JP2005228975A - Solar battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the energy conversion efficiency of a solar battery constituted from a p-type compound semiconductor layer and an n-type compound semiconductor layer. <P>SOLUTION: The solar battery comprises: a p-type compound semiconductor layer 3 containing a group Ib element, a group IIIb element, and a group VIb element; an n-type compound semiconductor layer 4 containing a group Ib element, a group IIIb group element, and a group VIb element; an n-type buffer layer 5; an n-type window layer 6; and an n-type transparent conductive layer 7. From a first energy difference (E<SB>C</SB>-E<SB>F</SB>) from a Fermi level E<SB>F</SB>to the bottom level E<SB>C</SB>of the conduction band in the n-type buffer layer 5, the difference reduced by the second energy difference (E<SB>C</SB>-E<SB>F</SB>) from a Fermi level E<SB>F</SB>to the bottom level E<SB>C</SB>of the conduction band in the n-type compound semiconductor layer 4 is made between -0.1 eV or more and 0.1 eV or less. The sum of half the value of an offset energy difference ΔE<SB>C</SB>from the minimum level to the maximum level of the band offset in the junction of the n-type buffer layer 5 and the n-type compound semiconductor layer 4, and the first energy difference is made between 0.3 eV or more and 0.9 eV or less. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a solar cell. In particular, the present invention relates to a solar cell with high energy conversion efficiency provided with a compound semiconductor layer containing a group Ib element, a group IIIb element, and a group VIb element as a light absorption layer.

Conventionally, a substrate type solar cell and a super straight type solar cell are known as a solar cell using a compound semiconductor film having a chalcopyrite structure such as a Cu (In, Ga) Se ₂ film as a light absorption layer. ing.

As a conventional substrate type solar cell, a metal layer such as a Mo film, a p-type Cu (In, Ga) Se ₂ layer, an n-type CuAl (S, Se) ₂ layer and an ITO layer are sequentially laminated on a glass substrate. The structure which did is known (for example, refer patent document 1). The pn junction in the above-mentioned conventional substrate type solar cell is composed of a heterojunction of a p-type Cu (In, Ga) Se ₂ layer and an n-type CuAl (S, Se) ₂ layer.

In addition, as a conventional super straight type solar cell, a transparent conductive layer, a ZnO: Al film, a CdS film formed by solution growth as an n-type window layer on a soda lime glass substrate, and a substrate as a p-type light absorption layer A configuration including a p-type CIS film (CuInSe ₂ film) vacuum-deposited at a temperature of 450 ° C. and an Au film as a back electrode is known (see, for example, Patent Document 2). The pn junction in the conventional super straight type solar cell is composed of a heterojunction of a p-type CIS film and an n-type CdS film.
Japanese Patent Laid-Open No. 11-298016 JP-A-7-263735

  Since the pn junction in the conventional typical substrate type solar cell and super straight type solar cell is composed of a heterojunction, the interface recombination current at the heterointerface is so large that it cannot be ignored, leading to a decrease in energy conversion efficiency. It was. Further, the carrier concentration of the p-type light absorption layer (p-type compound semiconductor layer), the carrier concentration of the n-type buffer layer, and the like have not been optimized. In particular, for the n-type buffer layer, only the high electrical resistance is noticed, and the resistance value and the carrier concentration remain unclear.

  Therefore, in the present invention, the pn junction of the solar cell is a homojunction or a pseudo-homojunction composed of a p-type compound semiconductor layer (p-type light absorption layer) and an n-type compound semiconductor layer, and the n-type compound semiconductor layer The energy conversion efficiency is improved by optimizing the combination with the n-type buffer layer.

In order to solve the above problems, a solar cell according to the present invention includes a substrate, a conductive layer formed on the substrate, and a group Ib element, a group IIIb element, and a group VIb element formed on the conductive layer. A p-type compound semiconductor layer, an n-type compound semiconductor layer containing a group Ib element, a group IIIb element, and a group VIb element formed on the p-type compound semiconductor layer, and an n-type compound semiconductor layer A n-type buffer layer, an n-type window layer formed on the n-type buffer layer, and an n-type transparent conductive layer formed on the n-type window layer. The difference obtained by subtracting the second energy difference from the Fermi level to the bottom level of the conduction band in the n-type compound semiconductor layer from the first energy difference to the bottom level of the conduction band in the shaped buffer layer is −0. Within the range of 1 eV to 0.1 eV, the n-type battery The sum of the half value of the offset energy difference between the minimum level and the maximum level of the band offset at the junction between the fur layer and the n-type compound semiconductor layer and the first energy difference is 0.3 eV or more and 0.9 eV or less. It is within the range (hereinafter also referred to as a substrate type solar cell). Here, “Fermi level” (E _F ), “bottom level of conduction band in n-type buffer layer”, “bottom level of conduction band in n-type compound semiconductor layer” and “conduction in n-type compound semiconductor layer” Each of the “bottom level of the band” is an energy level in a thermal equilibrium state or a short circuit state of the solar cell. The “bottom level of the conduction band” means the minimum energy level E _C that can be occupied by electrons in the conduction band. In the present specification, the names of the groups are named according to the IUPAC short-period periodic table. “Group Ib”, “Group IIIb” and “Group VIb” mean “Group 11”, “Group 13” and “Group 16” in the long-period periodic table recommended by IUPAC, respectively.

  In order to solve the above problems, a solar cell according to the present invention includes a substrate, an n-type transparent conductive layer formed on the substrate, and an n-type window layer formed on the n-type transparent conductive layer. An n-type buffer layer formed on the n-type window layer; an n-type compound semiconductor layer formed on the n-type buffer layer and containing an Ib group element, a IIIb group element, and a VIb group element; and an n-type compound A solar cell comprising a p-type compound semiconductor layer containing a group Ib element, a group IIIb element, and a group VIb element formed on the semiconductor layer, and a conductive layer formed on the p-type compound semiconductor layer. The first energy difference from the Fermi level to the bottom level of the conduction band in the n-type buffer layer is subtracted from the second energy difference from the Fermi level to the bottom level of the conduction band in the n-type compound semiconductor layer. Within the range of -0.1 eV to 0.1 eV The sum of the half value of the offset energy difference from the minimum level to the maximum level of the band offset at the junction between the n-type buffer layer and the n-type compound semiconductor layer and the first energy difference is 0.3 eV or more. It is characterized by being within a range of 0.9 eV or less (hereinafter also referred to as a super straight type solar cell). Here, it should be noted that the Fermi level and the bottom level of the conduction band of each layer are levels in a thermal equilibrium state or a short-circuit state, as in the case of the substrate type solar cell.

In the case of the solar cell of the present invention, the pn junction is a pseudo homojunction or a homojunction composed of a p-type compound semiconductor layer and an n-type compound semiconductor layer, and the first energy difference (n-type buffer) is also obtained. The difference obtained by subtracting the second energy difference (E _C -E _F in the n-type compound semiconductor layer) from E _C -E _{F in the} layer is in the range of −0.1 eV to 0.1 eV, and the n-type buffer The sum of the half value of the offset energy difference (difference from the minimum level of the band offset to the maximum level) and the first energy difference at the junction between the layer and the n-type compound semiconductor layer is 0.3 eV or more and 0.9 eV or less By making it within this range, solar cell characteristics such as energy conversion efficiency can be improved.

