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

In this work, we present the ion migration of CsPbIBr 2 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 CsPbIBr 2 and GA x Cs 1- x PbIBr 2 perovskites. The local lattice distortion introduced by GA could reduce the strain gradient in GA x Cs 1- x PbIBr 2 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 0.1 Cs 0.9 PbIBr 2 delivers comparable photovoltaic performance and improved stability compared to those of CsPbIBr 2 -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 CsPbIBr 2 counterpart shows an efficiency that is <30% of its initial value under the same test condition.