Residual strain relaxation suppresses hot carrier cooling in 2D halide perovskites

The extended hot-carrier (HC) lifetime boosts the efficiency of layered halide perovskites (LHPs) as photovoltaic devices. In this regard, residual structural strain plays a crucial role in carrier cooling dynamics, yet atomistic details of such influences remain largely unexplored in these complex hybrid materials. Here, we combine time-domain density functional theory, ab initio and non-adiabatic molecular dynamics (NAMD) to explore the impact of lattice strain relaxation on the HC dynamics in the stable Dion-Jacobson phase LHP (4F-PhDMA)PbBr₄. Despite marginal changes in static structural and electronic properties, strain relaxation extends the hot-hole (HH) lifetime by a substantial 62% in this perovskite under ambient conditions. This substantial change in HH dynamics despite minimal changes in static electronic properties highlights the importance of considering coupled electronic and lattice dynamics in strain-dependent HC relaxation. The detailed analyses reveal that diminished dynamic carrier-phonon interaction strength and faster quantum decoherence with lattice relaxation concertedly decelerate the HH cooling process. The suppressed wavefunction overlaps between participating valence band edge states and deactivated high-frequency phonon modes weaken the carrier-phonon coupling in relaxed LHPs. The tracked structural dynamics further identify that the enforced non-covalent coupling between the inorganic PbBr-layer and organic spacers under residual strain intricately controls the carrier cooling. Moreover, the greater extent of nuclear motion promotes rapid loss of decoherence, hindering the nonadiabatic intraband hole cooling in the relaxed perovskite. The study illustrates the importance of relaxing the in-built residual strain to strategically extend the HC lifetime in LHPs.