Intersystem Subpopulation Charge Transfer and Conformational Relaxation Preceding in Situ Conductivity in Electrochemically Doped Poly(3-hexylthiophene) Electrodes

Organic semiconductors are increasingly employed in electrochemical devices for energy conversion and storage and chemical sensing. In these systems, the conductivity can be modulated with electrochemical doping with substantial variation in electronic charge densities (10¹⁶ to 10²¹ cm^-3) stabilized by electromigration of counterions from the electrolyte phase. Herein, we focus on the model system of regioregular poly(3-hexylthiophene) to determine the structural evolution at the onset of conductivity arising from electrochemical doping, specifically targeting elucidation of structural relaxation that precedes volumetric swelling. Using spectroelectrochemical methods, a 20% electrochemical active fraction of the film volume comprised of a nanocrystallite subpopulation serves as a high doping efficiency charge nucleation site with an increase from 10¹⁶ to 10²⁰ carriers/cm^-3. A small carrier density window is observed where structural reversion of J-to-H aggregates occurs due to electrostatic repulsion of neighboring charges (bipolarons) on the nanocrystallites. After this conformational change, further increase in doping leads to generation of free volume for counterion diffusion in the nanocrystallites along with doping of the amorphous fraction and J-aggregate recovery. This result advances the structural knowledge of conductive polymer electrodes for electrochemical devices beyond what has been reported using X-ray scattering and provides a benchmark for synthetic structural changes to control hybrid electrical-ionic transport, emphasizing the need to control structural conformation relaxations in addition to volumetric swelling.