JP2000323733A - Solar cell - Google Patents

Abstract

(57) [Problem] To provide a solar cell having high conversion efficiency by suitably combining the electron affinity, the forbidden band width, and the material of a semiconductor layer. SOLUTION: A substrate 11, a lower electrode film 12, a p-type semiconductor layer 13 (a second semiconductor layer), an n-type semiconductor layer 14 (a first semiconductor layer), which are sequentially laminated on the substrate 11, and an upper portion It includes an electrode film 15, an antireflection film 16, and an extraction electrode 17 formed on the upper electrode film 15. The semiconductor layer 13 does not contain Cd, the semiconductor layer 14 is a light absorbing layer, and the forbidden band width Eg ₁ of the semiconductor layer 14 and the forbidden band width Eg ₂ of the semiconductor layer 13 are included.
Satisfy the relationship of Eg ₁ > Eg ₂ , and the electron affinity of the semiconductor layer 14 χ ₁ (eV) and the electron affinity of the semiconductor layer 13 χ
₂ (eV) satisfies the relationship 0 ≦ (χ ₂ −χ ₁ ) <0.5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solar cell,
In particular, for example, the present invention relates to a solar cell having a compound semiconductor layer containing a group Ib, a group IIIb, and a group VIb as a light absorbing layer.

[0002]

2. Description of the Related Art A CuInSe ₂ film (hereinafter sometimes referred to as a CIS film) or a Cu (In, Ga) film which is a compound semiconductor thin film (chalcopyrite structure semiconductor thin film) comprising a group Ib element, a group IIIb element and a group VIb element. Se ₂
A thin-film solar cell using a film (hereinafter sometimes referred to as a CIGS film) as a light absorbing layer has attracted attention because it exhibits high energy conversion efficiency and does not have a deterioration in efficiency due to light irradiation or the like.

In a solar cell, theoretically, when the forbidden band width of the light absorbing layer is in the range of 1.4 eV to 1.5 eV,
It is said that the highest conversion efficiency can be obtained. In the case of a solar cell using a CIGS film, the bandgap can be controlled by changing the ratio of Ga and In.
When the atomic ratio of a / (In + Ga) is 0.5 to 0.8, the forbidden band becomes 1.4 eV to 1.5 eV.

[0004]

However, in the current CIGS solar cell, the forbidden band width of the CIGS film is 1.2.
eV to 1.3 eV (which is Ga / (In + G
a) corresponds to a CIGS film having an atomic ratio of 0.2 to 0.3), the highest conversion efficiency can be obtained. Current CIG
The S solar cell has a problem that even if the Ga concentration is increased to widen the forbidden band width of the CIGS film, the conversion efficiency decreases, contrary to the theory.

[0005] In addition, currently reported high conversion efficiency C
The IGS solar cell includes a hetero junction of a CdS film serving as a window layer and a CIGS film serving as a light absorbing layer. On the other hand, in recent years, a CIGS solar cell not using CdS has attracted attention from the viewpoint of environmental pollution and the like, and ZnO has been used as a window layer instead of CdS as a window layer.
Some CIGS solar cells using a system semiconductor have been reported. However, there is a problem that the conversion efficiency is lower than when the CdS film is used. When a ZnO-based semiconductor is used for the window layer, the open-circuit voltage is particularly low.

In order to solve the above problem, the present invention provides a pn
An object is to provide a highly efficient solar cell in which a semiconductor layer forming a junction does not include a CdS film and has high efficiency.

[0007]

In order to achieve the above object, a first solar cell according to the present invention includes an n-type first semiconductor layer and a p-type second semiconductor layer, A solar cell in which the first semiconductor layer and the second semiconductor layer form a pn junction, wherein the first semiconductor layer does not contain Cd, and the second semiconductor layer is a light absorbing layer. And the first
The forbidden band width Eg ₁ of the semiconductor layer and the forbidden band width Eg ₂ of the second semiconductor layer satisfy the relationship of Eg ₁ > Eg ₂ , and the electron affinity 前 記₁ (eV) of the first semiconductor layer The electron affinity χ ₂ (eV) of the second semiconductor layer is 0 ≦ (χ ₂ −χ
₁ ) The relationship of <0.5 is satisfied. According to the above configuration, a highly efficient solar cell in which the semiconductor layer forming the pn junction does not contain CdS can be obtained.

In the first solar cell, it is preferable that the first semiconductor layer is formed closer to the light incident side than the second semiconductor layer. According to the above configuration, loss of incident light can be reduced.

In the first solar cell, a third semiconductor layer is further provided between the first semiconductor layer and the second semiconductor layer, and a forbidden band width Eg _{3 of} the third semiconductor layer is provided. And the forbidden band width Eg ₂ preferably satisfies the relationship of Eg ₃ > Eg ₂ . With the above configuration, a highly efficient solar cell can be obtained.

In the first solar cell, it is preferable that the third semiconductor layer is made of one of an n-type semiconductor and a high-resistance semiconductor. With the above structure, damage to the second semiconductor layer in the process of forming the first semiconductor layer can be reduced and a good pn junction can be provided, so that a solar cell with high conversion efficiency can be obtained.

In the first solar cell, the electron affinity χ ₃ (eV) of the third semiconductor layer and the electron affinity χ ₂ satisfy a relationship of (χ ₂ −χ ₃ ) ≧ 0.5, It is preferable that the thickness of the third semiconductor layer is 50 nm or less.
With the above structure, carriers are transported through the third semiconductor layer by tunneling, so that a solar cell with high conversion efficiency can be obtained.

In the first solar cell, the third semiconductor layer may be made of an oxide containing at least one element selected from the group consisting of Zn and IIIb elements, or Zn and IIIb.
Preferably, the chalcogen compound contains at least one element selected from group III elements.

[0013] In the first solar cell, an insulating layer is further provided between the first semiconductor layer and the second semiconductor layer, and a forbidden band width Eg _INS and a forbidden band width Eg of the insulating layer are provided. _Two
Preferably satisfy the relationship of Eg _INS > Eg ₂ .
With the above configuration, a highly efficient solar cell can be obtained.

In the first solar cell, the electron affinity χ _INS (eV) of the insulating layer and the electron affinity χ ₂ are (χ
It is preferable that the relationship of ₂ −χ _INS ) ≧ 0.5 is satisfied, and the thickness of the insulating layer is 50 nm or less. With the above structure, carriers are transported through the insulating layer,
A solar cell with high conversion efficiency can be obtained.

[0015] In the first solar cell, the insulating layer may include:
It is preferable to be made of at least one insulator selected from Al ₂ O ₃ , Ga ₂ O ₃ , Si ₃ N ₄ , SiO ₂ , MgF ₂ and MgO.

