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

We have envisaged the excitonic effect in Cs₃Bi₂X₉ (X=I, Cl, Br) based on first principles many body calculations using GW coupled with Bethe Salpeter Equation (BSE) formalism. Through the fine tuning of halide composition space, we could find a distinct crossover between Frenkel and Wannier-Mott Excitons in these vacancy-ordered mixed halide double perovskites. Our systematic electronic structure calculations considering relativistic spin-orbit coupling effect reveals long excitonic radiative lifetimes, which could be highly desirable for solar cells applications. The intriguing halide composition factor and the corresponding repercussion on the transition between Frenkel and Wannier-Mott Excitons could be correlated with the exciton radiative lifetime and photovoltaic efficiency variation in this exciting family of inorganic perovskites. Our theoretical investigation suggests the strong dependency of exciton binding energy on the halide compositional space, while it could be connected with the optical absorption spectra of the individual systems for the quest of lead-free stable and efficient next generation solar cells.