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.