Metal-insulator-semiconductor back contact has been employed for a perovskite organic lead iodide hetero junction solar cell,in which an ultrathin Al2O3 film as an insulating layer was deposited onto the CH3NH3PbI3 by atomic layer deposition technology.The light-to-electricity conversion efficiency of the devices is significantly enhanced from 3.30％ to 5.07％.Further the impedance spectrum reveals that this insulating layer sustains part of the positive bias applied in the absorber region close to the back contact and decreases the carrier transport barrier,thus promoting transportation of carriers.