Synthesis of Optimized CZTS Thin Films for Photovoltaic Absorber Layers by Sputtering from Sulfide Targets and Sulfuriza
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1268-EE03-04
SYNTHESIS OF OPTIMIZED CZTS THIN FILMS FOR PHOTOVOLTAIC ABSORBER LAYERS BY SPUTTERING FROM SULFIDE TARGETS AND SULFURIZATION H. Nukala1, J. L. Johnson2, A. Bhatia1, E. A. Lund3, W. M. Hlaing Oo1, M. M. Nowell4, L. W. Rieth2, and M. A. Scarpulla1,2 1
Materials Science & Engineering, University of Utah Electrical & Computer Engineering, University of Utah 3 Chemical Engineering, University of Utah 4 EDAX Inc., Draper, UT 2
ABSTRACT Cu2ZnSnS4 (CZTS) is a promising alternative for Cu(In,Ga)Se2 (CIGS) absorber layers in thin film solar cells and is comprised of commodity elements which will enable scale-up of chalcopyrite panel production unhindered by elemental supplies and costs. Various CZTS synthesis methods, especially sulfurization of stacked metal or metal sulfide layers, are being studied and have led to cell efficiencies up to 6.7 % [1]. Here we report our studies of CZTS thin film synthesis via room temperature co-sputtering from Cu2S, ZnS and SnS2 binary targets, followed by sulfurization between 500 °C – 650 °C using elemental sulfur vapor. Sputtering from sulfur-containing targets is designed to increase the sulfur content in the precursor films to promote stoichiometry. We report on the effects of processing including deposition on soda-lime and borosilicate glasses and deposition of Na-containing layers on film morphology, composition, phase, grain size, and electronic structure. INTRODUCTION Single-crystalline silicon remains the leading photovoltaic (PV) material, although thin-film technologies such as CIGS and CdTe are quickly advancing [2]. However these materials use rare and/or toxic elements. CZTS is derived from CIGS by replacing (In, Ga) atoms with Zn+Sn and Se with S thus using only environment-friendly commodity elements. It has a direct band gap of 1.5 eV and a high absorption coefficient allowing films ~1 µm thick to be used however its real enabling feature may be its predicted tolerance to native defects [3]. Further improvements to material and processes have the potential to raise the efficiency of CZTS PV cells into ranges useful for TW-scale terrestrial generation. EXPERIMENTAL CZTS sputtering Molybdenum layers were first sputtered on soda-lime (SLG) or borosilicate (BSG) glass substrates. Precursor CZTS films were co-sputtered from SnS2, Cu2S, and ZnS targets at different powers to control film stoichiometry [4]. Co-sputtering was chosen so as to incorporate as much sulfur as possible into the films and to suppress the long-range diffusion-mediated phase transitions required for synthesis from discrete stacks.
Sulfur annealing The sulfur content is still
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