JP2001203374A - Non-single-crystal thin-film solar cell and method for manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-single-crystal thin-film solar cell having a pin junction structure mainly composed of an amorphous silicon-based film and a method of manufacturing the same.

[0002]

2. Description of the Related Art Non-single-crystal thin-film solar cells using non-single-crystal thin films, especially amorphous silicon (hereinafter a-Si), which is a silicon-based amorphous thin film, and amorphous silicon containing a microcrystalline phase A non-single-crystal thin-film solar cell having a pin junction structure mainly composed of a non-single-crystal thin film such as a thin-film polycrystalline silicon (hereinafter referred to as μc-Si) has a larger area than a single-crystal silicon solar cell. In addition, since it can be manufactured at low temperature and at low cost, it is expected as a large-area thin-film solar cell for electric power.

In order to improve the efficiency of non-single-crystal solar cells, p
A p-type semiconductor layer (hereinafter referred to as p-type) which is a doped layer of an in-type solar cell.
An example in which μc-Si is applied as a material for an n-type semiconductor layer (hereinafter abbreviated as an n-layer) has been reported. By using μc-Si for the p-layer or n-layer on the light incident side, the short-circuit current density (hereinafter, referred to as Jsc) is increased by reducing the light absorption loss, and the open-circuit voltage (hereinafter, referred to as Voc) is increased by increasing the diffusion potential. Increase is achieved. On the other hand, by using the p-layer or the n-layer on the side opposite to light incidence, increase in Voc due to increase in diffusion potential, fill factor (hereinafter referred to as FF) due to decrease in contact resistance with the underlying electrode, and increase in Jsc can be seen. Can be Also,
FF, Js as tunnel junction layers when two or more layers are stacked
c is increased.

[0004]

However, depending on the preparation conditions of the μc-Si layer, there is a problem that a-Si is actually formed at an early stage of film formation. When this amorphous film is formed, there is a problem that Voc decreases due to low activation energy and Jsc decreases due to a large absorption coefficient. Further, since the conductivity is low, the resistance loss increases, and FF and Jsc decrease. In addition, the defect density at the interface between the amorphous film and the i-type semiconductor layer (hereinafter abbreviated as i-layer) increases, and the characteristics of the cell deteriorate.

[0005] As an attempt to suppress the amorphous film in the initial stage of film formation on the i-layer and to form a film containing microcrystals from the beginning, the surface of the i-layer before the μc film is formed is treated with hydrogen plasma. However, a certain effect has not yet been confirmed. Further, Pellaton et al. (Hereinafter referred to as CO2) on the a-Si i-layer.
The ₂ and referred) treatment by plasma, n-type [mu] c-Si for the purpose of tunnel junction layers of the tandem cell is reported that can be formed by the following film thickness 10 nm. (N. Pellaton Vaouch
er, B. Rech, D. Fischer, S. Dubail, M. Goetz, H. Ke
ppner, C. Beneking, O. Hadjadj, V. Shkllover and
A. Shah, Technical Digest of 9th Int. Photovoltaic
Science and Engineering Conf., Miyazaki, Nov. 11-
15, 1996, pp.651). However, it is difficult to control the composition and thickness of the layer formed at the interface by the CO ₂ plasma due to the preparation method, which causes problems in controllability and reproducibility, and damages the underlying a-Si layer. There's a problem. In addition, it does not disclose any applicability to a p-type μc film.

The inventors applied μc-Si to the p-layer, and formed an amorphous silicon oxide (hereinafter a-SiO) p / i interface layer having a thickness of 10 to 20 nm at the interface between the p-layer and the i-layer. By forming μc-Si at a low temperature of about 85 ° C. in the configuration of the pin type cell provided, the formation of the amorphous layer in the initial stage of film formation can be suppressed, and Voc uses a-SiO for the p layer. Report that it is better than if. (T. Sasaki, S. Fujikak
e, K. Tabuchi, T. Yoshida, T. Hama, H. Sakai and Y.
Ichikawa, J. Non-Cryst. Solids, 1999, to be publi
shed; T. Sasaki, S. Fujikake, K. Tabuchi, T. Yoshid
a, T. Hama, H. Sakai and Y. Ichikawa, Tech. Digest
of 11th Int. Photovoltaic Science and Engineering
Conf., Sapporo, Japan, Sep. 20-24, 1999, to be pub
However, compared to the case where a-SiO is used for the p layer, F
F and Jsc were low, and did not lead to an overall improvement in cell efficiency.

