Photosensitive materials, i.e., materials having characteristics that change when struck by electromagnetic radiation, have been of scientific and technological interest for many years. This interest has increased in recent years because of the realization that fossil fuels are being depleted and the prospect that photovoltaic solar energy conversion devices might be extensively used. Interest has been further stimulated in recent years because of the need for sensitive photodetectors, that is, optical radiation detectors, in optical communications systems. Many devices and materials have been considered as candidates and numerous approaches have been taken in fabricating such devices and materials.
One class of materials that is of particular interest for photosensitive devices, including photovoltaic devices, is formed by the chalcogenide, i.e., Group VIA, semiconductors, i.e., semiconducting compounds of sulfur, selenium and tellurium. This class of materials includes cadmium telluride, cadmium sulfide, cadmium selenide, lead selenide, etc. These compounds have been used in heterojunction solar cells, optical detectors, and more recently, semiconductor liquid junction solar cells. The latter cells are described in a review paper written by Heinz Gerischer and published in Electroanalytical Chemistry and Interfacial Electrochemistry, 58 pp. 263-274, (1975). Cadimum selenide is a leading candidate for use in such cells because its bandgap is not far from the theoretically most efficient bandgap for photovoltaic devices using sunlight.
Several methods have been used in the fabrication of cadmium selenide films for such cells. For example, the films have been electrodeposited both cathodically and anodically as well as electrolessly. The former method uses a selenious acid-cadmium ion solution and the latter method uses a cadmium anode in selenide medium. The methods are discussed in Nature, 261, pp. 403 (1976) and U.S. Pat. No. 4,127,449 issued on Nov. 28, 1978 to Adam Heller and Barry Miller, respectively. The electrodeposition methods are attractive candidates for use in photoelectrochemical cells, such as semiconductor liquid junction solar cells, because of the possibility of producing low cost and large area films.
Anodically deposited n-type cadmium selenide, CdSe, films are attractive candidates for use as photosensitive films because they have fairly good, as formed, photoresponses. However, the films are formed by anodizing cadmium in a selenium containing electrolyte and their growth is therefore restricted in thickness by the necessity of cadmium ion transport through the film. Cathodically deposited cadmium selenide films are not so limited in thickness because only electron transport to the solution interface is required. However, thermal treatment after electrodeposition or formation is required to develop adequate photoresponse.
The cathodic method reduces solutions of selenium oxide and selenious acid in the presence of relatively high concentrations of cadmium sulfate to yield a film of cadmium selenide at the cathode surface. A study of the reduction of selenious acid and cadmium selenide formation has shown that the initial cathodic reaction involves the formation of H.sub.2 Se. The desired deposition of cadmium selenide then proceeds through the reaction of this compound with Cd.sup.+2 ions to deposit cadmium selenide. However, the H.sub.2 Se formed may also undergo a reaction with the selenious acid to form elemental selenium. The rate of the latter reaction depends upon the concentration of the selenious acid but is sufficiently rapid at high acid concentrations so that elemental selenium is deposited with CdSe in significant amounts in cathodically produced films. Elemental selenium deposition can be minimized by plating at low selenious acid concentrations but the excess selenium in the film cannot be completely eliminated. The excess selenium is undesirable because the film initially has mixed n- and p-type behavior in its photocurrent spectra and the thermal treatment step previously mentioned betters the photocurrent spectra of the film by improving the initially poor crystallinity and vaporizing excess selenium.
Some of the above problems could be avoided by an electroless deposition method such as that disclosed in the Russian Journal of Inorganic Chemistry 14 (3) pp. 322-324 (1969) and discussed more fully in Thin Solid Films 60 pp. 55-59 (1979). Briefly, the method uses the decomposition of sodium selenosulfite in an alkaline solution of the corresponding cation. However, the reaction rate must be kept slow to avoid excessive precipitation. Further, while the initial growth rate may be as high as 200 Angstroms/minute, it decreases and ultimately falls to zero and thus provides a maximum achievable film thickness.
Thus, a process for the cathodic electrodeposition of metal-selenide films, such as CdSe, which does not require a thermal treatment step after deposition and which retains the desirable characteristics of CdSe films would be advantageous.