Reduced Thermal Conductivity and Improved Stability by B-Site Doping in Tin Halide Perovskites

Halide perovskites have attracted recent attention as thermoelectric materials due to their low thermal conductivity combined with good charge transport characteristics. The tin halide perovskites hold the highest zT within metal halide perovskites and offer lower toxicity than lead-containing perovskites that are well-known for photovoltaics. In this study, we partially substitute Sn (II) with Ge (II) to form mixed metal CsSn_1-xGe_xI₃ perovskite thin films that have substantially improved stability, remaining in the black orthorhombic phase after hours of ambient air exposure. We find Ge (II) at the surface seems to be oxidized in preference to Sn (II), and this retards oxidation of the bulk of the film. Moreover, Ge substitutions dramatically reduce the lattice thermal conductivity to 0.26 ± 0.01 Wm^-1K^-1 for CsSn_0.9Ge_0.1I₃ at 353 K. Density functional theory simulations show that Ge-doped Sn perovskites possess more low-frequency phonon modes than pristine CsSnI₃, which leads to stronger scattering among the acoustic phonons, resulting in lower phonon group velocity and reduced phonon lifetime. These findings make an important contribution to our understanding of the origin of the reduced lattice thermal conductivity and improved electrical stability of B-site doped perovskite materials.