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

Abstract Using first‐principles G 0 W 0 (G 0 is one‐electron Green’s function and W 0 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 Cs 3 Bi 2 X 9 (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 Cs 3 Bi 2 I 6 Cl 3 , Cs 3 Bi 2 I 6 Br 3 , Cs 3 Bi 2 Br 6 I 3 , Cs 3 Bi 2 Cl 6 Br 3 , Cs 3 Bi 2 Br 6 Cl 3 , and Cs 3 Bi 2 Cl 6 I 3 . This study reports the exciton radiative lifetimes of the vacancy‐ordered perovskites, revealing that these excitons exhibit long radiative lifetimes, particularly for Cs 3 Bi 2 Br 6 I 3 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 Cs 3 Bi 2 Cl 6 Br 3 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.