JP2003197935A - Solar battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar battery capable of reducing any defect in a semiconductor layer functioning as a window layer, and freely changing the forbidden band width of the window layer. <P>SOLUTION: This solar battery is provided with a p-type light absorbing semiconductor layer 13 and an n-type semiconductor layer 14 laminated on the semiconductor layer 13, and the semiconductor layer 14 is constituted of oxide expressed by a general formula; Zn<SB>1-</SB>ZCZO (in this case, the element C is at least one element selected from among Be, Mg, Ca, Sr, and Ba) as main components. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a solar cell,
In particular, for example, it relates to a solar cell in which a compound semiconductor layer containing a group Ib, a group IIIb, and a group VIb is used as a light absorption layer.

[0002]

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

In a solar cell, theoretically, when the forbidden band width of the light absorption layer is within 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, it is possible to control the forbidden band width 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 width is 1.4 eV to 1.5 eV.

[0004]

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

In addition, the high conversion efficiency C currently reported
The IGS solar cell has a heterojunction between a CdS film that is a window layer and a CIGS film that is a light absorption layer. On the other hand, in recent years, a CIGS solar cell that does not use CdS has been drawing attention from the viewpoint of environmental pollution, and ZnO is used as a window layer instead of CdS.
Some CIGS solar cells using a system semiconductor have been reported. However, there is a problem that the conversion efficiency is lower than that 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 problems, an object of the present invention is to provide a solar cell which has few defects in a semiconductor layer which functions as a window layer and which can freely change the forbidden band width of the window layer. .

[0007]

In order to achieve the above object, the solar cell of the present invention is a solar cell comprising a p-type light absorption layer and an n-type semiconductor layer laminated on the light absorption layer. And the semiconductor layer has the general formula Zn _1-Z C _Z O
The element C is at least one selected from Be, Mg, Ca, Sr and Ba, and 0 <Z <1. ) Is a main component of an oxide represented by. Second above
In the above solar cell, since there are few defects in the semiconductor layer functioning as the window layer and the band gap of the window layer can be freely changed, a solar cell with high conversion efficiency can be obtained.

In the above solar cell, it is preferable that the element C is Mg and the Z satisfies the relation of 0 <Z <0.5.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) In Embodiment 1, an example of the solar cell of the present invention will be described. In the first embodiment,
An example of a solar cell that generates an electromotive force by light incident from the side opposite to the substrate will be described.

FIG. 1 shows a sectional view of the solar cell of the first embodiment. With reference to FIG. 1, the solar cell 1 of Embodiment 1
Reference numeral 0 denotes the substrate 11, the lower electrode film 12, the semiconductor layer 13 (second semiconductor layer), the semiconductor layer 14 (first semiconductor layer), the upper electrode film 15, and the antireflection film, which are sequentially stacked 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.

The lower electrode film 12 may be a metal film made of Mo, for example.

The semiconductor layer 13 (second semiconductor layer) is a semiconductor layer that functions as a light absorption layer, and is a p-type semiconductor layer. The semiconductor layer 13 is arranged on the back surface side of the semiconductor layer 14. For the semiconductor layer 13, for example, a compound semiconductor layer containing an Ib group element, a IIIb group element and a VIb group element can be used. For example, CuInSe ₂ , C
u (In, Ga) Se ₂ , CuInS ₂ , Cu (In, G
a) S ₂ or the like can be used. The semiconductor layer 1
3 may have a surface semiconductor layer 13a on the surface on the side of the semiconductor layer 14 (the same applies in the following embodiments). A cross-sectional view of the solar cell 10a including the surface semiconductor layer 13a is shown in FIG. 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 the high resistance semiconductor layer, for example, Cu
An In ₃ Se ₅ and _{Cu (In, Ga) 3 Se} 5 and the like.

The semiconductor layer 14 (first semiconductor layer) is a layer that 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 include Cd (does not include as a constituent element or a dopant). For the semiconductor layer 14, for example, a compound containing Zn can be used. Examples of the compound containing Zn include oxides or chalcogen compounds containing Zn and a group IIa element, or 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 (provided that the element A is Be, Mg, Ca, Sr
And at least one selected from Ba and 0 <X
<1. ) Is the main component (containing 9
0 wt% or more) can be used. In this case, the content of the element A is preferably 0.1 at% or more. Further, an oxide represented by the general formula Zn _Y B _2-2Y O _3-2Y (where the element B is at least one selected from Al, Ga and In, and 0 <Y <1) is used. A compound having a main component (content of 90 wt% or more) may be used. In this case, the content rate of the element B is 5 at%
The above is preferable.

