The effect of molecular weight on organic photovoltaic device performance is investigated for a series of low-band-gap (ca. 1.65 eV) poly(3- hexadecylthienylene vinylene)s (C16-PTVs) prepared by acyclic diene metathesis (ADMET) polymerization. By utilizing monomers of varying cis:trans (Z:E) content, seven C16-PTVs were prepared with a number-average molecular weight range of 6-30 kg/mol. Polymers were characterized by size-exclusion chromatography, 1H NMR spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, wide-angle X-ray scattering, and differential scanning calorimetry. C16-PTVs were integrated into bulk-heterojunction (BHJ) solar cells with [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM), and conversion efficiency was found to increase with increasing molecular weight. This observation is attributable to an increase in polymer aggregation in the solid state and a corresponding increase in hole mobility. Finally, phase behavior and morphology of the C16-PTV:PCBM active layers were investigated by differential scanning calorimetry and atomic force microscopy, respectively.