JPH08130321A - Solar cell - Google Patents

Abstract

PURPOSE: To provide a solar cell whose conversion efficiency is high in the solar cell composed of an electrode with reference to a semiconductor light absorption layer, of a junction by the semiconductor light absorption layer and by a semiconductor window layer whose band gap is wider than the band gap of the semiconductor light absorption layer and whose conductivity type is different, of a transparent electrode with reference to the semiconductor window layer and of an inclined composition layer. CONSTITUTION: A semiconductor window layer 4/an inclined composition layer 6/a semiconductor light absorption layer 3 are composed of CdS/CdTe1- XSeX/ CdTe, CdS/CuIn1- XGaXSe2 /CuInSe or CdS/CuIn(Se1- XSX)2 /CuInSe2 . Then, the band gap of the inclined composition layer 6 is made small as the layer is away from a junction part to the semiconductor window layer 4, and the band gap of a part coming into contact with the semiconductor light absorption layer 3 is made equal to, or larger than, the band gap of the light absorption layer 3. Thereby, a recombination current is reduced, a high release voltage can be obtained, and the high conversion efficiency of a solar cell can be obtained.

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 conversion efficiency. More specifically, it relates to a heterojunction solar cell having a wide bandgap structure.

[0002]

2. Description of the Related Art Conventionally, many proposals have been made on the basic structure of solar cells. An ordinary heterojunction solar cell includes a semiconductor (window layer) 4 having a wide bandgap as shown in FIG. 2A and a semiconductor (optical absorption) having a bandgap of about 1.5 eV (or less). The layer 3 has a different type / different conductivity type junction and has a band profile as shown in FIG. For example, CdS
/ CdTe, CdS / CuInSe ₂ , and the like. Further, it has been proposed to introduce a graded composition layer into the structure of a solar cell in order to obtain high conversion efficiency. In many cases, it is introduced into a semiconductor (window layer) having a large band gap. For example, a-SiC / a-Si or AlGaA
This is the case with structures such as s / GaAs. These structures are structures in which a graded composition layer is introduced into a semiconductor (window layer) having a large band gap to obtain more photocurrent.

[0003]

However, even in the above-mentioned conventional heterojunction type solar cell, the light-power conversion efficiency is 12.2.
It was about 13.1%.

The object of the present invention was made in view of the problems of the prior art, and it is an object of the present invention to provide a solar cell having excellent performance.

[0005]

In order to solve the above problems, the solar cell of the present invention comprises an electrode for a semiconductor light absorption layer,
A solar cell comprising a semiconductor window layer having a bandgap wider than the bandgap of the semiconductor light absorption layer and a different conductivity type, which is in contact with the semiconductor light absorption layer at a junction, and a transparent electrode for the semiconductor window layer, The band gap between the semiconductor light absorbing layer and the semiconductor window layer becomes smaller as the distance from the junction with the semiconductor window layer becomes smaller, and the band gap of the portion in contact with the semiconductor light absorbing layer becomes the band gap of the semiconductor light absorbing layer. It is characterized in that it has a gradient composition layer that is equal to or larger than that of the depletion layer and has a thickness equal to the width of the depletion layer.

In the solar cell having the above structure, it is preferable that the semiconductor light absorption layer is made of a CdTe thin film and the graded composition layer is made of CdTe _1-X Se _X. Also,
In the solar cell having the above structure, the semiconductor window layer is made of a CdS thin film, and the semiconductor light absorption layer is made of a CdTe thin film.
The gradient composition layer is preferably made of CdTe _1-X Se _X.

In the solar cell having the above structure, the semiconductor
The light absorption layer is CuInSe₂ It is composed of a thin film and has the above-mentioned gradient composition.
Layer is CuIn_1-XGa_XSe₂It is preferable that
In the solar cell having the above structure, the semiconductor window layer is
CdS thin film, the semiconductor light absorption layer is CuInS
e₂ A thin film, the gradient composition layer being CuIn _1-XGa_X
Se₂It is preferable that

In the solar cell having the above structure, it is preferable that the semiconductor light absorption layer is made of a CuInSe ₂ thin film and the gradient composition layer is made of CuIn (Se _{1 -X} S _X ) ₂ .

