Cation-lattice interactions in hybrid organic-inorganic perovskites regulate optoelectronic properties and chemical stability of their highly dynamic lattice structures. Perovskite films have been shown to exhibit spatial heterogeneity in their optoelectronic responses. However, characterizing and controlling the underlying heterogeneity in cation-lattice interactions have remained elusive. Here, we apply infrared scattering scanning near-field optical microscopy to a triple cation (formamidinium-methylammonium-cesium) lead halide perovskite film, both at the ground and in the photo-induced transient state, by exploiting a vibrational resonance of formamidinium (FA) cation as a probe of its local chemical environments. The measured spatial variations in amplitude, peak position, and linewidth of the vibrational resonance reflect sub-micron-scale heterogeneity in composition, cation-lattice coupling, and its dynamics. A unique correlation observed between peak position and homogeneous linewidth reveals a varying reaction field that is accompanied by dynamical slowdown in the cation-lattice interactions, pointing to heterogeneous lattice contraction within the film. We attribute the spatial heterogeneity in lattice contraction to a non-uniform distribution of cesium cations, which have a major impact on the perovskite lattice structures and dynamics due to their small radii and highly localized charges. We further develop and apply visible-pump/infrared-probe vibrational nanoscopy to characterize a spatially varying transient vibrational resonance that reflects the heterogeneous polaron formation and its coupling to organic cations. The observed spatial heterogeneity in the cation-lattice interaction and cation-polaron coupling not only accounts for the previously observed heterogeneity in optoelectronic responses of the perovskites, but also provides for the molecular-scale insights that are critical for the engineering of the electronic, structural and dynamical disorder within the perovskite films.

Heterogeneous cation-lattice interaction and dynamics in a triple cation perovskite probed by ultrafast infrared vibrational nanoscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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