Intersystem sub-population charge transfer and conformational relaxation preceding in situ conductivity in electrochemically doped poly(3-hexylthiophene) (P3HT) 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 electro-migration of counterions from the electrolyte phase. Herein we focus on the model system of regioregular poly(3-hexylthiophene) (P3HT) 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 sub-population serves as high doping efficiency charge nucleation sites, 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 repul...