In the solar cell 10 of Embodiment 1, the semiconductor layer
13 forbidden band width Eg₂And the forbidden band width Eg of the semiconductor layer 14₁When
But Eg₁> Eg₂Meet the relationship. In addition, the semiconductor layer 13
Electron affinity χ₂(EV) and electron affinity of the semiconductor layer 14
χ₁(EV) is 0 ≦ (χ ₂−χ₁) <0.5 relationship
Fulfill.

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

The antireflection film 16 is a film that prevents incident light from being reflected at the interface of the upper electrode film 15. When the upper electrode film 15 is ITO or ZnO: Al, for example, MgF ₂ is used. be able to.

For the take-out electrode 17, for example, NiCr is used.
A metal film in which a film and an Au film are stacked can be used.

Next, an example of a method of 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 a vapor deposition method. After that, the semiconductor layer 13 is formed on the lower electrode film 12 by, for example, a vapor deposition method or a sputtering method. After that, the semiconductor layer 14 is formed on the semiconductor layer 13 by, for example, a chemical deposition method or a sputtering method. After that, the upper electrode film 15 is formed on the semiconductor layer 14 by, for example, a sputtering method. After that, 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, a vapor deposition method. In this way, 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 dipping method, a vapor deposition method or a vapor phase diffusion method.

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

Next, CI is applied to the semiconductor layer 13 which is a light absorption layer.
The function of the solar cell 10 will be described by taking a solar cell using a GS film as an example.

A method of increasing the forbidden band width of the CIGS film is an effective method for improving the efficiency of the solar cell using the CIGS film as the light absorption layer. However, in a conventional solar cell having a window layer made of CdS, When the band gap of the film is increased to 1.3 eV or more, the efficiency is lowered contrary to the theory. As one of the causes of this, a CIGS film which is a light absorption layer and a Cd which is a window layer
It has been pointed out that the energy difference (offset) of the conduction band at the heterojunction with the S film. Herberholtz (EH
erberholz) and the like are CdS when the atomic number ratio in the CIGS film is {Ga / (In + Ga)} <0.5.
The discontinuity of the band due to the offset between the conduction band of CGS and the conduction band of CIGS becomes a spike shape in which the conduction band of CdS rises and the conduction band protrudes near the junction, and the atomic number ratio {Ga /
For (In + Ga)}> 0.5, we have proposed a cliff-shaped model in which the conduction band of the CIGS film rises and a step is formed between the conduction band of CdS and the conduction band of CIGS (Solar Energy Materials and Solar
CELLS, Solar Energy Material
s and Solar Cells, 1997, 49, No. 3, 227). FIG. 4A shows a band diagram when the offset between CdS and CIGS has a spike shape.
FIG. 4B shows a band diagram when the offset between CdS and CIGS has a cliff shape. In this model, it is suggested that the band discontinuity of the conduction band becomes cliff-like, which increases the recombination at the heterojunction interface and in the vicinity of the interface, and decreases the conversion efficiency. C like this phenomenon
Considering the case where the forbidden band width of the IGS film is 1.2 eV to 1.3 eV, when the CdS film that is the window layer is replaced with the ZnO film, the band discontinuity of the conduction band between the ZnO film and the CIGS film is considered. Is expected to have a cliff shape in which the conduction band of the CIGS film is lifted.

Such band discontinuity in the conduction band of the heterojunction is caused by the difference between the electron affinity of the window layer and the electron affinity of the CIGS film which is the light absorption layer. In general, the band gap is different from n-type semiconductor and the p-type semiconductor, when the respective electron affinity and chi _n and chi _p, the chi _n <chi _p, the discontinuity of the conduction bands is spiky. On the other hand, when χ _n > χ _p , the conduction band discontinuity becomes a cliff shape. Where Ga
The relationship of electron affinity between the CuInSe ₂ film not containing CdS and the CdS film is such that CdS is smaller by about 0.2 eV to 0.3 eV. Therefore, when a heterojunction is formed, a spike occurs on the CdS side. However, as the Ga concentration increases, the electron affinity of CIGS decreases. Therefore, above a certain Ga concentration, the value becomes smaller than the electron affinity of CdS, and when a heterojunction is formed, C
Cliffs occur on the IGS side.

