Acidity-engineered benzenesulfonic acids achieve high-efficiency Brønsted-Lowry doping of electron-deficient conjugated polymers

Abstract

Conventional molecular charge-transfer (CT) doping is often inefficient in donor–electron-withdrawing (D–DEW) conjugated polymers, limiting their applications. Here, we systematically investigate Brønsted–Lowry acid doping in two D–DEW polymers, PIDF-BTF and PIDF-BTz, using a series of benzenesulfonic acids (BSAs) with tunable acidity enabled by functional-group substitution (–NH2, –CH3, –Cl, and –NO2). Spectroscopic and structural analyses reveal that the distribution of protonation sites governs charge transport. In PIDF-BTF, protonation occurs predominantly on the donor unit, generating delocalized polarons along the backbone, enhancing interchain coupling, and yielding a high electrical conductivity of 89.7 S cm−1. In contrast, PIDF-BTz is predominantly protonated at the high-affinity DEW bithiazole unit, producing highly localized charged states that suppress charge delocalization. Doping efficiency is further regulated by dopant acidity: Hall-effect measurements show that stronger acids produce higher carrier densities, consistent with DFT-calculated Gibbs free-energy differences for deprotonation. Consequently, NO2-BSA-doped PIDF-BTF achieves a power factor of 20.4 μW m−1 K−2. Overall, this work establishes acidity-controlled Brønsted–Lowry doping as an effective strategy to overcome the energy-level mismatch and restricted dopant diffusion inherent to molecular CT doping in electron-deficient D–DEW polymers.