  Organic-inorganic hybrid perovskites such as methylammonium lead iodide
(CH3NH3PbI3) are game-changing semiconductors for solar cells and
light-emitting devices owing to their exceptionally long carrier lifetime and
diffusion length. Determining whether the large dipole moment of the organic
cation and dynamic disorder benefit the optoelectronic properties of CH3NH3PbI3
has been an outstanding challenge. Herein, via transient absorption
measurements employing an infrared pump pulse tuned to a methylammonium
vibration, we observe slow, nanosecond-long thermal dissipation from the
selectively excited organic mode to the entire lattice. Resulting transient
electronic signatures, during the period of thermal-nonequilibrium when the
induced thermal motions are mostly concentrated on the organic sublattice,
reveal that induced motions of the organic cations do not alter absorption or
photoluminescence response of CH3NH3PbI3, beyond thermal effects. Our results
suggest that the attractive optoelectronic properties of CH3NH3PbI3 mainly
derive from the inorganic lead-halide framework.