The 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, Z
Since nO has an electron affinity larger than that of CdS by about 0.4 eV, when a heterojunction is formed even in a GaIn-free CuInSe ₂ film, a cliff may occur and a loss may occur.

Furthermore, when the electron affinity of the window layer is smaller than that of the light absorption 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 becomes 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 is a barrier for photoexcited carriers. In this case, carrier transportation is hindered, and photocurrent cannot be taken out to the outside. Therefore, the conversion efficiency is reduced. As described above, when spikes occur in the conduction band of the window layer and the light absorption layer, it can be seen that there is an optimum range of energy difference (offset) with which high efficiency can be obtained. The solar cell of Embodiment 1 is
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 optimum range. Therefore, the solar cell 10 of the first embodiment
Then, 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 solar cell which does not use CdS in the window layer and has high efficiency can be obtained. In the above embodiment, the case where the first semiconductor layer is arranged on the light incident side of the second semiconductor layer has been described, but the first semiconductor layer is arranged on the back surface 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.

A sectional view of the solar cell 20 of the second embodiment is shown in FIG. The solar cell 20 according to the second embodiment is different from the solar cell 10 according to the first embodiment only in that the semiconductor layer 21 is provided, and thus the duplicate description will be omitted.

The semiconductor layer 21 (third semiconductor layer) is arranged between the semiconductor layer 13 and the semiconductor layer 14. The forbidden band width Eg ₃ of the semiconductor layer 21 and the forbidden band width E of the semiconductor layer 13
With g ₂ , the relationship of Eg ₃ > Eg ₂ is satisfied. 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 can be used. Also,
SnO ₂ may be used as the semiconductor layer 21.

The electron affinity of the semiconductor layer 21, χ ₃ (e
V) and the electron affinity χ ₂ of the semiconductor layer 13 are (χ ₂ −
It is preferable to satisfy the relationship of χ ₃ ) ≧ 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) membranes can be used. Where Zn (O, S)
Is a compound consisting of Zn, O and S, and is Zn-O
It means 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 highly efficient 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.

A cross-sectional view of the solar cell 30 of the third embodiment is shown in FIG. The solar cell 30 according to the third embodiment is different from the solar cell 10 according to the first embodiment only in that an insulating layer 31 is provided, and thus redundant description will be omitted.

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 . Examples of the insulating layer 31 include Al ₂ O ₃ and 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 −χ _INS ) ≧
It is preferable to satisfy the relationship of 0.5. Also, the insulating layer 3
The film thickness of 1 is preferably 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 a vapor deposition method.

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

[0041]

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

In the calculation of Example 1, a Cu (In, Ga) Se ₂ film (CIGS film) having a forbidden band width Eg ₂ of 1.2 eV and an electron affinity of χ ₂ is used for the semiconductor layer 13 which is a light absorption layer. I was there. It was calculated that a Cu (In, Ga) ₃ Se ₅ layer as the surface semiconductor layer 13a was formed on the surface of this CIGS film. In addition, Zn
A semiconductor layer having a band gap (approximately 3.2 eV) substantially equal to O and an electron affinity of χ ₁ was used. Each film thickness is CI
Cu (In, Ga) that is a surface semiconductor layer with GS of 2 μm
_{The 3} Se ₅ layer has a thickness of 20 nm and the window layer has a thickness of 0.1 μm.

Then, in order to investigate the influence 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 is changed. The solar cell characteristics were calculated. In the calculation, Cu (In, Ga) ₃ which is the surface semiconductor layer 13a is used.
A defect is assumed at the interface between the Se ₅ layer and the semiconductor layer 14, and recombination occurs at the defect.

The calculation results are shown in FIGS. 7 and 8. 7A is a short-circuit current density (J _SC ), (b) is an open circuit voltage (V _OC ), FIG. 8A is a fill factor (FF), and (b) is a conversion efficiency (Efficiency). Shows.

