Backbone Torsion as the Limiting Factor in Charge Transport of Diketopyrrolopyrrole–Benzotriazole Donor–Acceptor Conjugated Polymer Films

Diketopyrrolopyrrole–benzotriazole (DPP–BTz)-based donor–acceptor copolymers are promising semiconductors for organic field-effect transistors (OFETs), yet the interplay between intrachain planarity and interchain π–π stacking remains insufficiently understood. We report a systematic study of dithienyl–DPP–BTz copolymers with varied DPP lactam side-chain composition (alkyl/fluoroalkyl = 10:0, 7:3, 5:5, 3:7) to decouple the effects of fluorination on molecular conformation and packing. Thin films were characterized via ultraviolet–visible spectroscopy, grazing incidence X-ray diffraction, and density functional theory simulations, while charge-transport properties, including the trap density of states, were evaluated in OFETs. Increasing the fluoroalkyl fraction shortens the π–π stacking distance but concurrently increases backbone torsion, highlighting a trade-off between enhanced interchain interactions and reduced intrachain conjugation. Despite the tighter π–π spacing, devices show progressively lower drain currents and mobilities with higher fluorination, establishing that backbone planarity and rigidity, rather than marginal π–π tightening, govern charge transport. These findings provide a practical design principle for donor–acceptor polymers: preserve backbone planarity through conformational locking while using side-chain chemistry to maintain processability without introducing torsional penalties. The structure–property insights presented here offer transferable guidelines for side-chain engineering beyond the DPP–BTz system.