Impacts of CsPbBr₃/PbSe Heterostructures on Carrier Cooling Dynamics at Low Carrier Density

Abstract Metal halide perovskite nanocrystals have recently emerged as a front‐runner material for high‐performance solar cells. However, slowing down the hot carrier (HC) cooling of perovskites at carrier densities comparable to the sun‐illumination level (≈10¹⁷ cm⁻³) is still a thriving challenge. A new strategy is presented to retard the HC cooling via charge localization at the CsPbBr₃/PbSe heterostructure interface. Ultrafast transient absorption study reveals two times slower HC relaxation time (from 770 fs to 1.4 ps) and much higher initial HC temperature, T_C (1663 K, compared to 900 K) for the heterostructure compared to the pure CsPbBr₃ at 3.64 eV excitation, under 1.86 × 10¹⁷ cm⁻³ carrier density. Furthermore, a combination of an electron‐phonon coupling model and first‐principles calculations confirm the slow relaxation through the Klemens channel due to a large energy gap between the longitudinal optical (LO) and longitudinal acoustic (LA) phonon modes. The localization of charge density near the heterojunction is responsible for the up‐conversion of LO modes to the higher energy and retards the HC relaxation. The findings highlight a new protocol for achieving long‐lived HCs in perovskites at low carrier densities, which would benefit the future of HC photovoltaics.