Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Patent
1998-03-13
1999-11-30
Chapman, Mark
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
136264, 427 74, 427 76, 4272552, 438761, 438763, H01L 3100
Patent
active
059946423
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for preparing a CdTe semiconductor film which is used mainly for solar cells, in particular, to a method for preparing the CdTe film by a close-spaced sublimation process.
BACKGROUND ART
In recent years, global environmental problems such as global warming due to carbon dioxide, depletion of the ozone layer, and the like have been highlighted. For that reason, expectation for development of new energy, in particular, that for solar cells have become glowingly great. However, for popularization of the solar cells, there are a number of problems which must be solved. In particular, an improvement in conversion efficiency of the solar cells and a reduction in price are desired.
A CdS/CdTe solar cell uses CdTe which has an forbidden band gap (=1.44 eV) being suitable for a light absorption layer, and thus it is one of the solar cells which are expected to yield a high conversion efficiency.
In the manufacturing process of the CdS/CdTe solar cell, a cadmium telluride (CdTe) film is formed on a surface of a cadmium sulfide (CdS) film in general. As a method for forming the CdTe film, the close-spaced sublimation process which can produce the CdTe film of high quality is attracting attention. The close-spaced sublimation process is a kind of the vapor deposition process. A CdTe solar cell which has a conversion efficiency of the world highest level (15.8%) at present is obtained by this process. The close-spaced sublimation process is disclosed in, for instance, "HIGH EFFICIENCY CdS/CdTe SOLAR CELLS FROM SOLUTION-GROWN CdS FILMS" (The Conference Record of the 22nd IEEE Photovoltaic Specialists Conference (1991) Vol. 2, p.952) by T. L. Chu et. al., or the like. According to this process, a material for forming the CdTe film (hereinafter referred to as a source) and a substrate are so placed as to face each other, with a gap of about 0.5-5 mm, and heated under a reduced pressure. In this manner, the source is caused to sublime and then deposit on the substrate.
According to this process, since the sublimed source is rearranged and crystallized on the substrate placed at a short distance as long as the mean free path level, a CdTe film which has a high crystallinity is obtained. Further, since the treatment is performed under the reduced pressure, the film-forming speed is high.
However, the above-mentioned conventional close-spaced sublimation process has the following problems.
In general, in the close-spaced sublimation process, as a source, a CdTe powder placed on a dish-shaped container so as to cover over it is used. In the above-mentioned literature for instance, a commercially-available polycrystal of CdTe with a purity of 5N, or a powder produced by pulverizing a polycrystal ingot of CdTe obtained by directly implanting a dopant as one of the constituting element, is used as the source.
In addition to the fact that this process uses an expensive CdTe powder, it has a low utilization efficiency. In this process, it is difficult to evenly place as much amount of the source for forming the film just once for covering over the container. For that reason, the same source is repeatedly used for forming the CdTe films. It is difficult to control the CdTe powder or the source since it changes in particle size, powder density, stoichiometric ratio and the like by the sublimation, and hence, with the repetition of the film-forming, dispersions in thickness and in quality of the CdTe film increases gradually. Therefore, the obtained solar cell has a large dispersion in performance. For that reason, in order to make the dispersion in performance of the solar cell small, of the source placed for covering, only about 10% is actually consumed for the film-forming, and the rest is discarded without being used.
Further, according to this process, since the film is formed under the reduced pressure, an equipment must once be stopped when the source is exchanged. Therefore, the production efficiency is also low.
When a CdTe film is formed by this process, on a subs
REFERENCES:
patent: 5248349 (1993-09-01), Foote et al.
patent: 5304499 (1994-04-01), Bonnet et al.
T.L. Chu et al., "High Efficiency Thin Film Cadmium Telluride Solar Cells", 11th E.C. Photovoltaic Solar Energy Conference, pp. 988-990 (1992).
Aramoto Tetsuya
Arita Takashi
Hanafusa Akira
Higuchi Hiroshi
Kumazawa Seiji
Chapman Mark
Matsushita Battery Industrial Co., Ltd.
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