First, the conduction band offset (Conduc)
When the band affinity is negative, that is, the electron affinity of the window layer is larger than the electron affinity of the CIGS film, J _SC gradually decreases as the offset increases negatively, but the decrease rate is small. On the other hand, V _OC and FF sharply decrease when the offset becomes negatively large. This is because when the offset becomes negative, the injected carriers stay longer at the interface between the window layer and the light absorption layer, and recombination via defects existing 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 absorption layer, V _OC decreases slightly as the offset increases. On the other hand, J _SC and FF sharply decrease when the offset becomes 0.5 eV or more. This has an offset of 0.5 eV
This is because, in the above case, the window layer becomes a barrier for photoexcited electron transport 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 having excellent characteristics can be obtained by setting the voltage to 5 eV or less.

Here, the offset of the conduction band between the actual window layer material and the CIGS film will be examined. When the 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 is obtained. On the other hand, when ZnO is used for the window layer, the offset is about -0.2e.
It can be seen that the value becomes V, which is reduced to about 70% when CdS is used.

Here, it is important that the electrons in the light absorption layer are
The absolute value of the affinity or electron affinity of the window layer, not the difference
Is. Therefore, a solar cell with high conversion efficiency is constructed.
The electron affinity is χ₂(EV) to the light absorption layer
Then 0 ≦ (χ₂−χ₁) <0.5 (preferably 0 ≦
(Χ₂−χ₁) <0.4) satisfying the electron affinity 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 is reduced. Therefore, the sunlight is most efficient
CIG having a forbidden band width that is converted into electrical energy
Electron affinity difference is 0 eV to 0.5 eV or less for S film
A highly efficient solar cell can be obtained by using a window layer that is
To be

Although a CIGS film having n-type Cu (In, Ga) ₃ Se ₅ thinly formed on the surface is assumed here as the p-type semiconductor serving as the light absorption layer, it 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 a Zn _{1 -X} Mg _X O film to be the semiconductor layer 14 (window layer) and its characteristics will be described. The Zn _1-X Mg _X O film was formed by binary co-sputtering of a ZnO target and a 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 prepared Zn _{1- X} Mg _X O film, X was 0.
3 (Zn _0.7 Mg _0.3 O) is a single phase strongly oriented along the c-axis, and X is 0.5 (Zn _0.5 Mg _0.5 O) up to Z.
Diffraction based on the structure of nO was strongly observed. When constructing an electronic device, it is generally advantageous to use a single-phase semiconductor or a dielectric because the loss of current or voltage is small. Therefore, Zn _{1 -X} is used in electronic devices.
The composition ratio when using the Mg _x O film 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 the Zn _{1 -X} Mg _X O films having different composition ratios. The calculated result is shown in FIG.
Shown in. In FIG. 9, υ represents the frequency of incident light, and α
Represents the light absorption coefficient. The optical band gap can be obtained from the extrapolation line of the data plotted for the film at each composition ratio. The optical bandgap of ZnO is about 3.24 eV, and the optical bandgap increases as the Mg content increases. Since the electron affinity decreases as the band gap increases, it can be seen that the electron affinity can be controlled by changing the Mg content.

Therefore, Zn_1-XMg_XThe Mg content of the O film
Electron affinity of the CIGS film and Zn when changed_1-X
Mg_XThe change in the difference from the electron affinity of the O film was calculated. Calculation
The results are shown in Fig. 10. The calculation was performed as follows. Well
First, using photoelectron spectroscopy (XPS), Zn_1-XM
g_XFor each of the O film and the CIGS film, the film core
Bell and valence band maximum (Valence Band M
Aximum) difference (E ^VBM _CL) Was measured. next,
From the measurement results, Zn_1-XMg_XCore of O film and CIGS film
Level difference (△ E_CL) Was asked. These values are
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 the following
According to the equation (2)_1-XMg_XO film and CIGS film
Electron affinity difference ΔE_CIs required. Note that Zn_1-XM
g_XBand gap Eg (ZnMgO) of the O film and CIG
The band gap Eg (CIGS) of the S film is a light transmission characteristic.
Of the solar cell depending on
It can be measured from the change in quantum efficiency. (1) △ E_V= E^VBM
_CL(CIGS) -E^VBM _CL(ZnMgO) -ΔE
_CL(2) △ E_C= Eg (ZnMgO) -Eg (CIG
S)-△ E_VIn addition, here, an electron using XPS measurement is used.
I showed how to calculate the difference in affinity.
It can also be calculated using spectroscopy (UPS)
It When using the UPS method, the conduction band level can be measured.
Therefore, the difference in electron affinity can be calculated directly.
It

