•Solar cell performance enhanced by adding dichloromethane to 1,2 o-dichlorobenzene.•The crystallization of P3HT increased in competing solvent evaporation process.•The film surface composition was tuned from P3HT-rich to more favored PCBM-rich.•The charge transfer at the active layer-cathode interface was optimized.In this work, the effects of mixed solvents on donor–acceptor vertical phase separation and light absorption was investigated. By using mixed orthogonal solutions of 1,2 o-dichlorobenzene (o-DCB) and dichloromethane (DCM), a PCBM([6,6]-phenyl-C61-butyric acid methyl ester)-rich top layer was induced in typical poly(3-hexylthiophene-2,5-diyl)(P3HT):PCBM bulk heterojunction structure. By carefully adjusting the o-DCB:DCM volume ratio, the contact between active layer and the Al cathode was significantly improved due to the precipitation of PCBM on the top surface, which resulting in an electron transport preferable interface between the active layer and cathode. Meanwhile, light absorption was also effectively improved due to the increased crystallinity of polymers under mixed solvents. Overall, the short circuit current was greatly increased, and the efficiency was improved from 3.07% in the control sample to 3.97% by adding 30% DCM. The detailed mechanism of the formation of PCBM-rich layer and enhanced light absorption with o-DCB:DCM solution was expatiated. Our findings suggest a facile spin coating method to fabricate efficient BHJ solar cells, which can pave the way for the large scale application of organic photovoltaic devices (OPVs) in the future.Graphical abstract