Antisite defect-mediated hot carrier cooling in halide double perovskite Cs2AgBiCl6: Microscopic role of electron–phonon coupling

The intrinsic antisite defects on the B-site (BiAg and AgBi) impose fundamental limits on the performance of silver-bismuth halide perovskite-based optoelectronic and photovoltaic devices. Through combining first-principles calculations and nonadiabatic molecular dynamics simulations, we systematically investigate how these defects modulate hot carrier cooling dynamics in Cs2AgBiCl6. Our results reveal that the BiAg antisite induces a significant octahedral distortion that reduces the degeneracy of the conduction band. The enhanced nonadiabatic couplings (NAC) and strengthened electron–phonon coupling ultimately accelerate electron cooling dynamics. As a donor-type defect, the BiAg antisite has limited influence on valence bands and hot-hole cooling process. Acting as a shallow acceptor, AgBi induces minimal structural distortion. Thus, the AgBi-defective system exhibits a similar hot-hole cooling process to its defect-free counterpart. Paradoxically, despite introducing low-frequency phonon modes (<200 cm−1) that can interact with hot electrons, the preserved energy gap and conduction band degeneracy suppress the NAC and electron–phonon couplings, thereby prolonging electron relaxation time. This study provides atomistic insights into defect-mediated carrier cooling processes and establishes defect-engineering strategies for optimizing hot carrier dynamics in double halide perovskites.