The photovoltaic effect was first observed in 1839 by Edmund Becquerel when he noted that a voltage appeared across two identical electrodes in a weak conducting solution that was subjected to light. This photovoltaic effect was first studied in solids such as selenium in the 1870's and by the 1880's, selenium photovoltaic cells were produced with 1 to 2% efficiency in converting light to electricity.
Since the initial experimentation with photovoltaics over a century ago, much work has been conducted in developing semiconductors for photovoltaic devices, i.e. solar cells. Much of the initial work was done with crystalline silicon which requires a relatively thick film such as on the order of about 100 microns and also must be of very high quality in either a single-crystal form or very close to a single crystal in order to function effectively. The most common process for making silicon cells is by the single-crystal cylinder process where a single-crystal silicon seed crystal is touched to a molten silicon melt and then withdrawn to provide a raised meniscus of molten silicon with both the seed crystal and the crucible holding the melt rotated oppositely to enhance radial growth. Suitable doping will make the cell either an N-type or a P-type semiconductor and upon slicing into a wafer of about 100 microns and formation of a junction will produce a solar cell or photovoltaic device. In addition, crystalline silicon can be made by casting of an ingot but its solidification is not as easily controlled as with single-crystal cylinders such that the resultant product is a polycrystalline structure. Direct manufacturing of crystalline silicon ribbons has also been performed with good quality as well as eliminating the necessity of cutting wafers to make photovoltaic devices. Another approach referred to as melt spinning involves pouring molten silicon onto a spinning disk so as to spread outwardly into a narrow mold with the desired shape and thickness. High rotational speeds with melt spinning increase the rate of formation but at the deterioration of crystal quality.
More recent photovoltaic development has involved thin films which have a thickness less than 10 microns so as to be an order of magnitude thinner than thick film semiconductors. Such thin film semiconductors include amorphous silicon, copper indium diselenide, gallium arsenide, copper sulfide and cadmium telluride. Amorphous silicon has been made into thin film semiconductors by plasma enhanced discharge, or glow discharge, as disclosed by U.S. Pat. No. 5,016,562. Other processes used to make thin film semiconductors include electrodeposition, screen printing and close-spaced sublimation. The close-spaced sublimation process has been used with cadmium telluride and is performed by inserting a glass sheet substrate into a sealed chamber that is then heated. The glass sheet substrate is supported at its periphery in a very close relationship, normally 2 to 3 mm, to a source material of cadmium telluride. After the heating has proceeded to about 450.degree.-500.degree. C., the cadmium telluride begins to sublime very slowly into elemental cadmium and tellurium and, upon reaching a temperature of about 650.degree.-725.degree. C., the sublimation is at a greater rate and the elemental cadmium and tellurium recombines at a significant rate as cadmium telluride on the downwardly facing surface of the peripherally supported glass sheet substrate. The heating is subsequently terminated prior to opening of the chamber and removal of the substrate with the cadmium telluride deposited on the substrate. Thus, the deposition of the cadmium telluride is at a varying temperature that increases at the start of the processing and decreases at the end of the processing. Furthermore, the largest area on which such close-spaced sublimation has previously been conducted is about 100 cm..sup.2 square. Increasing the size of the substrate can cause problems in maintaining planarity since the heated substrate which is supported at only its periphery will tend to sag at the center.
A more complete discussion of cadmium telluride processing is set forth in Chapter 11 of the book "Harnessing Solar Power-The Photovoltaics Challenge" by Ken Zweibel, published by Plenum Press of New York and London.
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