Unlocking the intermediate-phase evolution in perovskite crystallization with an operando infrared FEWS sensor.

The intermediate phase produced by the complexation of metal ions and solvent molecules usually occurs in the crystallization process of perovskite single crystal or film. Effective in situ monitoring of intermediate-phase evolution is beneficial to the control of crystal quality. However, it is difficult to realize. In this work, infrared fiber evanescent wave spectroscopy (FEWS) was raised to monitor the intermediate-phase evolution in real time and non-destructively using GeAsSeTe chalcogenide optical fibers. The vibrational and rotational dynamics of specific molecular functional groups was operando captured, reflecting a perovskite precursor of different states. Taking BM₂PbBr₄ (BM = benzimidazole) perovskite as an example, the shift of the stretching vibration of -C=O groups in DMF (N,N-dimethylformamide) toward low wavenumbers and then recovered toward original position probed the complexion of Pb²⁺ and carbonyl groups into (DMF)₂BMPbBr₃ intermediate phase and then decomplexing to precipitate BM₂PbBr₄ perovskite crystal. Some anomalous emergence of new vibrational bands associating with -C-N and -N-H bonds suggest the variation of DMF-BMBr hydrogen bonds during intermediate-phase evolution. This technique provides new, to the best of our knowledge, insights into the control of perovskite crystallization processes and pushes the development of high-quality perovskite materials for high-performance photovoltaic or optoelectronic devices.