency may decrease. On the other hand, it is preferable that the ratio C1 is greater than 0.80 from the viewpoint of forming a p-type light absorbing layer with an appropriate carrier concentration.

[0022] Furthermore, in the p-type light absorbing layer 103, the average value of the ratio C1 in the first region R1 is lower than the average value of the ratio C1 in the second region R2. In the example of Fig. 2, the average value of the ratio C1 in the first region R1 is 0.89, and the average value of the ratio C1 in the second region R2 is 0.94.

[0023] In this way, by lowering the ratio C1 on the surface on the second electrode layer 105 side, the thin-film solar cell 100 can repair damage caused by radiation.

[0024] Specifically, radiation is known to cause defects in Cu in the crystal structure of the p-type light absorbing layer 103. Examples of such defects include antisite defects, which are generated when Group III elements such as In and Ga are placed in Cu vacancies created by radiation. Antisite defects significantly degrade electrical characteristics. Antisite defects are passivated by being modified by other Cu vacancies, reducing the degradation of electrical characteristics. It is also known that Cu vacancies can transition within the p-type light absorbing layer even with relatively weak external energy, such as light irradiation or heating. By using this transition to promote the modification of antisite defects by Cu vacancies, the thin-film solar cell 100 can self-repair even if it is damaged, thereby repairing damage caused by radiation.

[0025] In the p-type light absorbing layer 103, by lowering the atomic ratio between Cu and group III elements on the surface on the second electrode layer 105 side, it is possible to increase the number of Cu vacancies that cause defect repair, thereby enabling sufficient self-repair and enabling the thin-film solar cell 100 to repair damage caused by radiation.

[0026] 3 to 5 show examples of the ratio G1 of the number of Ga atoms to the group III element atoms in the p-type absorber layer 103 in Examples A to C. The ratio G1 of the number of Ga atoms to the group III element atoms is calculated as the ratio (number of Ga atoms) / (number of group III element atoms).

[0027] In the p-type light absorbing layer 103, the average value of the ratio G1 in the first region R1 is lower than the average value of the ratio G1 in the second region R2. In the example of Fig. 3, the average value of the ratio G1 in the first region R1 in Example A is 0.13, and the average value of the ratio G1 in the second region is 0.45. In the example of Fig. 4, the average value of the ratio G1 in the first region R1 in Example B is 0.15, and the average value of the ratio G1 in the second region is 0.42. In the example of Fig. 5, the average value of the ratio G1 in the first region R1 in Example C is 0.09, and the average value of the ratio G1 in the second region is 0.48.

[0028] In the p-type light absorbing layer 103, the average value of the ratio G1 in the p-type light absorbing layer 103 is 0.2 or more and 0.4 or less. Figures 3, 4, and 5 show a case where the average value of the ratio G1 in the entire p-type light absorbing layer 103 is 0.29. By setting the average value of the ratio G1 to be 0.2 or more and 0.4 or less, it is possible to increase the photoelectric conversion efficiency.

[0029] In the p-type light absorbing layer 103, the absolute value of the rate of change of the ratio G1 from the first electrode layer 102 side toward the second electrode layer 105 side is greatest in the first region R1. In the example of FIG. 3, the absolute value of the rate of change of the ratio G1 reaches a maximum of 74.6% in the first region R1. In the example of FIG. 4, the absolute value of the rate of change of the ratio G1 reaches a maximum of 52.7% in the first region R1. In the example of FIG. 5, the absolute value of the rate of change of the ratio G1 reaches a maximum of 70.4% in the first region R1. More specifically, the absolute value of the rate of change of the ratio G1 reaches a maximum in the fourth region R4 within the first region R1 in all of FIGS. 3 to 5.

[0030] In this way, by making the position where the absolute value of the rate of change of the ratio G1 is maximum in the first region R1, which is the region on the second electrode layer 105 side, it is possible to promote electron movement to the surface on the second electrode layer 105 side by utilizing the band gradient of the conduction band of the p-type light absorbing layer 103.

[0031] In the first region R1, the ratio G1 in the p-type light absorbing layer 103 tends to decrease from the first electrode layer 102 side to the second electrode layer 105 side. This tendency is commonly shown in each of FIGS. 3, 4, and 5.

