Three-dimensional organic/inorganic perovskiteshave recently been suggested as a novel class of materials for dye-sensitized solar cells (DSSC) with improved photo-conversion efficiency.These compounds are modeledin this work within the density functional theory (DFT). The band-gaps aredominated by a giant spin-orbit coupling (SOC) in the conduction-band, which has been largely overlooked so far. Direct and isotropic optical transitions at room temperature are

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Based on density functional theory, the electronic and optical properties of hybrid organic/perovskite crystals are thoroughly investigated. We consider the mono-crystalline 4F-PEPI as material model and demonstrate the optical process is governed by three active Bloch states at the Γ point of the reduced Brillouin zone with a reverse ordering compared to tetrahedrally bonded semiconductors. Giant spin-orbit coupling effects and optical activities are subsequently inferred from symmetry analysis.

3D hybrid organic perovskites,CH 3 NH 3 PbX 3 (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. CsSnI 3 perovskite can also be used for hole conduction in DSSC.Density functional theory is used to compare lead and tin hybrid and all-inorganic perovskites. The band ordering is reversed as compared to conventional semiconductors. 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 holestates is small (0.17). The high temperature phase sequence of CsSnI 3 leading to the room temperature orthorhombic phase and the recently reported phases of CH 3 NH 3 MI 3 (where M=Pb, Sn)close to the room temperature, are also studied. Tetragonal distortionsfromthe ideal cubic phase,are analysed by a k.p perturbation, in