INVESTIGATION OF THE ELECTROCHEMICAL AGING OF POLY(3-HEXILTIOPHENE) USING IMPEDANCE SPECTROSCOPY R. Gonçalves, A.A. Correa, R. Pereira, E.C. Pereira* Chemistry Dept., Federal University of São Carlos, Mail Box 676, CEP 13565-905, São Carlos – SP, Brazil A R T I C L E I N F O Article history: Received 28 August 2015 Received in revised form 23 December 2015 Accepted 28 December 2015 Available online 31 December 2015 Keywords: poly(3-hexylthiophene) impedance spectroscopy electrochemical aging electrochemical degradation A B S T R A C T In this paper, the electrochemical ageing of poly(3-hexilthiophene) was studied using two different films electrodeposited under different experimental conditions. The first one has been was prepared in acetonitrile with 0.1 mol L À1 of monomer + 0.1 mol L À1 of LiClO 4 (Film I). The second one has been synthesized in the same solvent with 0.2 mol L À1 of monomer + 0.2 mol L À1 of LiClO 4 , plus the addition of 10 mmol L À1 of pure water (Film II). The electrochemical ageing has been carried out by the application of an overoxidation potential. At every 20 minutes, a cyclic voltammogram and electrochemical impedance experiments were measured. Each film has exhibited different pathway for the ageing process. Film II showed a smooth degradation of the electrochemical properties, mainly at the interface polymer chains and solution in the pores. The other one, Film I, has presented a two steps degradation process, been the first, up to 120 min, characterized by smooth changes in all the electrochemical parameters necessary to describe the polymer. After 120 min, the second degradation process occurs characterized by an abrupt change in in all the parameters, indicating an important change not only in the electrochemical degradation of the polymer but also in its morphology, measured by SEM micrographs. Furthermore, two electrode impedance electronic experiments confirmed an important degradation of the bulk polymer for Film I. ã 2015 Elsevier Ltd. All rights reserved. 1. Introduction Poly(3-hexylthiophene), P3HT is the most exploited Pth derivative[1–3] given its range of absorption, low bandgap, and high hole mobility being widelyused in bulk heterojunction solar cells. The durability of such organic devices is very limited compared with inorganic and hybrid ones. It is especially the case in those ones where the polymers are the active material and are submitted to successive oxidation/reduction cycles. One possibility to explain these results is the ion transport “in” and “out” of the polymer, which is necessary to compensate the generated charges in the polymer chains during the redox process. This intercalation can cause a mechanical stress in the material [4,5], which leads to premature ageing. Despite its importance, only a small number of papers investigate the impact of the ageing in the electrochemical properties of the polymers. Tourillon [6], for example, studied poly(3-alkylthiophenes) (P3ATs) films under different experimen- tal conditions in both absence or presence of water and at different pH values. The authors observed a strong effect of the solution pH on the electrochemical degradation, which they proposed to be related to a nucleophilic effect. In 1991, Harada et al [7], using cyclic voltammetry up to the superoxidation region, demonstrated that this process is irreversible, i.e., once the film degrades, it can no longer be recovered. A very important observation was made by Wang [8], who has described a relationship between the deactivation process of P3AT and the solvent pKa value. Pud, in 1994 [9], studied the stability and the anodic degradation of many conducting polymers. In that paper, the authors observed that the superoxidation does not disable all of the polymer properties, For example, the charge storage of polypirrole does not decrease after the process. Besides, the author observed that a morphological change occurs in the film due to the gas formation as byproduct of polymer superoxidation. In a previous work of our group, it was discussed the aging effect itself, focusing on the changes occurred on properties of polypirrole [10] and PANI/PSS self-assembled films [11]. In those works, it was used impedance spectroscopy and data were analyzed using transmission line models. The choice of the electrochemical impedance technique occurred because, with a single measurement, it is possible to separate the different processes that occur at the interface polymer/solution, the polymer chains and solution in the pores. It was observed an * Corresponding author. E-mail addresses: ernesto@ufscar.br, ernestopereira51@gmail.com (E.C. Pereira) . http://dx.doi.org/10.1016/j.electacta.2015.12.198 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved. Electrochimica Acta 190 (2016) 329–336 Contents lists available at ScienceDirect Electrochimica Acta journa l home page : www.e lsevier.com/loca te/ele cta cta