[0032] In the p-type light absorbing layer 103, the ratio G1 in the third region R3 is preferably 0.15 or less. More preferably, the ratio G1 is, for example, 0.13 or less. In the example of FIG. 3, the maximum value of the ratio G1 in the third region R3 is 0.10. In the example of FIG. 4, the maximum value of the ratio G1 in the third region R3 is 0.13. In the example of FIG. 5, the maximum value of the ratio G1 in the third region R3 is 0.06. Furthermore, in the first region R1, in a region at a depth of 300 nm from the surface on the second electrode layer 105 side, the ratio G1 is 0.1 or less.

[0033] In this way, by adjusting the ratio G1 throughout the p-type light absorbing layer 103 and the ratio G1 at the surface on the second electrode layer 105 side, the thin-film solar cell 100 can repair damage caused by radiation. Among the above-mentioned antisite defects, antisite defects in which Ga is located in a Cu vacancy are characterized by being difficult to self-repair by light irradiation or heating. By lowering the ratio of the number of Ga atoms to the number of Group III elements at the surface on the second electrode layer 105 side, as in the p-type light absorbing layer 103, it is possible to reduce the amount of Ga that causes defects that are difficult to self-repair, thereby suppressing the occurrence of defects and enabling the thin-film solar cell 100 to repair damage caused by radiation.

[0034] Finally, the p-type light absorbing layer 103 contains S as a Group VI element. More specifically, S may be contained in the first region R1. This adjusts the band gap of the p-type light absorbing layer 103 and increases the photoelectric conversion efficiency. In particular, by containing S in the first region R1, which has a reduced Ga ratio, it is possible to increase the photoelectric conversion efficiency while making it possible to repair damage caused by radiation.

[0035] <Manufacturing Method> Next, a method for manufacturing the thin-film solar cell 100 will be described with reference to FIGS. 6A to 6E.

[0036] First, as shown in FIG. 6A , a first electrode layer 102 is formed on a substrate 101. The thickness of the first electrode layer 102 may be, for example, 200 to 800 nm. The first electrode layer 102 is formed by DC sputtering using, for example, Mo as a material. The deposition conditions for the DC sputtering method are deposition pressure: 0.5 to 2.5 Pa, applied power: 1.0 to 3.0 W / cm. ２ It may also be possible to use the following.

[0037] Next, as shown in FIG. 6B, I-III-VI ２ A p-type light absorbing layer 103 containing a group compound is formed.

[0038] Examples of methods for forming the p-type light absorbing layer 13 include: (1) a method of forming a metal precursor film of a group I element and a group III element, and then forming a compound of the metal precursor film and a group VI element; and (2) a method of forming films of a group I element, a group III element, and a group VI element by vapor deposition.

[0039] First, a case where the p-type light absorbing layer 103 is formed using method (1) will be described. In method (1), a precursor film containing Ga and Cu is formed on the first electrode layer 102, then a precursor film containing In is formed, and then a precursor film containing Ga and Cu and a compound of the precursor film containing In with a group VI element (Se and S) are formed. Regarding Cu, which is a group I element, as described above, a precursor film containing a mixed crystal of Ga and Cu may be formed, or a precursor film containing Cu may be stacked on a precursor film containing Ga. The Cu concentration on the surface of the precursor film is controlled using the Se—S concentration in the reaction gas.

[0040] Each precursor film can be formed using a known film formation technique such as sputtering, etc. In this manner, the p-type absorber layer 103 having the atomic profile described with reference to FIGS.

[0041] In addition, the atomic ratio G1 of Ga to the group III element on the surface can be reduced by shortening the heat treatment time for forming a compound with the group VI element or by adding an alkali compound (for example, by adding it to the precursor film or diffusing it from the substrate).

[0042] The atomic ratio C1 of Cu to group III elements in the entire p-type light absorbing layer 103 can be adjusted by controlling the atomic numbers of Cu and group III elements in the precursor films to be stacked, for example, by changing the film thickness ratio of the Cu—Ga precursor film to the In precursor film or the film thickness ratio of the Cu—Ga precursor film to the Cu precursor film.

[0043] Next, a description will be given of the formation of the p-type light absorbing layer 103 by method (2). In method (2), Group I, III, and VI elements are vapor-deposited on the first electrode layer 102 using vapor deposition sources. Ga is vapor-deposited before In is vapor-deposited as a Group III element, thereby forming a semiconductor layer with a chalcopyrite structure, and thus forming the p-type light absorbing layer 103. In this manner, the p-type light absorbing layer 103 having the atomic profile described with reference to FIGS. 2 to 5 may be formed. The atomic ratio G1 of Ga to Group III elements on the surface can also be reduced by shortening the heat treatment time for vapor-depositing the Group VI element or by adding an alkali compound (vapor deposition using an alkali compound as a vapor deposition source, or diffusion of an alkali metal from the substrate).