SUMMARY OF THE INVENTION An object of the present invention is to provide a non-single-crystal thin-film solar cell which can improve the cell efficiency comprehensively while suppressing the formation of an amorphous film at the initial stage of film formation, without deteriorating characteristics such as FF and Jsc. It is to provide a manufacturing method thereof.

[0008]

In order to solve the above-mentioned problems, the present invention provides a non-single-crystal thin film having at least one pin junction formed by laminating a p-layer, a substantially intrinsic i-layer, and an n-layer. In a single crystal thin film solar cell, at least one p
The in-junction p-layer or n-layer is composed of a μc film,
1 to 8 between the film layer and the i-layer made of an amorphous film.
It has an a-SiO 2 interface layer with a thickness of nm.

[0009] Plasma CVD, thermal CVD, optical CVD
When a μc film, for example, a μc-Si film is formed by a method or the like, silicon hydride as a source gas is diluted with hydrogen. It is believed that the hydrogen atoms simultaneously activate the film formation surface and simultaneously etch the film. The etching rate of a-Si is faster than that of μc-Si, and becomes faster at lower temperatures. (HN Wanka and M.
B. Schubert, Mat. Res. Soc. Symp. Proc. 467, 1997,
pp.651) Therefore, when μc-Si is formed at low temperature, the etching action becomes stronger, and substantially no initial amorphous film is formed, and only the μc-film is formed. However, when the underlying i-layer is an amorphous film, the underlying layer is also etched and damaged, and the cell characteristics deteriorate.

Therefore, when a-SiO is provided at the interface, the etching rate of a-SiO is slow (H. Nozaki, N. Sakuma a.
nd H. Ito, Jpn. J. Appl. Phys. 28, 1989, pp.L170
8) By suppressing the etching and damage of the i-layer, a μc film in which the initial amorphous film is suppressed can be formed. However, since the band gap of the μc-film is smaller than that of a-SiO, aS
When iO is thick, Voc, Jsc, and FF are increased due to an increase in recombination current due to back diffusion of carriers generated in a-SiO into the μc-film.
And the cell characteristics deteriorate. Also, μc-film and a-SiO
When the thickness of a-SiO is large, carrier conduction at the interface is hindered by spikes and notches due to band discontinuity, Jsc and FF are reduced, and cell characteristics are deteriorated when the thickness of a-SiO is large. .

Conversely, if the thickness of a-SiO becomes thin, the function as a protective film becomes insufficient, and the cell characteristics deteriorate due to etching or damage of the i-layer. Therefore, a-SiO of the interface layer
There is an appropriate film thickness, and from the experiment described later, by setting the thickness to 1 to 8 nm, generation of carriers in a-SiO can be maintained while maintaining the function of the protective film at the time of etching. In addition, the cell characteristics can be improved by suppressing the conduction of carriers in the band discontinuous portion by the tunnel current.

The μc-film is made of silicon or a silicon alloy such as silicon oxide, silicon carbide, silicon nitride or the like. The effect is obtained when the μc-membrane is μc-Si, as demonstrated in the examples below. μc-SiO, μc-SiC, which have properties similar to μc-Si, and have large band gap and low absorption
Similar effects can be expected in the case of a silicon alloy such as μc-SiN.

The i-layer made of an amorphous thin film is made of silicon or a silicon alloy such as amorphous silicon germanium (hereinafter a-SiGe). It is demonstrated in the examples described below that a particularly advantageous effect is obtained when the i-layer is a-Si. Similar effects can be expected in the case of a silicon alloy such as a-SiGe having properties similar to a-Si.

In the method of manufacturing a non-single-crystal thin-film solar cell as described above, an a-SiO interface layer is formed on the i-layer, and a μc film is formed thereon in this order. By doing so, the underlying i-layer can be prevented from being etched by the a-SiO 2 interface layer during the formation of the μc film. Plasma CVD, thermal CV using mixed gas containing silicon hydride and carbon dioxide
An interface layer of a-SiO 2 is formed by either the D method or the photo-CVD method.