  As described above, the substrate type solar cell and superstrate type solar cell of the present embodiment include a substrate, a conductive layer, and a p-type compound semiconductor layer containing a group Ib element, a group IIIb element, and a group VIb element. , An n-type compound semiconductor layer containing an Ib group element, an IIIb group element, and a VIb group element, an n-type buffer layer, an n-type window layer, and an n-type transparent conductive layer. It should be noted that the stacking order of these layers differs between the substrate type solar cell and the superstrate type solar cell. Hereinafter, when simply referred to as a “solar cell”, both a substrate type solar cell and a super straight type solar cell are indicated. In the solar cell of the present embodiment, the pn junction is a pseudo-homojunction or a homojunction composed of a p-type compound semiconductor layer and an n-type compound semiconductor layer. Thereby, the interface defect in a pn junction interface is reduced, As a result, solar cell characteristics, such as energy conversion efficiency, improve.

  Further, the group Ib element of the p-type compound semiconductor layer and the group Ib element of the n-type compound semiconductor layer are the same kind of element, and the group IIIb element of the p-type compound semiconductor layer and the group IIIb element of the n-type compound semiconductor layer Are the same kind of element, and the VIb group element of the p-type compound semiconductor layer and the VIb group element of the n-type compound semiconductor layer are the same kind of element, the pn junction is a homojunction, and at the pn junction interface. Interface defects can be further reduced.

In the solar cell of the present embodiment, the second energy difference (E _C -E _F in the n-type compound semiconductor layer) is changed from the first energy difference (E _C -E _F in the n-type buffer layer). Since the reduced difference is within the range of −0.1 eV to 0.1 eV, electrons generated in the p-type compound semiconductor layer by light irradiation pass through the n-type buffer layer and the n-type window layer, and the n-type transparent conductivity Can be transferred to the layer well. Thereby, solar cell characteristics such as energy conversion efficiency are further improved.

The first energy difference (E _C -E _F in the n-type buffer layer) varies depending on the composition of the n-type buffer layer and the carrier concentration of the n-type buffer layer. Further, the second energy difference (E _C -E _F in the n-type compound semiconductor layer) varies depending on the composition of the n-type compound semiconductor layer and the carrier concentration of the n-type compound semiconductor layer. Therefore, by adjusting the composition and carrier concentration of the n-type buffer layer and the composition and carrier concentration of the n-type compound semiconductor layer, the difference obtained by subtracting the second energy difference from the first energy difference to a desired value. Can be controlled.

Further, in the case of the solar cell of the present embodiment, the half value of the offset energy difference (difference from the minimum level to the maximum level of the band offset) at the junction between the n-type buffer layer and the n-type compound semiconductor layer is further provided. And the first energy difference (E _C -E _F in the n-type buffer layer) is not less than 0.3 eV and not more than 0.9 eV, so that the electrons transferred to the n-type transparent conductive layer are n-type compound semiconductors. It is possible to satisfactorily prevent the layer from returning to the p-type compound semiconductor layer. In particular, the sum of the half and the first energy difference of the offset energy difference is preferably in the range of less than E _C -E _F in p-type compound semiconductor layer.

Here, it will be described that it is possible to prevent the return of electrons. The electrons once transferred to the n-type transparent conductive layer beyond the band offset at the junction between the n-type compound semiconductor layer and the n-type buffer layer are generally taken out of the solar cell, but some of them are thermally affected. Try to go back. However, since the kinetic energy of the returning electron is usually 0.3 eV or less, if the sum of the half value of the offset energy difference and the first energy difference is 0.3 eV or more, it is transferred to the transparent conductive layer. The ratio of electrons flowing back to the n-type compound semiconductor layer or the p-type compound semiconductor layer beyond the band offset at the junction between the n-type compound semiconductor layer and the n-type buffer layer is 6.1 × 10 ⁻⁶ or less. , Practically irreversible. Therefore, solar cell characteristics such as energy conversion efficiency are further improved.

  The band offset is uniquely determined by the composition of the n-type buffer layer and the composition of the n-type compound semiconductor. Therefore, the band offset is controlled by adjusting the composition of the n-type buffer layer and the composition of the n-type compound semiconductor layer.

In the solar cell of the present embodiment, the first energy difference (E _C -E _F in the n-type buffer layer) and the second energy difference (E _C -E _F in the n-type compound semiconductor layer) are substantially the same. A certain configuration is preferable. If the bottom level E _C of the conduction band in the n-type buffer layer is lower than the bottom level E _C of the conduction band in the n-type compound semiconductor layer, the interface recombination current at the junction interface becomes very large, and energy conversion efficiency, etc. This is because the solar cell characteristics are greatly reduced. Incidentally, as the bottom level E _C of the conduction band in n-type buffer layer is lower the than the bottom level E _C of the conduction band in n-type compound semiconductor layer, the interface recombination current increases. On the other hand, as the bottom level E _C of the conduction band in n-type buffer layer is higher the than the bottom level E _C of the conduction band in n-type compound semiconductor layer, electrons from the n-type compound semiconductor layer on the n-type buffer layer This is because the transfer efficiency is reduced. Here, “substantially the same” means the difference between the first energy difference (E _C −E _F in the n-type buffer layer) and the second energy difference (E _C −E _F in the n-type compound semiconductor layer). It means that the absolute value is 0.01 eV or less.

Furthermore, when the bottom level E _C of the conduction band in the n-type buffer layer is higher than the bottom level E _C of the conduction band in the n-type compound semiconductor layer, the carrier concentration of the n-type buffer layer is low and the resistance is high. As a result, it is necessary to reduce the thickness of the n-type buffer layer. However, when a thin n-type buffer layer is formed, the coverage of the n-type buffer layer with respect to the underlying layer becomes insufficient, and the leakage current between the n-type compound semiconductor layer and the n-type window layer increases (in the equivalent circuit). (Shunt resistance is reduced). Here, the underlayer means an n-type compound semiconductor layer in a substrate type solar cell, and an n-type window layer in a superstrate type solar cell. In particular, in a substrate type solar cell, the surface of the n-type compound semiconductor layer, which is a base layer, is likely to be an uneven surface, so that the influence appears remarkably. Accordingly, the first energy difference and the second energy are not reduced in order to prevent a decrease in solar cell characteristics such as energy conversion efficiency due to an increase in leakage current between the n-type compound semiconductor layer and the n-type window layer. It is preferable that the difference is substantially the same.

In the solar cell of the present embodiment, it is preferable that the sum of the half value of the offset energy difference and the first energy difference be 0.5 eV or less. With this configuration, the n-type window layer and the n-type transparent layer are not significantly reduced without significantly reducing the transfer efficiency of electrons from the n-type compound semiconductor layer and the p-type compound semiconductor layer to the n-type window layer and the n-type transparent conductive layer. This is because it is possible to satisfactorily prevent electrons from returning from the conductive layer to the n-type compound semiconductor layer and the p-type compound semiconductor layer. The half value of the offset energy difference (difference from the minimum level to the maximum level of the band offset) and the first energy difference (E in the n-type buffer layer) at the junction between the n-type compound semiconductor layer and the n-type buffer layer. _The larger the sum of ( _C− E _F ), the better the prevention of electron reversion, but the lower the electron transfer efficiency. Therefore, it is more preferable to optimize the sum of the half value of the offset energy difference and the first energy difference within a range of 0.3 eV to 0.5 eV.

