Solvent Swelling in Semiconducting Polymers Dictate Doping Ability

Semiconducting polymers are of interest due to their easy processibility and broad electronic applications. Electrochemistry allows for doping, through simply redox, at various potentials. When polymers are p-type doped to improve their conductivity, various positively charge carriers are created, including polarons (singly charged) and bipolarons (doubly charged). Carrier creation is accompanied by anion intercalation from the electrolyte for charge balance, and this insertion requires ion mobility. In previous studies, we found a direct relationship between ionic mobility and swelling of the polymer by the liquid electrolyte.

In this work, we used different regioregularities of poly(3-hexylthiophene) P3HT to understand the relationship between swelling and doping. We paired cyclic voltammetry, UV-Vis-NIR absorption spectroscopy, and grazing incidence wide angle X-ray scattering (GIWAXS) measurements to correlate doping level and structural change. We then used an in situ electrochemical quartz crystal microbalance (EQCM) to quantify the swelling of the polymers dynamically during cycling. Lastly, in situ conductivity measurements were used to measure the effect of swelling on the ionic and electronic conductivity. We also varied the counterion size to see its influence on crystallinity. Overall, we found that solvent swelling was required for bipolaron formation, and that it facilitated both the small structural changes need for polaron formation, and the disordering required for bipolaron formation.