Visualization of Ion Migration in an Inorganic Mixed Halide Perovskite by One-Photon and Multiphoton Absorption: Effect of Guanidinium A-Site Cation Incorporation

Abstract

In this work, we present the ion migration of CsPbIBr₂ under illumination and impede it by incorporating the large cations of guanidinium (GA). A series of "probe-set-probe" operations are applied to assess the photoluminescence (PL) behavior spectrally and spatially, which is correlated to the ion migration-induced phase separation, of CsPbIBr₂ and GA_<i>x</i>Cs_1-<i>x</i>PbIBr₂ perovskites. The local lattice distortion introduced by GA could reduce the strain gradient in GA_<i>x</i>Cs_1-<i>x</i>PbIBr₂ to inhibit the ion migration, leading to a stable PL spectrum and enhanced device stability under light stimulation. A solar cell with an optimized stoichiometric composition of GA_0.1Cs_0.9PbIBr₂ delivers comparable photovoltaic performance and improved stability compared to those of CsPbIBr₂-based perovskite solar cells, retaining 80% of its initial power conversion efficiency after being continuously bathed in light for 8 h under ambient conditions without encapsulation, while the CsPbIBr₂ counterpart shows an efficiency that is <30% of its initial value under the same test condition.