(508g) Photoinduced, Reversible Phase Transition in Methylammonium Lead Iodide

Metal halide perovskites (MHPs) are well known to undergo several structural phase transitions from lower to higher symmetry upon heating. While structural phase transitions have been investigated by a wide range of optical, thermal and electrical methods, most measurements are quasi-static and hence do not provide direct information regarding the fundamental timescale of phase transitions in these emerging semiconductors. Using cryogenic nanosecond transient absorption spectroscopy, we investigate the fundamental timescale of orthorhombic-to-tetragonal phase transition in the prototypical MHP, CH3NH3PbI3. By using mid-infrared pump pulses to impulsively heat up the material slightly below the phase-transition temperature and probing the transient optical response as a function of delay time, we observed a clean signature of transient, reversible orthorhombic-to-tetragonal phase transition. The forward phase transition is found to proceed in the tens of nanosecond timescale, after which a backward phase transition progresses at a timescale commensurate with heat dissipation from the film to the underlying substrate. In comparison to fully inorganic phase-change materials such as VO2, the orders of magnitude slower phase transition in CH3NH3PbI3 are attributed to the large energetic barrier associated with the strong hydrogen bonding between the organic cation and the inorganic framework. We discuss ongoing efforts in transient spectroscopy and imaging of phase transitions in other MHP compositions. Our work paves the way for unraveling phase transition dynamics in metal halide perovskites and other hybrid semiconducting materials.