Unraveling the structural phase transition and its effect on photophysical properties of mechanochemically synthesized CH₃NH₃PbBr₃

Abstract The mechanochemical synthesis route has become popular for the solvent-free synthesis of a wide range of hybrid halide perovskites. This work demonstrates that mechanochemically synthesized CH 3 NH 3 PbBr 3 is thermally more stable compared to chemically processed material and free from deep-level defects. Temperature-dependent structural study on mechanochemically synthesized material reveals that it exhibits a cubic phase at room temperature with a phase transition from cubic to tetragonal phase at 230 K. Below 150 K, it transforms into an orthorhombic phase, which belongs to an incommensurately modulated crystal structure (Imma(s00)00 γ ). Unlike the early reports, the orthorhombic Pnma structure is absent, and the modulated phase is extended over a wide range of temperatures from 145 K to 100 K. Temperature-dependent photoluminescence study shows that the emission spectrum comprises both the contribution from free and bound excitons. The blueshift of the bandgap with increasing lattice temperature can be explained by the large volume expansion coefficient (of the order 10 −4 ), which dominates over the contribution due to electron–phonon coupling. The linewidth broadening of the emission spectrum is mainly due to the interaction of the longitudinal optical phonon with the free electron. The photo-generated exciton exhibits high activation energy (∼100 meV) at room temperature, demonstrating its potential for light-emitting device applications.