Further, in the first semiconductor layer, the second semiconductor layer
The semiconductor layer preferably further includes an n-type semiconductor layer or a high-resistance semiconductor layer on the surface on the first semiconductor layer side. With the above structure, p is contained in the second semiconductor layer.
Since an n-junction is formed and the defect density at the junction interface can be reduced, a solar cell with high conversion efficiency can be obtained.

In the first solar cell, the second solar cell may be
Is preferably a compound semiconductor layer containing a group Ib element, a group IIIb element and a group VIb element. According to the above configuration, a solar cell using a chalcopyrite structure compound semiconductor with little light deterioration as a light absorbing layer can be obtained.

Further, in the first solar cell, the first
Is preferably made of a compound containing Zn. With the above structure, a solar cell in which the semiconductor layer forming the pn junction does not contain CdS and has particularly high conversion efficiency can be obtained.

In the first solar cell, the compound is an oxide containing Zn and a Group IIa element, or
It is preferably a chalcogen compound containing and a Group IIa element.

In the first solar cell, the compound is a compound represented by the general formula: Zn _1-X A _X O (where the element A is Be, M
It is at least one selected from g, Ca, Sr and Ba, and 0 <X <1. It is preferable that the oxide represented by ()) be the main component. With the above structure, by changing the elements A and X according to the second semiconductor layer, the electron affinity of the first semiconductor layer can be changed, and a solar cell with particularly high conversion efficiency can be obtained.

In the first solar cell, it is preferable that the element A is Mg and the X satisfies the relationship of 0 <X <0.5. As a result, a solar cell having higher characteristics can be obtained.

In the first solar cell, the compound is an oxide containing Zn and a Group IIIb element, or
A chalcogen compound containing n and a group IIIb element is preferred. With the above structure, a solar cell in which the semiconductor layer forming the pn junction does not contain CdS and has particularly high conversion efficiency can be obtained.

In the first solar cell, the compound is represented by the general formula: Zn _Y B _2-2Y O _3-2Y (where the element B is A
At least one selected from 1, Ga, and In, and 0 <Y <1. It is preferable that the oxide represented by ()) be the main component. With the above structure, by changing the elements B and Y according to the second semiconductor layer,
Changing the electron affinity of the first semiconductor layer;
A solar cell with a particularly high conversion efficiency can be obtained.

A second solar cell according to the present invention is a solar cell including a p-type light absorbing layer and an n-type semiconductor layer laminated on the light absorbing layer, wherein the semiconductor layer has a general formula of Zn _1-Z
C _Z O (however, the element C is at least one selected from Be, Mg, Ca, Sr and Ba, and 0 <Z <1
It is. ) As a main component. In the second solar cell, since there are few defects in the semiconductor layer functioning as the window layer and the forbidden band width of the window layer can be freely changed, a solar cell with high conversion efficiency can be obtained.

In the second solar cell, the element C is M
g, and the Z preferably satisfies the relationship of 0 <Z <0.5.

[0026]

Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 In Embodiment 1, an example of a solar cell of the present invention will be described. In the first embodiment,
An example of a solar cell in which an electromotive force is generated by light incident from the side opposite to the substrate will be described.

FIG. 1 shows a cross-sectional view of the solar cell of the first embodiment. Referring to FIG. 1, solar cell 1 of Embodiment 1
Numeral 0 denotes a substrate 11, a lower electrode film 12, a semiconductor layer 13 (second semiconductor layer), a semiconductor layer 14 (first semiconductor layer), an upper electrode film 15, and an anti-reflection film sequentially laminated on the substrate 11. 16 and an extraction electrode 17 formed on the upper electrode film 15. That is, the semiconductor layer 14 is arranged on the light incident side of the semiconductor layer 13.

For the substrate 11, for example, glass, stainless steel, a polyimide film or the like can be used.

As the lower electrode film 12, for example, a metal film made of Mo can be used.

The semiconductor layer 13 (second semiconductor layer) is a semiconductor layer functioning as a light absorbing layer, and is a p-type semiconductor layer. The semiconductor layer 13 is disposed on the back surface side of the semiconductor layer 14. As the semiconductor layer 13, for example, a compound semiconductor layer containing a group Ib element, a group IIIb element, and a group VIb element can be used. For example, CuInSe ₂ , C
u (In, Ga) Se ₂ , CuInS ₂ , Cu (In, G
a) it can be used as S _2. The semiconductor layer 1
3 may have a surface semiconductor layer 13a on the surface on the semiconductor layer 14 side (the same applies to the following embodiments). FIG. 2 is a cross-sectional view of a solar cell 10a including the surface semiconductor layer 13a. The surface semiconductor layer 13a is an n-type semiconductor layer or a semiconductor layer having a high resistance (resistivity of 10 ⁴ Ωcm or more). As a high-resistance semiconductor layer, for example, Cu
In ₃ Se ₅ and Cu (In, Ga) ₃ Se ₅ are mentioned.

The semiconductor layer 14 (first semiconductor layer) forms a pn junction with the semiconductor layer 13 and functions as a window layer. The semiconductor layer 14 is an n-type semiconductor layer. The semiconductor layer 14 does not contain Cd (not as a constituent element or a dopant). For the semiconductor layer 14, for example, a compound containing Zn can be used. As the compound containing Zn, for example, an oxide or a chalcogen compound containing Zn and a Group IIa element, or a compound containing Zn and II
An oxide containing a group Ib element or a chalcogen compound can be used. Specifically, for example, the general formula Z
n _1-X A _X O (where element A is Be, Mg, Ca, Sr
And at least one selected from Ba and 0 <X
<1. ) As the main component (content 9
0 wt% or more) can be used. In this case, the content of the element A is preferably 0.1 at% or more. In general formula Zn _Y B _2-2Y O _3-2Y (where element B of Al, at least one selected from Ga and In, 0 <Y <1.) The oxide represented by A compound having a main component (a content of 90 wt% or more) may be used. In this case, the content of the element B is 5 at%.
It is preferable that it is above.

In the solar cell 10 of Embodiment 1, the semiconductor layer
13 bandgap Eg_TwoAnd the band gap Eg of the semiconductor layer 14₁When
But Eg₁> Eg_TwoSatisfy the relationship. In addition, the semiconductor layer 13
Electron affinity of_Two(EV) and electron affinity of the semiconductor layer 14
χ₁(EV) and 0 ≦ (χ _Two−χ₁) <0.5
Fulfill.

The upper electrode film 15 is a transparent conductive film, for example, ZnO: Al in which ZnO is doped with Al,
ITO (Indium Tin Oxide) can be used.

The anti-reflection film 16 is a film for preventing incident light from being reflected at the interface of the upper electrode film 15, and when the upper electrode film 15 is made of ITO or ZnO: Al, for example, MgF ₂ is used. be able to.

