<p>Two donor-acceptor (D-A) type low bandgap (LBG) alternating conjugated copolymers containing larger conjugation planarity and stronger electro-withdrawing ability naphtho[1,2-<i>c</i>:5,6-<i>c</i>′]bis[1,2,5]thiadiazole (NT) unit, namely, poly[4,8-bis(5-(<i>n</i>-octylthio)thien-2-yl)-benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,6-diyl-<i>alt-</i>4,9-bis(4-(2-decyltetradecyl)thien-2-yl)naphtho- [1,2-<i>c</i>:5,6-<i>c</i>′]bis[1,2,5]thiadiazole-5,5′-diyl] (<b>PBDT-TS-DTNT-DT</b>) and poly[4,8-bis(triiso-propylsilylethynyl)benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,6-diyl-<i>alt</i>-4,9-bis(4-(2-decyltetradecyl)-thien-2-yl)naphtho[1,2-<i>c</i>:5,6-<i>c</i>′]bis[1,2,5]thiadiazole-5,5′-diyl] (<b>PBDT-TIPS-DTNT-DT</b>), were prepared by the palladium-catalyzed Stille polycondensation and characterized by gel permeation chromatography (GPC), UV-Vis absorption, thermal gravimetric analysis (TGA), cyclic voltammetry (CV) <i>etc</i>. <b>PBDT-TS-DTNT-DT</b> and <b>PBDT-TIPS-DTNT-DT</b> show the broader absorption and deeper highest occupied molecular orbital (HOMO) energy level approximately −5.45 and −5.62 eV, respectively. Bulk-heterojuction solar cells based on the resulted polymers and [6,6] phenyl-C<sub>61</sub> butyric acid methyl ester (PC<sub>61</sub>BM) blends, with the device configuration of ITO/PFN/polymer:PC<sub>61</sub>BM/MoO<sub>3</sub>/Ag were prepared and investigated. The results showed the power conversion efficiency (PCE) of 2.67% for <b>PBDT-TS-DTNT-DT</b>/PC<sub>61</sub>BM (w:w, 1:2) and 0.64% for <b>PBDT-TIPS-DTNT-DT</b>/PC<sub>61</sub>BM (w:w, 1:1), with relatively high open-circuit voltage (<i>V</i><sub>OC</sub>) of 0.86 and 1.05 V, small short-circuit current (<i>J</i><sub>SC</sub>) of 5.41 and 0.97 mA cm<sup>−2</sup> and moderate fill factor (<i>FF</i>) of 57.8% and 62.4%, under an AM1.5 simulator (100 mWcm<sup>−2</sup>), respectively.</p
