A STUDY OF SEMICONDUCTOR CANDIDATES FOR TERRESTRIAL THIN-FILM PHOTOVOLTAIC SOLAR ENERGY CONVERSION (ZINC-PHOSPHIDE, CELL)

This research program was divided into three parts, all dealing with semiconductors that are candidates for absorber layers for terrestrial thin-film solar cells.;In the second part, p-type cadmium telluride films deposited on glass by Menezes at the University of Mexico by hot-wall flash evaporation were studied. Typical films were highly resistive; attempts to lower the resistivity led to nonstoichiometric films. Observed electrical characteristics could be explained in terms of conductive tellurium pathways in a resistive CdTe matrix.;In the third and major part, the phenomena occurring at the surface of bulk polycrystalline zinc phosphide and at interfaces of zinc phosphide based devices were investigated so that photovoltaic performance could be improved.;Electrical measurements of magnesium/zinc phosphide junctions formed on annealed zinc phosphide surfaces, and analyses of similar surfaces by Rutherford backscattering and Auger electron spectroscopy, indicate that loss of phosphorus from a very thin surface region is sufficient to severely degrade diode characteristics.;In the first part, cadmium sulfide/indium phosphide heterostructures were fabricated by evaporating n-type cadmium sulfide onto p-type indium phosphide films grown at the Electronics Research Center of Rockwell International by metalorganic chemical vapor deposition (MOCVD) either on single crystal InP, or on zinc-doped gallium arsenide films deposited by MOCVD on molybdenum and glass. The best efficiencies obtained were 4.6% for the epitaxial films, and 1.4% for the polycrystalline films (compared with 7.5% for a junction formed on single crystal InP).;Surface photovoltage measurements show that at the surface of as-etched or as-polished zinc phosphide the energy bands are nearly flat, in contrast with p-InP and p-CdTe, which have a barrier at similarly prepared surfaces.;Passivating the zinc phosphide surface by depositing insulators proved difficult. High-frequency capacitance measurements of aluminum/aluminum oxide/zinc phosphide structures indicate that a high interface state density (greater than ten to the thirteenth states per electron volt per square centimeter) precludes barrier formation by severely pinning the surface Fermi level. MIS structures formed with a variety of insulators did not show improved diode characteristics not only because of Fermi level pinning, but also because of defects in the thin insulating layers.