It is demonstrated in the examples described later that the effect can be brought about by forming an a-SiO interface layer. When oxygen gas or nitrogen dioxide gas was used as the oxidizing agent, a good quality a-SiO 2 interface layer could not be formed. If a μc film is formed at a substrate temperature of 70 to 120 ° C,
The etching action becomes stronger, and substantially no initial amorphous film is formed, and a μc film containing a microcrystalline phase is formed from the beginning.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. [Example 1] An experimental test of a solar cell mainly composed of a-Si will be described. FIG. 1 is a sectional view of a non-single-crystal thin-film solar cell according to the present invention, and its manufacturing process will be described below.

As the substrate 1, a glass substrate was used. A silver (Ag) film having a thickness of 100 to 200 nm is formed as a metal electrode 2 on a glass substrate 1 by a sputtering method. Aluminum or the like can also be used. Next, a-Si n-layer 3, a-Si i-layer 4, and a / SiO p / i are formed by plasma CVD.
The interface layer 5 is formed sequentially. At that time, the substrate temperature is 60 ~
300 ° C., reaction pressure 13-130 Pa.

First, the n-layer 3 of a-Si is composed of a monosilane (hereinafter referred to as SiH ₄ ) gas of 10 to 200 ml / min, a phosphine (hereinafter referred to as PH ₃ ) gas of 0.1 to 2 ml / min, and hydrogen (hereinafter referred to as H). ₂ ) A mixed gas of 100 to 2000 ml / min is used to form a film having a thickness of 10 to 50 nm. Then a-Si i-layer 4
Are SiH ₄ gas 10 to 2000 ml / min, H ₂ gas 10
Using a mixed gas of ~ 2000ml / min, 100 ~ 500
Form nm. Then SiH the p / i interface layer 5 of a-SiO ₄
Gas 10-100ml / min, CO ₂ gas 1-50ml / min,
Using a mixed gas of 200 to 2000 ml / min of H ₂ gas,
1 to 8 nm is formed. The a-SiO of the p / i interface layer 5 has an oxygen concentration in the film of 5 to 20% and an optical gap of 1.9 to 2.0 eV.
belongs to. When oxygen gas or nitrogen dioxide gas was used as the oxidizing agent, a high quality a-SiO 2 p / i interface layer could not be formed.

On the p / i interface layer 5, a p layer 6 of μc-Si
The substrate temperature is 85 ° C., the reaction pressure is 13 to 130 Pa,
H ₄ gas 1 to 10 ml / min, (hereinafter referred to as B ₂ H ₆₎ diborane gas 0.001~0.1ml / min, H ₂ gas 100
Using a mixed gas of て 2000 ml / min, a film thickness of 10-60
Form nm. It is important that the μc-Si film is formed in a temperature range of 70 to 120 ° C. When the temperature was higher than 120 ° C., it was observed by transmission electron microscope observation that an amorphous layer was formed in the initial stage of film formation, and the characteristics of such a cell having μc-Si were deteriorated. When a microcrystalline thin film is formed at a temperature lower than 120 ° C., the etching action becomes strong, and substantially no initial amorphous film is formed, and only the microcrystalline thin film is formed. However, when the temperature was lower than 70 ° C., generation of vacancies in the microcrystalline thin film was observed with a transmission electron microscope, and the cell characteristics also deteriorated. In the range of 70 to 120 ° C., the generation of vacancies
The generation of an amorphous layer in the initial stage of film formation can be suppressed, and the cell characteristics are improved.

In this embodiment, since the substrate temperature is low, there is no problematic initial transition layer. The cross section was actually confirmed by transmission electron microscope observation. The content of the microcrystalline phase is 60 to 8
0%. Subsequently, ITO having a thickness of 60 to 80 nm is formed as the transparent electrode 7 by a sputtering method. Finally, a grid electrode 8 is formed on the transparent electrode 7 which can use ZnO or the like.

FIG. 2 is a characteristic diagram showing the relationship between the thickness of the p / i interface layer 5 of a-SiO and the cell characteristics. The vertical axis represents the cell characteristics [Voc, Jsc, FF, Eff], and the horizontal axis represents the thickness of the p / i interface layer 5. Voc is almost constant in the range of a-SiO film thickness of 1 to 12 nm, but drops sharply when it is thinner than 1 nm. Jsc is a
As the film thickness of -SiO is reduced, it increases to 1 nm and decreases at a film thickness smaller than that. FF increases up to 4 nm as the film thickness of a-SiO is reduced, and decreases at smaller film thicknesses.

