Real time mass change and mobility associated with the doping of poly(3-hexylthiophene) for high-performance organic electrochemical transistors*

Polythiophene systems have extensive applications in organic electronics especially in organic electrochemical transistors (OECTs) due to the polymer’s tunable conductivity and redox activity. For example, the conductivity and mobility of poly(3-hexylthiophene) (P3HT) is heavily dependent upon the doping level and the dopant type. Quantification of the doping levels in real time of P3HT or similar conjugated polymers associated with switching of electronic states (conductive vs insulation) is essential for high-performance devices. This study shows relationships among electrochemical doping, solvent uptake, and conductivity using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) and in-situ conductance measurements. Distinct mass transfer regions, quantified as a function of doping level are correlated with spectrochemical response and mobility. The mobility and charge carrier density are calculated, including the real time change in thickness. These experiments are complemented with electrochemical impedance spectroscopy coupled with EQCM-D to identify the time scale at which the doping reaction transitions from kinetic to diffusion control. This work gives valuable insight into the nature of mixed ion-electron transfer, including its time scale, as it relates to the electronic properties of P3HT.