Heterogeneous Cation–Lattice Interaction and Dynamics in Triple-Cation Perovskites Revealed by Infrared Vibrational Nanoscopy

Hybrid organic–inorganic perovskites exhibit extraordinary photovoltaic performance. This is believed to arise from almost liquid-like low-energy interactions among lattice ions and charge carriers. While spatial variations have recently been identified over multiple length scales in the optoelectronic response of perovskites, the relationship between the heterogeneity and the soft cation–lattice interactions has remained elusive. Here, we apply multivariate infrared vibrational nanoimaging to a formamidinium (FA)–methylammonium (MA)–cesium triple-cation perovskite by using the FA vibrational resonance as a sensitive probe of its local chemical environment. The derived correlation among nanoscale composition, cation–lattice coupling, and associated few-picosecond vibrational dynamics implies a heterogeneous reaction field and lattice contraction that we attribute to a spatially nonuniform distribution of cesium cations. The associated spatial variation in elasticity of the lattice leads to disorder in charge–phonon coupling and related polaron formation—the control of which is central to improving perovskite photovoltaics.