Electronic Properties, Screening, and Efficient Carrier Transport in NaSbS2

NaSbS2 is a semiconductor that was recently shown to have remarkable efficacy as a solar absorber indicating efficient charge collection even in material containing defects. We report first-principles calculations of properties that show (1) an indirect gap only slightly smaller than the direct gap, which may impede the recombination of photoexcited carriers, (2) highly anisotropic electronic and optical properties reflecting a layered crystal structure, (3) a pushed-up valence-band maximum due to repulsion from the Sb 5s states, and (4) cross-gap hybridization between the S p-derived valence bands and the Sb 5p states. This latter feature leads to enhanced Born effective charges that can provide local screening and, therefore, defect tolerance. These features are discussed in relation to the performance of the compound as a semiconductor with efficient charge collection.