Herein, we report a facile way to incorporate polyethylenimine (PEI) into a mixed BHJ solution of poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadizole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC 71 BM) to fabricate the simplified PSCs. The amine-containing PEI spontaneously migrated into the cathode buffer layer surface by one-step solution processing and thereby formed an ultrathin modification layer between the TiO2 and the active layer. The PEI additive are able to accumulate on the cathode buffer layer surface prior to the PCDTBT and PC71BM during spin-coating, because the amino of PEI can form hydrogen bonds with the hydroxyl groups on the TiO2 surface. By optimizing the doping concentration of PEI in the BHJ, the maximum PCE of 7.22% was achieved for the PCDTBT:PC71BM based device, leading to a 19.9% enhancement compared to the control device. The self-assembled PEI layer substantially reduces the work function and eliminates surface defects of TiO2, thereby significantly minimizing the electron extraction barrier and trap-assisted recombination. This work offers a new guidance to further enhance PSCs performance by using a simple and convenient interface engineering, which demonstrates a huge potential for application in large-scale fabrication. (C) 2018 Elsevier Ltd. All rights reserved.