The extraction electrode 17 is made of, for example, NiCr.
A metal film in which a film and an Au film are stacked can be used.

Next, an example of a method for manufacturing the solar cell 10 will be described.

First, the lower electrode film 12 is formed on the substrate 11 by, for example, a sputtering method or an evaporation method. After that, the semiconductor layer 13 is formed on the lower electrode film 12 by, for example, an evaporation method or a sputtering method. Thereafter, the semiconductor layer 14 is formed on the semiconductor layer 13 by, for example, a chemical deposition method or a sputtering method. Thereafter, the upper electrode film 15 is formed on the semiconductor layer 14 by, for example, a sputtering method. Thereafter, the extraction electrode 17 is partially formed on the upper electrode film 15 by, for example, an electron beam evaporation method.
To form After that, the antireflection film 16 is formed on the upper electrode film 15 by, for example, an evaporation method. Thus, the solar cell 10 can be formed. When an n-type semiconductor layer or a high-resistance semiconductor layer is formed on the surface of the semiconductor layer 13, it can be formed by, for example, a solution immersion method, a vapor deposition method, or a gas phase diffusion method.

FIG. 3 schematically shows a band diagram of an example of the solar cell 10. In FIG. 3, the semiconductor layer 1
Numeral 3 denotes Cu (In, Ga) Se ₂ , and Cu (In, Ga) which is a surface semiconductor layer 13 a is formed on the surface of the semiconductor layer 13.
_{The case where 3} Se ₅ is formed is shown.

Next, the semiconductor layer 13 serving as a light absorbing layer has a CI
The function of the solar cell 10 will be described using a solar cell using a GS film as an example.

An effective method for improving the efficiency of a solar cell using a CIGS film for the light absorbing layer is to increase the forbidden band width of the CIGS film. However, in a conventional solar cell having a window layer made of CdS, the CIGS If the forbidden band width of the film is increased to 1.3 eV or more, the efficiency decreases contrary to the theory. One of the causes is that the CIGS film as the light absorbing layer and the Cd film as the window layer
An energy difference (offset) in the conduction band at the heterojunction with the S film is pointed out. Herberholtz (EH
erberholz) and the like indicate that when the atomic ratio in the CIGS film {Ga / (In + Ga)} <0.5, CdS
The discontinuity of the band due to the offset between the conduction band of CdS and the conduction band of CIGS causes a spike shape in which the conduction band of CdS rises and the conduction band protrudes near the junction, and the atomic ratio ｛Ga /
When (In + Ga)｝> 0.5, a model is proposed in which the conduction band of the CIGS film is lifted and a step is formed between the conduction band of CdS and the conduction band of CIGS, forming a cliff-like model (Solar Energy Materials and Thora
CELLS, Solar Energy Material
s and Solar Cells, 1997, Vol. 49, No. 3, page 227). FIG. 4A shows a band diagram when the offset between CdS and CIGS takes a spike shape.
FIG. 4B shows a band diagram when the offset between CdS and CIGS has a cliff shape. This model suggests that the band discontinuity of the conduction band becomes cliff-like, increasing recombination at the heterojunction interface and near the interface, and lowering the conversion efficiency. Similar to this phenomenon, C
Considering the case where the forbidden band width of the IGS film is 1.2 eV to 1.3 eV, when the CdS film serving as the window layer is replaced with a ZnO film, the band gap of the conduction band between the ZnO film and the CIGS film is discontinuous. Is expected to be in the form of a cliff in which the conduction band of the CIGS film rises.

Such a discontinuity in the conduction band of the heterojunction is caused by a difference between the electron affinity of the window layer and the electron affinity of the CIGS film as the light absorbing layer. In general, assuming that the electron affinities of an n-type semiconductor and a p-type semiconductor having different forbidden band widths are χ _n and χ _p , when χ _n <帯_p , the conduction band discontinuity has a spike shape. On the other hand, when χ _n > χ _p , the conduction band discontinuity has a cliff shape. Where Ga
The relationship between the electron affinities of the CuInSe ₂ film containing no CdS film and the CdS film is about 0.2 eV to 0.3 eV smaller for CdS. Therefore, when a hetero junction is formed, a spike occurs on the CdS side. However, the electron affinity of CIGS decreases as the Ga concentration increases. Therefore, above a certain Ga concentration, the value becomes smaller than the electron affinity of CdS.
Cliffs occur on the IGS side.

The form of band discontinuity between the window layer and the CIGS film is also governed by the electron affinity between the window layer and the CIGS film. Here, comparing the CdS film and the ZnO film as the window layer,
Since nO has an electron affinity about 0.4 eV higher than CdS, when a heterojunction is formed even with a GaIn-free CuInSe ₂ film, a cliff may be generated and loss may occur.

Furthermore, when the electron affinity of the window layer is smaller than that of the light absorbing layer and a spike occurs in the conduction band, the energy difference in the conduction band is large, which affects the conversion efficiency of the solar cell. The energy difference between CdS and CIGS is about 0.
Although it is 2 eV to 0.3 eV and hardly acts as a barrier for carrier transport, for example, when ZnS is used for the window layer, the energy difference from CIGS is about 1.6 eV, which becomes a barrier for photoexcited carriers. In this case, since the transport of carriers is hindered, almost no photocurrent can be extracted to the outside. Therefore, the conversion efficiency decreases. Thus, when a spike occurs in the conduction band between the window layer and the light absorption layer, it can be seen that there is an optimum range of the energy difference (offset) at which high efficiency is obtained. The solar cell of the first embodiment includes:
The electron affinity and the forbidden band width of the semiconductor layer 13 (light absorbing layer) and the semiconductor layer 14 (window layer) are defined in consideration of the above optimum range. Therefore, the solar cell 10 of the first embodiment
In this case, recombination of carriers at the junction interface between the semiconductor layer 13 and the semiconductor layer 14 is reduced.

As described above, according to the solar cell 10 of the first embodiment, a highly efficient solar cell can be obtained without using CdS for the window layer. In the above embodiment, the case where the first semiconductor layer is disposed on the light incident side of the second semiconductor layer has been described. However, the first semiconductor layer is disposed on the back side of the second semiconductor layer. It may be arranged.

Embodiment 2 In Embodiment 2, another example of the solar cell of the present invention will be described.

FIG. 5 is a cross-sectional view of the solar cell 20 according to the second embodiment. The solar cell 20 according to the second embodiment is different from the solar cell 10 according to the first embodiment only in that a semiconductor layer 21 is provided.

The semiconductor layer 21 (third semiconductor layer) is arranged between the semiconductor layers 13 and 14. The forbidden bandwidth Eg ₃ of the semiconductor layer 21 and the forbidden bandwidth E of the semiconductor layer 13
The g _2, satisfy the relationship of Eg _3> Eg _2. As the third semiconductor layer, for example, an oxide containing at least one element selected from Zn and Group IIIb elements, or Zn
And a chalcogen compound containing at least one element selected from Group IIIb elements. Also,
As the semiconductor layer 21, SnO ₂ may be used.

