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

Abstract

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.