Effect of the purity of CdTe starting material on the impurity profile in CdTe/CdS solar cell structures

Abstract

We have investigated CdTe/CdS/In₂O₃:F/glass solar cell structures using quantitative SIMS for profiling the impurity distribution from the CdTe free surface through to the glass substrate. Ion implanted CdTe standards were used. The effect of the purity of the CdTe starting material was determined by studying two structures grown from 7N and 5N source materials. Particular emphasis was placed on the potentially electrically active impurities that may originate from the CdTe starting material, and are likely to affect the CdTe/CdS solar cell performance. It was shown that Cu, Zn, Sn, Sb and Pb profiles had the same level and shape in the CdTe layer regardless of the purity of the starting material used, and were therefore not originating from the starting material. Cl, O, Na and Si showed higher levels for structures grown using 5N purity CdTe compared to those from 7N, and may, at least in part, be due to the CdTe starting material used. It was also postulated that at least some impurities (in addition to Cl) may partially come from the CdCl₂ treatment, and/or from the TCO (In) and glass (for Si and Na). Te and S interdiffusion at the CdTe/CdS interface was also shown to be enhanced when 5N CdTe source material is used as compared to 7N.