Nanoscale Insights into Photovoltaic Hysteresis in Triple-Cation Mixed-Halide Perovskite: Resolving the Role of Polarization and Ionic Migration

Triple-cation mixed-halide perovskites of composition Cs-x(FA(y)MA(1-y))(1-x)Pb(IzBr1-z)(3) (CsFAMA) have been reported to possess excellent photovoltaic efficiency with minimal hysteresis; in this work, nanoscale insight is shed into the roles of illumination-induced polarization and ionic migration in photovoltaic hysteresis. By examining the concurrent evolution of ionic distribution and spontaneous polarization of CsFAMA under light illumination using dynamic-strain-based scanning probe microscopy, strong linear piezoelectricity arising from photoenhanced polarization is observed, while ionic migration is found to be not significantly increased by lightening. Nanoscale photocurrents are mapped under a series of biases using conductive atomic force microscopy, revealing negligible difference between forward and backward scans, and local IV curves reconstructed from principal component analysis show minimal hysteresis of just 1%. These observations at the nanoscale are confirmed in a macroscopic perovskite solar cell made of CsFAMA, exhibiting a high efficiency of 20.11% and with hysteresis index as small as 3%. Ionic migration, polarization, and photocurrent hysteresis are thus directly correlated at the nanoscale, and photoenhanced polarization in triple-cation mixed-halide perovskites is established, which does not contribute to the photovoltaic hysteresis.