Phase-change behavior of hot-pressed methylammonium lead bromide hybrid perovskites

The investigation of hot-pressed methylammonium lead bromide (MAPbBr₃) is reported. The as-prepared materials consisted of microcrystallites (30–500 μm size) with a non-centrosymmetric pseudo-cubic tetragonal crystalline structure (P4 mm space group). The thermal analysis under air showed a thermal stability of MAPbBr₃ perovskite till T = 250 °C and a broad endothermic event at T = 371 °C (melting point of solid PbBr₂). Under increasing pressure at room temperature, the MA–Pb–Br system retained the pseudo-cubic tetragonal structure, the optical adsorption spectra displayed a strong absorption edge between 570 and 572 nm, and the Tauc plots revealed a direct semiconducting behavior with band energy gaps around 2.20(± 0.01) eV. The steady-state emission (PL) photoluminescence spectra (excitation wavelengths λ_exc = 380 nm) showed an emission band at ~ 574 nm. With increase in the temperature (under constant pressure and treatment time), the MA–Pb–Br system turned into the higher cubic symmetry (space group \(Pm\, \bar{3} m\)), the absorption edges shifted to 571–575 nm, and at T = 120 °C, the absorbance profile begins to increase at higher wavelengths (> 600 nm). By increasing the pressure at constant temperature, the perovskite adopted the cubic structure and a slight decrease in the energy band gap (2.17 eV for T = 150 °C and P = 15 MPa) was recorded. With increase in the hot-pressing time at T = 150 °C and P = 20 MPa, the optical absorption edges remained at ~ 575 nm; however, the decrease in the absorbance can indicate a degradation of the materials. The changes in the emission mechanism(s) of the hot-pressed materials can be related to the observed change in the crystalline structure and morphology.