Roles of Pseudo-Closed s2 Orbitals for Different Intrinsic Hole Generation between Tl-Bi and In-Bi Bromide Double Perovskites.

Although metal halide double perovskites A2B(I)B(III)X6 are expected as nontoxic alternatives for lead halide perovskites, recent studies have shown that only Tl(I)-Bi(III) and In(I)-Bi(III) bromides are thermodynamically stable and possess optoelectronic properties suitable for photovoltaic absorbers. Here, we show, through density functional theory calculations, that Tl-Bi and In-Bi bromide double perovskites exhibit significantly different semiconducting behaviors due to the different energy levels of the highest-occupied pseudoclosed s2 orbitals of Tl(I) and In(I). While Tl-Bi double perovskites can exhibit semiconducting p-type properties, In-Bi bromide double perovskites exhibit metallic p-type ones regardless of the synthesis condition due to the extremely low formation energy of In vacancy. Such difference makes Tl-Bi bromide double perovskites suitable for optoelectronic applications, but not In-Bi bromide double perovskites. Furthermore, there is a high probability for In to substitute a Bi site, forming a local In-In bromide double perovskite structure with a lower local conduction band minimum, detrimentally affecting the open circuit voltage of In-Bi bromide double perovskite-based thin film solar cells.