Unusually stable and highly electrochemically reversible n-doping of regioregular alternate copolymer of dialkylthiophene and fluorenone

We describe detailed studies on the electrochemical activity of thin films of a new conjugated copolymer, namely, poly[(2,7-fluoren-9-one)-alt-(5,5′-(3,3′-di-n-octyl-2,2′-bithiophene))] (abbreviated PFDOBT-HH (HH = head-to-head)), which has been synthesized electrochemically by a mild oxidation process at low current density. The dynamics of both p- and n-dopings of PFDOBT-HH in sulfolane/TEABF4 solution was fully characterized by a combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopic (EIS) investigations. The development of a stable and highly electrochemically reversible n-doping capacity of the polymer film is presented as a function of the vertex potential in the CV curves of the n-doped PFDOBT-HH. Using EIS, clear evidence is furnished for the presence of trapped, negatively charged species in the polymer bulk as a consequence of prolonged, consecutive n-doping. Qualitative differentiation between the responses originating from mobile and trapped charge carriers has been established. In contrast to the cases of pristine polythiophene, polyfluorothiophene and nanoscale aryleneethynylene oligomers, n-doping of PFDOBT-HH, although leading to a partial trapping of the negatively charged carriers, does not result in any irreversible change in the electrochemical behavior of this polymer.