Methylammonium lead iodide (MAPbI3) perovskite nanocrystals (NCs) offer impressive optoelectronic properties with prospective utility in photovoltaics, lasers, and light-emitting diodes (LEDs). Sensitivity of MAPbI3 to structural rearrangements in response to photoexcitation has been of particular interest, as lattice distortions and phase transitions can influence carrier lifetime and diffusivity. In this report we use a combination of variable temperature X-ray diffraction (XRD) and transient X-ray diffraction (TRXRD) both to understand the thermal response and to examine photoinduced structural transformations following ultrafast optical excitation. MAPbI3 NCs are found to undergo a transient phase transition from the tetragonal to a pseudo-cubic phase under moderate and high fluence (0.02 – 4.18 mJ/cm2) photoexcitation which persists for the duration of the carrier recombination lifetime. Photoinduced non-thermal distortions of the lattice are also apparent during the first nanosecond after excitation. We find that there is symmetry breaking along the axial and equatorial axes during the recovery, and that full recovery of the structure exceeds timescales expected for thermal dissipation, pointing to meta-stability in the form of delayed recovery due to the structural phase transition. These findings are relevant for fundamental studies and applications that seek to utilize MAPbI3 NCs, as the altered phase and band structure of the excited materials will likely have implications for electronic or material properties critical for realizing the potential of this material class.

Transient phase transitions in CH3NH3PbI3 nanocrystals upon optical excitation

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