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The temperature dependence of the bandgap of perovskite semiconductor compound CsSnI 3 is determined by measuring excitonic emission at low photoexcitation in a temperature range from 9 to 300 K. The bandgap increases linearly as the lattice temperature increases with a linear coefficient of 0.35 meV K À1. This behavior is distinctly different than that in most of tetrahedral semiconductors. First-principles simulation is employed to predict the bandgap change with the rigid change of lattice parameters under a quasi-harmonic approximation. It is justified that the thermal contribution dominates to the bandgap variation with temperature, while the direct contribution of electron-phonon interaction is conjectured to be negligible likely due to the unusual large electron effective mass for this material. V

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We report on the synthesis and characterization of CsSnI3 perovskite semiconductor thin films deposited on inexpensive substrates such as glass and ceramics. These films contained polycrystalline domains with typical size of 300 nm. It is confirmed experimentally that CsSnI3 compound in its black phase is a direct band-gap semiconductor, consistent with the calculated band structure from the first principles. The band gap is determined to be ∼1.3 eV at Γ point at room temperature.