Counterion Influence on Thermoelectric Properties of Electrochemically Doped Regioregular and Regiorandom poly(3-hexylthiophene)

Doping of π-conjugated polymers is critical for tuning their properties and making them operable in many organic electronic devices including organic light emitting diodes, organic thermoelectrics, and organic electrochemical transistors. In both chemical doping and electrochemical doping an ionized dopant or counterion is required to balance the newly acquired charge on the doped π-conjugated polymer. The characteristics of this dopant or counterion, such as size, shape, and relative polarity, can significantly impact the optical, electronic, and thermoelectric properties of the resulting material. Here, we investigate how the counterion structure impacts the electrochemical doping ability, oxidation potential, ionization energy, and polaron absorbance of regioregular (rr) and regiorandom (rra) P3HT. We also probe how electrochemical doping with these different counterions leads to changes in conductivity and Seebeck coefficients influencing thermoelectric performance. The most prominent change in the polymer oxidation potential is observed for rr-P3HT with the large tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion. We propose that this large anion is excluded from the tightly packed crystalline regions and thus the oxidation potential is closer to that of the mostly amorphous rra-P3HT. The anions also result in significant differences in polaron absorbance and ionization energies as well as notable changes in peak broadening in epr spectra with increased doping. These results emphasize the important role that the counterion has in influencing the optical and electronic properties of doped π-conjugated polymers.