[0044] Cu, which is a group I element, may also be deposited as a vapor deposition film containing Cu and Ga or In, or as a vapor deposition source containing Cu and S or Se to form a p-type light absorbing layer.

[0045] Se or S, which is a group VI element, may be deposited as a Se film, an S film, or a film containing Se and S.

[0046] For example, the p-type light absorbing layer 103 is formed on the first electrode layer 102 by first evaporating Ga and Se, or Ga, In, and Se, then evaporating Cu, Se, and S, and finally evaporating In, Se, and S. Due to the heat generated when each element is evaporated, a compound of Cu, Se or S, and In and Ga is formed. After each element is evaporated, a heat treatment may be performed to form a compound of Cu, Se or S, and In and Ga. For the evaporation, a method such as a multi-source simultaneous evaporation method may be used.

[0047] Alternatively, the p-type light absorbing layer 103 may be formed by first depositing Cu and Ga (or a CuGa compound) on the first electrode layer 102, then depositing Cu and In (or a CuIn compound), and then depositing Se, S, and In (or an InSeS compound). A compound of Cu, Se or S, and In and Ga is formed by the heat generated when In (Se, S) is deposited. Alternatively, after depositing In (Se, S), a heat treatment may be performed to form a compound of Cu, Se or S, and In and Ga.

[0048] Alternatively, the p-type light absorbing layer 103 may be formed by first evaporating Cu and Ga (or a CuGa compound) on the first electrode layer 102, then evaporating Cu and In (or a CuIn compound), and then evaporating Se and S (or a SeS compound). A compound of Cu, Se or S, and In and Ga is formed by the heat generated when Se—S is evaporated. Alternatively, after Se—S is evaporated, a heat treatment may be performed to form a compound of Cu, Se or S, and In and Ga. This concludes the description of the process of forming the p-type light absorbing layer 103 using method (2).

[0049] Next, as shown in FIG. 6C , an electron transport layer 104 is formed on the p-type light absorbing layer 103. The electron transport layer 104 can be formed by solution growth, MOCVD, ALD, vapor deposition, or sputtering. Next, as shown in FIG. 6D , a second electrode layer 105 is formed on the electron transport layer 104. The second electrode layer 105 may be formed by sputtering or MOCVD. Next, as shown in FIG. 6E , a grid electrode 106 is formed on the second electrode layer 105. The grid electrode 106 may be formed by, for example, vapor deposition or by printing a paste-like conductive material on the second electrode layer 105.

[0050] 7A to 7D, the self-repair effect of the thin-film solar cell 100 will be described. 7A to 7D show the storage rate of the photoelectric conversion efficiency of the thin-film solar cell 100 (FIG. 7A), the open-circuit voltage V ｏｃ (FIG. 7B), the short-circuit current density J ｓｃ 7A to 7D are average values ​​of data from a plurality of thin-film solar cells 100, including the thin-film solar cells 100 of Examples A, B, and C.

[0051] The horizontal axis of Figures 7A to 7D is the proton fluence, which indicates the number of proton particles per unit area. In Figures 7A to 7D, the solid lines indicate the storage ratios when irradiated with a 300 keV proton beam, and the dashed lines indicate the storage ratios when irradiated with a 3 MeV proton beam. The storage ratio is the ratio of the value after irradiation to the value before irradiation. The examples of Figures 7A to 7D also show the storage ratios when light irradiation (light soaking: LS) ​​and heat light soaking (heat light soaking: HLS) are performed for self-repair of the thin-film solar cell 100 after irradiation. Light irradiation or heat light soaking promotes the migration of Cu defects in the p-type absorber layer 103, modifies antisite defects, and restores the performance of the thin-film solar cell 100. For comparison, the storage ratios when light irradiation or heat light irradiation is not performed are also shown.

[0052] 7A to 7D , even if the performance of the thin-film solar cell 100 is deteriorated by proton irradiation, for example, as shown in the case without repair, the respective characteristics are largely recovered by light irradiation or thermolight irradiation. Such recovery of characteristics is considered to be due to the fact that the p-type light absorbing layer 103 has the atomic profile described with reference to FIGS.

