JPH06232436A - Solar cell and manufacture thereof - Google Patents

Abstract

(57) [Abstract] [Purpose] The electron-hole pairs generated near the surface of the window layer are
Provided are a solar cell that can be efficiently extracted as a photocurrent and a method for manufacturing the same. [Structure] A transparent lower electrode 2 is formed on a glass substrate 1, and then a window layer 3 is formed on the lower electrode 2.
The window layer 3 is a ZnO thin film 3 doped with Al or In.
a, a ZnO thin film 3b with no doping and a CdS thin film 3c are sequentially laminated. Then, a CdTe thin film or Cu is formed on the window layer 3 as the light absorbing semiconductor layer 4.
An InSe ₂ thin film (p-type semiconductor) is formed, and a transparent upper electrode 5 is formed thereon. The lower electrode 2 is made of a material that does not react violently with ZnO even after a thermal process, such as ITO (indium-tin oxide) or SnO ₂ , and the upper electrode 5 is made of Au, Ni, or the like. The CdTe thin film or Cu as the absorbing semiconductor layer 4
It makes ohmic contact with the InSe ₂ thin film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell having high energy conversion efficiency and a method for manufacturing the solar cell.

[0002]

2. Description of the Related Art It is expected that energy supply will gradually become difficult in the near future, and high efficiency and low cost of solar cells have become major issues. Among them, thin-film solar cells, which can easily be made large in area, are thought to be able to significantly reduce costs.
It is strongly desired to improve the energy conversion efficiency. Currently, this thin film solar cell is a compound semiconductor (II
Is being has been developed extensively those using -VI group and I-III-VI ₂ group) thin film.

As shown in FIG. 4, a solar cell using a compound semiconductor thin film includes a compound semiconductor thin film as a window layer 6 having a wide band gap and transmitting light, and an absorbing layer having a narrow band gap and absorbing light. 7 has a heterostructure in which a compound thin film as 7 is laminated. In FIG. 4, 8 is a translucent insulating substrate, 9 is a lower electrode, and 10 is an upper electrode.

In this type of solar cell, the conditions required to obtain higher energy conversion efficiency are optimum optical design for obtaining more photocurrent and carrier re-creation at the interface or especially in the absorption layer. The goal is to form high quality heterojunctions and thin films without bonds.

The high-quality hetero interface is closely related to the combination of the window layer and the absorption layer, and is conventionally related to the CdS / CdTe system and CdS.
An excellent heterojunction has been obtained in the / CuInSe ₂ system.

Further, by using a semiconductor having a wider band gap, for example, ZnCdS or a transparent conductive oxide film (SnO ₂ , ITO (indium-tin oxide), ZnO, etc.) instead of CdS for the window layer, Attempts have also been made to improve the short wavelength sensitivity of light to improve energy conversion efficiency.

[0007]

However, the compound thin film heterojunction type solar cells configured as described above have one common problem. That is, the band gap of the window layer almost determines the short wavelength sensitivity in the solar cell.

FIG. 5 shows the wavelength dependence of the quantum efficiency in the vicinity of the band gap of the window layer of the solar cell having the structure shown in FIG. In FIG. 5, a solid line G indicates ZnCdS as a window layer.
The quantum efficiency of the solar cell using a thin film is shown. In this case, since the bandgap of the window layer is widened, the obtained photocurrent is improved as compared with the case where the CdS thin film is used as the window layer (dotted line H in FIG. 5), but the open-end voltage is slightly lowered. Due to the tendency, the energy conversion efficiency has not been significantly improved. The cause of this is ZnCdS
It is conceivable that the quality of the heterojunction formed by the thin film and the absorption layer will be lower than that of the CdS thin film and the absorption layer.

In any case, the diffusion length of the minority carriers of the compound thin film is very short in any case. Therefore, in the conventional structure, the electron-hole generated near the light incident surface, that is, near the window layer surface. It shows that few pairs have been acquired.

The present invention has been made in view of the above circumstances, and a solar cell and a method of manufacturing the same, in which electron-hole pairs generated near the surface of the window layer can be efficiently taken out as photocurrent. The purpose is to provide.

[0011]

To achieve the above object, a solar cell according to a first aspect of the present invention has a transparent lower electrode formed on a transparent insulating substrate, and the lower electrode. A window layer formed by sequentially stacking a ZnO thin film and a CdS thin film thereon, a light-absorbing semiconductor layer formed on the window layer, and a light-transmitting property formed on the light-absorbing semiconductor layer. And an upper electrode of at least.

