Abstract
Efficiency limiting mechanisms associated with CdS substrates, CdCl2 treated CdTe films, and Cu/Au contacts were investigated. It was found that heat treatment of CdS in hydrogen ambient prior to CdTe deposition removed the oxygen related defects from the CdS films, but created Cd deficient CdS surface, resulting in an optimum temperature of 450°C. Growth of CdTe films on the CdS under Te-rich conditions enhanced the interdiffusion between CdTe and CdS and reduced the CdTe bandgap to 1.47eV, whereas, growth of CdTe films under Cd-rich conditions retained the CdTe bandgap at 1.5 eV. CdCl2 treatment on CdTe improved the cell performance significantly by virtue of grain growth, reduced leakage current, and change in carrier transport mechanism. However, it also produced defects at Eν + 0.64 and Eν + 0.17 eV due to VCd-Cl complexes. An inverse correlation was found between the density of these defects and V ∞ Finally, a rapid initial degradation was observed in the higher efficiency (10-12%) CdTe cells with Cu/Au contacts but a bromine-methanol etch was found to restore the cell performance.
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