The p-type compound semiconductor layer is a semiconductor layer having at least one type Ib group element, at least one type IIIb group element, and at least one type VIb group element and having p-type conductivity. As the p-type compound semiconductor layer, Cu (copper) which is a group Ib element, at least one element selected from the group consisting of In (indium) and Ga (gallium), S (sulfur) and Se (selenium) And a structure containing at least one element selected from the group consisting of: More preferably, the p-type compound semiconductor layer is a p-type Cu (In, Ga) Se ₂ film (p-type CIGS film). This is because a solar cell with high energy conversion efficiency can be realized inherently compared to other configurations. The p-type compound semiconductor layer may further contain a p-type impurity.

The n-type compound semiconductor layer is a semiconductor layer having at least one type Ib group element, at least one type IIIb group element, and at least one type VIb group element and having n-type conductivity. As the n-type compound semiconductor layer, Cu that is a group Ib element, at least one element selected from the group consisting of In and Ga, and at least one element selected from the group consisting of S and Se are used. The structure to contain is preferable. More preferably, the n-type compound semiconductor layer is an n-type Cu (In, Ga) Se ₂ film (n-type CIGS film). This is because a solar cell with high energy conversion efficiency can be realized inherently compared to other configurations. The n-type compound semiconductor layer may further contain an n-type impurity.

  Here, an example of a method for forming a p-type compound semiconductor layer and an n-type compound semiconductor layer in a solar cell will be described. As a method of forming a p-type compound semiconductor layer and an n-type compound semiconductor layer in a substrate type solar cell, for example, after forming a p-type compound semiconductor layer, an n-type compound semiconductor layer is formed separately from the p-type compound semiconductor layer And a method of forming a p-type compound semiconductor crystal and then doping an n-type impurity into the surface layer of the p-type compound semiconductor crystal to convert the surface layer into an n-type compound semiconductor layer. When the n-type compound semiconductor layer is formed by the diffusion of the n-type impurity, the portion of the p-type compound semiconductor crystal where the diffusion of the n-type impurity is not diffused becomes the p-type compound semiconductor layer.

  On the other hand, as a method of forming the p-type compound semiconductor layer and the n-type compound semiconductor layer in the super straight type solar cell, for example, after forming the n-type compound semiconductor layer, the p-type compound semiconductor layer is separated from the n-type compound semiconductor layer. , A method of growing a n-type compound semiconductor crystal and then doping the surface layer of the n-type compound semiconductor crystal with a p-type impurity to convert it to a p-type compound semiconductor, and an n-type buffer containing the n-type impurity A p-type compound semiconductor crystal is grown on the layer, and an n-type impurity contained in the n-type buffer layer is diffused into the surface layer on the buffer layer side of the p-type compound semiconductor crystal to change the composition to an n-type compound semiconductor. A method is mentioned. Here, the n-type impurity contained in the n-type buffer layer means an element that functions as a donor when diffused into the p-type compound semiconductor film. When the p-type compound semiconductor layer is formed by the diffusion of the p-type impurity, the n-type compound semiconductor crystal in the portion where the diffusion of the p-type impurity is not diffused becomes the n-type compound semiconductor layer, and the n-type impurity is diffused by the n-type impurity diffusion. When the compound semiconductor layer is formed, the p-type compound semiconductor crystal in the non-diffused portion of the n-type impurity diffusion becomes the p-type compound semiconductor layer.

As the n-type buffer layer, a compound film containing Zn (zinc) (hereinafter referred to as a Zn-based compound film) is preferable. This is because, unlike Cd contained in an n-type buffer layer of a conventional solar cell, Zn is not harmful, and thus the solution used for forming the Zn-based compound film can be easily treated. Examples of the Zn-based compound film include a ZnO (zinc oxide) film, a ZnS (zinc sulfide) film, and a Zn (O, S) -based compound film. Here, as the Zn (O, S) -based compound film, for example, a Zn (O, S) film composed of a mixed crystal of ZnO and ZnS and ZnO, ZnS and Zn (OH) ₂ (zinc hydroxide) are used. Zn (O, S, OH) films composed of mixed crystals of Note that the Zn-based compound film may contain an n-type impurity for increasing the carrier concentration. Examples of the n-type impurity for the Zn-based compound film include elements such as B (boron) and Al (aluminum).

Examples of the n-type window layer include a ZnO film and a ZnMgO film. The bottom level E _C of the conduction band in the n-type window layer and the n-type window layer preferably further include an n-type impurity for increasing the carrier concentration. When the n-type window layer contains a slight amount of carriers, the n-type window layer does not have high resistance, that is, the resistance value of the series resistance in the equivalent circuit can be reduced, so that the energy conversion efficiency can be improved. Because.

In the solar cell of the present embodiment, the p-type compound semiconductor layer is a p-type Cu (In, Ga) Se ₂ film, the n-type compound semiconductor layer is an n-type Cu (In, Ga) Se ₂ film, and n Preferably, the shape buffer layer is a Zn (O, S) -based compound film, and the n-type window layer is a ZnO film. With this configuration, the pn junction of the solar cell can be a homojunction, and the offset energy at the junction between the n-type Cu (In, Ga) Se ₂ film and the Zn (O, S) -based compound film This is because the sum of the half value of the difference and the first energy difference (E _C −E _F in the n-type buffer layer) can be 0.3 eV or more. Further, by adjusting the composition and carrier concentration of the Zn (O, S) -based compound film, the first energy difference and the second energy difference (E _C -E _F in the n-type compound semiconductor layer) can be made the same. This is because it can also be a value.

Further, the n-type Cu (In, Ga) Se ₂ film includes a semiconductor crystal having the same composition as the Cu (In, Ga) Se ₂ crystal constituting the p-type Cu (In, Ga) Se ₂ film and its semiconductor A structure having an n-type impurity doped in the crystal is preferable. In this case, the pn junction can be surely made a homojunction, and interface defects at the pn junction interface can be reduced extremely well.

When the n-type compound semiconductor layer is an n-type Cu (In, Ga) Se ₂ film, a Zn (O, S) -based compound film is preferable as the n-type buffer layer. Here, the reason why the Zn (O, S) -based compound film is preferable will be described with reference to FIGS. 2 (a) to 2 (c). FIG. 2 is an explanatory diagram showing the band structure of the n-type compound semiconductor layer, the n-type buffer layer, and the n-type window layer in the solar cell of the present embodiment, and the band gap and conduction band for each material and composition. And the relative energy levels of the valence band. The n-type compound semiconductor layer is an n-type Cu (In, Ga) Se ₂ (n-type CIGS) film having a band gap of 1.12 eV, the n-type buffer layer is a Zn-based compound film, and the n-type window layer is a ZnO film. The case of being is illustrated. 2A shows a case where a ZnO film is used as the Zn-based compound film, and FIG. 2B shows a case where a Zn (O _0.78 S _0.22 ) film is used as the Zn-based compound film. (C) shows the case where a ZnS film is used as the Zn-based compound film. 2A to 2C, “E _V ” means the maximum energy level that can be occupied by electrons in the valence band.

