In this work, organic small molecule 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine, abbr. BCP) was selected as a buffer layer for interface engineering between active and electron-transporting layers (ETL) in a reversed perovskite solar cells. It was found that the device performance was significantly improved after THE deposition of a BCP layer between the ETL and the perovskite layer. It reveals that the spacing between the grains was effectively filled by the BCP by scanning electron microscope, which can be responsible to prevent the generation of defect state at perovskite interface. Further analysis of the charge accumulation in the device in combination with AC impedance measurement proved that the migration of ions and the photon-generated carrier recombination in the perovskite solar device were reduced after the deposition of BCP layer. As a result, the optoelectronic power-conversion-efficiency is improved from 15.7% to 17.4%.