The Significance of Polarons and Dynamic Disorder in Halide Perovskites

The development of halide perovskite semiconductors led to various technological breakthroughs in optoelectronics, in particular in the areas of photovoltaics and light-emitting diodes. Additionally, the study of their fundamental properties has uncovered intriguing puzzles that demand explanation. Polaronic effects associated with the coupling of electrons and holes to polar lattice vibrations are often invoked as a microscopic mechanism to explain various unusual experimental observations. While some form of polaronic behavior undoubtedly exists in these systems, several assumptions underlying standard models used to describe a polaron mechanism appear to be strongly violated in these materials. In this Perspective, we investigate the role of polaronic effects in halide perovskites and summarize signatures and failures of the polaron picture to explain physical characteristics of the materials. We highlight the importance of the complementary dynamic disorder concept that can rationalize various key properties of halide perovskites for which standard polaron and band-theory pictures of carrier transport fail.