JPH05259492A - Thin film solar cell and manufacture thereof - Google Patents

Abstract

PURPOSE:To raise conversion ratio and to reduce optical degradation by constituting i layer of the cell near the light incident side with amolphous silicon oxide represented by a specific general equation. CONSTITUTION:Relating to a thin film solar battely which has a structure where multiple cells having pin junctions are laminated and where the material of i layer 41 of the cell near the light incident side has a greater optical gap than that of a-Si forming an i layer 42 away from the light incident side, the i layer 41 of the cell near the light incident side is amolphous silicon oxide which is represented by angeneral equation a-Si(1-x)Ox, 0.02<x<0.15 (referred to as a-SiO:H). This kind of thin film solar battery is manufactured by resolving a-SiO:H with a gaseous mixture containing SiH4, CO2 and H2. The ratio between H2 and SiH4 in the gaseous mixture is set to be within the range 15-30.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film solar cell having a pin junction tandem structure made of an amorphous silicon (hereinafter abbreviated as a-Si) material and a method for manufacturing the same.

[0002]

2. Description of the Related Art A source gas is a plasma CVD method or an optical CVD method.
The a-Si solar cell formed by decomposing by a method has a feature that it can be easily thinned and has a large area, and is expected as a low-cost solar cell. The structure of this type of solar cell generally has a pin junction of an a-Si thin film. One method for improving the conversion efficiency of an a-Si solar cell is to stack a plurality of pin junction structures having different optical gaps in the i layer to form a tandem structure. For example, literature, H. Sannomiya, S. Moriuchi, Y.
Inoue, K.Nomoto, A.Yokota, M.Itoh, Y.Nakata and
As known in T. Tsuji "Proc. 5th PVSEC Kyoto (1990)" 387, there is an attempt to make a pin junction structure a three-layer structure. As shown in FIG. 1, this solar cell has a structure in which three pin junction cells are laminated on a transparent electrode 2 on a glass substrate 1 and a metal electrode 6 is provided on the uppermost layer, and light is incident through the glass substrate 1. To do. The i layers 41, 42, and 43 sandwiched between the p layer 3 and the n layer 5 of the pin structure of each cell are made of a material whose optical gap gradually decreases from the light incident side, so that the effective light is obtained. It is not used, and in this case i
Layer 41 is amorphous silicon carbide (a-SiC),
The i layer 42 of the middle cell is made of a-Si, and the i layer 43 of the bottom cell is made of amorphous silicon germanium (a-SiGe). This lamination is a very effective means for suppressing the photodegradation peculiar to a-Si solar cells.

[0003]

Here, the p-layer 3 and the i-layer 4 are provided.
If the light-incident-side cell composed of the 1 and n layers 5 is regarded as one single cell, a-SiC having a wider optical gap than a-Si is used for the i-layer 41.
An open circuit voltage of 0.9 or higher, which is higher than that of a cell using Si for the i layer, can be obtained. a-SiC is formed by decomposing a mixed gas of SiH ₄ , C ₂ H ₂ and H ₂ . However, defects and photoinduced defects in the film increase, which significantly reduces the fill factor and reliability. For example, the optical gap
Regarding a single cell in which a-SiC of Eg = 1.9 eV to 2.1 eV is applied to the i layer, it was extremely difficult to suppress photodegradation to 10% or less even if the i layer was thinned to 100 nm. This accounted for the large weight of the photodegradation of the tandem cell.

An object of the present invention is to use a material having a larger optical gap than a-Si for the i layer of the cell on the light incident side of the tandem cell to enhance the conversion efficiency and to produce a thin film solar cell with little photodegradation and its manufacture. To provide a method.

[0005]

[Means for Solving the Problems]
For the purpose of the present invention, a plurality of cells having a pin junction are stacked.
The material of the i-layer of the cell that has the structure
It is more optical than a-Si that forms the i-layer of the cell farther from the incident side.
Close to the light-incident side in thin-film solar cells with large caps
The i layer of the cell has the general formula a-Si_(1-x)O_xIs expressed as 0.02 <
Amorphous silicon oxide with x <0.15 (hereinafter
a-SiO: H). And that
Such a thin-film solar cell manufacturing method uses a-SiO: H as SiH
_Four, CO ₂And H₂Formed by decomposition of mixed gas containing
Shall be achieved. In addition, H in the mixed gas₂And SiH
_FourIt is effective that the ratio of is in the range of 15 to 30.

[0006]

Operation: represented by a-Si _(1-x) O _x , 0.02 <x <0.15
The thin film is a thin film with an optical gap of 1.9eV to 2.1eV and has very good film characteristics. A cell applying this to the i-layer of the pin junction has less photodegradation, and the initial characteristics and reliability of the tandem cell are low. It can be used as an excellent top cell.

