On the origins of impurities in CdTe-based thin film solar cells

We present a multi-element study of impurities in CdTe/CdS photovoltaic cells, aimed at understanding their origins. Our investigation was based on calibrated secondary ion mass spectrometry (SIMS) depth profiling, with O, Na, Si, S, Cl, Cu, Zn, In, Sn, Sb and Pb being studied quantitatively in the CdTe. The solar cell structures were grown by sputtering and close-space sublimation using 99.99999% (7N) purity CdTe, and some of them were further processed (CdCl2 anneal and Br2-methanol etch). SDMS results from both the back and front surfaces are reported. It was shown that all of the impurity species studied (except Na and Pb) are introduced at levels > 1.5×1015 during the growth itself, with their concentrations being between 2 and 1000 times the maxima that could possibly be present in the source material. Processing caused increases in the levels of O, S, Cl, Na, Si and Pb in the CdTe. The strongest increase, Na (× 100), but also a minor increase in Sb, were ascribed to impurities in the CdCl2. © 2007 Materials Research Society.