Dynamic Screening and Slow Cooling of Hot Carriers in Lead Halide Perovskites

Among the exceptional properties of lead halide perovskites (LHPs) is the ultraslow cooling of hot carriers. Carrier densities below the Mott density for large polarons (≤ ≈10¹⁸ cm^-3 ) are focused on here. As in other semiconductors, a nascent hot electron distribution initially cools down via emission of longitudinal optical (LO) phonons on the 10^-14 -10^-13 s timescale. What distinguishes LHPs from conventional semiconductors is the exceptionally efficient screening in the former. The dielectric screening in LHPs on the 10^-13 s timescale results in an order-of-magnitude reduction in the Coulomb potential upon the formation of a large polaron, likely with ferroelectric-like local ordering. Further LO-phonon emission is inhibited, and this leads to partial retention of hot electron energy on the 10^-12 s timescale, more so in hybrid LHPs than in their all-inorganic counterparts. Further cooling of hot polarons occurs on the 10^-10 s timescale, and this can be attributed to the slow diffusion of heat out of the large polaron volume due to the low thermal conductivity of LHPs. Like other carrier properties, slow hot carrier cooling in LHPs can be intimately related to efficient screening in a soft, anharmonic, and dynamically disordered lattice.