[0007]

Example A single cell as shown in FIG. 2 was produced.
That is, first, a transparent electrode ₂ such as SnO ₂ is formed on a glass substrate 1, and a p-layer 3 made of a-SiO: H, an i-layer 4 made of a-SiO: H and an a-Si are formed on the transparent electrode 2. And n layers having a thickness of 12 nm, 100 nm, and 15 nm, respectively.
Among these, the method for forming the i-layer based on the present invention is as follows. Monosilane was used as a raw material gas in a film forming chamber containing the glass substrate 1 on which the transparent electrode 2 and the p layer 3 had already been formed.
A mixed gas of (SiH ₄ ), carbon dioxide (CO ₂ ) and hydrogen (H ₂ ) is introduced. The gas pressure is 0.5 to 1 Torr
To The substrate temperature is maintained at 150 to 200 ° C., high frequency power is applied between the electrodes, and a-SiO: H film is formed by glow discharge decomposition. The composition of the a-SiO: H film is SiH ₄ and CO ₂ ,
It can be changed by changing the gas ratio of SiH ₄ and H ₂ . a-SiO: H a is represented by the general formula _{a-Si (1-x)} O x, for example, H ₂ / SiH ₄ = 25, the values of CO ₂ / SiH ₄ gas ratio and x when the substrate temperature 170 ° C. There is a linear relationship between them as shown in FIG. The optical gap increases as the value of x increases. For example, when x = 0.03, the optical gap becomes 1.95 eV, and when x = 0.15, the optical gap becomes 2.1 eV. FIG. 4 shows the optical gap of the i layer of a single cell, the open circuit voltage V _OC, and the fill factor FF.
Shows the relationship. Focusing on the open-circuit voltage, it gradually increases with an increase in the optical gap in the region of 2.05 eV or less. On the other hand, regarding the fill factor, it can be seen that the optical gap sharply decreases at an optical gap of 2.0 eV or more. By combining these two, it can be seen that if the optical gap is 2.0 eV or less, a relatively good characteristic with a fill factor of 0.6 or more can be obtained.

FIG. 5 shows that H ₂ / SiH ₄ = 25, CO ₂ / SiH ₄
= 0.13, substrate temperature 170 ° C, a-Si with optical gap 1.95 eV
The time-dependent change of each characteristic when a single cell in which an O: H film is applied to the i layer is subjected to a light irradiation test for 1000 hours is shown. In the figure, J _SC represents the short-circuit current density per cm ² , and EFF represents the conversion efficiency. The artificial sunlight used in this test is AM1.5, 100m
W / cm ² . Looking at this figure, a remarkable increase in V _OC is seen until about 10 hours after the light irradiation. Also, the light deterioration after that is very gradual. Comparing the initial value and the value after 1000 hours, V _OC increased by about 5%, J _SC deteriorated by about 4%, and FF deteriorated by about 6%, but the deterioration rate of conversion efficiency Is as small as about 5%. The value of V _OC is 0.873 V in the initial stage and 0.916 V after 1000 hours, which is a large value for a single cell.
From the above, it can be seen that this cell acts very effectively as a top cell of a multi-layer tandem cell, especially a three-layer tandem cell.

Therefore, as an embodiment of the present invention, the top layer i layer 41 has the structure shown in FIG.
V a-SiO: H, a-Si with an optical gap of 1.75 eV is applied to the i-layer 42 of the middle cell, and a-SiGe with an optical gap of 1.5 eV is applied to the i-layer 43 of the bottom cell to improve the long wavelength sensitivity. Manufactured tandem cells. And so far, V _OC = 2.51V, J _SC = 6.87mA / cm ² , FF = 0.720
, EFF = 12.4%, and a thin film solar cell with little change with time even after a light irradiation test for 1000 hours is obtained.

[0010]

According to the present invention, SiH_Four, H₂In addition to the
CO₂CO of raw material gas mixed with gas ₂/ SiH_FourGas ratio
Since the oxygen ratio in the film can be easily changed by changing
A-SiO: H film for i-layer of top cell
Higher conversion efficiency of 12.4% in case of 3-layer tandem
It is possible to manufacture thin film solar cells with
Came.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of a three-layer tandem cell according to one embodiment of the present invention.

FIG. 2 is a cross-sectional view of a single cell manufactured to investigate the effect of the present invention.

FIG. 3 is a relationship diagram of the CO ₂ / SiH ₄ gas ratio and the oxygen ratio in the film.

FIG. 4 is a relationship diagram of the optical gap and open circuit voltage of the single cell shown in FIG.

FIG. 5 is a time-dependent change diagram of each characteristic of the single cell shown in FIG.

[Explanation of symbols]

 1 glass substrate 2 transparent electrode 3 p layer 4 i-type a-SiO: H film 41 top cell i layer 42 middle cell i layer 43 bottom cell i layer 5 n layer 6 metal electrode

Claims (3)

[Claims] 1. A structure in which a plurality of cells having a pin junction are stacked, and the material of the i layer of the cell closer to the light incident side is optical than amorphous silicon forming the i layer of the cell farther from the light incident side. In the case of a large gap, the i layer of the cell near the light incident side is represented by the general formula a-Si _(1-x) Ox, and 0.02 <x <
A thin-film solar cell characterized by being 0.15 amorphous silicon oxide. 2. Amorphous silicon oxide is Si
The method for producing a thin film solar cell according to claim 1, which is formed by decomposing a mixed gas containing H ₄ , CO _2, and H ₂ . 3. The ratio of H ₂ to SiH _{4 in} the mixed gas is 15 to.
The method for producing a thin-film solar cell according to claim 2, wherein the range is 30.