Multiple-stage structure transformation of organic-inorganic hybrid   perovskite CH3NH3PbI3
  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. 
Qiong Chen, Henan Liu, Hui-Seon Kim, Yucheng Liu, Mengjin Yang, Naili   Yue, Gang Ren, Kai Zhu, Shengzhong Liu, Nam-Gyu Park...
cond-mat.mtrl-sci physics.optics