Band alignment of Cs2BX6 double halide perovskites and TiO2 using electron affinity rule

Cs2BX6 double halide perovskites have attracted broad attention in the field of materials science, physics, and optoelectronics. They are promising as potential candidates for light-absorber layers in solar cells and ultrafast scintillators. This is due to their remarkable preferences such as low-cost and simple processability, tunable bandgaps, large-absorption coefficient, high mobility and fast respond under excitation. To provide an initial information for designing multijunction devices made from Cs2BX6, the band alignment of Cs2BX6 is necessary to determine. Thus, we investigated the surface physics of Cs2BX6, where B = Ti, Zr, Hf and X = Cl, Br, I, by using electron affinity rule and extracted all the natural band offsets among Cs2BX6 and TiO2. We found that the band alignment of Cs2BX6 is type-I, whether they were grouped by the same transition-metal or same halide. The positions of conduction band minimum of Zr - and Hf - compounds are relatively close to each other due to the position of Zr - and Hf - d states. The shift in valence band is more influential cause of bandgap variation in Ti - compounds than the change in conduction band. We hope that the band alignment of Cs2BX6 will be benefit not only for the heterostructure design but also for understanding the behavior of tuning bandgaps of alloys among Cs2BX6 either by mixing B or X atoms.