Solvent Swelling in Semiconducting Polymers Dictate Doping Ability

Semiconducting polymers are of interest due to their easy processibility and various electronic applications. In the pristine form, they are not intrinsically conducting because of their wide bandgap, however, their conductivity can be improved through simple redox doping. Electrochemical methods allow for doping at various potentials, provides an easy way to control the redox energy, and therefore the doping level. Removing electrons (p-type doping) by oxidation creates positive charge carriers. Both polarons (singly charged) and bipolarons (doubly charged) can be created at different potentials. Creation of p-polarons is accompanied by anion intercalation from the electrolyte into the polymer matrix for charge balance. The system requires ion mobility to facilitate this insertion to occur. In previous studies we found a direct relationship between this ionic mobility and swelling of the polymer by the liquid electrolyte. In the literature, many researchers address the importance of swelling in electrochemically doped polymers through side-chain engineering, or controlled cross-linking. In electrochemical systems, however, swelling is not static, and should change dynamically during polymer doping. This dynamic swelling has been shown for electrochemical systems doped in aqueous media, but it is not well understood for fully hydrophobic polymers doped in organic solvents, such as common battery electrolytes.
In this work, we studied the electrochemical behavior of the model conjugated polymer poly(3-hexylthiophene) [P3HT], using three different regioregularities of P3HT to understand the factors that control swelling. In our work, we first paired cyclic voltammetry and UV-Visible-NIR absorption spectroscopy to show how the electronic properties of polymers are influenced by the doping potential. We found that polarons are always formed at low potential, and then biopolarons start to form at higher applied potentials, but the starting crystallinity of the neutral polymer controls the specific potential for each reaction. Through grazing incidence wide angle X-ray scattering (GIWAXS) measurements, we observed significant structural changes with doping—the most crystalline polymers lose some of the overall crystallinity when counter-anions and solvent enter the polymer network, while the regiorandom polymer undergoes dopant-induced crystallinity. We then used an in situ electrochemical quartz crystal microbalance (EQCM) to directly visualize how the doping level dynamically affects the swelling of semiconducting polymers of differing regioregularities, and in different solvents. Regardless of the regioregularity, polaron formation does not result in significant swelling, but bipolaron formation does. Lastly, in situ measurements of electronic and ionic conductivities show that the polymer that swells the most (RRa P3HT), has the highest ionic conductivity, while the most regioregular, most crystalline shows the highest electronic conductivity. We also varied the salt in battery electrolytes in an effort to understand the role of anion size on electrochemical doping. We find that as the anion size gets smaller, the polaron formation energy goes up, while the bipolaron formation energy goes down. Anion size was also found to influence the extent to which polymer crystallinity was lost upon bipolaron formation.