A Theoretical Study of Structure Property Relations in Photovoltaic Perovskites

Perovskite materials are a great example of systems with a multitude of structure-property relationships. For example, metal insulator transitions in the nickelates and manganites can be linked to breathing and Jahn-Teller distortions which allow for charge and orbital ordering, respectively. This study examines the relationship between atomic structure and electrical band-gaps in the promising photovolatics material A2Au2X6 (A = Cs, Rb, K, X = I, Cl), through first principles calculations based on DFT. We find that there is a complex competition between various structural degrees of freedom in the material, which can be manipulated through chemistry and pressure, and that each of the structural modes can strongly tune the band gap. For example, in the Rb2Au2I6 double perovskite, we predict that, contrary to expectation, hydrostatic pressure produces a polar phase. We argue that this is due to a) the cooperative coupling of the Jahn-Teller and tilt modes which are both reduced with pressure, and b) the competitive coupling of tilting and polar modes. We finally discuss the effect of each of these modes and chemical changes on the band gap, and how the polar mode could be helpful to separate photo-excited carriers via the photoferroic effect.