[0053] 7A to 7D, the thermal irradiation was performed by irradiating light for one hour at a temperature of 150° C. This corresponds to approximately 16 to 69 days at a temperature of 60° C. Therefore, the thin-film solar cell 100 can sufficiently self-repair even in an actual space environment.

[0054] 8 shows the retention rate of photoelectric conversion efficiency when the thin-film solar cell 100 according to this embodiment and other thin-film solar cells are irradiated with a 300 keV proton beam and self-repaired by thermal irradiation. The data for the thin-film solar cell 100 in FIG. 8 is an average value of data from a plurality of thin-film solar cells 100, including the thin-film solar cells 100 of Examples A, B, and C. FIG. 8 shows the retention rate RF1 of the photoelectric conversion efficiency of the thin-film solar cell 100, the retention rate RF2 of the photoelectric conversion efficiency of the thin-film solar cell described in Non-Patent Document 1, and the retention rate RF3 of the photoelectric conversion efficiency of the thin-film solar cell (GaAs-based) described in Non-Patent Document 2. The thin-film solar cell 100 is shown to have a higher self-repair effect than other thin-film solar cells.

[0055] In typical thin-film solar cells deployed in outer space, protective measures such as a cover glass protecting the light-absorbing layer of the thin-film solar cell are attached to the thin-film solar cell with an adhesive to prevent degradation of cell performance. However, space-use cover glass and adhesives are very expensive, and the use of such a cover glass and adhesive increases the overall weight of the power generation device, making this undesirable. Therefore, by enabling the thin-film solar cell itself to fully self-repair, as in the thin-film solar cell 100, the need for a cover glass and adhesive is eliminated. This allows for a lightweight battery module including the thin-film solar cell 100, reducing launch costs and the cost of the number of components. Furthermore, because the thin-film solar cell 100 can self-repair damage caused by radiation, it can be used as a long-life battery in outer space or the atmosphere above the stratosphere. Furthermore, because the thin-film solar cell 100 is radiation-resistant, it can be used as a battery with a longer life than conventional thin-film solar cells, even in a radiation environment. It can also be used as a battery with a longer life in the troposphere, where radiation levels are lower than in outer space. For example, it can be attached to the surface (roof, window, wall, etc.) of a building, mobile object, aircraft, etc., and used as a long-life power generation device.

[0056] The thin-film solar cell 100 according to this embodiment has been described above. The thin-film solar cell 100 includes a substrate 101, a first electrode layer 102 provided on the substrate 101, and a thin-film solar cell 100 having a structure including I-III-VI layers provided on the first electrode layer 102. ２ The p-type light absorbing layer 103 includes a p-type light absorbing layer 103 containing a Group I compound, and an n-type second electrode layer 105 provided on the p-type light absorbing layer 103, wherein the p-type light absorbing layer 103 has a first region R1 on the second electrode layer 105 side when the p-type light absorbing layer 103 is divided into two equal parts in the depth direction of the p-type light absorbing layer 103, and a second region R2 other than the first region R1. The p-type light absorbing layer 103 contains Cu as a Group I element and Ga and In as Group III elements, and the average atomic ratio of Cu to the Group III elements in the first region R1 is lower than the average atomic ratio of Cu to the Group III elements in the second region R2. The average value of the atomic ratio of Ga to group III elements in the first region R1 is lower than the average value of the atomic ratio of Ga to group III elements in the second region R2, and the average value of the atomic ratio of Ga to group III elements in the p-type light absorbing layer 103 is 0.2 or more and 0.4 or less. In the p-type light absorbing layer 103, the absolute value of the rate of change of the atomic ratio of Ga to group III elements from the first electrode layer 102 side toward the second electrode layer 105 side is greatest in the first region R1, and the atomic ratio of Ga to group III elements in the p-type light absorbing layer tends to decrease from the first electrode layer 102 side toward the second electrode layer 105 side in the first region R1.

