Epitaxial thin films of CdTe (1–5 μm) have been grown directly onto (001) InSb substrates or onto intermediate buffer layers of InSb (0.25–0.5 μm) by molecular beam expitaxy. Cross-sectional transmission electron microscopy and high-resolution transmission electron microscopy have been used to characterize the film and interfacial microstructures. Inferences about film quality were also compared with single-crystal x-ray rocking curve data and agreed well. Resulting microstructural features were correlated with various experimental growth parameters and substrate cleaning procedures. Results show that near-perfect CdTe films can be grown on InSb substrates, but film quality is critically dependent upon substrate cleaning. Other factors observed to influence defect formation in the films include growth rate, total growth time, or a change in growth rate during film growth. Extended defects which form include twins, line dislocations, or looplike defects. Lattice imaging has demonstrated the lattice matching across the InSb film/InSb substrate interface, despite the formation of In precipitates during the heat cleaning procedure.
Photovoltaic materials are reviewed with regard to their possible use in systems that could provide very large amounts of electric power from the Sun before the end of the century. The key is taken to be the cost of the solar cells which are considered to be presently about two orders of magnitude too high. Only silicon, in single crystal or ribbon form, or CdS in thin-film form are thought to be sufficiently developed to permit their possible large-scale exploitation by the last decade of the century.Silicon is considered to have the advantage over CdS at present for large-scale use because of the higher performance levels and the broader existing technology base. CdS thin films are considered to have greater potential if selected improvements can be effected in design and performance, because of lower projected cost and the ease of automating manufacture.
Techniques have been developed for separating the Cu2S layer from Cu2S/CdS thin-film solar cells by slowly dissolving away the CdS. Free-standing Cu2S layers, cemented-down Cu2S layers, Cu2S layers exposed after potting and cross sectioning, and replicas of Cu2S layers have been prepared and examined in the scanning electron microscope. The Cu2S morphology so disclosed is a complex composite of a conformal coating of the upper surface of the texture-etched CdS film and more or less vertical penetrations into the CdS film along grain boundaries. The texture etch of the CdS film opens deep [about (10–15) ×10−4 cm] etch pits from which a substantial portion of the Cu2S penetration into grain boundaries occurs. The thickness of the Cu2S layer of conventional CdS thin-film solar cells is about 1000 Å, though the thickness of some of the deeper penetrations may be less than 500 Å.
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