As a result, Eff increases up to 4 nm when the film thickness of a-SiO is reduced, and decreases from around 2 nm when the film thickness is smaller than that. Therefore, a-Si as the p / i interface layer 5
It can be seen that there is an optimum region for the thickness of O, and the cell efficiency decreases on both upper and lower sides. The maximum value of Eff is 7.5% when the thickness of a-SiO is 4 nm.

In addition to the solar cell of the example of the present invention in which the thickness of a-SiO was 4 nm, four solar cells were prototyped including three having different structures for comparison. The breakdown is as shown in Table 1. FIGS. 3, 4, and 5 are cross-sectional views showing the cell structures of the solar cells of Comparative Examples 1, 2, and 3, respectively. Comparative Example 1
This is a cell in which a p / i interface layer 5 is provided and a p-layer 9 of a-SiO is laminated on the p / i interface layer 5. The a-SiO of the p layer 9 differs from the a-SiO for the p / i interface layer in that the oxygen concentration in the film is 20 to 25% and the optical gap is 2.0 to 2.15 eV. In Comparative Examples 2 and 3, the p-layer was set to μc without providing the p-i interface layer of a-SiO.
-Si. The difference between Comparative Examples 2 and 3 is that the surface of the i-layer is CO ₂ before the p-layer of a-SiO is laminated.
This is the point of the plasma treatment.

[0024]

[Table 1] In Comparative Example 1, the efficiency (Eff) was 7.19%. In Comparative Example 2, Voc, Jsc, and FF are lower than in Comparative Example 1. In particular, Voc is significantly reduced, and Eff is as low as 6.24%. This is probably because the underlying i-layer was etched or damaged during the μc-Si film formation, increasing the number of interface defects.

In Comparative Example 3, Voc was particularly low similarly to Comparative Example 2, and Eff was 6.16%, which was lower than Comparative Examples 1 and 2.
No improvement in cell characteristics was observed by the O ₂ plasma treatment. The broken line in the diagram of Eff in FIG. 2 is Eff in Comparative Example 1. In the range of the a-SiO film thickness of the interface layer from 1 to 8 nm, Eff is comparative example 1.
It can be seen that the value is higher. Comparative Example 4 had a cell structure similar to that of the embodiment of FIG. 1, and the thickness of the p / i interface layer of a-SiO was 12 nm. Although Voc was improved and exceeded the value of Comparative Example 1, Jsc and FF were lower than Comparative Example 1, and Eff was 6.66.
% Stayed.

As a result, by applying the present invention, the maximum output (Pmax) under the optimum load condition is 1.13 of Comparative Example 4.
As compared with Comparative Example 1, it was improved by 1.04 times. [Embodiment 2] A p-layer of a-Si, a-Si
A non-single-crystal thin-film solar cell was fabricated in which an i-layer of 1 nm, a-SiO of 1 to 8 nm, and an n-layer of μc-Si were formed in this order. Again,
Similar improvement in cell characteristics was observed.

The a-SiO acts as an etching protection film, thereby suppressing the formation of an amorphous layer at the initial stage of forming a microcrystalline thin film, and suppressing carrier recombination due to reverse diffusion and improving carrier conduction, thereby improving cell characteristics. It is thought that it was done. As the p-layer or the n-layer of the amorphous film (μc film) containing a microcrystalline phase, in addition to μc-Si described in the embodiment, a microcrystalline silicon alloy is used to further reduce optical absorption loss. Can be.
Specifically, μc-SiO 2, μc-SiC, and μc-SiN are mentioned.

As the i-layer of the amorphous thin film, an amorphous silicon alloy (a-Si alloy) can be used in addition to a-Si described in the embodiment. In particular, as the a-Si alloy, a-SiGe having a large absorption coefficient is desirable. a-SiO is usually prepared using SiH
_4. Silicon hydride such as disilane (Si ₂ H ₆ ) and CO ₂
Is used, but may be diluted with H ₂ or a rare gas as the case may be. Film formation is plasma CVD, thermal CV
This is performed by the D method or the photo CVD method.

As described above, in this embodiment, a substrate type solar cell using a light transmitting substrate such as a glass substrate has been described. However, even when an opaque substrate is used, μc-Si
The same phenomenon is observed when is applied, and the present invention is effective.

