A-Cation-Dependent Excited State Charge Carrier Dynamics in Vacancy-Ordered Halide Perovskites: Insights from Computational and Machine Learning Models

Lead-free vacancy-ordered halide perovskites (VOHPs) are emerging as promising materials for environmentally friendly next-generation optoelectronic devices. However, a detailed atomistic understanding of charge carrier dynamics under ambient conditions ( 300 K) is lacking. Thus, attempts to refine the optoelectronic performance of VOHPs through material selection remain challenging. Here, we combine non-adiabatic molecular dynamics, time-domain density functional theory, and an unsupervised machine learning (ML) model for in-depth analyses of prevailing nonradiative carrier recombination and its entropy correlation with real-time structural dynamics. Our key findings illustrate that the thermal fluctuations are intricately linked to the performance-limiting nonradiative recombination in these perovskites. The detailed ML model-based analyses reveal the complex nonlinear correlations between photophysical properties and several structural features that dominantly influence excited state carrier dynamics. The atomistic insights unambiguously demonstrate that the organic methylammonium (MA) cation as the A cation in the lattice significantly impacts thermal fluctuations of the isolated inorganic metal halide octahedral subsystem. The stronger electron-phonon interactions introduce substantially faster nonradiative carrier relaxation over time. The inorganic elements, cesium and rubidium (A cations), largely restrict the lattice dynamics, weakening instantaneous nonradiative electron-phonon coupling in VOHPs. These inorganic VOHPs exhibit longer carrier lifetimes, depicting suppressed nonradiative carrier recombination processes. Our study emphasizes the decisive impacts of complex dynamic structure-excited state property correlations on the charge carrier dynamics in a group of primarily isostructural halide perovskites. This study provides valuable insights into the strategically designed lead-free perovskites for optoelectronic device applications.