Abstract
We achieved annealing-induced optical-bandgap widening in Cu2ZnSnS4 (CZTS), which is a promising material for use in high-efficiency thin-film solar cells, and we performed synchrotron-radiation X-ray absorption fine structure (XAFS) measurements to study the reasons underlying the widening. CZTS thin-film samples were fabricated at a low temperature of 230 °C via sputtering with a CZTS ceramic target followed by annealing at up to 550 °C. Optical absorption spectroscopy revealed optical bandgap widening when the annealing temperature was ≥350 °C. Compositional changes, phase separations, and lattice strains were excluded as possible reasons. XAFS measurements revealed that the radial distribution functions (RDFs) around the Zn K-absorption edge almost completely corresponded to those of a kesterite structure. In addition, the RDFs indicated that the disorder of atomic positions within 1 nm from zinc atoms tended to decrease with increasing annealing temperature, which we conclude is the primary reason for the optical-bandgap widening.
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