Boosting Stability of Cesium/Formamidinium Based Perovskite Solar Cells via Eliminating Intermediate Phase Transition and X-Anion Vacancy

Boosting the stability of cesium/formamidinium (Cs/FA) based perovskite solar cells (PSCs) has received tremendous attention. However, the crystallization of perovskites usually undergoes complex intermediate phase transitions and ion loss processes, which seriously degrade the efficiency and stability of PSCs. Herein, iodine monobromide (IBr, an interhalogen) is incorporated into the precursor solution to regulate the perovskite crystallization process. IBr can directly induce the formation of perovskite crystal nuclei in the intermediate film, avoiding a complex phase transformation (2H-4H-3C). This leads to a reduction in the impurity phase, an increase in grain size, and an improvement in crystal quality. Furthermore, IBr can effectively compensate X-anion vacancy, thereby reducing defect density and nonradiative recombination, which enhances device performance. Thus, the efficiency of the optimal device is 24.82%. Simultaneously, the device demonstrated excellent stability. After 400 h of continuous operation, the efficiency value of the unencapsulated PSCs still retains 89% of its initial value. This study provides an effective strategy for manufacturing PSCs with excellent efficiency and stability.