The electron affinity 半導体₃ (e
V) and the electron affinity χ ₂ of the semiconductor layer 13 are (χ ₂ −
χ ₃ ) It is preferable to satisfy the relationship of ≧ 0.5. Also,
The thickness of the semiconductor layer 21 is preferably 50 nm or less. As the semiconductor layer 21, for example, Zn (O,
S) A film can be used. Here, Zn (O, S)
Is a compound consisting of Zn, O and S, and is Zn-O
A compound containing a bond and a Zn-S bond.

The solar cell 20 can be manufactured by the same method as the solar cell 10 described in the first embodiment. The semiconductor layer 21 can be formed by, for example, a chemical deposition method or a vapor deposition method.

According to the solar cell 20 of the second embodiment,
A high-efficiency solar cell can be obtained without using CdS as the window layer.

Embodiment 3 In Embodiment 3, another example of the solar cell of the present invention will be described.

FIG. 6 is a cross-sectional view of the solar cell 30 according to the third embodiment. The solar cell 30 according to the third embodiment is different from the solar cell 10 according to the first embodiment only in that the solar cell 30 includes an insulating layer 31.

The forbidden band width Eg _INS of the insulating layer 31 and the forbidden band width Eg ₂ of the semiconductor layer 13 satisfy the relationship of Eg _INS > Eg ₂ . As the insulating layer 31, for example, Al ₂ O ₃ , Ga ₂
An insulating layer made of at least one insulator selected from O ₃ , Si ₃ N ₄ , SiO ₂ , MgF ₂ and MgO can be used.

Note that the electron affinity χ _INS of the insulating layer 31 and the electron affinity χ ₂ of the semiconductor layer 13 are (χ ₂ −INS _INS ) ≧
It is preferable to satisfy the relation of 0.5. Also, the insulating layer 3
It is preferable that the film thickness of No. 1 is 50 nm or less.

The solar cell 30 can be manufactured by the same method as the solar cell 10 described in the first embodiment. The insulating layer 3
1 can be formed by, for example, a sputtering method or an evaporation method.

According to the solar cell 30 of the third embodiment,
A high-efficiency solar cell can be obtained without using CdS as the window layer.

[0058]

EXAMPLES Example 1 In Example 1, the solar cell characteristics when the offset between the conduction band of the semiconductor layer 13 and the conduction band of the semiconductor layer 14 was changed with respect to the solar cell 10a described in the first embodiment. An example of calculating is shown. The band structure of the solar cell used for the calculation is the same as in FIG.

In the calculation of the first embodiment, a Cu (In, Ga) Se ₂ film (CIGS film) having a band gap Eg ₂ of 1.2 eV and an electron affinity of χ ₂ is used for the semiconductor layer 13 as the light absorbing layer. Was. The calculation was performed on the assumption that a Cu (In, Ga) ₃ Se ₅ layer as the surface semiconductor layer 13a was formed on the surface of this CIGS film. The semiconductor layer 14 serving as a window layer includes Zn
O and has a substantially equal band gap (about 3.2 eV), electron affinity using a semiconductor layer is chi _1. Each film thickness is CI
GS is 2 μm, and Cu (In, Ga) which is a surface semiconductor layer
_{The 3} Se ₅ layer is 20 nm, and the window layer is 0.1 μm.

Then, in order to investigate the effect of the offset (χ ₂ −χ ₁ ) between the conduction band of the semiconductor layer 13 and the conduction band of the semiconductor layer 14, the difference in electron affinity between the semiconductor layer 13 and the semiconductor layer 14 was changed. Calculation of solar cell characteristics was performed. In the calculation, Cu (In, Ga) ₃ which is the surface semiconductor layer 13a is used.
Assuming that a defect is present at the interface between the Se ₅ layer and the semiconductor layer 14, recombination occurs at the defect.

The calculation results are shown in FIGS. 7A shows the short-circuit current density (J _SC ), FIG. 7B shows the open-circuit voltage (V _OC ), FIG. 8A shows the fill factor (FF), and FIG. 8B shows the conversion efficiency (Efficiency). Is shown.

First, the conduction band offset (Conduc)
tion Band Offset) is negative, that is, focusing on the case the electron affinity of the window layer is greater than the electron affinity of the CIGS film, J _SC is decreased gradually as the offset increases in the negative, the decrease rate is small. On the other hand, V _OC and FF decrease sharply when the offset becomes negatively large. This is because, when the offset becomes negative, the injected carrier stays at the interface between the window layer and the light absorbing layer for a longer time, and recombination via defects present at the interface increases. Next, when the offset is positive, that is, when the electron affinity of the window layer is smaller than the electron affinity of the light absorbing layer, V _OC slightly decreases with an increase in the offset. On the other hand, J _SC and FF sharply decrease when the offset becomes 0.5 eV or more. This is because the offset is 0.5 eV
This is because the window layer becomes a barrier for photoexcited electron transport in the above case, and electrons do not flow. From the above,
The offset between the conduction band of the window layer and the conduction band of the light absorption layer is set to 0.
It can be seen that a solar cell with high characteristics can be obtained by setting it to 5 eV or less.

Here, the actual window layer material and the offset of the conduction band of the CIGS film will be examined. When a CdS film is used for the window layer, the offset is 0.2 eV to 0.3 eV,
It is within the range where high conversion efficiency can be obtained. In contrast, when ZnO is used for the window layer, the offset is about -0.2 e.
V, which is about 70% lower than when CdS was used.

Here, what is important is that electrons in the light absorbing layer are important.
Not the absolute value of the affinity or electron affinity of the window layer, but the difference
It is. Therefore, a solar cell with high conversion efficiency can be constructed.
To achieve this, the electron affinity is χ_Two(EV) for the light absorbing layer
Then, 0 ≦ (χ_Two−χ₁) <0.5 (preferably, 0 ≦
(Χ_Two−χ₁) <0.4) electron affinity that satisfies the relationship χ ₁
It is necessary to select a window layer having For example, CIG
Changing the Ga concentration of the S film increases the forbidden band width, and
The electron affinity decreases. Therefore, sunlight is most efficient
With a forbidden band width that converts to electrical energy
Electron affinity difference between S film and 0 eV-0.5 eV
By using a window layer that is
Can be

Here, a CIGS film in which n-type Cu (In, Ga) ₃ Se ₅ is thinly formed on the surface is assumed as a p-type semiconductor to be a light absorbing layer, but is covered with the n-type CIGS film. Similar results were obtained when a p-type CIGS film or a p-type CIGS film whose surface was covered with a high-resistance Cu (In, Ga) S ₂ layer was used.

