The　introduction　of　Cl　into　CH3NH3PbI3　precursors　is　reported　to　enhance　the　performance　of　CH3NH3PbI3　solar　cell,　which　is　attributed　to　the　significantly　increased　diffusion　lengths　of　carriers　in　CH3NH3Pb(I,Cl)(3)　solar　cell.　It　has　been　assumed　but　never　experimentally　approved　that　the　defect　density　in　CH3NH3Pb(I,Cl)(3)　solar　cell　should　be　reduced　according　to　the　higher　carrier　lifetime　observed　from　photoluminescence　(PL)　measurement.　We　have　fabricated　CH3NH3Pb(I,Cl)(3)　solar　cell　by　adding　a　small　amount　of　Cl　source　into　CH3NH3PbI3　precursor.　The　performance　of　CH3NH3Pb(I,Cl)(3)　solar　cell　is　significantly　improved　from　15.39%　to　18.60%.　Results　from　scanning　electron　microscopy　and　X-ray　diffraction　indicate　that　the　morphologies　and　crystal　structures　of　CH3NH3PbI3　and　CH3NH3Pb(I,Cl)(3)　thin　films　remain　unchanged.　Open　circuit　voltage　decay　and　admittance　spectroscopy　characterization　jointly　approve　that　Cl　plays　an　extremely　important　role　in　suppressing　the　formation　of　defects　in　perovskite　solar　cells.　Published　by　AIP　Publishing.