Multilayered polymer solar cells were fabricated by a combination of spin-coating and layer-by-layer (LbL) deposition techniques. A light-harvesting layer was precisely designed by LbL assembly of poly(p-phenylenevinylene) (PPV) and an anionic PPV derivative, poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene] (MPS-PPV). The remaining hole-transporting and electron-transporting layers were prepared by the spin-coating method. In order to improve optical properties of the MPS-PPV LbL film, dimethylsulfoxide (DMSO) was added to an aqueous solution of MPS-PPV. As a result, the absorption of PPV/MPS-PPV LbL films increased at around 500 nm compared with that of the reference LbL films fabricated from an aqueous solution of MPS-PPV. The short-circuit current density of the multilayered polymer solar cell was improved by > 10% compared to the reference cell with PPV/MPS-PPV LbL layers fabricated from the aqueous solution. Furthermore, the external quantum efficiency at around 500 nm was also improved, suggesting the increase in the light-harvesting efficiency of the LbL layer fabricated from the water/DMSO mixed solution of MPS-PPV.