Slow cooling of hot polarons in halide perovskite solar cells

TL;DR

This paper investigates the slow cooling process of hot polarons in halide perovskite solar cells, revealing the roles of polaron states, phonon bottlenecks, and the material's thermal properties in thermalisation kinetics.

Contribution

It provides a detailed theoretical explanation of the thermalisation process and identifies the impact of polaron overlap and low thermal conductivity on cooling dynamics.

Findings

Initial cooling rate of 78 meVps in CH3NH3PbI3

Identification of a phonon bottleneck at high polaron densities

Cooling limited by ultra-low thermal conductivity

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 Fr\"ohlich 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.