Thiojulolidine Functionalized Benzimidazoline Dopant with Polarizability-Driven Counterion Coulombic Mitigation for High-Performance n-Doped Polymer Thermoelectrics

One of the fundamental limitations of n-doped thermoelectric polymers lies in Coulombic trapping between dopant counterions and polymer polarons, which severely restricts carrier generation and transport. Herein, we address this through a heteroatom engineering strategy that leverages atomic polarizability to mitigate counterion Coulombic attraction, using the benzimidazoline dopant 1,3-dimethyl-2-(julolidine-9-yl)-2,3-dihydro-1H-benzo[d]imidazole (JLBI) and its sulfur-substituted analogue as a model system for doping the n-type polymer ThDPP-CNBTz. Specifically, replacing 1,7-dimethylene in julolidine with sulfur atoms converts JLBI into 1,3-dimethyl-2-(1,7-dithiajulolidin-9-yl)-2,3-dihydro-1H-benzo[d]imidazoline (S-JLBI). This substitution at the meta positions transforms the localized surface electrostatic potential (ESP) of the JLBI cation into an extended potential in S-JLBI cation due to the dielectric shield from polarizable 3p valence electron cloud of sulfur, thereby weakening the dopant counterion Coulombic trap. The weakened trapping is evidenced by reduced conductivity activation energy across doping concentrations. Consequently, the S-JLBI-doped ThDPP-CNBTz films achieve a maximum power factor of 43.91 μW m–1 K–2 with a conductivity of 37.83 S cm–1, outperforming the JLBI-doped counterpart (34.10 μW m–1 K–2, 20.80 S cm–1) at the optimized doping level. Overall, this work establishes a blueprint for manipulating cation ESP and resulted counterion Coulombic trap via sulfur heteroatom substitution and highlights S-JLBI as a promising n-dopant for high-performance polymer thermoelectrics.