A series of novel soluble donor-acceptor low-bandgap-conjugated polymers consisting of different oligothiophene (OTh) coupled to electron-accepting moiety 2-pyran-4-ylidenemalononitrile (PM)-based unit were synthesized by Stille or Suzuki coupling polymerization. The combination of electron-accepting PM building block with varied OThn (the number of thiophene unit increases from 3 to 5) results in enhanced pi-pi stacking in solid state and intramolecular charge transfer (ICT) transition, which lead to an extension of the absorption spectra of the copolymers. Cyclic voltammetry measurements and molecular orbital distribution calculations indicate that the highest occupied molecular orbitals (HOMO) energy levels could be fine-tuned by changing the number of thiophene units of the copolymers, and the resulting copolymers possessed relatively low HOMO energy levels promising good air stability and high-open circuit voltage (V-oc) for photovoltaic application. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)phenyl C-61-butyric acid methyl ester as acceptor. It was found that the highest V-oc, reached 0.94 V, and the short circuit currents (J(sc)) were improved from 1.78 to 2.54 mA/cm(2), though the power conversion efficiencies of the devices were measured between 0.61 and 0.99% under simulated AM 1.5 solar irradiation of 100 mW/cm(2), which indicated that this series copolymers can be promising candidates for the photovoltaic applications. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2765-2776, 2010