A series of new 2,7-dioctyl-substituted dibenzo[a,c]phenazine (BPz) derivatives were designed and synthesized as electron-deficient units, which were copolymerized with an electron-rich indacenodithiophene (IDT) to construct narrow-bandgap copolymers PIDT-OHBPz, PIDT-OFBPz, and PIDT-OBPQ via Stale polycondensation. The 2,7-dioctyl substituents enhanced solubility and offered a new approach for developing various BPz derivatives. All copolymers showed high hole mobilities above 0.01 cm(2) V-1 s(-1) as measured by field effect transistors. The best performance of polymer solar cell was achieved based on PIDT-OFBPz with inverted device structure of ITO/ZnO/PFN/polymer:PC61BM/MoO3/Al, which showed an open-circuit voltage (V-oc) of 0.97 V, a short-circuit current (J(sc)) of 8.96 mA/cm(2), and a fill factor (FF) of 58.99%, leading to a high power conversion efficiency (PCE) of 5.13%. These results indicate that 2,7-alkyl-substituted BPz derivatives can be used as an excellent electron-deficient building block for the construction of high-performance organic electronic materials.