Cation Disorder Tailors Electron–Phonon Coupling and Prolongs the Hot Carrier Lifetime in Cs ₂ AgInX ₆ Double Perovskites

B-site cation disorder, which has been proven to have significant influence on optoelectronic properties, usually occurs in halide double perovskites. Through first-principles calculations and non-adiabatic molecular dynamics simulations, we systematically elucidate the effects of cation disorder on the optoelectronic and carrier dynamics properties of Cs₂AgInCl₆ double perovskites. Our results reveal that cation disorder induces defect-dominated band edges, leading to significant band gap reduction and the absence of deep-level defects. Despite more low-frequency phonons interacting with electrons, the non-adiabatic coupling shows only a slight enhancement due to compensating effects from increased interband energy gaps and spatial decoupling of band-edge wave functions. Our results also reveal that hot carrier cooling dynamics are slowed in the case of cation disordering. This work provides fundamental insights into how cation disorder regulates the defect properties and carrier dynamics in lead-free halide double perovskites, offering valuable guidance for future applications.