Nuclear Quantum Effects Accelerate Hot Carrier Relaxation but Slow Down Recombination in Metal Halide Perovskites

Inorganic semiconductors are composed of heavy elements whose vibrational motions are well described by classical mechanics. Heavy elements, such as Pb and I, support charge carriers in metal halide perovskites. Nevertheless, the soft structure and strong coupling between the organic and inorganic components create conditions in which nuclear quantum effects (NQEs) can play important roles. By combining ab initio, ring-polymer, and nonadiabatic molecular dynamics approaches with time-domain density functional theory, we demonstrate how NQEs influence structural and electronic properties and electron-vibrational dynamics in hybrid organic-inorganic (MAPbI₃) and all-inorganic (CsPbI₃) perovskites. Quantum zero-point fluctuations enhance structural disorder, reduce the band gap, and accelerate elastic electron-vibrational scattering responsible for coherence loss. NQEs have opposite influences on intraband carrier relaxation and interband recombination. These inelastic scattering events are governed by the product of the overlap-like electron-phonon matrix element and atomic velocity. NQEs reduce the overlap and increases the velocity. The intraband carrier relaxation involves many states. Reduction of overlap between some states is offset by other pathways, while an increased velocity makes intraband relaxation faster. Electron-hole overlap in band-edge states plays a key role in the recombination, and its reduction by NQEs-enhanced disorder makes the recombination slower. This phenomenon is seen with both MAPbI₃ and CsPbI₃ and is much more pronounced when a light organic component is present. This study offers a detailed understanding of the role of NQEs in the carrier relaxation processes of perovskites, offering important theoretical insights into hot carriers and carrier recombination that govern the performance of solar cells and other optoelectronic devices.