Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1999-10-07
2001-02-13
Diamond, Alan (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192150, C204S192170, C204S192130, C136S252000, C136S256000, C136S258000, C136S265000, C438S057000, C438S085000, C438S096000, C438S098000
Reexamination Certificate
active
06187150
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a thin film photovoltaic device, and, particularly, to a method for improving the performance of a thin film photovoltaic device. It should be noted that in the present specification the terms “crystalline” and “microcrystalline” are intended to also mean partially amorphous material.
In recent years, photovoltaic devices using a semiconductor thin film have been developed extensively. The object of the development of these photovoltaic devices is to obtain a high quality semiconductor thin film formed on an inexpensive substrate in a low temperature process which affords the compatibility of low manufacturing costs with a high performance. Such a photovoltaic device is expected to be used in various applications such as solar cells and photo-sensors.
As an example of the photovoltaic device, a device in which a transparent conductive film, a thin film photovoltaic unit, a back transparent conductive film and a back metal electrode are successively laminated on a substrate has been known. The back transparent conductive film arranged between the thin film photovoltaic unit and the back metal electrode is formed to remove thermal strain caused by a difference in the coefficient of thermal expansion between the semiconductor thin film and the metal electrode thereby increasing the adhesion strength of the metal electrode to the thin film photovoltaic unit. Further the back transparent conductive film is formed also to prevent metal atoms of the metal electrode from diffusing into the photovoltaic unit. It is generally required for this back transparent conductive film to have a thickness of 10 nm to 1 &mgr;M with high transmittance and a low resistance as low as 1.5×10
−3
&OHgr;cm or less.
Conventionally, the back transparent conductive film which meets such requirements has been formed, for example, by sputtering under conditions of a pressure of 2×10
−2
Torr or less, a substrate temperature of 100 to 450° C. and a discharge power of 500 to 1500 mW/cm
2
.
However, it has been found that the resulting photovoltaic device is not sufficiently improved in the photovoltaic characteristics when the back transparent conductive film is formed under the sputtering conditions as aforementioned.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for easily manufacturing a thin film photovoltaic device improved in photovoltaic characteristics.
The present inventors have made it clear that if the back transparent conductive film is formed under the above described conventionally used sputtering conditions, the photovoltaic unit is damaged by negative ions (e.g., O
−
) produced during sputtering. Specifically, the surface of the photovoltaic unit is oxidized with the result that the photovoltaic characteristics of the resulting photovoltaic device cannot be sufficiently improved.
The present inventors have found that excellent photovoltaic characteristics can be obtained if the back transparent conductive film is formed on the photovoltaic unit by sputtering comprising steps of forming an initial thin back transparent conductive film under a low damage condition only at the initial stage and thereafter forming a main back transparent conductive film with the remainder thickness under conditions ensuring that a high quality film is obtained.
According to a first aspect of the present invention, there is provided a method for manufacturing a thin film photovoltaic device comprising a transparent conductive film, a thin film photovoltaic unit, and a back transparent conductive film and a back metal electrode which are successively formed on a substrate, wherein the back transparent conductive film is formed by sputtering comprising steps of forming an initial back transparent conductive film under a pressure of 5×10
−2
Torr or more for 1 to 30 seconds in the initial stage and forming a main back transparent conductive film having the remainder thickness under a pressure reduced to {fraction (1/10)} the initial pressure or less.
According to a second aspect of the present invention, there is provided a method for manufacturing a thin film photovoltaic device comprising a transparent conductive film, a thin film photovoltaic unit, and a back transparent conductive film and a back metal electrode which are successively formed on a substrate, wherein the back transparent conductive film is formed by sputtering comprising steps of forming an initial back transparent conductive film at a discharge power of 80 to 500 mW/cm
2
for 1 to 30 seconds in the initial stage and forming a main back transparent conductive film having the remainder thickness at a discharge power increased to 4 times the initial discharge power or more.
According to a third aspect of the present invention, there is provided a method for manufacturing a thin film photovoltaic device comprising a transparent conductive film, a thin film photovoltaic unit, and a back transparent conductive film and a back metal electrode which are successively formed on a substrate, wherein the back transparent conductive film is formed by sputtering comprising steps of forming an initial back transparent conductive film at a temperature of 100° C. or less for 1 to 30 seconds in the initial stage and forming a main back transparent conductive film having the remainder thickness at a temperature of 100° C. or more.
In the present invention, although a material of the back transparent conductive film is not particularly limited as long as it is a transparent conductive oxide (TCO), it is preferable to use a film comprising primarily ZnO.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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patent: 5078803 (1992-01-01), Pier et al.
patent: 5078804 (1992-01-01), Chen et al.
patent: 5116479 (1992-05-01), Nakamura et al.
patent: 5578501 (1996-11-01), Niwa
patent: 5605610 (1997-02-01), Ishibashi
patent: 6040521 (2000-03-01), Kushiya et al.
patent: 5-13789 (1993-01-01), None
patent: 5-82814 (1993-04-01), None
patent: 5-82815 (1993-04-01), None
Yamamoto Kenji
Yoshimi Masashi
Diamond Alan
Hogan & Hartson LLP.
Kaneka Corporation
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