Acid vapor doping of polar polythiophenes for high electrical conductivity

An ideal doping process should be highly efficient and avoid the disruption of molecular packing to achieve the synergistic optimization of carrier concentration and mobility. Herein, we synthesized two polythiophene derivatives with oligo ethylene glycol as side chains and thienothiophene (bithiophene) as unit, denoted Pg₃2T-TT (Pg₃2T-2T). A strong acid, trifluoromethanesulfonic acid (TfOH), was used as dopant to elucidate the effects of two doping methods, solution- and vapor-doping without ultrahigh vacuum, on the polar polythiophenes' electrical properties. Notably, the σ of vapor-doped films were two times higher than those of solution-doped counterparts. Specifically, TfOH vapor-doped Pg₃2T-TT achieved a remarkable σ value of up to 1173.9 S cm^-1, among the highest values of polar polythiophenes. The high σ of vapor-doped films was attributed to the increased carrier concentrations without degradation of mobilities, which indicates that vapor doping enables more effective doping reactions with maintained crystallinity, compared with solution doping, because acid vapor molecules directly diffuse into the polar side chains of pure polymers as confirmed by grazing-incidence wide-angle X-ray scattering. Moreover, TfOH-doped films exhibited exceptional air stability stored in air for over one month. This work demonstrates that TfOH vapor doping without ultrahigh vacuum conditions represents a promising approach for improving the electrical conductivity and stability of conductive polymers for flexible electronics.