Core Structures of Dislocations within CdTe Grains
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Core Structures of Dislocations within CdTe Grains Chen Li1,2, Timothy J. Pennycook3,1,#, Donovan N. Leonard1,*, Kim Jones4, Zhiwei Wang4, Mowafak Al-Jassim4, Naba Paudel5, Yanfa Yan5 and Stephen J. Pennycook1 1
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA 2 Department of Chemistry, Vanderbilt University, Nashville, TN 37235 USA 3 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 USA 4 The Measurements and Characterization Group, National Renewable Energy Laboratory, Golden, CO 80401 USA 5 Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606 USA # Now at SuperSTEM Facility, Daresbury, UK. * Now at Evans Analytical Group, Raleigh, NC 27606 USA ABSTRACT CdTe is well known as an excellent photovoltaic material for high efficiency solar cell applications because it has a direct band-gap, low fabrication cost and high optical absorption coefficient. However, the nonradiative recombination and low average minority carrier lifetime caused by the defects in CdTe solar cells limit its efficiency. So far, grain boundaries (GB) have been considered to be the major origin of the nonradiative recombination. However, we show that CdTe grains contain many dislocations that could limit device efficiency. Scanning transmission electron microscopy (STEM) was used to determine the atomic structure of intrinsic and extrinsic stacking faults and their terminating partial dislocation cores. Z-contrast images are sensitive to atomic number and are able to distinguish Cd and Te atomic columns. Unpaired Cd and Te atomic columns were found to form the partial dislocation cores, suggesting the presence of dangling bonds. These defects are likely to be electrically active, and may be the origin of the low minority carrier lifetime. INTRODUCTION Cadmium telluride is a II-VI compound semiconductor which is the basis for one of the leading thin film solar cell technologies [1,2]. Its band-gap of ~1.48 eV gives a ShockleyQueisser (SQ) efficiency limit of 32%. For several years the record efficiency for a CdTe/CdS thin film solar cell was 16.7%, achieved by the National Renewable Energy Laboratory [3], but in the past year, First Solar Inc. and GE company continued to update the world record to 18.7 % [4]. However there is still a large gap between the current world record and the theoretical efficiency. In order to understand the critical problems limiting the efficiency of CdTe solar cells, we have developed a program combining aberration-corrected STEM & electron energy loss spectroscopy (EELS), electron beam induced current (EBIC), cathodoluminescence (CL) and theoretical calculations to correlate structure to electrical and optical properties, including the effect of CdCl2 heat treatment. Aberration correction provides higher resolution, or alternatively, it can provide higher current, which therefore generates higher EELS, CL and EBIC signal levels. This combination system will provide a direct correlation between the atomic structur
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