Anisotropy of thermal diffusivity in lead halide perovskite layers revealed by thermal grating technique

Heat management of optoelectronic devices is of critical significance in lead halide perovskites due to the intrinsically low thermal conductivity of this material. Despite its importance, thermal conductivity remains understudied, particularly in polycrystalline perovskite layers with different halides. Here, we employ a novel method for investigation of thermal properties in perovskite layers, which is based on a light-induced transient diffraction grating technique. We demonstrate the applicability of thermal grating technique by determining in an all-optical way the thermo-optic coefficient, speed of sound, and thermal conductivity in vapor-deposited polycrystalline layers of MAPbX₃ (X = Cl, Br, I), MAPbBr₂I, and MAPbCl₂Br perovskites. We reveal the spatial anisotropy of thermal conductivity, which is noticeably lower in the direction along the layer surface (0.2-0.5 W/(m K)) if compared to that across the layer (0.3-1.1 W/(m K)). Finally, we demonstrate that for both directions the thermal conductivity scales linearly with the average speed of sound in the perovskite layers.