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

In Example 2, the solar cell 20 of Embodiment 2 was used.
An example of the actual fabrication of will be described. In addition, Example 2
In the solar cell of, the surface semiconductor layer 1 is formed on the surface of the semiconductor layer 13.
Cu (In, Ga) Se containing Cd as 3a
_{Has two} layers. In addition, 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 formed on the glass substrate.
An electrode film was formed, and a Cu (In _0.8 Ga _0.2 ) Se ₂ (CIGS) film to be the semiconductor layer 13 (light absorption layer) was formed thereon. The Mo film and the Cu (In, Ga) Se ₂ film are
It was created by the following method (Japanese Journal)
al of Applied Physics, 199
5 years, 34 volumes, see L1141). First, the Mo film is Ar
It was formed by a sputtering method in a gas atmosphere. The film thickness was about 1 μm. Next, a Cu (In, Ga) Se ₂ film was formed by a three-step evaporation method. First, as the first stage,
(In, Ga) ₂ S on the Mo film at a substrate temperature of 350 ° C.
e ₃ film is formed, and then, as the second step, the substrate temperature is set to 50
The temperature was raised to 0 ° C. or higher and Cu and Se were vapor-deposited to form a Cu (In, Ga) Se ₂ film having an excessive Cu composition. Finally, in the 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 slight excess of (In, Ga). Cu
The thickness of the (In, Ga) Se ₂ film was about 2 μm.

Next, the CIGS film is dipped in an aqueous solution of cadmium nitrate and ammonia to form a film on the surface of the CIGS film.
A surface semiconductor layer made of Cu (In, Ga) Se ₂ doped with Cd was formed. Next, a Zn (O, S) film (film thickness 10 nm) to be the semiconductor layer 21 (buffer layer) was formed on the semiconductor layer 13 by the 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 -X} M having different Mg contents is formed as a semiconductor layer 14 (window layer) on the Zn (O, S) film.
A g _x O film (film thickness 0.1 μm) was formed. Zn _1-X Mg _X
The O film was formed by the method described above. Then, the semiconductor layer 14
An ITO film (having a film thickness of 0.1 μm) is formed on the upper electrode film 15 as an upper electrode film 15.
m) was formed by the sputtering method. Further, the extraction electrode 17 and the antireflection film 16 are formed on the upper electrode film 15.
Of MgF ₂ (film thickness 0.12 μm) was formed by electron beam evaporation. In this way, the solar cell 20 was manufactured.

FIG. 11 shows current-voltage characteristics when a Zn _0.97 Mg _0.03 O film having X of 0.03 is used as the window layer.
At this time, a conversion efficiency of 16.0% was obtained. This value is comparable to the efficiency of a solar cell using a CdS film for the window layer. Next, FIG. 12 shows the change in conversion efficiency with respect to the Mg content. The addition of Mg improves the efficiency of the 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 ZnO / Zn (O, S) interface. Therefore, the conversion efficiency of the solar cell is reduced. On the other hand, when Mg is added, the band gap increases and the electron affinity decreases as shown in FIG. 9, so the offset (χ ₂ −χ ₁ ) with the CIGS film becomes positive and the interface recombination rate decreases. And the conversion efficiency is improved. In the solar cell of Example 2, the efficiency hardly changed even if the Mg content was increased to around 0.2. It is considered that this is 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 in the window layer.

In Example 2, the Zn _1-x Mg _x O film was formed by the binary simultaneous sputtering method of ZnO and MgO, but any MgO was added in advance (ZnO
+ MgO) sintered body target, 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 do not change the crystal structure of ZnO to Zn _1-X Mg _X O. Further, here, as the semiconductor layer 21 (buffer layer) having a small electron affinity,
A Zn (O, S) film, which is a high resistance n-type semiconductor, is used.
The same effect can be obtained by using a ZnS film having an offset of 1.3 eV or more with respect to 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 with 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 example using the Zn _1-x Mg _{x 0} film as the layer has higher efficiency.

