Chlorine-assisted recovery of performance loss in MAPbI3 solar cells caused by low purity of PbI2

We report an exciting discovery that the performance loss in MAPbI3 solar cells due to low purity of PbI2 can be fully recovered by adding a small amount of Cl source (MACl). The averaged efficiency of MAPbI3 solar cells using low-purity PbI2 is 14.16%, whereas that using high-purity PbI2 is 16.39%. Excitingly, the averaged efficiency of MAPbI3 solar cells using low-purity PbI2 is boosted to 16.58% by adding a small amount of MACl into MAPbI3 precursor, which is comparable to that from high-purity PbI2. Results from scanning electron microscopy and x-ray diffraction indicate that the morphologies and structures of MAPbI3 thin films are not affected by the purity of PbI2 and the addition of small amount of Cl source. Further investigations using open-circuit voltage decay and admittance spectroscopy (AS) reveal that low purity of PbI2 leads to lower electron lifetime due to higher defect density, whereas the addition of Cl can suppress the defect density, resulting into higher electron lifetime. This discovery may offer a great opportunity for fabricating low-cost, high-efficiency perovskite solar cells using low-purity feedstocks, which would definitely expedite the development of environment-friendly perovskite solar cells.