Subdiffraction Infrared Imaging of Mixed Cation Perovskites: Probing Local Cation Heterogeneities

Compositional engineering has led to dramatic improvements in hybrid perovskite-based solar cell stabilities and performance. Mixed cation perovskites have emerged as champion photovoltaic materials with power conversion efficiencies exceeding 22%. However, there has been relatively little work done to explore local cation-related compositional inhomogeneities in mixed cation perovskite films. Such studies are necessary because hybrid perovskite optical properties and, consequently, their photovoltaic performance strongly depend on composition. Here, we perform spatially resolved, subdiffraction infrared photo thermal heterodyne imaging measurements to probe cation-specific compositional distributions within FA(x)MA(1-x)PBI(3) perovskite films. Our measurements reveal that these perovskites possess large compositional spatial heterogeneities with cation distributions varying on average similar to 20% from expected ensemble stoichiometries. Correlated emission measurements show intrafilm emission energies differing by over 30 meV due to these compositional differences. These measurements thus reveal cation stoichiometric heterogeneities and their direct impact on local photovoltaic response-determining opticalproperties of mixed cation perovskites.