In the solar cell having the above structure, the semiconductor window layer is made of a CdS thin film, the semiconductor light absorption layer is made of a CuInSe ₂ thin film, and the gradient composition layer is made of Cu.
It is preferably composed of In (Se _{1 -X} S _X ) ₂ .

[0010]

The solar cell of the present invention comprises a semiconductor window layer having an electrode for the semiconductor light absorption layer, a band gap wider than the band gap of the semiconductor light absorption layer in contact with the semiconductor light absorption layer at a junction, and a different conductivity type. And a transparent electrode for the semiconductor window layer, wherein the band gap becomes smaller between the semiconductor light absorption layer and the semiconductor window layer as the distance from the junction with the semiconductor window layer increases, and The band gap of the portion in contact with the absorption layer has a graded composition layer which is equal to or larger than the band gap of the semiconductor light absorption layer, and the thickness thereof is equal to the width of the depletion layer. With this configuration, the recombination current of the heterojunction portion is reduced, a high release voltage can be obtained, and the light-power conversion efficiency can be improved.

Further, in the above structure of the solar cell, the semiconductor light absorption layer is made of a CdTe thin film, and the gradient composition layer is made of CdTe.
According to the preferred structure of dTe _{1 -X} Se _{X, the} recombination current of the heterojunction is reduced, a higher open-circuit voltage can be output as compared with the conventional heterojunction, and the conversion efficiency is superior to that of the conventional solar cell. Can be obtained.

Further, in the above construction of the solar cell, the semiconductor window layer is made of a CdS thin film, and the semiconductor light absorption layer is made of CdS.
It is made of a Te thin film, and the gradient composition layer is CdTe _1-X Se.
_Since it consists of _X , by using a CdS thin film, it is possible to maintain a higher quality junction, reduce the recombination current, and output a higher open circuit voltage than the conventional junction, which is superior to conventional solar cells. It is possible to obtain high conversion efficiency.

In the solar cell having the above-mentioned structure, according to a preferred mode in which the semiconductor light absorption layer is made of CuInSe ₂ thin film and the gradient composition layer is made of CuIn _{1 -X} Ga _X Se ₂ , The coupling current is reduced, and it is possible to output a higher open-ended voltage than the conventional heterojunction,
The conversion efficiency superior to that of the conventional solar cell can be obtained.

In the solar cell having the above structure,
The semiconductor window layer is made of a CdS thin film, and the semiconductor light absorption layer
Is CuInSe₂ It is composed of a thin film, and the gradient composition layer is Cu.
In _1-X Ga_XSe₂According to a preferred embodiment,
The use of dS thin film maintains a higher quality joint,
It also reduces the recombination current and is higher than conventional junctions.
It can output a high open-circuit voltage and is superior to conventional solar cells.
The conversion efficiency can be obtained.

In the solar cell having the above structure, since the semiconductor light absorption layer is made of CuInSe ₂ thin film and the graded composition layer is made of CuIn (Se _{1 -X} S _X ) ₂ , the heterojunction part The coupling current is reduced, a higher open circuit voltage can be output as compared with the conventional heterojunction, and the conversion efficiency superior to that of the conventional solar cell can be obtained.

In the solar cell having the above structure, the semiconductor window layer is made of a CdS thin film, the semiconductor light absorption layer is made of a CuInSe ₂ thin film, and the gradient composition layer is made of Cu.
Since it is made of In (Se _{1 −X} S _X ) ₂ , by using a CdS thin film, a higher quality junction can be maintained and the recombination current can be reduced, resulting in a higher open-circuit voltage than the conventional junction. It is possible to output and obtain a conversion efficiency superior to that of the conventional solar cell.

That is, in the solar cell of the present invention, since the graded composition layer is introduced as described above, the recombination current of the heterojunction is reduced without affecting the wavelength dependence of the quantum efficiency, A solar cell that can output a higher open-circuit voltage than that of a conventional heterojunction can be realized, and at the same time, the photocurrent is comparable to that of a heterojunction solar cell with a conventional configuration.

[0018]

Embodiments of the solar cell of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 is a diagram showing an outline of one embodiment of the present invention. 1A shows a cross-sectional view of the structure of the solar cell of the present invention, FIG. 1B shows a band diagram of a heterojunction portion (including a graded composition layer), and a broken line in FIG. 1B. FIG. 4 is a band diagram of a junction in the case of a heterojunction solar cell having a conventional structure (without a gradient composition layer). A region p in which band bending occurs indicates a depletion layer.