A second structure of the solar cell according to the present invention is that a transparent lower electrode formed on a transparent insulating substrate, and a light absorbing semiconductor layer formed on the lower electrode. ,
At least a window layer formed by sequentially stacking a CdS thin film and a ZnO thin film on the light absorbing semiconductor layer and a translucent upper electrode formed on the window layer are provided.

In the first or second structure, Zn
It is preferable to introduce an n-type dopant on the electrode side of the O thin film. In the first or second structure, the light absorption semiconductor layer is preferably made of a II-VI group compound semiconductor, and particularly preferably made of CdTe.

In the first or second structure, the light absorbing semiconductor layer is preferably made of chalcopyrite type semiconductor, and particularly preferably made of CuInSe ₂ .

The first method for manufacturing a solar cell according to the present invention is to use ITO as a transparent lower electrode on a transparent insulating substrate.
A step of forming an (indium-tin oxide) film, a step of forming a ZnO thin film on the transparent lower electrode; a step of heat-treating the translucent insulating substrate at 300 ° C. or higher; It comprises at least a step of sequentially laminating a CdS thin film, a light absorbing semiconductor layer, and a translucent upper electrode.

A second method of manufacturing a solar cell according to the present invention comprises a step of forming a light-transmitting lower electrode on a light-transmitting insulating substrate, a light-absorbing semiconductor layer, CdS on the lower electrode.
Laminated film of thin film and ZnO thin film, as a transparent upper electrode I
The step of sequentially stacking TO films and the translucent insulating substrate
And at least a step of heat treatment at 00 ° C. or higher.

[0017]

According to the first or second structure of the present invention, an internal electric field is generated in the window layer, and as a result, the CdS thin film and Z
Electrons generated by sunlight near the interface of the nO thin film −
Since the hole pairs can be efficiently taken out to obtain a larger amount of photocurrent, the energy conversion efficiency of the solar cell can be significantly improved.

In the first or second structure of the present invention, according to the preferable structure in which the n-type dopant is introduced into the electrode side of the ZnO thin film, the resistance value of the ZnO thin film is sufficiently low due to the action of the n-type dopant. And as a result,
Since the exchange of carriers between the ZnO thin film and the CdS thin film is smoothly performed, the energy conversion efficiency of the solar cell can be dramatically improved. Further, according to this preferable configuration, the electron-hole pairs can be efficiently taken out while maintaining the quality of the interface between the ZnO thin film and the CdS thin film.

Further, in the first or second structure of the present invention, according to a preferable structure in which the light absorption semiconductor layer is made of a II-VI group compound semiconductor or a chalcopyrite type semiconductor, a CdS thin film is formed. Since a bonding interface with excellent quality can be formed, a high quality solar cell can be manufactured.

According to the first or second manufacturing method of the present invention, In in ITO thermally diffuses into the ZnO thin film and acts as an n-type dopant, so that it is not necessary to separately form an n-type dopant layer. As a result, the manufacturing process can be simplified.

[0021]

EXAMPLES The present invention will be described in more detail below with reference to examples. 1 (A) and 1 (B) are sectional views showing an embodiment of the solar cell according to the present invention.

The essential difference between the two is whether sunlight is incident from the lower electrode 2 side (FIG. 1 (A)) or the upper electrode 5 side (FIG. 1 (B)). There is no. Therefore, only the configuration of FIG. 1A will be described below.

The glass substrate 1 is used as a translucent insulating substrate. First, the translucent lower electrode 2 is formed on the glass substrate 1, and then the window layer 3 is formed on the lower electrode 2. The window layer 3 has a structure in which a ZnO thin film (hereinafter referred to as “n-type dopant layer”) 3a doped with Al or In, a ZnO thin film 3b with no doping, and a CdS thin film 3c are sequentially stacked. Here, the film thicknesses of the n-type dopant layer 3a, the ZnO thin film 3b, and the CdS thin film 3c are about 0.7 μm, 0.1 μm, and 0.2 μm, respectively. The n-type dopant layer 3a and the ZnO thin film 3b were formed by a sputtering method at a substrate temperature of 150 ° C. and a vacuum degree of 10 ⁻² T.
The CdS thin film 3c is formed by a vacuum evaporation method at a substrate temperature of 150 ° C. and a vacuum degree of 10 ⁻⁶ Torr.

