Inactive (PbI 2 ) 2 RbCl stabilizes perovskite films for efficient solar cells.

In halide perovskite solar cells the formation of secondary-phase excess lead iodide (PbI 2 ) has some positive effects on power conversion efficiency (PCE) but can be detrimental to device stability and lead to large hysteresis effects in voltage sweeps. We converted PbI 2 into an inactive (PbI 2 ) 2 RbCl compound by RbCl doping, which effectively stabilizes the perovskite phase. We obtained a certified PCE of 25.6% for FAPbI 3 (FA, formamidinium) perovskite solar cells on the basis of this strategy. Devices retained 96% of their original PCE values after 1000 hours of shelf storage and 80% after 500 hours of thermal stability testing at 85°C.