Energy-offset and dopant-location driven molecular doping of polar polyselenophene with high electrical conductivity toward flexible piezoresistive sensor

Molecular doping is an effective approach to modulate the electrical properties of polymer semiconductors. However, doping efficiency is often limited by incomplete charge transfer between polymers and dopants. Herein, we designed three polymers, Pg₃2T-Se, Pg₃2T-T, P2T-Se, with tailored highest occupied molecular orbital (HOMO) levels by combining selenophene/thiophene backbone and alkyl/oligoethylene glycol side chains. It is found that the degree of charge transfer is strongly dependent on the energy offset between polymer and dopant (F4TCNQ). Pg₃2T-Se with a shallowest HOMO level shows completely integer charge transfers (ICT) state, even dopant dianions (double doping) is detected at a low dopant concentration, while both ICT state and charge transfer complex (CTC) state are observed in doped P2T-Se with a deepest HOMO level. A remarkably electrical conductivity of Pg₃2T-Se up to 1135.9 S cm⁻¹ is obtained via single-solution doping, which is attributed to its high carrier concentration. The microstructure evolution of polymer packing upon doping further indicate that the dopants insert into the side chain will facilitate the generation of ICT states. Our results demonstrate that a large energy offset, together with dopant located in the side chains (away from the backbone), promotes high doping level, thereby achieving high conductivity. Moreover, high-conductivity polymer film is attractive for sensor due to a low power consumption and an enhanced sensitivity. F4TCNQ-doped Pg₃2T-Se acts as active layer in flexible piezoresistive sensor and shows high sensitivity and cyclic stability, demonstrating its promising potential in wearable devices.