Unraveling A-Site Cation Control of Hot Carrier Relaxation in Vacancy-Ordered Halide Perovskites Through Quantum...

The rapid thermalization of photoexcited hot carriers (HCs) through ultrafast electron-phonon scattering represents a fundamental efficiency limitation in optoelectronic devices. Vacancy-ordered halide perovskites (VOHPs) emerge as promising candidates for HC harvesting, where quantum confinement within discrete metal halide octahedral units creates phonon bottlenecks that can substantially prolong HC lifetimes. Here, state-of-the-art nonadiabatic molecular dynamics (NAMD) with time-domain density functional theory (TDDFT) and machine learning (ML) investigates HC dynamics in A₂SnBr₆ (A = Rb, Cs, methylammonium (MA)). While quantum confinement establishes discrete energy states near band edges, thermal motion of polar MA cations breaks this phonon bottleneck through enhanced non-adiabatic coupling, strongly accelerating HC cooling. Contrarily, non-polar inorganic cations (Rb, Cs) exhibit longer HC lifetimes due to suppressed lattice dynamics and weakened electron-phonon interactions. Shapley additive explanations (SHAP) reveal that MA cation-induced inter-octahedral dihedral distortions emerge as dominant structural features accelerating hot electron cooling in MA2SnBr6. However, thermally induced geometric variations play a minor role in altering the electronic and non-adiabatic processes in inorganic A-site cation-based VOHPs. These insights establish fundamental principles for strategic A-site cation engineering in lead-free perovskites, tailoring HC dynamics for high-efficiency optoelectronics.