Molecular weight is among the most important properties governing polymer behavior. In the rapidly developing field of polymer solar cells, understanding molecular weight effects is necessary to maximizing device efficiency and realizing commercialization. Semicrystalline poly(3-hexadecyl-2,5-thienylene vinylene) (P3HDTV), with a low band gap of 1.65 eV, was synthesized by acyclic diene metathesis polymerization with tuned chain length. Neat polymer samples of various molecular weight were characterized by size-exclusion chromatography, cyclic voltammetry, differential scanning calorimetry, UV-Vis spectroscopy, and wide-angle X-ray scattering. P3HDTV was blended with the electron acceptor methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) to form the bulk heterojunction (BHJ) photovoltaic cell active layer. Completed cells were characterized with diode measurements in the dark and under simulated solar illumination, and with monochromatic external quantum efficiency (EQE) measurements. BHJ cells were optimized for active layer thickness and P3HDTV/PCBM composition. In a series of 100 nm thick, constant composition devices, increasing molecular weight produced a modest drop in cell open circuit voltage (Voc), saturating at 0.40 V. This was overcome by large increases in cell short circuit current (Jsc) with increasing molecular weight, resulting in increased power conversion efficiency (PCE) with increased molecular weight. Cells incorporating high molecular weight P3HDTV showed PCE of ∼1%.