Example 2 In Example 2, first, a method of manufacturing and a characteristic of a Zn _{1 -X} Mg _X O film serving as a semiconductor layer 14 (window layer) will be described. The Zn _1-x Mg _x O film was formed by dual simultaneous sputtering of a ZnO target and an MgO target. The composition ratio of Zn and Mg was controlled by changing the high frequency power applied to the two targets. From the X-ray diffraction measurement of the produced Zn _{1- X} Mg _X O film, X was found to be 0.
3 until (Zn _0.7 Mg _0.3 O) is a single phase strongly oriented to c-axis, X until _0.5 (Zn 0.5 Mg 0.5 O) is, Z
Diffraction based on the structure of nO was strongly observed. In the case of configuring an electronic device, it is generally advantageous to use a single-phase semiconductor or dielectric because current or voltage loss is small. Therefore, Zn _1-X
When the Mg _X O film is used, the composition ratio is 0 <X <
A range of 0.5 is preferred.

Next, the relationship between the light absorption coefficient and the photon energy was calculated by measuring the transmittance of Zn _1-x Mg _x O films having different composition ratios. FIG. 9 shows the calculated result.
Shown in In FIG. 9, υ represents the frequency of incident light, and α
Represents a light absorption coefficient. The optical band gap can be determined from an extrapolation line of the data plotted for the film at each composition ratio. The optical band gap of ZnO is about 3.24 eV, and the optical band gap increases as the content of Mg increases. Since the electron affinity decreases due to the increase in the band gap, it can be seen that the electron affinity can be controlled by changing the Mg content.

Therefore, Zn_1-XMg_XMg content of O film
The electron affinity of the CIGS film and Zn_1-X
Mg_XThe change in difference from the electron affinity of the O film was calculated. Calculation
The results are shown in FIG. The calculation was performed as follows. Ma
Instead, using photoelectron spectroscopy (XPS), Zn_1-XM
g_XFor each of the O film and CIGS film,
Bell and the maximum of the valence band (Value Band M
maximum) (E) ^VBM _CL) Was measured. next,
From the measurement results, Zn_1-XMg_XCore of O film and CIGS film
Level difference (△ E_CL). Use these values as
By applying to (1), Zn_1-XMg_XO film and CI
Level difference of valence band from GS film ΔE_V(Valence
 Band Offset) is required. Then,
According to the equation (2), Zn_1-XMg_XO film and CIGS film
Electron affinity difference ΔE_CIs required. Note that Zn_1-XM
g_XBand gap Eg (ZnMgO) and CIG of O film
The band gap Eg (CIGS) of the S film is the light transmission characteristic.
Or reflection characteristics, and the solar cell's
It can be measured from the change in quantum efficiency. (1) △ E_V= E^VBM _CL(CIGS) -E^VBM _CL(Zn
MgO)-△ E_CL (2) △ E_C= Eg (ZnMgO) -Eg (CIG
S)-△ E_V Here, the difference in the electron affinity using the XPS measurement was calculated.
The calculation method was shown, but photoelectron spectroscopy using ultraviolet light
It can also be calculated using (UPS). UPS method
When using, the conduction band level can be measured.
The difference in child affinity can be calculated directly.

Next, the characteristics of the solar cell when the Mg content was changed were examined.

In Example 2, the solar cell 20 of Embodiment 2 was used.
A description will be given of an example of actually manufacturing the. Example 2
The solar cell of the present invention has a surface semiconductor layer 1 on the surface of the semiconductor layer 13.
Cu (In, Ga) Se containing Cd as 3a
_{It has two} layers. In the solar cell of Example 2, a Zn _1-X Mg _X O film was used as the semiconductor layer 14 (window layer).

In Example 2, first, Mo was placed on a glass substrate.
An electrode film was formed, and a Cu (In _0.8 Ga _0.2 ) Se ₂ (CIGS) film serving as the semiconductor layer 13 (light absorbing layer) was formed thereon. The Mo film and the Cu (In, Ga) Se ₂ film are:
Created by the following method (Japanese Journal
al of Applied Physics, 199
5 years, 34 volumes, L1141). First, the Mo film is Ar
It was formed by a sputtering method in a gas atmosphere. The thickness was about 1 μm. Next, a Cu (In, Ga) Se ₂ film was formed by a three-stage evaporation method. First, as the first stage,
(In, Ga) ₂ S at a substrate temperature of 350 ° C. on the Mo film
e ₃ film is formed, and then, as a second step, a substrate temperature of 50
The temperature was raised to 0 ° C. or higher, and Cu and Se were deposited to form a Cu (In, Ga) Se ₂ film having a Cu excess composition. Finally, as a third step, while maintaining the substrate temperature, In, Ga and Se were simultaneously vapor-deposited to form a Cu (In, Ga) Se ₂ film having a composition with a slight excess of (In, Ga). Cu
The thickness of the (In, Ga) Se ₂ film was about 2 μm.

Next, the CIGS film is immersed in an aqueous solution of cadmium nitrate and ammonia, and the surface of the CIGS film is
A surface semiconductor layer made of Cu (In, Ga) Se ₂ doped with Cd was formed. Next, a Zn (O, S) film (thickness: 10 nm) serving as the semiconductor layer 21 (buffer layer) was formed on the semiconductor layer 13 by a chemical deposition method. This Zn
The electron affinity of the (O, S) buffer layer is smaller than the electron affinity of the CIGS film by 0.5 eV or more, and the conduction band level of the Zn (O, S) film is at an energy position higher than the conduction band level of the CIGS film.

After that, Zn ₁ -XM having different Mg contents as a semiconductor layer 14 (window layer) on the Zn (O, S) film.
g _X O film (thickness 0.1 [mu] m) was formed. Zn _1-X Mg _X
The O film was formed by the method described above. Then, the semiconductor layer 14
On top of this, an ITO film (film thickness 0.1 μm) is formed as the upper electrode film 15.
m) was formed by a sputtering method. Further, on the upper electrode film 15, the extraction electrode 17 and the antireflection film 16 are formed.
MgF ₂ (film thickness 0.12 μm) was formed by electron beam evaporation. Thus, the solar cell 20 was produced.

