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 mu s$\umu \mathrm{s}$ at 300 K and 24 mu s$\umu \mathrm{s}$ 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 mu s$\umu \mathrm{s}$ at 300 K and 334 ns at 5 K. Due to their long exciton lifetime, these materials present interesting opportunities for photovoltaic applications.