Triple cation perovskites (TCPs) are organic-inorganic hybrid materials that first rose to prominence as efficient photovoltaic materials, yet also hold promise for other applications like lasing, exciton condensation, single photon emitter, photodetectors, or photocatalysis. Their high performance continues to surprise considering the nano- and microscale heterogeneities and high defect densities of the typically polycrystalline thin films used. It is believed that the soft dynamically deformable lattice and mobile cations have the unique ability to stabilize charge carriers by polaron formation. However, the material science of perovskites has remained largely empirical with a lack of spectroscopic access to the elementary processes defined at the low-energy scale of the electron and lattice dynamics in the infrared. The relevant information with its inter- and intragrain heterogeneity in composition and structure is lost in conventional spatially averaged spectroscopy or static imaging.
Here a STROBE team from CU Boulder in collaboration with researchers from imo-imomec (Belgium) combined three nano-imaging modalities developed through STROBE previously, and in the application to an important hybrid perovskite photovoltaic material, provide for the first time a real space view of composition, lattice structure, and carrier dynamics simultaneously. Mid-infrared nano-spectroscopy of the ground state vibrational response probing composition and the static lattice parameter was correlated with excited state spectroscopy resolving both ps- to ns- polaron relaxation and associated coupled lattice dynamics. The researchers could watch for the first time the transient lattice deformation and cation-lattice coupling as the polaron forms, grows, and evolves into the long-lived carriers giving rise to the photovoltaic response. Our work shows how correlated ground and excited state structural and dynamics nano-imaging could guide optimization of composition and thin film preparation to transform the field from the conventional trial and error approach to a targeted material design.