Two novel dibromo monomers consisting of the isomers of 5-alkylphenanthridin-6(5H)-one (PN) and 6-alkoxylphenanthridine (PO) were synthesized through alkylation of the precursor 3,8-dibromophenanthrindi-6(5H)-one, where the molecular structures were confirmed by NMR spectroscopy. The medium bandgap conjugated polymers PDBTPN and PDBTPO were constructed by utilizing such two isomers PN and PO as the electron-donating units and dithiophenebenzo[ 2,1,3] diathiazole as the electron-accepting unit. The resulting polymers exhibited analogous absorption profiles with optical bandgap of 1.90 eV, while PDBTPO showed slightly higher absorption coefficiency. Cyclic voltammetry measurements revealed that these polymers had relatively deep highest occupied molecular orbital levels of about 25.70 eV. Polymer solar cells based on such two polymers showed relatively high open-circuit voltage of about 0.90 V. All devices exhibited moderate performances with the best power conversion efficiency of 3.77% achieved based on PDBTPO. Devices based on PDBTPO showed slightly higher power conversion efficiency than those based on PDBTPN, which can be ascribed to higher hole mobility and more favorable film morphology of the former. (C) 2016 Wiley Periodicals, Inc.