Impact of Small Phonon Energies on the Charge-Carrier Lifetimes in Metal-Halide Perovskites

Solar cells based on metal-halide perovskite absorber layers have resulted in outstanding photovoltaic devices with long non-radiative lifetimes as a crucial feature enabling high efficiencies. Long non-radiative lifetimes occur if the transfer of the energy of the electron-hole pair into vibrational energy is slow, due to, e.g., a low density of defects, weak electron phonon coupling or the release of a large number of phonons needed for a single transition. Here, we discuss the implications of the known material properties of metal-halide perovskites (such as permittivities, phonon energies and effective masses) and combine those with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI$_3$ lead to a strong dependence of recombination rates on trap position, which can be readily deduced from the underlying physical effects determining non-radiative transitions. Here, we show that this is important for the non-radiative recombination dynamics of metal-halide perovskites, as it implies that these systems are rather insensitive to defects that are not at midgap energy. This can lead to long lifetimes, which indicates that the low phonon energies are likely an important factor for the high performance of optoelectronic devices with metal halide perovskites.