How charge trapping affects the conductivity of electrochemically doped poly(3-hexylthiophene) films

Electrochemical doping is an elegant method of controlling the doping level and charge carrier densities of conjugated polymer films and enhancing their thermoelectric figure of merit. Applying this doping technique to films of poly(3-hexylthiophene) (P3HT) results in conductivities with values as high as 200 S/cm. The stability of the doped films in the solid state can be probed by UV-vis-NIR spectroscopy. We found that the choice of the conducting salt in the liquid electrolyte exerts a strong influence over the conductivity. Using TBAPF₆ and LiClO₄ provides highest conductivities for P3HT films, while LiTFSI and TBABF₄ show overall lower performance. This effect is also reflected in cyclic voltammetry measurements coupled with in situ spectroscopy. Overall lower reversibility upon multiplex cycling in LiTFSI and TBABF₄ electrolytes suggests strong charge trapping effects, which one might attribute to a considerable fraction of charges (holes/ions) remaining in the films after charge/discharge cycles. The salts with stronger charge irreversibility in the electrochemistry experiments show the poorer solid state conductivities. Our conclusion is that one should carefully choose the electrolyte to ensure good percolation pathways and delocalized charge transport throughout doped films.