Synthesis and Properties of Soluble Fused Thiophene Diketopyrrolopyrrole-Based Polymers with Tunable Molecular Weight

It is challenging to realize both a fully conjugated rigid polymer backbone and high molecular weight at the same time. Previously, we reported a DPP-FT4 polymer with molecular weight up to 30 kDa. A new design and synthesis was required to overcome this limitation. Here, we report the successful synthesis of a conjugated semiconducting polymer with tunable molecular weight over a wide range. Through molecular design and synthesis control, our new polymer can be selectively prepared with number-averaged molecular weight (M n) ranging from approximately 20 to 100 kDa, realizing both high molecular weight and high solubility at the same time. Four polymers within this range were investigated, with particular emphasis on M n of 50 kDa (P2) and 97 kDa (P4). The relationships between molecular weight and polymer properties, molecular packing, and electrical behavior are explored in detail. All the polymers in this series are fully soluble in nonchlorinated solvents at room temperature, which is promising for large-area advanced electronic device applications. The effect of molecular weight on the charge-transport performance of our new polymer was investigated using bottom-gate/top-contact field-effect transistor devices. Stable device characteristics with high on/off ratios up to 107 were obtained. Of particular interest is the discovery that the hole mobility of P2 (lower M n) is higher than that of P4 (higher M n). This is mainly due to morphological manipulation as demonstrated by atomic force microscopy and grazing-incidence X-ray diffraction.