Conformational Disordering\nof Organic Cations during\nMoisture-Induced Structural Collapse in Organic–Inorganic Hybrid\nPerovskites

Moisture-induced structural collapse is an important\nfactor hindering\nthe commercialization of organic–inorganic hybrid perovskites,\nwhere organic cations play a crucial role. However, owing to the softness\nand complexity of organic cations, very few techniques can be used\nto characterize the collapse process in situ and in real time. In\nthis study, we used a phenylpropylamine (PPA)-based 2D perovskite\nas a model and demonstrated that femtosecond sum frequency generation\nvibrational spectroscopy (SFG-VS) provides a powerful probe for the\nconformational disordering evolution of organic cations during perovskite\ncollapse. Photoluminescence (PL) spectroscopy was used to monitor\nperovskite structural collapse. Following the collapse of the perovskite\nfrom a 2D confinement effect to a 1D confinement effect, the effective\nin-plane charge carrier mobility (<i>φμ</i>)\nand 2D exciton ratio of the films decreased. The decreases in the\n2D exciton ratio and <i>φμ</i> are strongly\ncorrelated with the relative conformational disordering degree of\nthe organic cations, revealing the structural origin of perovskite\ncollapse. Coating the perovskite film surface with a Teflon layer\ncan effectively retard structural collapse and maintain the conformational\nordering of organic cations.