Then, the light absorbing semiconductor layer 4 is formed on the window layer 3.
As a thin film, a CdTe thin film or a CuInSe ₂ thin film (p-type semiconductor) is formed with a film thickness of about 3 μm, and the translucent upper electrode 5 is formed thereon. Here, the CdTe thin film or the CuInSe ₂ thin film is formed by a vacuum evaporation method at a substrate temperature of 4
It is formed at 00 ° C. and a vacuum degree of 10 ⁻⁶ Torr.

As the light-absorbing semiconductor layer 4, the quality of the junction interface with the CdS thin film 3c is improved, and a high-quality solar cell can be manufactured. Therefore, a II-VI group compound semiconductor such as CdTe or CuInSe ₂ is used. Such chalcopyrite type semiconductors are preferable. Examples of II-VI group compound semiconductors include ZnTe, CdZnTe, and Cd in addition to CdTe.
HgTe and the like are useful. As the chalcopyrite semiconductor, other CuInSe _2, mixed crystal of CuInSe ₂ and CuInS ₂ and CuInTe ₂ it is useful.

For the lower electrode 2, a material that does not react violently with ZnO even after a thermal process, such as ITO (indium-tin oxide) or SnO _2, is used, and the substrate temperature is 100 ° C. by a sputtering method and the degree of vacuum is ; 10 ^-2 Tor
It is formed by r. Further, as the upper electrode 5, Au, Ni
And the like are used to make ohmic contact with the CdTe thin film or the CuInSe ₂ thin film as the light absorbing semiconductor layer 4.

FIG. 2 shows a band diagram of the solar cell (FIG. 1 (A)) configured as described above. As shown in FIG.
By providing the junction between the CdS thin film 3c and the ZnO thin film 3b in the window layer 3, an internal electric field is generated in the window layer 3, and as a result, sunlight is mainly generated near the interface between the CdS thin film 3c and the ZnO thin film 3b. Since it is possible to efficiently take out the electron-hole pairs generated by, and obtain more photocurrent,
The energy conversion efficiency of the solar cell can be greatly improved.

The lower electrode 2 side of the ZnO thin film 3b (C
By introducing the n-type dopant only in the vicinity of the side opposite to the dS thin film interface), Zn acts by the action of the n-type dopant.
The resistance value of the O thin film 3b becomes sufficiently low, and as a result, Zn
Since carriers can be exchanged smoothly between the O thin film 3b and the CdS thin film 3c, the energy conversion efficiency of the solar cell can be dramatically improved.

By introducing the n-type dopant only in the vicinity of the lower electrode 2 side of the ZnO thin film 3b in this way, the electron-hole pairs can be efficiently taken out while maintaining the quality of the interface between the ZnO thin film 3b and the CdS thin film 3c. Is preferred, which is preferable.

FIG. 3 shows the result of comparing the characteristics of the solar cell manufactured as described above with the conventional solar cell. Figure 3
In the middle, dotted lines E and F respectively show the case where the absorption layer of the conventional solar cell shown in FIG. 4 is CdTe and the case where it is CuInSe ₂ , and the energy conversion efficiency is about 10%. Further, in FIG. 3, solid lines C and D respectively show the case where the light absorbing semiconductor layer of the solar cell having the structure of the present embodiment is CdTe and the case where CuInSe ₂ is used, and the window layer is made of Zn.
Although the photocurrent is about the same as that using the CdS thin film, the solar cell having the configuration of this example can obtain a larger open-ended voltage. As a result, the energy conversion efficiency of solar cells of any system shows 13%,
It was possible to confirm the usefulness of the present invention.

The solar cell having the above structure can also be manufactured by the following method. That is, ITO is used as the lower electrode 2 and a ZnO thin film 3b with no doping is formed thereon with a film thickness of about 0.8 μm. Then, the glass substrate 1 is preliminarily heat-treated at a temperature of 300 ° C. or higher, and then ZnO is formed. The above-mentioned Cd on the thin film 3b
The S thin film 3c, the light absorbing semiconductor layer 4, and the upper electrode 5 are sequentially stacked. If such a method is adopted, In in ITO thermally diffuses into the ZnO thin film 3b and acts as an n-type dopant, so that it is not necessary to separately form a thin film as the n-type dopant layer 3a, as shown in FIG. The above configuration can be realized, and as a result, the manufacturing process can be simplified.

The same thing can be said for the configuration of FIG. 1 (B). In this case, I is used as the upper electrode 5.
The glass substrate 1 may be heat-treated at a temperature of 300 ° C. or higher after forming the upper electrode 5 using TO.

