Photothermally induced, reversible phase transition in methylammonium lead triiodide

Metal halide perovskites (MHPs) undergo structural phase transitions, from lower to higher symmetry, upon heating. Although phase transitions have been investigated by optical, thermal, and diffraction methods, most measurements are quasi-static and do not inform on the fundamental timescale of their phase transitions. Here, we investigate the timescale of the orthorhombic-to-tetragonal phase transition in methylammonium lead triiodide using nanosecond transient absorption spectroscopy. Using mid-infrared pulses to impulsively heat the material held right below the phase-transition temperature, we observe a clean signature of transient, reversible orthorhombic-to-tetragonal transition on the tens-of-nanoseconds timescale. A high degree of transient phase transition is observed accounting for one-third of the steady-state counterpart. Compared with inorganic phase-change materials, the orders-of-magnitude-slower phase transition in MAPbI3 suggests a large energy barrier for activating the hydrogen-bond network and the accompanying cooperative octahedral rotations. Our approach paves the way for unraveling phase-transition dynamics in MHPs and other hybrid materials.