[0030]

As described above, according to the present invention, a non-single-crystal thin-film solar cell having at least one pin junction formed by laminating a p-layer made of an amorphous thin film, a substantially intrinsic i-layer, and an n-layer. In the battery, at least one p-layer or n-layer of the pin junction is a μc film containing a microcrystalline phase,
1 to 8 nm between the film layer and the i-layer made of an amorphous film
By providing an a-SiO 2 interface layer having a thickness of 1 mm, the reduction of FF and Jsc, which had been a problem when a conventional thicker a-SiO 2 interface layer was applied, was greatly improved.
Can be greatly improved. Further, the p-layer is a-SiO
The cell characteristics can be improved as compared with the non-single-crystal thin-film solar cell described above.

In particular, the μc film is formed at a substrate temperature of 70 to 12
By forming the film at 0 ° C., generation of voids and an amorphous layer in the initial stage of film formation can be suppressed. A-SiO of the interface layer is
Plasma C using a mixed gas containing silicon hydride and CO ₂
It can be formed by a VD method, a thermal CVD method or a photo CVD method. Therefore, the present invention relates to the structure and manufacturing method of a high efficiency non-single-crystal thin film solar cell, and is an important invention.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a non-single-crystal thin-film solar cell according to a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a-SiO 2 thickness of a p / i interface layer and cell characteristics.

FIG. 3 is a cross-sectional view of a non-single-crystal thin-film solar cell of Comparative Example 1.

FIG. 4 is a cross-sectional view of a non-single-crystal thin-film solar cell of Comparative Example 2.

FIG. 5 is a cross-sectional view of a non-single-crystal thin-film solar cell of Comparative Example 3.

[Explanation of symbols]

 1. Glass substrate 2. Metal electrode 3. 3. n-layer (a-Si) 4. i-layer (a-Si) 5. p / i interface layer (a-SiO 2) 6. p-layer (μc-Si) Transparent electrode 8. Grid electrode 9. p-layer (a-SiO)

Claims (8)

[Claims] 1. A pin having a stacked structure of a p-type semiconductor layer made of an amorphous thin film, a substantially intrinsic i-type semiconductor layer, and an n-type semiconductor layer.
In a non-single-crystal thin-film solar cell having at least one junction, at least one p-type semiconductor layer or n-type semiconductor layer of a pin junction is formed of an amorphous film including a microcrystalline phase,
A non-single-crystal thin film having 1 to 8 nm thick amorphous silicon oxide between an amorphous semiconductor layer containing the microcrystalline phase and an i-type semiconductor layer made of an amorphous film. Solar cells. 2. The non-single-crystal thin-film solar cell according to claim 1, wherein the amorphous film containing a microcrystalline phase is made of silicon or a silicon alloy. 3. The non-single-crystal thin-film solar cell according to claim 1, wherein the semiconductor layer containing a microcrystalline phase is a silicon alloy and is one of silicon oxide, silicon carbide, and silicon nitride. 4. The non-single-crystal thin-film solar cell according to claim 1, wherein the i-type semiconductor layer made of an amorphous thin film is made of silicon or a silicon alloy. 5. The non-single-crystal thin-film solar cell according to claim 1, wherein the i-type semiconductor layer is made of amorphous silicon germanium. 6. A pin formed by laminating a p-type semiconductor layer made of an amorphous thin film, a substantially intrinsic i-type semiconductor layer, and an n-type semiconductor layer.
In a non-single-crystal thin-film solar cell having at least one junction, at least one p-type semiconductor layer or n-type semiconductor layer of a pin junction is formed of an amorphous film including a microcrystalline phase,
Fabrication of a non-single-crystal thin-film solar cell having an amorphous silicon oxide layer having a thickness of 1 to 8 nm between an amorphous semiconductor layer containing the microcrystalline phase and an i-type semiconductor layer made of an amorphous film A method for manufacturing a non-single-crystal thin-film solar cell, comprising: forming an interface layer of silicon oxide on an i-type semiconductor layer; and forming an amorphous film containing a microcrystalline phase thereon. 7. A plasma CVD method, a thermal CVD method or an optical CVD method using a mixed gas containing silicon hydride and carbon dioxide.
The method for producing a non-single-crystal thin-film solar cell according to claim 6, wherein the interface layer of silicon oxide is formed by any one of the methods. 8. An amorphous film containing a microcrystalline phase is formed at a substrate temperature of 70.degree.
The method of manufacturing a non-single-crystal thin-film solar cell according to claim 6, wherein the non-single-crystal thin-film solar cell is formed at a temperature of from −120 ° C.