[0057] In the p-type light absorbing layer 103, the amount of Cu vacancies that repair defects caused by radiation can be increased on the second electrode layer 105 side, allowing the thin-film solar cell 100 to repair damage caused by radiation. Furthermore, by lowering the atomic ratio of Ga to group III elements on the surface on the second electrode layer 105 side, the amount of Ga, which causes defects that are difficult to self-repair, can be reduced, thereby suppressing the occurrence of defects and making it easier for the thin-film solar cell 100 to repair damage caused by radiation. By locating the position where the absolute value of the rate of change in the atomic ratio of Ga to group III elements is maximized in the first region R1, electron migration to the surface on the second electrode layer 105 side can be promoted by utilizing the band slope of the conduction band of the p-type light absorbing layer 103, thereby improving electrical characteristics. These factors enable the thin-film solar cell 100 to repair damage caused by radiation.

[0058] The p-type light absorbing layer 103 may also contain S as a Group VI element, which can adjust the band gap of the p-type light absorbing layer 103 and increase the photoelectric conversion efficiency. In particular, if S is contained in the first region R1, which has a reduced Ga ratio, it is possible to increase the photoelectric conversion efficiency while making it possible to repair damage caused by radiation.

[0059] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The elements of the embodiments, as well as their arrangement, materials, conditions, shapes, sizes, etc., are not limited to those illustrated and can be modified as appropriate. Furthermore, configurations shown in different embodiments can be partially substituted or combined with each other.

[0060] 100... Thin film solar cell, 101... Substrate, 102... First electrode layer, 103... P-type light absorbing layer, 103, 104... Electron transport layer, 104, 105... Second electrode layer, 106... Grid electrode

Claims

1. A substrate; a first electrode layer provided on the substrate; and a I-III-VII layer provided on the first electrode layer. ２ a p-type light absorbing layer containing a Group I compound, and an n-type second electrode layer provided on the p-type light absorbing layer, wherein the p-type light absorbing layer has a first region on the second electrode layer side when the p-type light absorbing layer is divided into two equal parts in a depth direction of the p-type light absorbing layer, and a second region other than the first region, the p-type light absorbing layer contains Cu as a Group I element and Ga and In as Group III elements, an average value of the atomic ratio of Cu to the Group III elements in the first region is lower than an average value of the atomic ratio of Cu to the Group III elements in the second region, an average value of the atomic ratio of Ga to the Group III elements in the first region is lower than an average value of the atomic ratio of Ga to the Group III elements in the second region, an average value of the atomic ratio of Ga to the Group III elements in the p-type light absorbing layer is 0.2 or more and 0.4 or less, a thin-film solar cell, wherein an absolute value of a rate of change of a ratio of the number of atoms of Ga to the group III element from the first electrode layer side to the second electrode layer side in the p-type light absorbing layer is maximum in the first region, and the ratio of the number of atoms of Ga to the group III element in the p-type light absorbing layer tends to decrease from the first electrode layer side to the second electrode layer side in the first region.

2. A thin-film solar cell according to claim 1, wherein the first region has a third region on the second electrode layer side when the first region is divided in half in the depth direction, and a fourth region other than the third region, and the ratio of the number of atoms of Ga to a group III element in the third region is 0.15 or less.

3. A thin-film solar cell according to claim 2, wherein the absolute value of the rate of change is maximum in the fourth region.

4. A thin-film solar cell according to claim 1, wherein the p-type light absorbing layer contains S as a Group VI element.

5. A thin-film solar cell according to claim 4, wherein said S is included in said first region.

6. A first step of providing a first electrode layer on a substrate; and forming a first electrode layer on the first electrode layer. ２ the second step of providing a p-type light absorbing layer containing a Group I compound; and a third step of providing an n-type second electrode layer on the p-type light absorbing layer, wherein the p-type light absorbing layer has a first region on the second electrode layer side when the p-type light absorbing layer is divided into two equal parts in a depth direction of the p-type light absorbing layer, and a second region other than the first region, the p-type light absorbing layer contains Cu as a Group I element and Ga and In as Group III elements, and the second step comprises: an average value of a ratio of the number of atoms of Cu to the Group III elements in the first region is lower than an average value of a ratio of the number of atoms of Cu to the Group III elements in the second region; an average value of a ratio of the number of atoms of Ga to the Group III elements in the first region is lower than an average value of a ratio of the number of atoms of Ga to the Group III elements in the second region; and an average value of a ratio of the number of atoms of Ga to the Group III elements in the p-type light absorbing layer is 0.2 or more and 0.4 or less, a first region of the p-type light absorbing layer having a first atomic number ratio of Ga to Group III element, the first region being a maximum, and a second region of the p-type light absorbing layer having a first atomic number ratio of Ga to Group III element, the second region being a maximum, and

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