Influence of Grain Size on Phase Transitions in Halide Perovskite Films

Abstract

Grain size in polycrystalline halide perovskite films is known to have an impact on the optoelectronic properties of the films, but its influence on their soft structural properties and phase transitions is unclear. Here, we use temperature-dependent X-ray diffraction, absorption, and macro- and micro-photoluminescence measurements to investigate the tetragonal to orthorhombic phase transition in thin methylammonium lead iodide films with grain sizes ranging from the micron scale down to the tens of nanometre scale. We show that the phase transition nominally at ~150 K is increasingly suppressed with decreasing grain size and, in the smallest grains, we only see the first evidence of a phase transition at temperatures as low as ~80 K. With decreasing grain size, we also see an increasing magnitude of the hysteresis in the structural and optoelectronic properties when cooling to, and then upon heating from, 100K. Our work reveals the remarkable sensitivity of the optoelectronic, physical and phase properties to the local environment of the perovskite structure, which will have large ramifications for phase and defect engineering in operating devices.EPSRC NanoDTC Royal Society ERC Starting Gran