Two donor-p-acceptor (D-p-A) type low bandgap (LBG) conjugated polymers, namely, PT-DTNT-DT and PTT-DTNT-DT, consisting of thiophene (T) and/or thieno[3,2-b] thiophene (TT) as donor moiety, two flanking 2-decyltetradecylthienyl as pi-bridges and large planarity and strong electron-withdrawing naphtho[1,2-c:5,6-c']bis[1,2,5] thiadiazole (NT) as acceptor moiety have been developed and systematically investigated. Both copolymers exhibited strong absorption in the range of 300-850 nm, whilst possessed the low-lying highest occupied molecular orbital (HOMO) energy level of ca. -5.38 eV and the matched lowest unoccupied molecular orbital (LUMO) energy level of -3.90 eV with PC71BM. Compared with NT-based star material PNTz4T using 2,2'-bithiophene (2T) as the linker, the HOMO energy level of the resultant two polymers have been significantly lowered while the absorption spectra range were obviously broadened by replacing of 2T with T and TT. PTT-DTNT-DT showed better coplanarity, more pronounced aggregation tendency in chlorobenzene and higher hole mobility of space-charge-limited current (SCLC) model than those of PT-DTNT-DT. Inverted bulk heterojunction polymer solar cells (PSCs), with these NT-based copolymers as electron donors, respectively, and PC71BM as electron acceptor were fabricated. The optimized cell based on PT-DTNT-DT exhibited the power conversion efficiency (PCE) of 5.10%, with an open-circuit voltage (V-OC) of 0.76 V, promising short-circuit current (J(SC)) of 11.24 mA cm(-2) and fill factor (FF) of 59.88%, whereas, the device based on PTT-DTNT-DT showed relatively higher PCE of 6.88% which was slightly higher than that of PNTz4T (6.72%), with an increased JSC of 14.74 mA cm(-2) and FF of 64.00%, despite the slightly decreased V-OC of 0.73 V, under the same experimental conditions. Therefore, these results indicated the oligothiophene-NT based copolymers were promising candidates aimed at achieving efficient PSCs. (C) 2017 Elsevier Ltd. All rights reserved.