As shown in FIG. 2A, when the n-type buffer layer is a ZnO film and the n-type window layer is a ZnO film, the bottom level E _C of the conduction band in the ZnO film of the n-type buffer layer. Is lower by 0.2 eV than the bottom level E _C of the conduction band in the n-type compound semiconductor (n-type CIGS) layer, so that recombination is easy at the junction interface, and the energy conversion efficiency is lowered.

As shown in FIG. 2C, when the n-type buffer layer is a ZnS film and the n-type window layer is a ZnO film, the bottom level E _C of the conduction band in the ZnS film is n It becomes higher by 1.4 eV than the bottom level E _C of the conduction band in the type CIGS film. If the carrier concentration of the ZnS film is low and the resistance is high, the thickness of the ZnS film needs to be 10 nm or less. In this case, particularly in a substrate type solar cell, if the surface of the n-type CIGS film is a flat surface, the coverage of the ZnS film with respect to the n-type CIGS film is rarely insufficient, but the surface of the n-type CIGS film If it is a surface with even irregularities, the coverage of the ZnS film will be insufficient. As a result, an increase in leakage current between the n-type CIGS film and the ZnS film (lowering of the shunt resistance of the equivalent circuit) causes a decrease in energy conversion efficiency.

When the Zn (O, S) film is represented by a general chemical formula Zn (O _1-x S _x ) film, the bottom level E _C of the conduction band in the Zn (O _1-x S _x ) film as X increases. As the carrier concentration increases, the bottom level E _C of the conduction band in the Zn (O _1−x S _x ) film decreases. Here, X in the general chemical formula Zn (O _1-x S _x ) is a real number satisfying 0 <X <1. Therefore, when a Zn (O, S) film is used for the n-type buffer layer and a ZnO film is used for the n-type window layer, the bottom level E _C of the conduction band in the Zn (O, S) film is at least shown in FIG. It is possible to control to an arbitrary energy level between the bottom level E _C of the conduction band in the ZnO film shown in FIG. 2 and the bottom level E _C of the conduction band in the ZnS film shown in FIG. it can. That is, the band offset between the n-type buffer layer and the n-type compound semiconductor layer can be arbitrarily controlled.

For example, as shown in FIG. 2B, when a Zn (O _0.78 S _0.22 ) film is used, the band offset with respect to the n-type CIGS film is about 0.2 eV.

2 (a) and 2 (c), the bottom level E _C of the conduction band in the ZnO film can take an energy level lower than the bottom level E _C of the conduction band in the n-type CIGS film, and the ZnO film bottom level E _C of the conduction band in that it can be seen that take a higher energy level than the bottom level E _C of the conduction band in n-type CIGS film. Further, the bottom level E _C of the conduction band in the Zn (O, S) film is changed to an arbitrary energy level between the bottom level E _C of the conduction band in the ZnO film and the bottom level E _C of the conduction band in the ZnS film. 2 (a) to 2 (c) show that the conduction band in the Zn (O, S) film is controlled by controlling the composition and carrier concentration of the Zn (O, S) film. show that it is possible for the bottom level E _C in the same energy level and the bottom level E _C of the conduction band in n-type CIGS film. Therefore, it is preferable to form the n-type buffer layer with a Zn (O, S) film having an intermediate composition (mixed crystal composition) between the ZnO film and the ZnS film.

In the solar cell of the present embodiment, when the n-type compound semiconductor is a Cu (In, Ga) Se ₂ film, the Zn (O, S) -based compound film is a Zn (O, S) film or Zn (O, S, OH) film is preferable. As described above with reference to FIGS. 2A to 2C, the first energy difference (in the n-type buffer layer) is adjusted by adjusting the composition of the Zn (O, S) -based compound film. E because _C -E _F) as possible to ensure the same E _C -E _F) and the second energy difference (n-type compound semiconductor layer.

When the n-type compound semiconductor is an n-type Cu (In, Ga) Se ₂ film and the n-type buffer layer is a Zn (O, S) -based compound film, a ZnO film is preferable as the n-type window layer. If the n-type window layer is a ZnO film, as can be seen from FIGS. 2A to 2C, the bottom level E _C of the conduction band in the ZnO film is the conduction in the Zn (O, S) -based compound film. This is because the electrons from the Zn (O, S) -based compound film can be transferred to the ZnO film satisfactorily because it is lower than the bottom level E _{C of the} band.

Here, a preferable range for the carrier concentration of the n-type Cu (In, Ga) Se ₂ film and a preferable range for the carrier concentration of the n-type buffer layer will be described with reference to FIGS. FIG. 3 shows the correlation between the carrier concentration and the second energy difference (E _C −E _F ) in the n-type Cu (In, Ga) Se ₂ film (n-type compound semiconductor layer) of the solar cell of this embodiment. FIG. FIG. 3 shows the case where the band gap (E _g ) of the n-type Cu (In, Ga) Se ₂ film is 1.12 eV. FIG. 4 is a correlation diagram showing the correlation between the composition of the Zn-based compound film and the first energy difference (E _C -E _F ) in the Zn-based compound film.

As shown in FIG. 3, when the carrier concentration of the n-type Cu (In, Ga) Se ₂ film (n-type CIGS film) changes from 1 × 10 ¹⁴ cm ⁻³ to 1 × 10 ¹⁵ cm ⁻³ , n The second energy difference (E _C -E _F ) in the CIGS film varies from 0.22 eV to 0.17 eV. For example, when the carrier concentration is 2 × 10 ¹⁴ cm ⁻³ (black dot in FIG. 3), the second energy difference (E _C −E _F ) in the n-type CIGS film is 0.21 eV.

Further, as shown in FIG. 4, when the composition of the Zn (O _1-x S _x ) film changes from the ZnO film (X = 0) to the ZnS film (X = 1), Zn (O _1-x S _x) first energy difference film (E _C -E _{F) is, Zn (O 1-x S} x) 0 from 0.26eV in the case of the carrier concentration of the membrane 1 × 10 ¹⁴ cm ^-3. When the carrier concentration is 5 × 10 ¹⁴ cm ⁻³ , it changes from 0.22 eV to 0.24 eV, and when the carrier concentration is 1 × 10 ¹⁵ cm ⁻³ , it changes from 0.21 eV to 0.2 eV. When the carrier concentration is 5 × 10 ¹⁵ cm ⁻³ , it changes from 0.17 eV to 0.18 eV, and when the carrier concentration is 1 × 10 ¹⁶ cm ⁻³ , it changes from 0.15 eV to 0.1. It varies from 17EV, if the carrier concentration of 5 × 10 ¹⁶ cm ^-3 is variable from 0.11eV to 0.13eV To. As can be seen from FIG. 4, in the case of a Zn (O _1-x S _x ) film having the same composition (X = constant), the higher the carrier concentration of the Zn (O _1-x S _x ) film, the first When the energy difference (E _C -E _F ) is small and the carrier concentration of the Zn (O _1-x S _x ) film is the same, the larger the value of X (closer to the ZnS composition), the first Energy difference (E _C −E _F ) becomes larger. Note that the carrier concentration in the case of a Zn (O _1-x S _x ) film (X = constant) can be controlled by the content of n-type impurities doped in the Zn (O _1-x S _x ) film.

