Photoluminescence Studies on Cu and O Defects in Crystalline and Thin-film CdTe
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Photoluminescence Studies on Cu and O Defects in Crystalline and Thin-film CdTe Caroline R. Corwine,1 Timothy A. Gessert,2 James R. Sites,1 Wyatt K. Metzger,2 Pat Dippo,2 Jingbo Li,2 Anna Duda,2 and Glenn Teeter 2 1 Colorado State University, Fort Collins, CO 80523 2 National Renewable Energy Laboratory, Golden, CO 80401 ABSTRACT Polycrystalline thin-film CdTe is one of the leading materials used in photovoltaic solar cells. One way to improve device performance and stability is through understanding how various process steps alter defect states in the CdTe layer. Low-temperature photoluminescence (PL) studies show a 1.456-eV PL peak in single-crystal CdTe that is likely due to a Cui-OTe defect complex. A similar peak, observed in as-deposited glass/SnO2:F/CdS/CdTe thin-film structures, strongly suggests a common origin. The 1.456-eV peak is also seen in a thin-film sample after performing the CdCl2 treatment needed for high efficiencies. INTRODUCTION CdTe is of special interest in the photovoltaic (PV) industry because of its nearly ideal band gap of ~1.47 eV at room temperature. Advancement in the efficiency of polycrystalline CdTe/CdS solar cells has occurred mostly through refinement of processing techniques rather than by understanding how the processing techniques are affecting the defect states in the CdTe, and thus enhancing current transport in the device. The effects of copper- and oxygen-related impurities are of prime interest in CdTe PV devices because these elements have been intentionally introduced into device processing techniques to improve device performance. In most devices, Cu is intentionally incorporated during the back-contact process. However, even before this final process step, Cu is present in the CdTe layer as a native impurity, as well as introduced during the CdCl2 treatment. Thus, even if Cu were eliminated from the final contacting process, Cu-related defects would still be present in the device and would influence the overall device performance. Oxygen is included in the chamber atmosphere during the CdTe film growth. This has been shown to reduce the pinhole density [1], so that the subsequent backcontact process does not shunt the device, and to stimulate the growth of smaller grains in the film [2]. Studies on completed devices show that solar cells made with O2 present during the film growth have a higher efficiency than those that do not incorporate oxygen [3]. Additionally, it has been noted that an oxygen-containing ambient during CdCl2 processing improves device efficiency [4]. Oxygen and copper are introduced simultaneously into thin-film CdTe solar cells during the CdTe growth stage: oxygen in the ambient gas and copper as a residual source impurity. It is very likely that these two species will interact to form defect complexes involving both Cu and O. Most previous defect studies on CdTe have investigated the effects of Cu or O separately [5]. Those studies that include both Cu and O either did so unintentionally [6] and did not consider that both species were p
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