FIG. 11 shows current-voltage characteristics when a Zn _0.97 Mg _0.03 O film in which X is 0.03 is used for the window layer.
At this time, 16.0% was obtained as the conversion efficiency. This value is comparable to the efficiency of a solar cell using a CdS film as a window layer. Next, FIG. 12 shows a change in the conversion efficiency with respect to the Mg content. The addition of Mg improves the efficiency of a solar cell using only ZnO by about 30%. The difference (χ ₂ −χ ₁ ) between the electron affinity χ ₁ (eV) of the ZnO film used here and the electron affinity χ ₂ (eV) of the CIGS film is about −0.1.
(EV), and since the conduction band level of the window layer ZnO is low, recombination increases at the interface of ZnO / Zn (O, S). Therefore, the conversion efficiency of the solar cell is decreasing. On the other hand, when Mg is added, the band gap increases and the electron affinity decreases as shown in FIG. 9, so that the offset (χ ₂ −χ ₁ ) from the CIGS film becomes positive and the interface recombination speed decreases. And the conversion efficiency is improved. In the solar cell of Example 2, the efficiency hardly changes even if the Mg content is increased to around 0.2. This is probably because the value of the offset (χ ₂ −χ ₁ ) is within the range of 0.5 eV or less. As described above, it was found that the conversion efficiency of the solar cell can be improved by using the Zn _1-x Mg _x O film capable of controlling the electron affinity for the window layer.

In the second embodiment, the Zn ₁ -x Mg _x O film is formed by the dual simultaneous sputtering method of ZnO and MgO, but any MgO is added in advance (ZnO
+ MgO) using Zn _1-X Mg _X
An O film can be formed. Further, the same effect can be obtained by adding a small amount of impurities such as Al ₂ O ₃ which does not change the crystal structure of ZnO to Zn _1-X Mg _X O. Here, as the semiconductor layer 21 (buffer layer) having a small electron affinity,
Although a Zn (O, S) film which is a high resistance n-type semiconductor was used,
The same effect can be obtained by using a ZnS film or the like having an offset of 1.3 eV or more from the CIGS film. Also, here
Although an extremely thin n-type surface semiconductor layer is formed on the surface of the p-type CIGS film by doping Cd, even when this surface semiconductor layer is not formed, the window is larger than that of a conventional solar cell using a ZnO film. The solar cell of this embodiment using a Zn _1-x Mg _x O film as a layer has higher efficiency.

(Example 3) In Example 3, an example of manufacturing the solar cell 10a of Embodiment 1 will be described.
In the third embodiment, a glass substrate is used as the substrate 11, a Mo film is used as the lower electrode film 12, and a C layer is used as the semiconductor layer 13 (light absorbing layer).
u (In, Ga) Se ₂ , as a semiconductor layer 14 (window layer), a Zn _Y Al _2-2Y O _3-2Y film (0 <
Y <1), ITO as upper electrode film 15, antireflection film 1
6, MgF ₂ was used. Note that Cu (In, Ga) is formed on the surface of the semiconductor layer 13 as a surface semiconductor layer 13 a.
Se ₂ was formed.

In the solar cell of Example 3, since Al ₂ O ₃ has a smaller electron affinity than ZnO, it can be expected that the electron affinity can be controlled by adding Al ₂ O ₃ to the ZnO film. Zn _Y Al _2-2Y O _3-2Y film ZnO
It was formed by dual simultaneous sputtering of a target and an Al ₂ O ₃ target. The composition ratio between Zn and Al was controlled by the high frequency power applied to the two targets. FIG. 13 shows a change in the conversion efficiency of the solar cell with respect to the Al content.
The conversion efficiency on the vertical axis in the figure is normalized by the conversion efficiency when ZnO is used. When the value of the atomic ratio {Al / (Zn + Al)} in the Zn _Y Al _2-2Y O _3-2Y film is up to 0.1,
The conversion efficiency is lower than when a ZnO film to which Al is not added is used. This small amount of Zn _Y Al _2-2Y O _3-2Y film by adding Al is a low resistance, is believed to flow leakage current. Next, Al / (Zn +
When the value of (Al) is 0.2 to 0.7, the efficiency is improved, and the value is almost constant. This is because the electron affinity of the film becomes smaller than that of the CIGS film by increasing the amount of Al ₂ O ₃ added. Next, Al / (Zn + A
When the value of l) is 0.7 or more, the efficiency sharply decreases. This is because the difference (χ ₂ −χ ₁ ) between the electron affinity χ ₁ of the Zn _Y Al _{2 -2 Y} O 3 _{-2 Y} film and the electron affinity χ ₂ of the CIGS film is 0.
Becomes more 5 eV, since the Zn _Y Al _2-2Y O _3-2Y film becomes a barrier, believed to be due to longer flows carriers photoexcited in the CIGS film. Thus, by changing the content of Al, Zn _Y Al _2-2Y O _3-2Y can electron affinity control membrane, Al / (Zn + Al) values in the range of 0.2 to 0.7 It was found that the conversion efficiency could be improved by.

The ratio of the number of atoms in the CIGS film (Ga /
Since the electron affinity of the CIGS film changes by changing (In + Ga)), the range of the Al composition ratio in which the conversion efficiency is improved changes. In the third embodiment,
Was used Zn _Y Al _2-2Y O _{3-2 Y} film as the window _{layer, Zn Y Ga 2-2Y O 3-2Y (} 0 using IIIb group elements instead of Al
The same effect can be obtained in <Y <1) and the like. However, the optimum range of the group IIIb element composition ratio for improving the conversion efficiency differs depending on the element used.

(Example 4) In Example 4, an example of manufacturing the solar cell 30 of Embodiment 3 will be described.
In the solar cell of Example 4, a glass substrate was used as the substrate 11,
Mo film as the lower electrode film 12 and Cu as the semiconductor layer 13
(In, Ga) Se ₂ , Z as the semiconductor layer 14 (window layer)
An n _0.9 Mg _0.1 O film, ITO as the upper electrode film 15, MgF ₂ as the antireflection film 16, and an Al ₂ O ₃ film as the insulating layer 31 (buffer layer) were used. Note that Cu (In, Ga) Se ₂ containing Cd was formed on the surface of the semiconductor layer 13 as the surface semiconductor layer 13 a.

[0080] Here, the semiconductor layer 14 the difference between the electron affinity chi ₂ electron affinity chi ₁ and the semiconductor layer 13 of the (window layer) (light absorbing layer) (χ ₂ -χ ₁₎ is the 0~0.5eV Within range. Further, the electron affinity χ _INS (eV) and the electron affinity χ ₂ (eV) of the insulating layer 31 are (χ ₂ −χ _INS ) ≧ 0.5 (eV).
Satisfy the relationship. Each manufacturing method is the same as that of the second embodiment. Note that the Al ₂ O ₃ film is CIG by electron beam evaporation.
It was formed on the S film.

FIG. 14 shows the change in the conversion efficiency of the solar cell when the thickness of Al ₂ O ₃ is changed. Here, the efficiency on the vertical axis is normalized by the efficiency when the film thickness of Al ₂ O ₃ is 0 (when there is no Al ₂ O ₃ film). Al ₂ O ₃ film thickness of 10 n
The efficiency was highest at about m, the efficiency was reduced by increasing the film thickness, and the output was significantly reduced at 50 nm or more. The reason will be described below. First, when the film thickness of Al ₂ O ₃ is 10
nm, the Al ₂ covering the CIGS film
The coverage of the O ₃ film is low, and the ZnS partially covers the CIGS film surface.
_It is considered that sputtering damage occurs due to the collision of flying acceleration particles or ionized gas molecules when forming the _1-X Mg _X O film, the defect density at the interface of the CIGS film increases, and the conversion efficiency decreases.

