Impact of composition engineering on charge carrier cooling in hybrid perovskites: computational insights

Abstract

Mixed A-cation halide perovskites have emerged as one of the most promising materials for next-generation optoelectronic applications due to such factors as attractive charge carrier transport properties and enhanced stability under operating conditions. However, the influence of A-cation mixing on the excited state charge carrier dynamics and, particularly, on ultrafast hot-carrier relaxation processes, is yet to be studied in sufficient detail. We combine nonadiabatic molecular dynamics and time-domain density functional theory methods to establish the impact of formamidinium (FA)–cesium (Cs) mixing on the subpicosecond-scale hot-charge carrier cooling processes in FA_1−xCs_xPbI₃ (x ≤ 0.5) materials. Our ab initio study illustrates that the partial substitution of organic FA species with inorganic Cs cations substantially extends the hot-electron and hot-hole relaxation times. Observed increases in the hot-carrier lifetimes indicate better performance of FA_1−xCs_xPbI₃ compared to parent FAPbI₃ in the field of hot carrier solar cells. The atomistic details of lattice dynamics reveal that FA–Cs cation mixing partially suppresses thermal fluctuations in the structure, weakening the carrier–phonon interaction under ambient conditions. Increased structural rigidity and weakened carrier–phonon interactions in turn lower the rates of intraband nonadiabatic transitions of hot-carriers and enhance their excited state lifetimes. The in-depth understanding of the relationship between the dynamic structure and carrier relaxation allows us to further propose rational design principles that can enhance the hot-carrier lifetimes in photoactive materials. The computational guides will help to realize photovoltaic devices that efficiently harvest hot-carriers and exhibit an improved power conversion performance compared to traditional single-junction solar cells.