P(BDT-TCNT) and P(DTBDAT-TCNT), which has an extended conjugation length, were designed and synthesized for applications in organic solar cell (OSCs). The solution absorption maxima of P(DTBDAT-TCNT) with the extended conjugation were red-shifted by 5-15 nm compared with those of P(BDT-TCNT). The optical band gaps and highest occupied molecular orbital (HOMO) energy levels of both P(BDT-TCNT) and P(DTBDAT-TCNT) were similar. The structure properties of thin films of these materials were characterized using grazing-incidence wide-angle X-ray scattering and tapping-mode atomic force microscopy, and charge carrier mobilities were characterized using the space-charge limited current method. OSCs were formed using [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as the electron acceptor and 3% diphenylether as additive suppress aggregation. OSCs with P(BDT-TCNT) as the electron donor exhibited a power conversion efficiency (PCE) of 4.10% with a short-circuit current density of J(SC)=9.06 mA/cm(2), an open-circuit voltage of V-OC=0.77 V, and a fill factor of FF=0.58. OSCs formed using P(DTBDAT-TCNT) as the electron donor layer exhibited a PCE of 5.83% with J(SC)=12.2 mA/cm(2), V-OC=0.77 V, and FF=0.62. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3182-3192