  We demonstrate synthesis of BaZr(S,Se)3 chalcogenide perovskite alloys by
selenization of BaZrS3 thin films. The anion-exchange process produces films
with tunable composition and band gap without changing the orthorhombic
perovskite crystal structure or the film microstructure. The direct band gap is
tunable between 1.5 and 1.9 eV. The alloy films made in this way feature 100x
stronger photoconductive response and a lower density of extended defects,
compared to alloy films made by direct growth. The perovskite structure is
stable in high-selenium-content thin films with and without epitaxy. The
manufacturing-compatible process of selenization in H2Se gas may spur the
development of chalcogenide perovskite solar cell technology.
