Effect of molecular weight in diketopyrrolopyrrole-based polymers in transistor and photovoltaic applications

ABSTRACT 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, ĐM, 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 ĐM 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. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2245–2253 While much is now known regarding the effects of Mn and ĐM on rr‐P3HT and PBTTT, much less is known about the new class of donor–acceptor type polymers synthesized using step‐growth techniques. The use of preparatory size exclusion chromatography affords precise separation of rigid DPP‐based polymers into fractions with low dispersity. These fractions reveal a large effect on the performance metrics understood through the investigation of morphology and crystallinity in the thin‐film devices.