When the n-type compound semiconductor layer is an n-type CIGS film corresponding to the black dot in FIG. 3, the first energy difference is the second energy difference (E _C −E _F = 0.21 eV in the n-type compound semiconductor layer). ) to the same value and the value of the first energy difference becomes _{0.21eV Zn (O 1-x S} x) composition of the membrane (X in FIG. 4) and Zn (O _1-x S _x ) The carrier concentration of the film is selected. Specifically, for example, when 1 × 10 ¹⁵ cm ⁻³ is selected as the carrier concentration of the Zn (O _1-x S _x ) film, Zn (O ₁ ) as shown by the black circles in FIG. _{0. 78} S _0.22) selecting (1-X = 0.78). Note that there are a plurality of combinations of carrier concentration and composition for a Zn (O _1-x S _x ) film having a first energy difference of 0.21 eV.

As described above, the carrier concentration of the Zn (O _1-x S _x ) film and the Zn (O _1-x S _x ) so that the first energy difference and the second energy difference have the same value. Although it is ideal to strictly control the composition (value of X) of the film, it is actually difficult to strictly control them. However, the second energy to the _{Zn (O 1-x S x} ) changes in the second energy difference with respect to a change in the carrier density of the film and _{Zn (O 1-x S x} ) changes in the composition of the membrane (change in X) Since the change in the difference is moderate in both cases, the carrier concentration is in the range of 8 × 10 ¹⁴ cm ^{−3 to} 3 × 10 ¹⁵ cm ⁻³ and 0.75 ≦ 1-X ≦ 0.8. If the composition is within the range, the second energy difference (E _C -E _F ) in the Zn (O _1-x S _x ) film is in the vicinity of 0.21 eV (± 0.01 eV) and is strictly controlled. There is no big difference from the case. In consideration of suppressing the interface recombination current, the second energy difference (E _C −E _F ) in the Zn (O _1-x S _x ) film is set to 0.21 eV or more and 0.23 eV or less. Is preferred.

  In the above description, the Zn (O, S) film has been described. However, even in the case of a Zn (O, S, OH) film, the content of OH groups is reduced by annealing, and a slight amount of OH groups is contained. Since it becomes a Zn (O, S, OH) film, it is qualitatively equivalent to a Zn (O, S) film.

In the solar cell of the present embodiment, when the n-type compound semiconductor is an n-type Cu (In, Ga) Se ₂ film and the n-type buffer layer is a Zn (O, S) -based compound film, the n-type Cu ( The average carrier concentration in the vicinity of the surface layer on the n-type buffer layer side in the In, Ga) Se ₂ film is in the range of 1 × 10 ¹⁴ cm ^{−3 to} 3 × 10 ¹⁴ cm ⁻³ and Zn (O, S) The average carrier concentration in the vicinity of the surface layer on the n-type Cu (In, Ga) Se ₂ film side in the system compound film is in the range of 1 × 10 ¹⁵ cm ⁻³ or more and 3 × 10 ¹⁵ cm ⁻³ or less. Is preferred. With this configuration, the first energy difference (E _C −E _F in the n-type buffer layer) and the second energy difference (E _C −E _F in the n-type compound semiconductor layer) are surely substantially the same, This is because the sum of the first energy difference and the half value of the offset energy difference (ΔE _{C at} the junction between the n-type buffer layer and the n-type compound semiconductor layer) is reliably 0.3 eV or more. Note that “in the vicinity of the surface layer” means a layer having a depth of 0.05 μm from the surface.

In the solar cell of the present embodiment, it is preferable that the n-type Cu (In, Ga) Se ₂ film has a thickness in the range of 0.3 μm or less. In order to suppress the recombination of holes generated by light absorbed by the n-type Cu (In, Ga) Se ₂ film, it is preferably within the hole diffusion length. Therefore, if the thickness of the n-type Cu (In, Ga) Se ₂ film is within a range of 0.3 μm or less, hole recombination can be suppressed in order to satisfy the condition within the hole diffusion length. It is.

  In the solar cell of the present embodiment, when the n-type buffer layer is a Zn (O, S) compound film, the thickness of the Zn (O, S) compound film is in the range of 0.08 μm to 0.1 μm. It is preferable to set it as the structure which is inside. If the thickness of the Zn (O, S) -based compound film is within this range, the coverage of the n-type buffer layer with respect to the underlayer is sufficient, which is caused by the occurrence of pinholes in the n-type buffer layer. This is because a decrease in energy conversion efficiency can be suppressed. In particular, in the case of a substrate type solar cell including an n-type compound semiconductor layer having an uneven surface, if the thickness of the Zn (O, S) -based compound film is less than 0.08 μm, the n-type compound semiconductor layer This is because it is difficult to form an n-type buffer layer with good coverage and pinholes are easily generated in the n-type buffer layer, and in the formation of an n-type window layer to which a sputtering method is applied, an n-type compound is formed. This is because the semiconductor layer and the p-type compound semiconductor layer are sputter damaged. On the other hand, if the film thickness is larger than 0.1 μm, the resistance in the n-type buffer layer is increased, that is, the series resistance in the equivalent circuit is increased, and the solar cell characteristics such as energy conversion efficiency are lowered. .

  Therefore, the film thickness of the n-type buffer layer in the substrate type solar cell is 0.08 μm or more and 0.1 μm or less depending on the surface shape of the n-type compound semiconductor layer including the n-type compound semiconductor layer and the p-type compound semiconductor layer. It is preferable to select appropriately from the range. Specifically, the film thickness of the n-type buffer layer is increased if the unevenness on the surface of the compound semiconductor layer is large, and the film thickness of the n-type buffer layer is decreased if the unevenness is small.

  On the other hand, in the case of a super straight type solar cell, since the n-type buffer layer is formed on the n-type window layer having a flat surface compared to the n-type compound semiconductor layer, the thickness of the n-type buffer layer is 0.08 μm. It may be the following.

  The substrate in the solar cell of the present embodiment may be made of any known material. In addition, as a board | substrate of a super straight type solar cell, in order to receive sunlight from the board | substrate side, it is necessary to use the board | substrate which has a light transmittance. The substrate of the substrate type solar cell contains at least one alkali metal element selected from the group consisting of group Ia elements (alkali metal elements), particularly Na (sodium), K (potassium) and Li (lithium) The substrate to be used is preferable. “Group Ia” means “Group 1” in the long-period periodic table recommended by IUPAC. If the substrate contains an Ia group element, when the p-type compound semiconductor crystal is formed on the conductive layer, the Ia group element of the substrate diffuses into the p-type compound semiconductor crystal through the conductive layer. This is because the crystallinity of the semiconductor crystal is improved.

  The conductive layer in the solar cell of the present embodiment may be made of any known material, but a metal such as a Mo (molybdenum) film, a Cr (chromium) film, an Au (gold) film, or Pt (platinum). A membrane is preferred.

