Polymer solar cells with a low-bandgap dye molecule are demonstrated by employing ternary blends of poly(3-hexylthiophene) (P3HT), fullerene derivative (PCBM), and silicon phthalocyanine derivative (SiPc) under different annealing conditions. The solvent annealing induces selective allocation of SiPc molecules at amorphous P3HT domains that would be formed at the interface between P3HT and PCBM crystalline domains, and furthermore effectively suppresses diffusion and aggregation of low molecular weight components which degrade the performance of the thermally annealed devices. As a result, the solvent-annealed ternary blend solar cells exhibited an enhanced power conversion efficiency of 4.5%.