Example 3 In Example 3, an example of producing the solar cell 10a of the first embodiment will be described.
In Example 3, the substrate 11 is a glass substrate, the lower electrode film 12 is a Mo film, and the semiconductor layer 13 (light absorbing layer) is C.
u (In, Ga) Se ₂ , a semiconductor layer 14 (window layer) as a Zn _Y Al _2-2Y O _3-2Y film (0 <
Y <1), ITO as the upper electrode film 15, antireflection film 1
MgF ₂ was used as 6. In addition, on the surface of the semiconductor layer 13, as the surface semiconductor layer 13a, Cu (In, Ga)
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 is ZnO
The target and the Al ₂ O ₃ target were formed by two-source simultaneous sputtering. The composition ratio of Zn and Al was controlled by the high frequency power applied to the two targets. The change in conversion efficiency of the solar cell with respect to the Al content is shown in FIG.
The conversion efficiency on the vertical axis of the figure is standardized by the conversion efficiency when ZnO is used. When the value of the atomic number 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 that when a ZnO film not added with Al is used. This is _presumably because the Zn _Y Al _2-2Y O _3-2Y film has a low resistance due to the addition of a small amount of Al, and a leakage current flows. Next, Al / (Zn +
When the value of Al) is from 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 as the added amount of Al ₂ O ₃ increases. Next, Al / (Zn + A
When the value of l) is 0.7 or more, the efficiency drops sharply. This, Zn _Y Al _{2 -2Y} O difference between the electron affinity chi ₂ of _3-2Y film electron affinity chi ₁ and the CIGS film (χ ₂ -χ ₁₎ is 0.
It is considered that 5 eV or more and the Zn _Y Al _2-2Y O _3-2Y film _serves as a barrier, and the carriers optically excited in the CIGS film _stop flowing. Thus, by changing the Al content, the electron affinity of the Zn _Y Al _2-2Y O _3-2Y film can be controlled, and the value of Al / (Zn + Al) is within the range of 0.2 to 0.7. It was found that the conversion efficiency can be improved by.

The ratio of the number of atoms in the CIGS film (Ga /
Since the electron affinity of the CIGS film is changed by changing (In + Ga)), the range of the Al composition ratio in which the conversion efficiency is improved is changed. In addition, in Example 3,
Although a Zn _Y Al _2-2Y O _{3-2 Y} film was used as the window layer, Zn _Y Ga _2-2Y O _3-2Y (0
The same effect can be obtained even with <Y <1). However, the optimum range of the composition ratio of the group IIIb element for improving the conversion efficiency depends 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 as the substrate 11,
Mo film as the lower electrode film 12, 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, an ITO film as the upper electrode film 15, a MgF ₂ film as the antireflection film 16, and an Al ₂ O ₃ film as the insulating layer 31 (buffer layer) were used. On the surface of the semiconductor layer 13, Cu (In, Ga) Se ₂ containing Cd was formed as the surface semiconductor layer 13a.

[0063] 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 It is within the 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)
Meet the relationship. Each manufacturing method is the same as that of the second embodiment. In addition, the Al ₂ O ₃ film is formed by CIG by electron beam evaporation.
It was formed on the S film.

FIG. 14 shows changes in conversion efficiency of the solar cell when the film thickness of Al ₂ O ₃ was 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). The film thickness of Al ₂ O ₃ is 10n
The efficiency became the highest at about m, the efficiency decreased as the film thickness increased, and the output significantly decreased at 50 nm or more. The reason for this will be described below. First, the film thickness of Al ₂ O ₃ is 10
When it becomes smaller than nm, Al ₂ which covers the CIGS film
The coverage of the O ₃ film is low and Zn is partially formed on the surface of the CIGS film.
_It is considered that sputtering damage occurs due to collision of flying acceleration particles or ionized gas molecules when forming the _1-X Mg _X O film, the defect density at the CIGS film interface increases, and the conversion efficiency decreases.

Next, the thickness of the insulating layer is larger than 10 nm.
Conversion efficiency decreases, and sharply decreases above 50 nm
Describe the reason for doing so. Insulator layer with low electron affinity
Al ₂O₃Is CIGS film and Zn_1-XMg_XFormed with O film
It becomes a barrier of the pn junction. However, if the film thickness is thin,
The rear tunnels through this barrier to the n-type window layer.
On the other hand, as the film thickness increases, the barrier thickness also increases.
Further, the tunnel current is extremely reduced and the efficiency is reduced. Shi
Therefore, the electron affinity is higher than that of CIGS.
Use an insulating layer smaller than 0.5 eV as a buffer layer
If it is, the film thickness is preferably 50 nm or less, and it is optimal.
There is a wide range of film thickness.

