Slow Cooling of Hot Polarons in Halide Perovskite Solar Cells

High Resolution Image Download MS PowerPoint Slide Halide perovskites show unusual thermalization kinetics for above-bandgap photoexcitation. We explain this as a consequence of excess energy being deposited into discrete large polaron states. The crossover between low-fluence and high-fluence “phonon bottleneck” cooling is due to a Mott transition where the polarons overlap ( n ≥ 10 18 cm –3 ) and the phonon subpopulations are shared. We calculate the initial rate of cooling (thermalization) from the scattering time in the Fröhlich polaron model to be 78 meV ps –1 for CH 3 NH 3 PbI 3 . This rapid initial thermalization involves heat transfer into optical phonon modes coupled by a polar dielectric interaction. Further cooling to equilibrium over hundreds of picoseconds is limited by the ultralow thermal conductivity of the perovskite lattice.