Effect of interdiffusion and impurities on thin film CdTe/CdS photovoltaicjunctions

We have used low temperature photoluminescence (PL) to study thin film CdTe /CdS solar cell structures. The devices were produced by close space sublim ation (CSS) and have undergone a post-growth treatment, a vital step in inc reasing device efficiency. The treatment consisted of evaporating a thin la yer of CdCl2 onto the back CdTe surface and heat treating in air at 400 deg reesC for between 10 and 120 min. This produced a range of device efficienc ies from 2% to 9%. The efficiency improvements are the result of a complex interaction between the CdCl2, impurities and sulfur interdiffusion. The st ructures were prepared for PL by a chemical bevel etching technique which a llows the luminescence emission to be studied as a function of depth throug hout the sample. The main features in the PL spectra have been identified a s being due to the CI-A center and the Te-dislocation-related Y luminescenc e band. Using PL we have quantified the S diffusion into the CdTe which has a maximum of 20% at the interface in the most efficient samples. We have a lso obtained the profiles of recombination and non-radiative recombination centers in the device. We observe correlations between impurity centers and device efficiency which can help explain the effects of the CdCl2 treatmen t on the optoelectronic properties of the CdTe/CdS junction. (C) 2000 Kluwe r Academic Publishers.