Band-Like Transport in Low-Dimensional Ternary Chalcogenides

Chalcogenides have long been appealing for photovoltaics becasue of their stability, composition of Earth-abundant, non-regulated elements, and processability by scalable growth methods. Recently, these materials have gained renewed interest for the development of nontoxic alternatives to lead-halide perovskites. However, a significant challenge has been carrier localization, which limits the performance of these materials. Carrier localization has been so widely found among heavy pnictogen-based perovskite-inspired materials that it is being labelled a hallmark of these materials. In this talk, I will discuss our recent discovery and rationalization of band-like transport in CuSbSe₂, a structurally 2D material. Through optical pump terahertz probe spectroscopy, along with temperature-dependent mobility measurements, we verify that this material avoids carrier localization, exhibiting instead large polaron formation. Making use of detailed computations, we reveal the underlying factors are 1) the presence of regular free volume in this layered material to relax distortions from the propagation of acoustic phonon modes, 2) quasi-bonding between layers increasing the electronic dimensionality, and 3) Born effective charges that are not anomalously high on Sb3+. The former two factors favour low acoustic coupling constants, while the latter favours a lower Fröhlich coupling constant, together enabling band-like transport. We anticipate these principles to be generalizable, and could enable the development of perovskite-inspired materials with longer diffusion lengths.