Kesterites and Chalcopyrites: A Comparison of Close Cousins
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Kesterites and Chalcopyrites: A Comparison of Close Cousins Ingrid Repins1, Nirav Vora1, Carolyn Beall1, Su-Huai Wei1, Yanfa Yan1, Manuel Romero1, Glenn Teeter1, Hui Du1, Bobby To1, Matt Young1, Rommel Noufi1 1 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, U.S.A. ABSTRACT Chalcopyrite solar cells based on CuInSe2 and associated alloys have demonstrated high efficiencies, with current annual shipments in the hundreds of megawatts (MW) range and increasing. Largely due to concern over possible indium (In) scarcity, a related set of materials, the kesterites, which comprise Cu2ZnSnS4 and associated alloys, has received increasing attention. Similarities and differences between kesterites and chalcopyrites are discussed as drawn from theory, depositions, and materials characterization. In particular, we discuss predictions from density functional theory, results from vacuum co-evaporation, and characterization via x-ray diffraction, scanning electron microscopy, electron beam-induced current, quantum efficiency, secondary ion mass spectroscopy, and luminescence. INTRODUCTION Chalcopyrite solar cells have progressed from first proof of concept on melt-grown crystals [1] to high-efficiency thin-film laboratory devices [2,3] to manufacturing presently in the hundreds of MW range. Manufacturing volumes are expected to push into the gigawatt (GW) range in coming years due to the combination of relatively high efficiency (compared to CdTe or α-Si) and potentially low processing costs (compared to single-crystal Si). As production volumes increase, concern has arisen as to whether limits in the world supply of In will restrict the amount of chalcopyrite photovoltaics that can be manufactured at low cost. While the current cost of In adds only 1 to 10 ͆/Watt to the price of module manufacturing [4], a tenfold increase in In price would be problematic for the necessary sub$1/W manufacturing cost. Estimates of when In scarcity will become important to module prices range in CuIn1-x,GaxSe2 (CIGS) manufacturing volumes from 4 GW/yr to over 100 GW/yr [4,5,6,7]. If estimates of 4 GW/yr are correct, In scarcity could impact the industry within 10 years. If estimates of over 100 GW/yr are correct, and multiple technologies continue to be close in cost, In scarcity may not impact the photovoltaic industry. To mitigate possible future effects of In scarcity, it has been proposed that CIGS can be replaced by Cu2ZnSn(S,Se)4 in the kesterite structure, in which every two group III (In or Ga) atoms in chalcopyrite structure are replaced by a Zn (group II) and Sn atom (group IV), thus maintaining the octet rule. To date, solar cells with up to 9.7% efficiency [8] have been made using these In-free kesterite absorbers. In our work with the co-evaporation of chalcopyrites and kesterites, we have observed both similarities and differences between absorbers made using these closely related crystal structures. Several aspects of this comparison are discussed in the sections below.
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PREDICTIONS FROM DENSITY FUNCTIONA
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