Is Cs2TiBr6 a promising Pb-free perovskite for solar energy applications?

In a quest for Pb-free perovskites suitable for solar energy applications, Cs2TiBr6 has recently been reported as a promising compound, with appropriate optical and electrical properties as well as high stability under environmental stresses. In this study, we pursue investigation on this compound, demonstrating phase pure Cs2TiBr6 powder formation using solution synthesis and providing complementary experimental characterization and theoretical calculations. An experimental absorption onset of around 2.0 eV is extracted and a weak broad photoluminescence is measured. Density functional theory calculations predict an indirect bandgap, parity-forbidden for both the direct and indirect transitions, which explains the weak and Stokes shifted luminescence. Additionally, we highlight the strong instability of Cs2TiBr6 powder in ambient atmosphere. Therefore, our experimental results supported by theoretical calculations differ from previous results and raise doubts on the suitability of Cs2TiBr6 in its pristine form for solar energy applications.