  Metal halide perovskites (MHPs) are 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 this emerging class of semiconductors. Here we investigate the
timescale of the orthorhombic-to-tetragonal phase transition in the
prototypical metal halide perovskite, methylammonium lead triiodide (CH3NH3PbI3
or MAPbI3) using cryogenic nanosecond transient absorption spectroscopy. By
using mid-infrared pump pulses to impulsively heat up the material at slightly
below the phase-transition temperature and probing the transient optical
response as a function of delay time, we observed a clean signature of a
transient, reversible orthorhombic-to-tetragonal phase transition. The forward
phase transition is found to proceed at tens of nanoseconds timescale, after
which a backward phase transition progresses at a timescale commensurate with
heat dissipation from the film to the underlying substrate. A high degree of
transient phase transition is observed accounting for one third of the
steady-state phase transition. In comparison to fully inorganic phase-change
materials such as VO2, the orders of magnitude slower phase transition in
MAPbI3 can be attributed to the large energy barrier associated with the strong
hydrogen bonding between the organic cation and the inorganic framework. Our
approach paves the way for unraveling phase transition dynamics in MHPs and
other hybrid semiconducting materials.
