To achieve ideal phase separation in P3HT:PCBM (1:1, w/w) blend films, a selective second solvent, DMF, which cannot dissolve P3HT but slightly dissolves PCBM, was added into 1,2-dichlorobenzene. Addition of 10% DMF led to better charge transport, better morphology and average power conversion efficiency (PCE) enhancement from 3.75% to 4.29%. To form an acceptor rich layer near the cathode and achieve effective vertical phase separation, PCBM solution in DMF was spin-coated on the top surface of the P3HT:PCBM active layer. The PCBM rich layer enhanced hole blocking and electron transport, leading to an average PCE improvement from 4.29% to 4.83% (for the PCBM rich layer formed by a 2 mg ml(-1) PCBM solution). Finally, to obtain more uniform and smooth films with better contact of Ca/PCBM and BHJ/PCBM interfaces, the films were thermally annealed at 120 ° C for 10 min after spin coating the PCBM rich layer, leading to an average PCE enhancement from 4.83% to 5.17% (the best PCE was 5.31%).