Does CdTe Deposition Affect the Impurity Profile in Sputtered CdS Window Layers?
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Does CdTe Deposition Affect the Impurity Profile in Sputtered CdS Window Layers? Mahieddine Emziane, Ken Durose, Douglas P. Halliday, Alessio Bosio1, and Nicola Romeo1 Department of Physics, University of Durham, South Road, Durham, DH1 3LE, U. K. 1 Department of Physics, University of Parma, Parco Area delle Scienze 7a, 43100 Parma, Italy. ABSTRACT We report a multi-element study of impurities in CdS window layers by dynamic and quantitative SIMS. Two CdS/TCO/glass samples, grown separately using nominally the same conditions, were considered. In2O3:F grown on soda lime glass was used as TCO followed by a sputtered CdS layer. One of the samples was subsequently used as a substrate for growth of CdTe by CSS. SIMS was carried out on both samples, and O, Na, Si, Cl, Sb, In, Zn, Sn, Pb, Cu, Te, S and Cd were depth profiled. It was shown that before CdTe growth, most of the impurity elements showed flat levels in the CdS ranging from 2-3×1020 cm-3 for Zn and O to 2-3×1017 cm-3 for Na, Cl, Sb and Te. Si was found to segregate at the CdS/TCO interface with a maximum level of 1018 cm-3. However, following CdTe growth, the impurities in the CdS layer showed higher concentrations and different profile shape compared to those before CdTe growth. Some of the impurities also showed a diffusion-like profile following the CdTe growth as compared to before. Possible explanations of these changes are discussed in terms of the purity of the starting materials and the growth environments, as well as the diffusion from the TCO and glass. INTRODUCTION The performance of polycrystalline thin film CdTe/CdS photovoltaic cells critically depends on impurities and interfaces as these are among the main difficulties encountered in fabricating electronic devices. Insufficient experimental data is available in the literature regarding impurities and their behaviour in this kind of solar cell devices. The focus to date was mainly on impurities that were related to the back contact and their diffusion into the CdS and CdTe and their effect on the device, and very little was done to investigate impurities that may originate from the starting materials and environments used for the fabrication and processing of entire devices. The effect on the CdTe/CdS solar structures, of impurities present in the CdS window layer has not been extensively investigated and it is yet not understood in terms of device stability, lifetime and efficiency. CdS thin films are often grown by chemical bath deposition (CBD) rather than the physical growth techniques such as sputtering [1, 2] and close space sublimation (CSS) [3]. It has been shown that CBD grown CdS layers intrinsically contain impurities [3-6], and in particular oxygen that is believed to be beneficial in terms of solar cell efficiency [3]. Intentional doping was also shown to be effective in some cases. For instance, the photovoltaic characteristics of CdTe/CdS solar cells fabricated using boron doped CdS film as the window layer were improved due to the increase of the electrical conductivity
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