A short review on the strategy towards development of π-conjugated polymers with highly reversible p- and n-doping

In order to increase the n-doping ability and ensure the high stability of the reduced forms of π-conjugated polymers, we compare two alternative concepts for designing such polymers, based on polythiophene. The first is based on the introduction of an electron-withdrawing group in the 3-position of the thiophene ring, whereas the second approach involves the synthesis of subunits that possess high electron-withdrawing abilities and polymers comprising bi- or quatro-thiophene subunits in the polymeric backbone. Among the four polymeric films reported in this paper, exceptionally good results were obtained with polymethyl octyl thiopene oxadiozole (PMOThOD) film electrodes, which are composed of quatro-thiophene and oxadiazole subunits. The remarkable redox activity at low potentials of this polymer and the absence of charge trapping, as well as the charge trapping phenomena occurring with the other polymeric systems studied, are explained and discussed in light of both the cyclic voltammetric and electrochemical impedance spectroscopic studies of these systems. The conclusions reached are supported by modeling the impedance of the films' electrodes, which are subjected to charge trapping, either because of geometric or energetic non-homogeneities.