Crossover of Frenkel and Wannier-Mott Excitons Through Halide Composition Tuning in Mixed Halide Perovskites

Using first-principles G₀W₀ (G₀ is one-electron Green's function and W₀ is the dynamical screening Coloumb potential) coupled Bethe-Salpeter equation (BSE) calculations with spin-orbit coupling, exceptionally strong excitonic effects are identified in several bismuth-based vacancy-ordered mixed halide double perovskites. These perovskites are thermodynamically stable with negative formation energy. For Cs₃Bi₂X₉ (X = Cl,Br,I) double perovskites, both the bandgap and excitonic binding energy decrease as the size of the halogen atom increases. The excitonic effects can be tuned in mixed halide perovskites such as Cs₃Bi₂I₆Cl₃, Cs₃Bi₂I₆Br₃, Cs₃Bi₂Br₆I₃, Cs₃Bi₂Cl₆Br₃, Cs₃Bi₂Br₆Cl₃, and Cs₃Bi₂Cl₆I₃. This study reports the exciton radiative lifetimes of the vacancy-ordered perovskites, revealing that these excitons exhibit long radiative lifetimes, particularly for Cs₃Bi₂Br₆I₃ with 11141 $$ at 300 K and 24 $$ at 5 K. The long radiative lifetimes are linked to the delocalization of the exciton (Wannier-Mott type) in real space, whereas the more localized exciton (Frenkel type) in Cs₃Bi₂Cl₆Br₃ results in shorter radiative lifetimes of 155 $$ at 300 K and 334 ns at 5 K. Due to their long exciton lifetime, these materials present interesting opportunities for photovoltaic applications.