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.