Next, the thickness of the insulating layer is larger than 10 nm.
Conversion efficiency decreases when it becomes smaller, and sharply decreases above 50 nm.
I will explain why. Insulator layer with low electron affinity
Al _TwoO_ThreeIs the CIGS film and Zn_1-XMg_XFormed with O film
Pn junction barrier. However, if the film thickness is small,
The rear tunnels through this barrier to the n-type window layer.
In contrast, as the film thickness increases, the barrier thickness increases.
Tunnel current is extremely reduced, and efficiency is reduced. I
Therefore, the electron affinity is higher than that of CIGS.
Using an insulating layer smaller than 0.5 eV as a buffer layer
In this case, the film thickness is preferably 50 nm or less, and
There is a wide range of film thickness.

Note that, instead of using the insulating layer, the high-resistance semiconductor layer (a semiconductor layer whose electron affinity is larger than the electron affinity of the light absorption layer by 0.5 eV or more) described in Embodiment 2 can be used.
Similar results are obtained. In addition, Al ₂ O ₃
Similar results can be obtained by using Ga ₂ O ₃ , Si ₃ N ₄ , SiO ₂ , MgF ₂ or the like instead of.

Example 5 In Example 5, another example of the solar cell 20 described in Embodiment 2 will be described. In Example 5, the change in the conversion efficiency of the solar cell when the semiconductor layer 14 (window layer) was fixed and the electron affinity of the semiconductor layer 13 (light absorption layer) was changed was measured. In the solar cell of Example 5, a glass substrate is used as the substrate 11, a Mo film is used as the lower electrode film 12, and a CuIn (Se _1−X S _x ) ₂ film (0 ≦ X ≦ 1) in which S is dissolved as the semiconductor layer 13 is used. A Zn _0.8 Mg _0.2 O film as the semiconductor layer 14 (window layer), ITO as the upper electrode film 15, and MgF as the antireflection film 16.
_{2. A} ZnS film (10 nm thick) was used as the semiconductor layer 21 (buffer layer). In addition, on the surface of the semiconductor layer 13, Cu containing Cd is used as the surface semiconductor layer 13a.
_{In (Se 1-X S X} ) to form a ₂ layer.

CuInS ₂ has an electron affinity about 0.4 eV smaller than CuInSe ₂ . Therefore, by changing the solid solution rate X of S, the electron affinity of the semiconductor layer 13 changes. FIG. 15 shows a change in conversion efficiency with respect to the solid solution rate X of S. The conversion efficiency on the vertical axis is CuInSe ₂
This is a value normalized by the conversion efficiency when the film (X = 0) is used.

As shown in FIG. 15, the solid solution ratio X of S is 0 to
While it is 0.8, the conversion efficiency hardly changes,
When X> 0.8 or more, the efficiency decreases. This is for the following reason. Window layer Zn _0.8 Mg _0.2 O for CuInSe ₂
The electron affinity of the film is about 0.3 eV smaller. Therefore, X
Is 0.8 or less, the electron affinity of the window layer and the electron affinity of the light absorption layer satisfy the condition of the solar cell of the present invention that can obtain high conversion efficiency. On the other hand, when the solid solution rate X of S increases, the electron affinity of the CuIn (Se _1−X S _X ) ₂ film decreases. At this time, the electron affinity of Zn _0.8 Mg _0.2 O χ ₁ (e
V) and the electron affinity of CuIn (Se _1-X S _X ) ₂ χ ₂ (e
V), the efficiency does not change in a range satisfying the relationship of 0 ≦ χ ₂ −χ ₁ <0.5. However, when the solid solution rate X of S further increases, (χ ₂ −χ ₁ ) <0 (eV), and the efficiency is reduced due to the influence of interface recombination. As can be seen from Examples 1 and 2, when the electron affinity of the light-absorbing layer is reduced, it is necessary to form a window layer having an electron affinity suitable for it. As described above, it is preferable to use a Zn _1-x Mg _x O film whose electron affinity can be controlled as the window layer.

Here, CuIn is used as the light absorbing layer.
Although a (Se _{1 -X} S _X ) ₂ film was used, Cu (In _{1 -X} Ga _X ) was used.
Similar results are obtained with the Se ₂ film (0 ≦ X ≦ 1). In this case, since the electron affinity of CuGaSe ₂ is smaller than that of CuInSe ₂ by about 0.6 eV, the electron affinity greatly changes depending on the Ga solid solubility X. However, Cu (In
_{Even if the X of the 1-X} Ga _X ) Se ₂ film changes, high conversion efficiency can be obtained by applying a window layer having an electron affinity suitable for it. Similarly, Cu (In _1-x Ga _x ) (S
In the e _1-Y S _Y ) ₂ film, the electron affinity varies depending on the solid solution rate X of Ga and the solid solution rate Y of S. By preparing a window layer having an electron affinity suitable for it, the conversion efficiency is improved. Solar cell with high Furthermore, even in a graded light absorption layer in which the solid solution rate of Ga and the solid solution rate of S change in the film thickness direction, the electron affinity is higher than the electron affinity of the light absorption layer in the depletion layer. Similarly, high conversion efficiency can be obtained by using a window layer having a smaller value of 0.5 eV.

Although the embodiments of the present invention have been described above by way of examples, the present invention is not limited to the above embodiments, but can be applied to other embodiments based on the technical idea of the present invention.

For example, in the above-described embodiment, a solar cell that generates power by light incident from the side opposite to the substrate is described, but a solar cell that generates power by light incident from the substrate side may be used.

[0090]

As described above, according to the first solar cell of the present invention, the forbidden band width and the electron affinity of the first semiconductor layer (window layer) and the second semiconductor layer (light absorbing layer) are obtained. By defining the relationship, a solar cell with high conversion efficiency can be obtained. This is because by using a window layer having an electron affinity within the above range, recombination at the junction interface can be suppressed, and a window layer and a light absorption layer that do not act as a barrier for photocarriers can be provided. Further, according to the first solar cell, a solar cell having high conversion efficiency without using CdS as the window layer can be obtained.

In the second solar cell of the present invention, the window layer has the general formula Zn _{1 -Z} CzO (where the element C is Be, Mg,
It is at least one selected from Ca, Sr and Ba, and 0 <Z <1. ), The window layer has few defects, and the band gap and the electron affinity of the window layer can be freely changed. Therefore, a solar cell with high conversion efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one example of a solar cell of the present invention.

