DFT and k . p modelling of the phase transitions of lead and tin halide perovskites for photovoltaic cells

3D hybrid organic perovskites, CH3NH3PbX3 (X = halogen), have recently been used to strongly improve the efficiency of dye sensitized solar cells (DSSC) leading to a new class of low-cost mesoscopic solar cells. CsSnI3 perovskite can also be used for hole conduction in DSSC. Density functional theory and GW corrections are used to compare lead and tin hybrid and all-inorganic perovskites. The room temperature optical absorption is associated to electronic transitions between the spin-orbit split-off band in the conduction band and the valence band. Spin-orbit coupling is about three times smaller for tin. Moreover, the effective mass of relevant band edge hole states is small (0.17). The high temperature phase sequence of CsSnI3 leading to the room temperature orthorhombic phase and the recently reported phases of CH3NH3MI3 (where M = Pb, Sn) close to the room temperature, are also studied. Tetragonal distortions from the ideal cubic phase are analysed by a k p perturbation, including spin-orbit effect. In addition, the non-centrosymmetric phases of CH3NH3MI3 exhibit a splitting of the electronic bands away from the critical point. The present work shows that their physical properties are more similar to conventional semiconductors than to the absorbers used in DSSC. [GRAPHICS] (c) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim