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Effects of Impurities in CdTe/CdS Structures: Towards Enhanced Device Efficiencies

We report on a comprehensive study of the effect of impurities in thin film CdTe/CdS PV structures. Dynamic, quantitative SIMS and ICPMS spectrometry have been used to analyze starting materials, device layers and final device structures. Source materials of different purity were used: CdCl2 (5N to 95%), CdTe (7N to 5N), CdS (4N) and TCO (5N). Structures grown on glass have been compared with those grown on sapphire. Impurity profiles were obtained for O, Na, Si, Cl, Cu, Zn, Sn, Sb, In, Pb, S, Cd and Te. The profiles for Cu, Zn, Sn and Pb were found to be independent of the CdTe starting material and post-growth CdCl2 treatment. Cl, O, Na and Si had higher levels in structures grown with 5N CdTe source material. Interdiffusion of Te and S was enhanced in devices grown with 5N CdTe. In was found to originate from the TCO. Na originated principally from the glass but was also introduced by the use of lower grade CdCl2. The Si profile was independent of the substrate used. Intentional incorporation of 62 during the CdTe growth led to an enhanced concentration of both O and Cl in finished devices. This resulted in an improvement in all device parameters and an increase in efficiency from 11.5% to 14%