Structural evolution, optical gap and thermoelectric properties of CH3NH3SnBr3 hybrid perovskite, prepared by mechanochemistry

Abstract

Direct bandgap semiconductors of the hybrid-perovskite family CH₃NH₃PbX₃ (X = I, Br, Cl) exhibit outstanding light absorption properties and are the materials of choice for solar energy applications. As an alternative to poisonous Pb, tin-containing perovskites would show a lower effective mass thus exhibiting a higher charge carrier mobility. An auspicious candidate is CH₃NH₃SnBr₃, with an estimated band gap of 1.902 eV, anticipating applications in photovoltaic devices for the visible to ultra-violet wavelength region. We describe that this perovskite can be prepared by ball milling in a straightforward way, yielding specimens with a superior crystallinity. A structural investigation from synchrotron X-ray powder diffraction (SXRD) data was essential to revisit the successive phase transitions this compound experiences down to 120 K, guided by specific heat capacity and DSC measurements. From the cubic structure identified at RT and 270 K, there is a gradual evolution of the patterns, analysed as a phase admixture between the cubic and the low-symmetry phase present at 160 K. This corresponds to an orthorhombic Pmc2₁ superstructure; this acentric space group enables polarization along the c-axis where there is a twofold screw axis, evidenced in the distribution of Sn–Br distances. Furthermore, there are two conspicuous changes in the orthorhombic framework, yet keeping the Pmc2₁ space group, which agree with the main calorimetric events (observed at 224 and 147 K). We interpret these changes as an interplay between the tilting of the SnBr₆ octahedra of the inorganic framework and the breaking and reconstruction of H-bond interactions with the organic CH₃NH⁺₃ unit. The stereochemical effect of the lone electron pair of the Sn²⁺ ion is clear in the SnBr₆ octahedral distortion. Diffuse reflectance UV/Vis spectroscopy yields an optical gap of ∼2.1 eV, in agreement with ab- initio calculations. A Seebeck coefficient of ∼2000 μV K⁻¹ is determined near RT, which is one order of magnitude higher than those reported for other halide perovskites.