Anomalous Electron-Phonon Coupling in Cesium-Substituted Methylammonium Lead Iodide Perovskites

Most metal halide perovskites (MHPs) which under ambient conditions crystallize in a tetragonal or cubic phase exhibit a fairly linear increase of the fundamental gap with increasing temperature, in frank contrast to other conventional semiconductors. Though valid in general for MHPs, it has been shown for the archetypal perovskite MAPbI(3) (MA stands for methylammonium) that such a behavior is due to an almost equal footing of the effects of thermal expansion on the electronic states and the temperature-induced gap renormalization due to the electron-phonon interaction. Here, the influence of the incorporation of small amounts of Cs on the gap renormalization was investigated in two Cs(x)MA(1-x)PbI(3) single crystals (x = 0.05 and 0.1). Strikingly, from ambient to a temperature T-on of 250-280 K, the slope of the linear temperature dependence of the energy gap for both Cs-containing samples is almost half that for pure MAPbI(3). Below T-on and within the stability range of the tetragonal phase, the slope recovers the value of MAPbI(3). Based on density functional theory calculations, this surprising behavior is explained as being due to the appearance of an additional electron-phonon coupling mechanism ascribed to dynamic tilting of the PbI6 octahedrons. The latter is activated by the translational dynamics of the Cs cations between equivalent potential minima inside the inorganic cage voids, which is unfolded above the onset temperature T-on. These results provide further evidence of the key role that the A-site cation dynamics plays in the optoelectronic properties of this fascinating class of materials.