Defect Physics of CuInSe 2 for Photovoltaic Applications Using Extended X-ray Absorption Fine Structure (EXAFS)
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Defect Physics of CuInSe2 for Photovoltaic Applications using Extended X-ray Absorption Fine Structure (EXAFS) Theanne Schiros,1 Scott Calvin,2 P.E. Stallworth,1 Faisal Alamgir,1 J.-F. Guillemoles,3 S.G. Greenbaum,1 and M.L. den Boer.1 1 Hunter College of the City University of New York, NY, NY 10021, USA 2 Naval Research Laboratory, Washington D.C. 20375, USA 3 Laboratoire d’Electrochimie et de Chimie Analytique ENSCP, 75231 Paris, FRANCE ABSTRACT Native point defects in undoped samples of CuInSe2 (CIS) have been identified by a multiple-edge refinement of the extended x-ray absorption fine structure of the copper, indium and selenium absorption edges. Ab initio theoretical models for the pure compound and for various defect structures were constructed and carrier-type statistics were predicted by simultaneously fitting multiple absorption sites to these models. As expected, a model of our measurements based on pure compounds with no defects does not yield a good fit to the data. We find that a best fit requires a significant population of defects. Preliminary quantitative analysis suggests a 15 % vacancy in Cu, a 2-12% population of Cu-Se anti-sites and 15% In-Se anti-sites. INTRODUCTION Current research has focused on ternary chalcopyrite semiconductor CuInSe2 to elucidate the puzzling interplay between the compound’s exceptional photovoltaic properties and the native point defects which control them [1-7]. Although extensive theoretical calculations [1-3, 5] and experimental measurements[5-9] have been directed toward the investigation of different energy levels attributed to the defects, including vacancies, interstitials and anti-sites, the nature and extent of these defects are poorly understood. A better understanding of the defect structure in CuInSe2 is critical to the development of thin film technology and of value to basic science. Extended x-ray absorption fine structure spectroscopy (EXAFS) studies provide element-specific characterization of the local environment of materials, including interatomic distances, site occupancy factors, and disorder parameters, often unattainable with other techniques such as X-ray diffraction. Since the CIS thin film structure often differs greatly from the bulk and local distortions are of paramount importance to photovoltaic performance, EXAFS spectroscopy is suited to contribute invaluable pieces of information to this puzzle. Spectra were collected on all three elemental K edges and ab initio theoretical standards were built for each absorption site for: (i) defect-free (stoichiometric) CuInSe2, (ii) Cu vacancies, (iii) Cu-Se anti-sites, and (iv) In-Se antisites. By exploiting the redundancy of information offered by multiple absorption sites and taking full advantage of the well-parametrized theory of EXAFS in our analysis, we hope to demonstrate that EXAFS can provide a reliable technique for identifying
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