Observation of lower defect density brought by excess PbI2 in CH3NH3PbI3 solar cells

The introduction of excess PbI2 into CH3NH3PbI3 precursors has been reported to boost the efficiency of CH3NH3PbI3 solar cell. It was assumed that the excess PbI2 helped to reduce the defect density in CH3NH3PbI3 solar cell. In this work, by adding non-equimolar PbI2 into CH3NH3PbI3 precursor solution, PbI2-rich CH3NH3PbI3 solar cells have been fabricated. The efficiency of CH3NH3PbI3 solar cell was significantly improved from 14.14% to 16.80%. Results obtained from scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicate that the excess PbI2 does not affect the morphological and crystal properties of CH3NH3PbI3 thin film. Based on time-resolved photoluminescence (TRPL) measurement, it was found that the carrier lifetime of PbI2-rich perovskite thin film was significantly increased. Lower defect density was observed in PbI2-rich CH3NH3PbI3 solar cell by admittance spectroscopy (AS) characterization, indicating PbI2 can suppress the formation of defects in CH3NH3PbI3 solar cells.