Semicrystalline poly(3-hexyl-2,5-thienylene vinylene) (P3HTV) with a low band gap of 1.65 eV has been synthesized by acyclic diene metathesis polymerization and incorporated into bulk heterojunction (BHJ) organic solar cells. The polymer was thermally characterized by differential scanning calorimetry and thermogravimetric analysis and was blended with the electron acceptor methanofullerene [6,6]-phenyl Cöi-butyric acid methyl ester (PCBM) to make a light-harvesting charge-transfer thin film. The properties of P3HTV/PCBM blends were studied as a function of PCBM composition by wide-angle X-ray scattering, atomic force microscopy, transmission electron microscopy, UV-vis absorption spectroscopy, and charge-transport and photovoltaic measurements. The PCBM solubility limit, that is, the phase separation point, was estimated to be 50 wt % PCBM. The phase behavior of the blend was directly correlated with electrical transport behavior in a fieldeffect transistor testbed. At the phase separation point, charge carrier transport switches from hole only to ambipolar (both electron and hole) due to the formation of an electron-transporting percolating network of PCBM domains. BHJ solar cells were constructed with P3HTV films blended with varying weight fractions of PCBM. In these cells, spun-cast films of P3HTV/PCBM mixtures were sandwiched between poly(3,4ethylene dioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)-coated ITO and Al electrodes. The best performance of polymer solar cells was observed at 50-60% PCBM, near the phase separation point at which power conversion efficiencies of 0.80-0.92% were measured under AM 1.5, 100 mW/cm 2 illumination.