Conformational Disordering of Organic Cations during Moisture-Induced Structural Collapse in Organic–Inorganic Hybrid Perovskites

Moisture-induced structural collapse is an important factor hindering the commercialization of organic–inorganic hybrid perovskites, where organic cations play a crucial role. However, owing to the softness and complexity of organic cations, very few techniques can be used to characterize the collapse process in situ and in real time. In this study, we used a phenylpropylamine (PPA)-based 2D perovskite as a model and demonstrated that femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) provides a powerful probe for the conformational disordering evolution of organic cations during perovskite collapse. Photoluminescence (PL) spectroscopy was used to monitor perovskite structural collapse. Following the collapse of the perovskite from a 2D confinement effect to a 1D confinement effect, the effective in-plane charge carrier mobility (φμ) and 2D exciton ratio of the films decreased. The decreases in the 2D exciton ratio and φμ are strongly correlated with the relative conformational disordering degree of the organic cations, revealing the structural origin of perovskite collapse. Coating the perovskite film surface with a Teflon layer can effectively retard structural collapse and maintain the conformational ordering of organic cations.