In FIG. 1, reference numeral 1 is an insulating substrate such as a glass substrate, on which a Mo thin film having a film thickness of about 1 μm is formed as a lower electrode 2, and an ITO thin film as a transparent upper electrode is formed on the lower electrode 2. 5 is formed to have a film thickness of about 0.5 μm. The structure of the semiconductor window layer 4 / gradient composition layer 6 / semiconductor light absorption layer 3 that forms the basis of the solar cell of the present invention is CdS.
Semiconductor window layer 4 (film thickness 0.1 μm) / CdTe _{1 −X} Se _X gradient composition layer 6 / CdTe semiconductor light absorption layer 3 (film thickness 3 μm)
It is composed by. In this structure, the CdTe _1-X Se _X layer in contact with the CdS layer has an X value of 0.20, and the CdTe _1-X Se _X layer in contact with the CdTe layer has an X value of 0.0. Further, the film thickness of the CdTe _1-X Se _X layer is, for example, 0.2 to so that it is almost the same as the depletion layer width p.
Optimal design for 1.0 μm.

When the film thickness of the gradient composition layer 6 of CdTe _{1 -X} Se _X is thinner than that of the depletion layer region, short-circuit photocurrent cannot be sufficiently obtained, and when it is thicker than that of the depletion layer region, the open-end voltage is increased. Tends to decline. As described above, CdTe
A conventional CdTe _1-X Se _X gradient was prepared by using a multi-source deposition apparatus to fabricate a solar cell having the structure of the present invention in which the film thickness of the _1-X Se _X graded composition layer 6 was optimized. The performance was compared with a solar cell without a composition layer.

The CdTe _1-X Se _X gradient composition layer 6 produced here was p-type and had a hole concentration of about 1 × 10 ¹⁵ cm ^-3 as in the CdTe thin film, so that CdTe _1-X Se _{X was formed.} The film thickness of the gradient composition layer was set to about 0.7 μm.

In FIG. 3, the wavelength dependence of the quantum efficiency of the solar cell according to this example is shown by A ', and the wavelength dependence of the quantum efficiency by the solar cell of the conventional example is shown by A for comparison. There is no difference in quantum efficiency between the two, and short-circuit photocurrents of almost the same level can be expected.

FIG. 4 shows current-voltage characteristics measured under a solar simulator of AM 1.5 and 100 mW / cm ² . As compared with the characteristic A of the solar cell having the conventional structure, the open-ended voltage of the characteristic A ′ of the solar cell of the present invention was higher. This effect shows that by introducing the graded composition layer, the recombination current of the heterojunction portion is reduced, and a solar cell capable of outputting a higher open-ended voltage than that of the conventional heterojunction can be realized. The obtained conversion efficiency was 12.2% (Jsc = 22.5mA / cm ² , Voc = 0.750 for the conventional solar cell).
V, FF = 0.720), 14.1% (Jsc = 2) in the solar cell of this example.
It was 2.2 mA / cm ² , Voc = 0.890 V, FF = 0.715).

(Example 2) Similar to the structure of the solar cell of Example 1, the lower electrode 2 is formed on the insulating substrate 1 such as a glass substrate.
As a thin film of Mo, a film having a thickness of about 1 μm is formed, and on this lower electrode 2, an ITO thin film 5 having a film thickness of about 0.5 μm is formed as a transparent upper electrode. On top of this, the layers of semiconductor window layer 4 / gradient composition layer 6 / semiconductor light absorption layer 3 which form the basis of the solar cell of the present invention are formed. Also in this embodiment, the semiconductor window layer 4 is made of CdS and has a film thickness of 0.
It is formed to 1 μm.

However, in this embodiment, the graded composition layer 6 is made of C
uIn_1-XGa_XSe₂ And the semiconductor light absorption layer 3 is formed from
CuInSe₂ From the point that it is formed to a film thickness of 2 μm from
This is different from the first embodiment. In addition, the CdS half of this embodiment
CuIn in contact with the conductor window layer 4 _1-XGa_XSe₂Gradient composition layer
The value of X of 6 is 0.15, and CuInSe₂Touch the layers
CuIn_1-XGa_XSe₂The value of X in the layer is 0.0.
In addition, CuIn_1-XGa_XSe₂ The layer thickness is the depletion layer width p
It is necessary to optimize the design so that it is almost the same as. Cu
In_1-XGa_XSe₂ The layer thickness is thinner than the depletion layer region
If the short circuit photocurrent cannot be obtained, the depletion region
If it is thicker than the area, the open-circuit voltage tends to decrease.
You. Therefore, the film thickness of the gradient composition layer 6 is, for example, 0.
2 to 1.0 μm.

