Phase segregation in mixed halide perovskite by post-treatment of methylammonium halides

Mixed halide perovskites are considered to have great application prospects in the field of optoelectronics due to their bandgap tunability. However, their applications are limited by phase instability under illumination. Here, we have used PL measurement to investigate phase segregation in mixed halide perovskite films with and without post-treatment using methylammonium halide solutions. Our findings reveal the PL evolution is strongly dependent on the morphology of perovskite films: the PL peak from small I-rich clusters is observed at the initial time in the films with unreacted lead halides, accompanied by faster PL evolution with increase of the concentration of methylammonium halide solution; adding excess methylammonium halide solution to the perovskite film leads to the disappearance of that PL peak. We attribute the difference of PL evolution in various films to the different evolution of local electric fields, which are induced by carrier trapping at grain boundaries. Our results indicate the local electric fields play an essential role in the halide segregation process.