Thermochemical and Kinetic Aspects of Cadmium Telluride Solar Cell Processing
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Thermochemical and Kinetic Aspects of Cadmium Telluride Solar Cell Processing
Brian E. McCandless Institute of Energy Conversion University of Delaware Newark, Delaware 19716 USA ABSTRACT A chemical and kinetic basis underlying processing strategies for thin film polycrystalline CdTe/CdS solar cell fabrication is presented. The processing conditions employed for moderate and high conversion efficiency CdTe/CdS solar cells fall within a consistent framework based on temperature and concentration of CdCl2 and O2 species during film deposition or thermal treatment. Detailed microstructural and compositional results are compared for thin-film CdTe/CdS structures with CdTe deposited by physical vapor deposition, electrodeposition and close-space sublimation. X-ray diffraction coupled with a model for diffusion of CdS into CdTe is used to determine the effect of treatment conditions on bulk and grain boundary diffusion coefficients. The effects of CdS diffusion, oxides, recrystallization and grain growth with respect to device efficiency, stability and process control over large area are discussed.
INTRODUCTION Processing highly efficient CdTe thin-film solar cells in a superstrate configuration involves exposure of CdTe/CdS couples to high temperature and reactive chemical species, in particular CdCl2 and O2 [1]. Although the role of CdCl2 and O2 is not quantitatively understood, processing sequences have been combined to yield small-area cells with conversion efficiency, η, up to ~13% [2]. Obtaining the highest efficiencies, greater than 15%, is achieved by depositing CdTe by one specific method, close-space sublimation, at temperatures near 600°C, and deposition in O2 partial pressure has been reported to be advantageous [3,4]. These devices exhibit open circuit voltages, Voc ~ 830 mV and short circuit current, Jsc ~ 25 mA/cm2 and are sensitive to small processing deviations. For example, to obtain Jsc > 25 mA/cm2 under AM 1.5 illumination requires CdS thickness less than 100 nm in the operating device. Obtaining high Voc in the same device requires lateral continuity in the CdS film to avoid occurrence of parallel junctions between CdTe and the underlying transparent oxide (TO). This is because CdTe/TO junctions exhibit higher dark diode current, Jo, than CdTe/CdS junctions, and a surprisingly low area density of CdS voids, < 0.01%, can reduce the Voc of the entire device by ~100 mV. A key processing challenge is thus to minimize CdS film thickness while maintaining CdS film integrity during thermal and chemical processing steps such as CdTe deposition, post-deposition treatment and contact formation. This mandate has enormous impact for process control when fabricating large-area CdTe/CdS cells at high efficiency. This paper presents the relationship between the CdTe-CdS-CdCl2-O2 chemical system, film deposition and treatment processes, and film structure and diffusion. The impact of processing variations on electrical device operation and implications for stability and large area manufacture are addressed. H1.6.1
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