The n-type transparent conductive layer in the solar cell of the present embodiment may be made of any known material, but it is an ITO film, SnO ₂ film, In ₂ O ₃ film, ZnO: Al film, ZnO: B film. Etc. are preferred. Further, the bottom level E _C of the conduction band in the n-type transparent conductive layer is preferably an energy level lower than the bottom level E _C of the conduction band in the n-type window layer. This is because the transfer efficiency of electrons from the n-type window layer to the n-type transparent conductive layer is improved.

(Embodiment 1)
The substrate type solar cell of the first embodiment will be described with reference to FIG. 1 and FIG. FIG. 1 is a schematic cross-sectional view showing the structure of the solar cell according to the first embodiment.

A substrate type solar cell shown in FIG. 1 includes a substrate 1, a conductive layer 2 formed on the substrate 1, a p-type CIGS film 3 (p-type compound semiconductor layer) formed on the conductive layer 2, and An n-type CIGS film 4 (n-type compound semiconductor layer) formed on the p-type CIGS film 3, and a Zn (O, S) film 5 (n-type buffer layer) formed on the n-type CIGS film 4; , A ZnO film 6 (n-type window layer) formed on the Zn (O, S) film 5 and an n-type transparent conductive layer 7 formed on the ZnO film 6. In the substrate type solar cell shown in FIG. 1, the thickness of the n-type CIGS film 4 is within a range of 0.3 μm or less, and the average carrier concentration in the vicinity of the surface layer of the n-type CIGS film 4 is 1 × 10. ¹⁴ cm and ^-3 3 × 10 ¹⁴ cm ^-3 in the range, Zn (O, S) the thickness of the film 5 was set within the following 0.1μm or 0.08μm, Zn (O, S) layer The average carrier concentration in the vicinity of the surface layer 5 is in the range of 1 × 10 ¹⁵ cm ^{−3 to} 3 × 10 ¹⁵ cm ⁻³ .

  In the substrate type solar cell shown in FIG. 1, the substrate 1, the conductive layer 2, and the n-type transparent conductive layer 7 may be made of any known material.

If substrate type solar cell shown in FIG. 1, Zn (O, S) E in the membrane 5 _C -E _F (first energy difference) E in the n-type CIGS film 4 _C -E _F (No. 2) and the half value of the offset energy difference at the junction between the n-type Zn (O, S) film 5 and the n-type CIGS film 4 and the Zn (O, S) film 5 E _C −E _F (first energy difference) in the range of 0.3 eV to 0.5 eV.

  Here, the energy band structure of the solar cell according to Embodiment 1 having the structure shown in FIG. 1 will be described with reference to FIG. FIG. 5 is an explanatory diagram for explaining an energy band structure in a thermal equilibrium state or a short-circuit state of the solar cell according to the first embodiment. FIG. 5 shows only the energy band structure of the p-type CIGS film 3, the n-type CIGS film 4, the Zn (O, S) film 5, the ZnO film 6 and the n-type transparent conductive layer 7. At the bottom of 5, the boundary between these layers is shown. Further, an enlarged view of the band offset in the vicinity of the interface between the n-type CIGS film 4 and the Zn (O, S) film 5 is shown in the upper part of FIG.

As shown in FIG. 5, the bottom level E _C and Zn (O, S) in n-type CIGS film 4 and the bottom level E _C in the film 5 is substantially the same energy level, Zn (O, S) Each of the bottom levels E _{C in the} film 5, the ZnO film 6 and the n-type transparent conductive layer 7 sequentially becomes a low energy level, and the n-type CIGS film 4 and the Zn (O, S) film 5 The sum of the offset energy difference ΔE _C of the band offset generated at the junction and the first energy difference (E _C −E _F in the Zn (O, S) film 5) is the third energy difference (p-type CIGS film 3). because it is E _C -E _F) or less, the electrons generated in the p-type CIGS film 3 with the light receiving sunlight, n-type CIGS film 4, Zn (O, S) the film 5 and ZnO film 6 sequentially in Can be transferred to the n-type transparent conductive layer 7 in a good manner. Further, the offset energy difference ΔE _C at the junction between the n-type CIGS film 4 and the Zn (O, S) film 5 and the first energy difference (E _C −E _F in the Zn (O, S) film 5) Since the sum is 0.3 eV or more, the electrons once reaching the ZnO film 6 and the n-type transparent conductive layer 7 exceed the band offset at the junction between the n-type CIGS film 4 and the Zn (O, S) film 5. Returning to the n-type CIGS film 4 and the p-type CIGS film 3 can be satisfactorily suppressed. This is because the band offset at the junction between the n-type CIGS film 4 and the Zn (O, S) film 5 functions as a barrier. Therefore, the solar cell of the first embodiment shown in FIG. 1 is a solar cell with a small leakage current and high energy conversion efficiency.

In the solar cell of the first embodiment, the pn junction is a homojunction composed of the p-type CIGS film 3 and the n-type CIGS film 4, and the Zn (O, S) film 5 with respect to the n-type CIGS film 4 is used. Since the coverage is good, it is possible to realize a junction characteristic in which the shunt resistance in the equivalent circuit is 1 kΩ / cm ² or more and the diode index (n value) is about 1.5.

  A method for manufacturing the substrate type solar cell shown in FIG. 1 will be described. First, the substrate 1 is prepared. As the substrate 1, a substrate containing a group Ia element such as Na is preferable. This is because the crystallinity of the p-type CIGS film 3 and the n-type CIGS film 4 can be improved with this configuration.

  Next, a conductive layer 2 is formed on the substrate 1. Any known method such as sputtering may be used for forming the conductive layer 2.

  Next, a p-type CIGS crystal is grown on the conductive layer 2. For the formation of the p-type CIGS crystal, any known method such as a vacuum deposition method or a selenization method may be used. Finally, a part of the p-type CIGS crystal is converted into the n-type CIGS film 4 (n-type compound semiconductor layer), and the other part becomes the p-type CIGS film 3 (p-type compound semiconductor layer).

Next, a Zn (O, S) film 5 (n-type buffer layer) is formed on the p-type CIGS crystal with a film thickness of 0.08 μm or more and 0.1 μm or less. Any known method such as a solution growth method may be used to form the Zn (O, S) film 5, but the carrier concentration of the Zn (O, S) film 5 is 1 × 10 ¹⁵ cm ⁻³ or more 3 It adjusts so that it may become in the range of * 10 < ¹⁵ > cm <^-3> or less.

Here, the case where the Zn (O, S) film 5 is formed by applying the solution growth method will be specifically described. First, using water as a solvent, a substance containing an n-type impurity that will function as a donor in the Zn salt, sulfide, complexing agent and Zn (O, S) film 5 is dissolved in a solvent such as water. An alkaline material solution for forming the Zn (O, S) film 5 is prepared. An example of the Zn salt is ZnSO ₄ , an example of the sulfide is SC (NH ₂ ) ₂ , and an example of the complexing agent is NH ₃ .

Next, a laminate composed of the substrate 1, the conductive layer 2, and the p-type CIGS crystal is immersed in a material solution maintained at a predetermined temperature to deposit Zn, O, S, etc. on the surface of the p-type CIGS crystal. . Thereby, a Zn (O, S, OH) film is formed on the p-type CIGS crystal. The solution growth of the Zn (O, S, OH) film proceeds based on the reaction of Zn salt, sulfide and complexing agent in the alkaline solution. Here, it should be noted that O and H in the Zn (O, S, OH) film are supplied from at least water (H ₂ O) which is a solvent of the material solution.

