Side-Chain Length Matching Enhances Aggregation and Crystallization in Conjugated Polymers

Side-chain engineering is a powerful approach for tuning the microstructure of conjugated polymers to optimize the device performance. Here, we report an anomalous side-chain length dependency of aggregation and crystallization in a series of diketopyrrolopyrrole-thieno[3,2-b]thiophene (PDPPTT-Cx, x = 5-12) copolymers. Instead of the common solubility-crystallinity trade-off induced by side-chain length, PDPPTT-Cx with intermediate side-chain lengths (x = 7-9), particularly PDPPTT-C8, showed significantly enhanced solution-state aggregation, ordered packing, and crystallinity in the solid state. First-principles modeling and calculations attributed this behavior to a side-chain length matching effect, which maximized interchain contact and van der Waals interactions. With the enhanced packing, PDPPTT-C8 achieved a hole mobility of 0.091 cm² V^-1 s^-1 and a conductivity of 10.3 S cm^-1 after doping, over one magnitude higher than those of its shorter or longer side-chain analogues. These findings highlight the sensitivity of conjugated polymer microstructure to subtle structural modifications, providing guidance for precise side-chain engineering in high-performance organic electronics.