Partial lead substitution as a tool to manage the light-induced recrystallization and photostability of mixed-cation lead halide perovskites

Substitution of Pb<sup>2+</sup> ions in lead halide perovskite Cs<sub>0.12</sub>FA<sub>0.88</sub>PbI<sub>3</sub> (FA is formamidinium cation) was investigated to address the issues of its phase stability and photostability. The incorporation of optimally selected Nd<sup>3+</sup>, Tb<sup>3+</sup> and Er<sup>3+</sup> cations into Cs<sub>0.12</sub>FA<sub>0.88</sub>Pb<sub>0.99</sub>M<sub>0.02/<em>n</em></sub>I<sub>3</sub> films significantly suppressed the light-induced recrystallization and photodegradation, maintaining the morphology, optical properties and phase composition after 9000 h of light exposure, and improved the efficiency and operational stability of perovskite solar cells.