  After forming the Zn (O, S, OH) film, an annealing process (heating process) is immediately performed in a nitrogen gas atmosphere or in air. Thereby, OH groups are removed from the Zn (O, S, OH) film as much as possible, and the Zn (O, S) film 5 (n-type buffer layer) is formed. Further, by this annealing treatment, Zn contained in the Zn (O, S, OH) film is diffused into the p-type CIGS crystal, and the surface layer of the p-type CIGS crystal on the Zn (O, S) film 5 side. Becomes the n-type CIGS film 4 (n-type compound semiconductor layer), and the part of the p-type CIGS crystal where Zn has not diffused becomes the p-type CIGS film 3 (p-type compound semiconductor layer).

  When the solution growth method is applied as described above, the composition of the finally formed Zn (O, S) film 5 is such that the concentration of the Zn salt dissolved in the material solution, the pH of the material solution, Zn (O , S, OH) film is controlled by adjusting the temperature of the material solution when the film is deposited. The carrier concentration of the Zn (O, S) film 5 is such that the concentration of the n-type impurity dissolved in the material solution, the pH of the material solution, and the material solution when the Zn (O, S, OH) film is deposited. It is controlled by adjusting the temperature. If a desired carrier concentration can be realized without doping the Zn (O, S) film 5 with an n-type impurity, the n-type impurity does not have to be dissolved in the material solution, but a solution growth method is applied. Since the Zn (O, S) film 5 formed in this manner is likely to have high electric resistance, it is preferable to dissolve n-type impurities in the material solution.

When the n-type CIGS film 4 is formed by Zn diffusion as described above, the thickness and carrier concentration of the n-type CIGS film 4 are controlled by adjusting the processing temperature and processing time in the annealing process. That is, the Zn diffusion profile is controlled so that the thickness of the n-type CIGS film 4 is within a range of 0.3 μm or less, and the average carrier concentration in the vicinity of the surface layer of the n-type CIGS film 4 is 1 × 10 ¹⁴ cm ^−3. Adjust to the range of 3 × 10 ¹⁴ cm ⁻³ or less. Note that the thickness and carrier concentration of the n-type CIGS film 4 also depend on the Zn content in the Zn (O, S, OH) film.

  The Zn (O, S, OH) film formed by applying the solution growth method has much less pinholes than those formed by other methods, and the surface of the p-type CIGS crystal having irregularities. The coverage against is also good. Thereby, finally, the coverage of the Zn (O, S) film 5 with respect to the n-type CIGS film 4 is also improved. In addition, a Zn (O, S, OH) film can be formed at a temperature of 100 ° C. or lower. Since the film can be formed at a temperature of 100 ° C. or less, there is an advantage that the structure of the solution growth apparatus is simple and the temperature control is easy. Moreover, the damage given to the CIGS film at the time of film formation becomes very small. Further, when the laminate is immersed in the material solution, the surface of the p-type CIGS crystal is modified by slightly etching the surface of the p-type CIGS crystal because the material solution is alkaline. The surface modification improves the crystallinity of the Zn (O, S, OH) film and reduces interface defects at the interface between the p-type CIGS crystal and the Zn (O, S, OH) film. Thereby, finally, the interface defect between the n-type CIGS film 4 and the Zn (O, S) film 5 is reduced.

  Next, a ZnO film 6 is formed on the Zn (O, S) film 5. Any known method such as sputtering may be used to form the ZnO film 6. The ZnO film 6 may be a high resistance film, but a film having a slight n-type carrier (donor) is preferable. This is because if the ZnO film 6 is a high resistance film, the resistance value of the series resistance in the equivalent circuit of the solar cell is increased, and thus the solar cell characteristics such as energy conversion efficiency are lowered. As the n-type impurity for adjusting the carrier concentration of the ZnO film 6, Al, In, or the like is preferable. Further, since the Zn (O, S) film 5 is formed with a film thickness of 0.8 μm or more and 0.1 μm or less, the p-type CIGS film 3 and the n-type film are formed even if the ZnO film 6 is formed by applying the sputtering method. Sputter damage hardly occurs in the CIGS film 4.

  Next, an n-type transparent conductive layer 7 is formed on the ZnO film 6. Any known method such as sputtering may be used to form the n-type transparent conductive layer 7.

  In Example 1, an example of the solar cell according to Embodiment 1 will be described more specifically. For convenience, FIG. 1 is referred to.

  The solar cell of Example 1 includes a soda lime glass substrate as a substrate 1, a Mo film as a conductive layer 2, a p-type CIGS film 3 as a p-type compound semiconductor layer, and an n-type compound semiconductor layer. The structure includes a CIGS film 4, a Zn (O, S) film 5 as an n-type buffer layer, a ZnO film 6 as an n-type window layer, and an ITO film as an n-type transparent conductive layer 7.

A method for manufacturing the solar cell of Example 1 will be described. First, a Mo film is formed as a conductive layer 2 on a soda lime glass substrate as the substrate 1 to a film thickness of about 0.4 μm by sputtering. For example, the soda lime glass substrate is not heated, and the film is formed under the film forming conditions where the input power is 1.5 kW and the atmospheric pressure is 1 Pa. Next, a p-type CIGS crystal is deposited on the Mo film to a thickness of about 2 μm by vapor deposition. As a deposition source, a deposition apparatus including a Cu deposition source, an In deposition source, a Ga deposition source, and a Se deposition source is used. First, Cu / (In + Ga) = 0.85 to 0.9, Ga / (In + Ga) = 0. .25 at a soda lime glass substrate temperature of 350 ° C., then a soda lime glass substrate temperature of 500 ° C., Cu / (In + Ga) = 0.7, Ga / (In + Ga) = 0.25 Film formation was performed at the setting. Next, a Zn (O, S, OH) film is obtained by immersing a laminate having a soda lime glass substrate, a Mo film and a p-type CIGS film in a material solution maintained at a temperature of 80 ° C. as an n-type buffer layer. Was formed to a film thickness of about 0.1 μm. The material solution is a mixed solution of ZnSO ₄ , SC (NH ₂ ) ₂ , and NH _3. The ZnSO ₄ concentration is 0.16 M (mol), the SC (NH ₂ ) concentration is 0.6 M, and the NH ₃ concentration is 7. 5M. The mixed solution was 0.01 mg of Al as a dopant and sufficiently dissolved. Next, the laminated body on which the Zn (O, S, OH) film was further formed was annealed at 200 ° C. for 10 minutes in a nitrogen gas atmosphere. As a result, a Zn (O _0.78 S _0.22 ) film 5 substantially free of OH groups is formed, and an n-type CIGS film 4 (n-type compound semiconductor layer) containing Zn in the surface layer of the p-type CIGS crystal Formed. Next, a ZnO film 6 is formed as an n-type window layer on the Zn (O, S) film 5 to a thickness of about 0.1 μm by sputtering. Without heating the soda lime glass substrate, a film was formed under the conditions of an input power of 200 W and an atmospheric pressure of 3 Pa. Next, an ITO film is formed as a n-type transparent conductive layer 7 on the ZnO film 6 to a thickness of about 0.4 μm by sputtering. Without heating the soda lime glass substrate, a film was formed under the conditions of an input power of 1.5 kW and an atmospheric pressure of 0.3 Pa. Thereby, the manufacture of the solar cell of Example 1 having the structure shown in FIG. 1 was completed.