The heat treatment temperature of the glass substrate 1 is preferably 450 ° C. or lower. This is because when the temperature is 450 ° C. or higher, mutual diffusion between layers occurs, and the characteristics of the solar cell deteriorate.

[0034]

As described above, according to the first or second structure of the present invention, an internal electric field is generated in the window layer, and as a result, the solar field is mainly present near the interface between the CdS thin film layer and the ZnO thin film. Since the electron-hole pairs generated by light can be efficiently taken out and more photocurrent can be obtained, the energy conversion efficiency of the solar cell can be significantly improved.

In the first or second structure of the present invention, Z
According to a preferable configuration in which the n-type dopant is introduced into the electrode side of the nO thin film, Z acts by the action of the n-type dopant.
The resistance of the nO thin film becomes sufficiently low, and as a result, ZnO
Since carriers are smoothly exchanged between the thin film and the CdS thin film layer, the energy conversion efficiency of the solar cell can be dramatically improved. Further, according to this preferable configuration, the electron-hole pairs can be efficiently taken out while maintaining the quality of the interface between the ZnO thin film and the CdS thin film.

Further, in the first or second structure of the present invention, according to a preferable structure in which the light absorption semiconductor layer is made of a II-VI group compound semiconductor or a chalcopyrite type semiconductor, the quality of the light absorption semiconductor layer and the CdS thin film is improved. It is possible to form a high-quality solar cell because the excellent junction interface can be formed.

According to the first or second manufacturing method of the present invention, In in ITO thermally diffuses into the ZnO thin film and acts as an n-type dopant, so that it is not necessary to separately form an n-type dopant layer. As a result, the manufacturing process can be simplified.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a solar cell according to the present invention.

FIG. 2 is a band diagram of an example of a solar cell according to the present invention.

FIG. 3 is a characteristic diagram of a solar cell according to the present invention and a conventional solar cell.

FIG. 4 is a cross-sectional view showing a conventional solar cell.

FIG. 5 is a diagram showing wavelength dependence of quantum efficiency in the vicinity of a band gap of a window layer of a conventional solar cell.

[Explanation of symbols]

 1 glass substrate 2 lower electrode 3 window layer 3a n-type dopant layer 3b ZnO thin film 3c CdS thin film 4 light absorption semiconductor layer 5 upper electrode

Claims (9)

[Claims] 1. A transparent lower electrode formed on a transparent insulating substrate, a window layer formed by sequentially stacking a ZnO thin film and a CdS thin film on the lower electrode, and a window layer above the window layer. A solar cell comprising at least a light-absorbing semiconductor layer formed on the light-absorbing semiconductor layer, and a translucent upper electrode formed on the light-absorbing semiconductor layer. 2. A translucent lower electrode formed on a translucent insulating substrate, a light absorbing semiconductor layer formed on the lower electrode, and a CdS thin film on the light absorbing semiconductor layer. Zn
A solar cell comprising at least a window layer formed by sequentially stacking O thin films, and a translucent upper electrode formed on the window layer. 3. The solar cell according to claim 1, wherein an n-type dopant is introduced on the electrode side of the ZnO thin film. 4. The solar cell according to claim 1, wherein the light absorbing semiconductor layer is made of a II-VI group compound semiconductor. 5. The solar cell according to claim 4, wherein the light absorbing semiconductor layer is made of CdTe. 6. The solar cell according to claim 1, 2 or 3, wherein the light absorbing semiconductor layer is made of chalcopyrite type semiconductor. 7. The solar cell according to claim 6, wherein the light absorbing semiconductor layer is made of CuInSe ₂ . 8. A step of forming an ITO (indium-tin oxide) film as a transparent lower electrode on a transparent insulating substrate; a step of forming a ZnO thin film on the transparent lower electrode; Manufacturing a solar cell comprising at least a step of heat-treating an optically insulating substrate at 300 ° C. or higher, and a step of sequentially laminating a CdS thin film, a light absorbing semiconductor layer, and a transparent upper electrode on the ZnO thin film. Method. 9. A step of forming a translucent lower electrode on a translucent insulating substrate, and a light absorbing semiconductor layer on the lower electrode,
A method of manufacturing a solar cell, which comprises at least a step of sequentially laminating a CdS thin film and a ZnO thin film, an ITO film as a translucent upper electrode, and a step of heat-treating the translucent insulating substrate at 300 ° C. or higher. .