Slow cooling of hot polarons in halide perovskite solar cells

Abstract

Halide perovskites show unusual thermalisation kinetics for above bandgap photo-excitation. We explain this as a consequence of excess energy being deposited into discrete large polaron states. The cross-over between low-fluence and high-fluence `phonon bottleneck' cooling is due to a Mott transition where the polarons overlap ($n\ge 10^{18}/{\mathrm{cm}}^3$) and the phonon sub-populations are shared. We calculate the initial rate of cooling (thermalisation) from the scattering time in the Fröhlich polaron model to be 78 meVps${}^{-1}$ for ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. This rapid initial thermalisation 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 ultra-low thermal conductivity of the perovskite lattice.

7 pages, 1 schematic, 3 figures