In the p-type CIGS film 3 of the solar cell of Example 1, the contents of Cu, In, Ga, and Se are 25 atomic%, 18 atomic%, 7 atomic%, and 50 atomic%, respectively, and the energy gap is The carrier concentration was 1 × 10 ¹⁶ cm ⁻³ . In the n-type CIGS film 4 of the solar cell of Example 1, the contents of Cu, In, Ga, and Se are 25 atomic%, 18 atomic%, 7 atomic%, and 50 atomic%, respectively. The energy gap is 1.12 eV, the second energy difference (E _C −E _F ) is 0.21 eV, the average carrier concentration in the vicinity of the surface layer is 2 × 10 ¹⁴ cm ⁻³ , and the film thickness is about It was 0.3 μm. Moreover, in the Zn (O, S) film 5 of the solar cell of Example 1, the contents of Zn, O, and S are 50 atomic%, 39 atomic%, and 11 atomic%, respectively. The energy difference (E _C −E _F ) was 0.21 eV, and the carrier concentration was 1 × 10 ¹⁵ cm ⁻³ . In the ZnO film 6, the carrier concentration due to the n-type impurity was 1 × 10 ¹⁵ cm ⁻³ .

  Here, each composition (content rate) was measured by EDX. Each carrier concentration was obtained by Hall measurement or CV measurement. The film thickness of the n-type compound semiconductor layer 4 was measured by analyzing a Zn diffusion profile by SIMS or the like.

  In the solar cell of Example 1, the sum of the first energy difference and the offset energy difference was 0.2 eV. The band offset is obtained by XPS or UPS.

When the solar cell characteristics were measured under simulated sunlight with an AM (air mass) of 1.5 and an intensity of 100 mW / cm ² with respect to the solar cell of Example 1, the open-circuit voltage (V _OC ) Is 0.67V, the short-circuit current (J _SC ) is 38 mA / cm ² , the fill factor (FF) is 0.73, the energy conversion efficiency is 18.5%, and the diode index (n value) Was 1.5. As can be seen from these measured values, the solar cell of Example 1 was a solar cell with extremely excellent solar cell characteristics such as energy conversion efficiency.

  The present invention can be used to improve characteristics such as energy conversion efficiency of solar cells.

3 is a schematic cross-sectional view showing the structure of a solar cell according to Embodiment 1. FIG. It is explanatory drawing for demonstrating the energy band structure of the n-type compound semiconductor, n-type buffer layer, and n-type window layer in the solar cell of this Embodiment. It is a correlation diagram showing the correlation between the n-type compound semiconductor layer of the solar cell of this embodiment the energy difference carrier concentration and the first in the (n-type CIGS film) (E _C -E _F). It is a correlation diagram showing the correlation between the n-type buffer layer of the solar cell of this embodiment (Zn compound layer) composition and second energy difference in (E _C -E _F). It is explanatory drawing for demonstrating the energy band structure in the thermal equilibrium state or short circuit state of the solar cell which concerns on Embodiment 1. FIG.

Explanation of symbols

1 substrate 2 conductive layer 3 p-type CIGS film (p-type compound semiconductor layer)
4 n-type CIGS film (n-type compound semiconductor layer)
5 Zn (O, S) film (n-type buffer layer)
6 ZnO film (n-type window layer)
7 n-type transparent conductive layer

Claims (9)

A substrate,
A conductive layer formed on the substrate;
A p-type compound semiconductor layer formed on the conductive layer and containing a group Ib element, a group IIIb element, and a group VIb element;
An n-type compound semiconductor layer formed on the p-type compound semiconductor layer and containing a group Ib element, a group IIIb element, and a group VIb element;
An n-type buffer layer formed on the n-type compound semiconductor layer;
An n-type window layer formed on the n-type buffer layer;
An n-type transparent conductive layer formed on the n-type window layer;
A solar cell comprising:
A second energy difference from the Fermi level to the bottom level of the conduction band in the n-type compound semiconductor layer from the first energy difference from the Fermi level to the bottom level of the conduction band in the n-type buffer layer. Is within a range of −0.1 eV or more and 0.1 eV or less,
The sum of the half value of the offset energy difference from the minimum level to the maximum level of the band offset at the junction between the n-type buffer layer and the n-type compound semiconductor layer and the first energy difference is 0.3 eV or more. A solar cell characterized by being in a range of 0.9 eV or less. A substrate,
An n-type transparent conductive layer formed on the substrate;
An n-type window layer formed on the n-type transparent conductive layer;
An n-type buffer layer formed on the n-type window layer;
An n-type compound semiconductor layer containing an Ib group element, a IIIb group element, and a VIb group element formed on the n-type buffer layer;
A p-type compound semiconductor layer containing a group Ib element, a group IIIb element and a group VIb element formed on the n-type compound semiconductor layer;
A conductive layer formed on the p-type compound semiconductor layer;
A solar cell comprising:
A second energy difference from the Fermi level to the bottom level of the conduction band in the n-type compound semiconductor layer from the first energy difference from the Fermi level to the bottom level of the conduction band in the n-type buffer layer. Is within a range of −0.1 eV or more and 0.1 eV or less,
The sum of the half value of the offset energy difference from the minimum level to the maximum level of the band offset at the junction between the n-type buffer layer and the n-type compound semiconductor layer and the first energy difference is 0.3 eV or more. A solar cell characterized by being in a range of 0.9 eV or less.   The solar cell according to claim 1 or 2, wherein the first energy difference and the second energy difference are substantially the same.   The solar cell according to claim 1 or 2, wherein a sum of a half value of the offset energy difference and the first energy difference is 0.5 eV or less. The p-type compound semiconductor layer is a p-type Cu (In, Ga) Se ₂ film;
The n-type compound semiconductor layer is an n-type Cu (In, Ga) Se ₂ film;
The n-type buffer layer is a Zn (O, S) -based compound film;
The solar cell according to claim 1, wherein the n-type window layer is a ZnO film.   The solar cell according to claim 5, wherein the Zn (O, S) -based compound film is a Zn (O, S) film or a Zn (O, S, OH) film. The average carrier concentration in the vicinity of the surface layer on the n-type buffer layer side in the n-type Cu (In, Ga) Se ₂ film is in the range of 1 × 10 ¹⁴ cm ⁻³ or more and 3 × 10 ¹⁴ cm ⁻³ or less. ,
The average carrier concentration in the vicinity of the surface layer on the n-type Cu (In, Ga) Se ₂ film side in the Zn (O, S) -based compound film is 1 × 10 ¹⁵ cm ⁻³ or more and 3 × 10 ¹⁵ cm ⁻³ or less. The solar cell according to claim 5, which falls within the range of The solar cell according to claim 5, wherein a thickness of the n-type Cu (In, Ga) Se ₂ film is in a range of 0.3 μm or less.   The solar cell according to claim 5, wherein a thickness of the Zn (O, S) -based compound film is in a range of 0.08 μm to 0.1 μm.

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