As described above, the solar cell having the structure of the present invention in which the film thickness of the CuIn _{1 -X} Ga _X Se ₂ gradient composition layer is optimized was produced by using a multi-source vapor deposition apparatus, and similarly produced by multi-source vapor deposition. The performance was compared with that of a conventional solar cell having no CuIn _{1 -X} Ga _X Se ₂ gradient composition layer. CuIn _1-X G produced here
The a _x Se ₂ gradient composition layer is p-type and its hole concentration is Cu.
Since it was about 5 × 10 ¹⁵ cm ⁻³ like the InSe ₂ thin film, the film thickness of the CuIn _{1 -X} Ga _X S ₂ gradient composition layer was 0.3 μm.
It was about m.

Regarding the solar cell of this embodiment and the conventional example,
The wavelength dependence of the quantum efficiency is shown in B and B '(B: conventional structure, B': structure of the present invention) in FIG. There is no difference between the two and almost the same short-circuit photocurrent is obtained.

FIG. 4 shows current-voltage characteristics measured under a solar simulator of AM 1.5 and 100 mW / cm ² . The characteristic B ′ of the solar cell of the present invention showed a higher open circuit voltage than the characteristic B of the solar cell of the conventional structure. This effect shows that by introducing the graded composition layer, the recombination current of the heterojunction portion is reduced, and a solar cell capable of outputting a higher open-ended voltage than that of the conventional heterojunction can be realized. The conversion efficiency obtained was 13.1% (Jsc = 40.5mA / cm ² , Voc =
0.455V, FF = 0.710), 1 in the solar cell according to the present embodiment.
It was 5.3% (Jsc = 40.4mA / cm ² , Voc = 0.528V, FF = 0.715).

(Embodiment 3) Similar to the structure of the solar cell of Embodiment 1, the lower electrode 2 is formed on the insulating substrate 1 such as a glass substrate.
As a thin film of Mo, a film having a thickness of about 1 μm is formed, and on this lower electrode 2, an ITO thin film 5 having a film thickness of about 0.5 μm is formed as a transparent upper electrode. On top of this, the layers of semiconductor window layer 4 / gradient composition layer 6 / semiconductor light absorption layer 3 which form the basis of the solar cell of the present invention are formed. Also in this embodiment, the semiconductor window layer 4 is formed of CdS to have a film thickness of 0.1 μm, as in the above embodiment.

However, in this embodiment, the graded composition layer 6 is C
This is different from Embodiments 1 and 2 in that it is formed of uIn (Se _{1 -X} S _X ) ₂ and the semiconductor light absorption layer 3 is formed of CuInSe ₂ to a film thickness of 2 μm. Also,
CuIn (Se _1-X S _X ) in contact with the CdS layer of this example
The X value of the _two- gradient composition layer 6 is 0.20, and CuInS
e ₂ CuIn (Se _1-X S) in contact with the semiconductor light absorption layer 3
_{The X} value of _X ) ₂ is 0.0. In addition, CuIn (Se
_It is necessary to optimally design the film thickness of _1−X S _X ) ₂ so as to be almost the same as the depletion layer width p. CuIn (Se _1-X S _X )
_When the film thickness of the _two layers is smaller than that of the depletion layer region, sufficient short-circuit photocurrent cannot be obtained, and when it is thicker than that of the depletion layer region, the open circuit voltage tends to decrease. Therefore, the film thickness of the gradient composition layer 6 is, for example, 0.2 to 1.0 μm.

As described above, CuIn (Se _1-X S _X ) ₂
The conventional CuIn (Se _{1 -X} S _X ) ₂ gradient composition prepared by using a multi-source deposition apparatus to fabricate a solar cell having the structure of the present invention in which the film thickness of the gradient composition layer 6 The performance was compared with a layerless solar cell. CuI made here
The n (Se _{1 −X} S _X ) ₂ gradient composition layer 6 is p-type and has a hole concentration of about 2 × 10 ¹⁵ cm ⁻³ as in the CuInSe ₂ thin film.
Therefore, the film thickness of the CuIn _{1 -X} Ga _X Se ₂ gradient composition layer 6 was set to about 0.3 μm.