In place of the insulating layer, the high-resistance semiconductor layer described in Example 2 (a semiconductor layer having an electron affinity larger than the electron affinity of the light absorption layer by 0.5 eV or more) may be used.
Similar results are obtained. In addition, as an insulating layer, 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 conversion efficiency of the solar cell was measured when the semiconductor layer 14 (window layer) was fixed and the electron affinity of the semiconductor layer 13 (light absorption layer) was changed. In the solar cell of Example 5, a glass substrate as the substrate 11, a Mo film as the lower electrode film 12, and a CuIn (Se _1-X S _X ) ₂ film (0 ≦ X ≦ 1) in which S is solid-solved as the semiconductor layer 13. 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 (film thickness 10 nm) was used as the semiconductor layer 21 (buffer layer). In addition, on the surface of the semiconductor layer 13, Cu containing Cd was used as the surface semiconductor layer 13a.
An In (Se _1-X S _X ) ₂ film was formed.

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

As shown in FIG. 15, the solid solution rate X of S is 0 to 0.
While it is 0.8, the conversion efficiency hardly changes,
If 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 smaller by about 0.3 eV. Therefore, X
When 0.8 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 CuIn (Se _1-X S _X ) ₂ electron affinity χ ₂ (e
The efficiency does not change in a range where V) and 0 ≦ χ ₂ −χ ₁ <0.5. However, when the solid solution rate X of S is further increased, (χ ₂ −χ ₁ ) <0 (eV), which is strongly influenced by the interfacial recombination and the efficiency is reduced. As can be seen from Examples 1 and 2, when the electron affinity of the light absorption layer becomes small, it is necessary to form a window layer having an electron affinity suitable for it. Thus, as the window layer, it is preferable to use the Zn _{1 -X} Mg _X O film whose electron affinity can be controlled.

Here, CuIn is used as the light absorption layer.
A (Se _1-X S _X ) ₂ film was used, but Cu (In _1-X Ga _X ) ₂ was used.
Similar results are obtained with a 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 solution rate X. However, Cu (In
_{Even if 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 changes depending on the solid solution rate X of Ga and the solid solution rate Y of S, but the conversion efficiency can be improved by preparing a window layer having an electron affinity suitable for it. A solar cell with high efficiency can be obtained. Further, even in the graded type light absorption layer in which the solid solution rate of Ga and the solid solution rate of S are changed in the film thickness direction, the electron affinity is higher than that of the light absorption layer in the depletion layer. Similarly, high conversion efficiency is obtained by using a window layer having a small value of 0.5 eV.

Although the embodiments of the present invention have been described above with reference to examples, the present invention is not limited to the above-described embodiments and can be applied to other embodiments based on the technical idea of the present invention.

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

[0073]

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 absorption layer). By defining the relationship of, 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, it is possible to suppress recombination at the junction interface and to provide a window layer and a light absorption layer that do not serve as a barrier for photocarriers. Furthermore, according to the said 1st solar cell, the solar cell which does not use CdS as a window layer and has high conversion efficiency is obtained.

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

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an 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 view showing an example of a band diagram of the solar cell shown in FIG.

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

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

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

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

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

FIG. 9 shows Zn _1-X Mg _X O films with different composition ratios.
It is a graph which shows the 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 for an example of the solar cell of the present invention.
It is a graph which shows a voltage characteristic.

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 changes 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 changes in normalized conversion efficiency when the film thickness of an Al ₂ O ₃ film that is a buffer layer is changed.

FIG. 15 is a light absorption layer of CuIn (Se _1-X S _X ) ₂
It is a graph which shows the change of standardization conversion efficiency when changing the solid-solution rate X of a film.

[Explanation of symbols]

10, 10a, 20, 30 Solar cells 11 board 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

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shigeo Hayashi             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. F-term (reference) 5F051 AA10 CB15 DA20 FA04 FA06                       GA03 HA02 HA19

Claims (2)

[Claims] 1. 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 the general formula Zn _{1 -Z} C _Z O , 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 following as a main component. 2. The element C is Mg and the Z is 0 <
The solar cell according to claim 1, wherein the relationship of Z <0.5 is satisfied.