Uncovering the precise effect of the conjugated polymer chain length on the semiconducting properties in thin-film devices is confounded by the step-growth polymerization techniques typically used. Here, we use preparatory size exclusion chromatography to isolate fractions of two diketopyrrolopyrrole-thiophene based co-polymers with low molar-mass dispersity, D-M, and number average molecular weights up to 180 kDa. We find that the charge carrier mobility can vary over three orders of magnitude in the range from 9 to 70 kDa, while a factor of 3-4 increase in photovoltaic performance was noted over the same range. The effect of D-M was found to be most drastic when the largest chains were mixed with the shortest. The study of the thin-film morphology and crystallinity by GIWAXS give further insights into the origin of these effects. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2245-2253