On the Origins of Impurities in CdTe-Based Thin Film Solar Cells
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1012-Y03-34
On the Origins of Impurities in CdTe-Based Thin Film Solar Cells Mahieddine Emziane1,2, Douglas P. Halliday1, Ken Durose1, Nicola Romeo3, and Alessio Bosio3 1 Department of Physics, Durham University, South Road, Durham, DH1 3LE, United Kingdom 2 Physics Department, Oxford University, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom 3 INFM - Phyiscs Department, University of Parma, Parco Area delle Scienze, Parma, 7A-43100, Italy ABSTRACT 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 closespace sublimation using 99.99999% (7N) purity CdTe, and some of them were further processed (CdCl2 anneal and Br2-methanol etch). SIMS 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. INTRODUCTION The potential of impurities to affect the performance, stability and lifetime of polycrystalline thin film devices is widely appreciated, but the details of the behavior of specific impurities has only recently been the subject of major investigation. We have recently reported an extensive SIMS study of the distribution of impurities in CdTe/CdS solar cell structures grown on In2O3:F-coated glass substrates [1-4]. This paper concentrates on the detailed SIMS investigation of similar sample structures with quantitative profiling being done from both sides i.e. with SIMS cratering from both the back and front of the structures. The aim is to elucidate the most likely origins of impurities which are relevant to the performance of CdTe-based solar cells. Here the introduction of impurities from the source material is investigated by a process of elimination - that is by using 99.99999% (7N) (i.e. highly pure) CdTe for the growth. Also, the role of processing in impurity introduction was studied by comparing an as-grown control with a processed sample. Nine impurities were shown to be introduced by growth, while eight were increased in concentration upon processing. EXPERIMENTAL The solar cell structure studied was CdTe/CdS/In2O3:F/glass. Both the In2O3:F and CdS layers were sputtered with typical substrate temperatures of 500∞C and 200∞C. A Cerac 99.99% pure CdS target was used. The CdTe starting material was specially prepared to 99.99999% (7N) standard by GEC Marconi InfrañRed (UK). The CdTe layers were deposited by close-space
sublimation (CSS) at
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