Polarons in Metal Halide Perovskites

The peculiar optoelectronic properties of metal-halide perovskites, partly underlying their success in solar cells and light emitting devices, are likely related to the complex interplay of electronic and structural features mediated by formation of polarons. In this paper the current status of polaron physics in metal-halide perovskites is reviewed based on a first-principles computational perspective, which has delivered hitherto noaccessible insights into the electronic and structural features associated with polaron formation in this materials class. The role of organic (dipolar) versus inorganic (spherical) A-site cations is extensively analyzed, these cations are related to modulation of the energetics and structural extension of polarons in lead-halide perovskites. Further tuning of polaron energetics is achieved by individual variations in metal (e.g., Pb -> Sn) and halide (e.g., I -> Br), showing a transition from a semilocalized to a localized polaron regime in which charge holes can be trapped at isolated Sn centers. The vastly varying and tunable nature of charge lattice interactions represents a peculiarity of metal-halide perovskites that should be taken into account when designing novel materials or targeting specific compositional engineering of existing perovskites.