The　open-circuit　voltage　(V　(oc))　is　one　of　the　important　parameters　that　influence　the　power　conversion　efficiency　(PCE)　of　polymer　solar　cells.　Its　value　is　mainly　determined　by　the　energy　level　offset　between　the　highest　occupied　molecular　orbital　(HOMO)　of　the　donor　and　the　lowest　unoccupied　molecular　orbital　(LUMO)　of　the　acceptor.　Therefore,　decreasing　the　HOMO　value　of　the　polymer　could　lead　to　a　high　V　(oc)　and　thus　increasing　the　cell　efficiency.　Here　we　report　a　facile　way　to　lower　the　polymer　HOMO　energy　level　by　using　methoxyl　substituted-benzodithiophene　(BDT)　unit.　The　polymer　with　the　methoxyl　functionl　group　(POBDT(S)-T1)　exhibited　a　HOMO　value　of-5.65　eV,　which　is　deeper　than　that　(-5.52　eV)　of　polymer　without　methoxyl　unit　(PBDT(S)-T1).　As　a　result,　POBDT(S)-T1-based　solar　cells　show　a　high　V　(oc)　of　0.98　V　and　PCE　of　9.2%.　In　contrast,　PBDT(S)-T1-based　devices　show　a　relatively　lower　V　(oc)　of　0.89　V　and　a　moderate　PCE　of　7.4%.　The　results　suggest　that　the　involvement　of　methoxyl　group　into　conjugated　copolymers　can　efficiencly　lower　their　HOMO　energy　levels.