  By performing spatially resolved Raman and photoluminescence spectroscopy
with varying excitation wavelength, density, and data acquisition parameters,
we have achieved a unified understanding towards the spectroscopy signatures of
the organic-inorganic hybrid perovskite, transforming from the pristine state
(CH3NH3PbI3) to fully degraded state (i.e., PbI2) for samples with varying
crystalline domain size from mesoscopic scale (approximately 100 nm) to
macroscopic size (cm), synthesized by three different techniques. We show that
the hybrid perovskite exhibits multiple stages of structure transformation
occurring either spontaneously or under light illumination, with exceptionally
high sensitivity to the illumination conditions (e.g., power, illumination time
and interruption pattern). We highlight four transformation stages (Stage 1 -
4, with Stage 1 being the pristine state) along a primary structure degradation
path exhibiting distinctly different Raman spectroscopy features at each stage,
and point out that previously reported Raman spectra in the literature reflect
degraded structures of either Stage 3 or 4. Additional characteristic optical
features of partially degraded materials under the joint action of spontaneous
and photo degradation are given. This study offers reliable benchmark results
for understanding the intrinsic material properties and structure
transformation of this unique category of hybrid materials, and a
straightforward method to monitor the structure degradation after the material
is used in a device or characterized by other techniques. The findings are
pertinently important to a wide range of potential applications where the
hybrid material is expected to function in greatly different environment and
light-matter interaction conditions.
