J. Phys. Chem. Lett., Just Accepted Manuscript
DOI: 10.1021/acs.jpclett.6b02691
Publication Date (Web): January 4, 2017
Copyright © 2017 American Chemical Society
Vibrational modes in the terahertz (THz) frequency range are good indicators of lead halide perovskite's crystallization phase. We performed real-time THz spectroscopy to monitor the crystallization kinetics in lead halide perovskite films. First, THz absorptance was measured while the perovskite film was annealed at different temperatures. During this process, we observed the dynamical phase transition of the perovskite structure from an intermediate phase to the tetragonal phase. By analyzing the Avrami exponent, we observed an abrupt dimensionality switch (of interfacial growth from 1D to 2D) with increasing temperature starting at approximately 90°C. More interestingly, we monitored the laser-induced enhancement of the crystallinity of the pre-annealed perovskite film. The THz absorptance increased initially, then subsequently decayed over a couple of hours, although the enhancement factor varies depending on the film crystallinity. In particular, the Avrami analysis implied that the enhanced crystallization upon light illumination was assisted by the 1D diffusion processes. The perovskite film conductivity increased by the light irradiation more dramatically than the thermal annealing due to percolation threshold behaviors. The activation energy for light-induced crystallization was measured of 2.3 eV, which indicated that enhanced crystallization originated from the photo-induced structural change of residual lead iodide at the grain boundary.
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