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Polycrystalline CdSe and CdTe layers were fabricated by putting liquid Cd in contact with Se or Te vapors under constant Ar flow. The crystalline structure, surface properties, and semiconducting properties of these materials have been determined. Both materials were found to be w-type semiconductors. The results show that, under the proper experimental conditions, the liquid metal-vapor reaction enables the synthesis of polycrystalline CdSe photoelectrodes with a 6.9% energy conversion efficiency when used in an electrochemical photovoltaic cell under 80 mW/cm 2 of white light illumination. This efficiency rates amongst the highest ones measured under similar conditions using polycrystalline CdSe. These CdSe layers have a majority charge carrier density of ND = 2.6 x 10 17 cm~ 3 and possess a highly textured surface which is assumed to be mainly responsible for the high photovoltaic efficiency. The highly textured CdTe samples obtained by this process, however, show a photovoltaic conversion efficiency of only 0.2%, and this is seen to be mainly due to their high majority charge carrier density of ND = 7.8 x 10 19 c m " 3 .

I. INTRODUCTION Interest in the use of regenerative electrochemical photovoltaic cells for low-cost solar energy conversion has resulted in a large body of literature expounding the preparation and characterization of polycrystalline CdX (X = S, Se, Te) semiconductor electrodes. Because CdSe and CdTe have appropriate bandgaps for matching the solar spectrum, these compounds have been the focus of much of this research. Polycrystalline photoelectrodes with acceptable performance, sometimes approaching that of single crystals, have been produced by such techniques as vacuum evaporation, electroplating, screen-printing, chemical bath deposition, spray pyrolysis, and pressure sintering.1"15 Studies based on the work of Iwanov and Nanev concerning the direct synthesis of epitaxial II-VI films on single crystal Cd and Zn substrates16 have led to the development of another promising technique for the lowcost production of polycrystalline II-VI layers. This particular method, referred to in the past as the tarnishing reaction17 or the gas-solid process,18 involves the reaction under constant argon flow of a chalcogen vapor with the surface of a heated polycrystalline metal substrate. Throughout these preliminary studies, the metal substrate was always kept at a temperature below its melting point and the reaction proceeded as the chalcogen (X) vapor "'Current address: Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A1. b)Address correspondence to this author. J. Mater. Res., Vol. 6, No. 6, Jun 1991

was brought in contact with the solid metal (M) surface according to the general expression • -MX( so iid)

•^(solid) + -X"( vapor)

Ar flow

The thickness of the II-VI layer thus obtained was shown to vary with reaction time, giving a maximum thickness of ~ 3 /xm after 4 h.17"19 The best photovoltaic energy conversion efficiency for II-VI materials produced under these condit