FIG. 2 is a cross-sectional view showing another example of the solar cell of the present invention.

FIG. 3 is a schematic diagram showing an example of a band diagram of the solar cell shown in FIG.

FIG. 4 is a schematic view showing an example of a band diagram of a conventional solar cell.

FIG. 5 is a sectional view showing another example of the solar cell of the present invention.

FIG. 6 is a sectional view showing still another example of the solar cell of the present invention.

FIG. 7 is a graph showing (a) a short-circuit current density and (b) an open-circuit voltage for an example of the solar cell of the present invention.

FIG. 8 is a graph showing (a) a fill factor and (b) a conversion efficiency of an example of the solar cell of the present invention.

FIG. 9 shows Zn _1-X Mg _X O films having different composition ratios.
5 is a graph showing a relationship between a light absorption coefficient and photon energy.

Is a graph showing the change in the difference between the electron affinity of the electron affinity and the CIGS film of Zn _1-X Mg _X O film when [10] varying Mg content of Zn _1-X Mg _X O film .

FIG. 11 shows an example of the current-
5 is a graph showing voltage characteristics.

FIG. 12 is a graph showing a change in conversion efficiency with respect to a change in Mg content of a Zn _1-X Mg _X O film.

FIG. 13 is a graph showing a change in normalized conversion efficiency when the value of Y in the Zn _Y Al _2-2Y O _3-2Y film is changed.

FIG. 14 is a graph showing a change in normalized conversion efficiency when the thickness of an Al ₂ O ₃ film serving as a buffer layer is changed.

FIG. 15 shows a light absorption layer of CuIn (Se _1-X S _X ) ₂
5 is a graph showing a change in normalized conversion efficiency when a solid solution rate X of a film is changed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10, 10a, 20, 30 Solar cell 11 Substrate 12 Lower electrode film 13 Semiconductor layer (second semiconductor layer) 13a Surface semiconductor layer 14 Semiconductor layer (first semiconductor layer) 15 Upper electrode film 16 Antireflection film 17 Extraction electrode 21 semiconductor layer 31 insulating layer

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Shigeo Hayashi 1006 Kazuma Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. F-term (reference) 5F051 AA07 AA10 DA03 DA11

Claims (19)

[Claims] An n-type first semiconductor layer and a p-type second semiconductor layer, wherein the first semiconductor layer and the second semiconductor layer form a pn junction A solar cell, wherein the first semiconductor layer does not contain Cd, the second semiconductor layer is a light absorbing layer, a forbidden band width Eg ₁ of the first semiconductor layer, and the second semiconductor layer of the forbidden band width Eg ₂ is met Eg _1> Eg ₂ of relationship, the first electron affinity chi _first semiconductor layer and (eV) and the second electron affinity chi ₂ (eV) of the semiconductor layer is , 0 ≦ (χ ₂
-Χ ₁ ) A solar cell characterized by satisfying the relationship of <0.5. 2. The solar cell according to claim 1, wherein the first semiconductor layer is formed on a light incident side of the second semiconductor layer. 3. A semiconductor device further comprising a third semiconductor layer between the first semiconductor layer and the second semiconductor layer, wherein a forbidden band width Eg ₃ and a forbidden band width Eg of the _third semiconductor layer are provided.
g ₂ is claim 1 or 2 satisfying the relation of Eg _3> Eg ₂
The solar cell according to 1. 4. The solar cell according to claim 3, wherein the third semiconductor layer is made of one of an n-type semiconductor and a high-resistance semiconductor. 5. The electron affinity χ of the third semiconductor layer.
₃ (eV) and the electron affinity χ ₂ are (χ ₂ −χ ₃ ) ≧
The solar cell according to claim 3, wherein the relationship of 0.5 is satisfied, and the thickness of the third semiconductor layer is 50 nm or less. 6. The method according to claim 1, wherein the third semiconductor layer is formed of Zn and III.
The solar cell according to claim 5, which is an oxide containing at least one element selected from group b elements or a chalcogen compound containing at least one element selected from Zn and group IIIb elements. 7. An insulating layer is further provided between the first semiconductor layer and the second semiconductor layer, wherein the forbidden band width Eg _INS and the forbidden band width Eg _{2 of the} insulating layer are Eg _INS. > the solar cell according to claim 1 or 2 satisfying the relation of Eg _2. 8. The electron affinity of the insulating layer χ _INS (eV)
8. The solar cell according to claim 7, wherein the electron affinity χ ₂ satisfies the relationship of (膜厚₂ −χ _INS ) ≧ 0.5, and the thickness of the insulating layer is 50 nm or less. 9. 9. The method according to claim 1, wherein the insulating layer is made of Al ₂ O ₃ , Ga ₂ O ₃ , S
i ₃ N _4, a solar cell according to claim 8 consisting of SiO _2, at least one insulating material selected from MgF ₂ and MgO. 10. The solar cell according to claim 1, wherein the second semiconductor layer further includes an n-type semiconductor layer or a high-resistance semiconductor layer on a surface on the first semiconductor layer side. 11. The method according to claim 11, wherein the second semiconductor layer comprises a group Ib element,
The solar cell according to any one of claims 1 to 10, which is a compound semiconductor layer containing a group IIIb element and a group VIb element. 12. The solar cell according to claim 1, wherein the first semiconductor layer is made of a compound containing Zn. 13. The solar cell according to claim 12, wherein the compound is an oxide containing Zn and a Group IIa element, or a chalcogen compound containing Zn and a Group IIa element. 14. The compound represented by the general formula Zn _1-X A _X O
(However, the element A is Be, Mg, Ca, Sr and Ba
13. The solar cell according to claim 12, wherein the main component is an oxide represented by at least one selected from the group consisting of 0 <X <1). 15. The element A is Mg, and X is 0.
The solar cell according to claim 14, which satisfies a relationship of <X <0.5. 16. The solar cell according to claim 12, wherein the compound is an oxide containing Zn and a Group IIIb element, or a chalcogen compound containing Zn and a Group IIIb element. 17. The compound represented by the general formula: Zn _Y B _2-2Y O
_{13. The solar cell according} to claim 12, wherein an oxide represented by _3-2Y (where, the element B is at least one selected from Al, Ga, and In, and 0 <Y <1) is a main component. battery. 18. A solar cell comprising a p-type light absorbing layer and an n-type semiconductor layer laminated on the light absorbing layer, wherein the semiconductor layer has a general formula Zn _{1 -Z} C _Z O (wherein , Element C
Is at least one selected from Be, Mg, Ca, Sr and Ba, and 0 <Z <1. A solar cell comprising an oxide represented by the formula (1) as a main component. 19. The element C is Mg and Z is 0.
The solar cell according to claim 18, wherein a relationship of <Z <0.5 is satisfied.