Regarding the solar cell of this embodiment and the conventional example,
The wavelength dependence of the quantum efficiency is shown in C and C '(C: conventional structure, C': structure of this embodiment) in FIG. There is no difference between the two and almost the same short-circuit photocurrent is obtained.

FIG. 4 shows current-voltage characteristics measured under a solar simulator of AM 1.5 and 100 mW / cm ² . As compared with the characteristic C of the solar cell having the conventional structure, the characteristic C ′ of the solar cell of the present example showed a higher open circuit voltage. This effect shows that by introducing the graded composition layer, the recombination current of the heterojunction portion is reduced, and a solar cell capable of outputting a higher open-ended voltage than that of the conventional heterojunction can be realized. The obtained conversion efficiency is
13.1% (Jsc = 40.5mA / cm ² , Voc
= 0.455V, FF = 0.710), with the solar cell according to the present embodiment 15.
It was 1% (Jsc = 40.2mA / cm ² , Voc = 0.555V, FF = 0.675).

[0034]

As described above, according to the solar cell of the present invention, the band gap between the semiconductor light absorption layer and the semiconductor window layer becomes smaller as the distance from the junction with the semiconductor window layer increases, and the semiconductor light Since the bandgap of the portion in contact with the absorption layer has a graded composition layer which is equal to or larger than the bandgap of the semiconductor light absorption layer, and the film thickness thereof is equal to the depletion layer width, The recombination current of the junction is reduced and a high release voltage can be obtained,
Energy conversion efficiency can be increased.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a solar cell of the present invention.

FIG. 2 is a cross-sectional view showing the structure of a conventional solar cell.

FIG. 3 is a diagram showing wavelength dependence of quantum efficiency of conventional and inventive solar cells.

FIG. 4 is a diagram showing solar cell characteristics (current-voltage characteristics) of conventional and inventive solar cells.

[Explanation of symbols]

 1 Insulating Substrate 2 Lower Electrode 3 Semiconductor Light Absorbing Layer 4 Semiconductor Window Layer 5 Transparent Conductive Layer 6 Gradient Composition Layer

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takahiro Wada 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (7)

[Claims] 1. An electrode for a semiconductor light absorption layer, a semiconductor window layer having a band gap wider than the band gap of the semiconductor light absorption layer contacting the semiconductor light absorption layer at a junction and having a different conductivity type, and the semiconductor window. A solar cell comprising a transparent electrode for the layer, wherein the band gap between the semiconductor light absorption layer and the semiconductor window layer becomes smaller as the distance from the junction with the semiconductor window layer increases, and the portion in contact with the semiconductor light absorption layer. The solar cell is characterized in that it has a graded composition layer having a bandgap equal to or larger than the bandgap of the semiconductor light absorption layer, and has a film thickness equivalent to the depletion layer width. 2. The solar cell according to claim 1, wherein the semiconductor light absorption layer is made of a CdTe thin film, and the graded composition layer is made of CdTe _{1 −X} Se _X. 3. The semiconductor window layer comprises a CdS thin film,
The solar cell according to claim 1, wherein the semiconductor light absorption layer is made of a CdTe thin film, and the graded composition layer is made of CdTe _1-X Se _X. 4. The semiconductor light absorption layer is made of a CuInSe ₂ thin film, and the gradient composition layer is made of CuIn _{1 —X} Ga _X Se ₂
The solar cell according to claim 1, which comprises: 5. The semiconductor window layer is made of a CdS thin film,
The solar cell according to claim 1, wherein the semiconductor light absorption layer is made of a CuInSe ₂ thin film, and the graded composition layer is made of CuIn _{1 -X} Ga _X Se ₂ . 6. The semiconductor light absorption layer is made of a CuInSe ₂ thin film, and the gradient composition layer is made of CuIn (Se _{1 −X} S _X ).
The solar cell according to claim 1, which is composed of ₂ . 7. The semiconductor window layer comprises a CdS thin film,
The solar cell according to claim 1, wherein the semiconductor light absorption layer is made of a CuInSe ₂ thin film, and the gradient composition layer is made of CuIn (Se _{1 -X} S _X ) ₂ .