UNCORRECTED PROOF EA 10360 1–9 Electrochimica Acta xxx (2004) xxx–xxx Electrochemical overoxidation of poly(3,4-ethylenedioxythiophene)— PEDOT studied by means of in situ ESR spectroelectrochemistry 3 4 A. Zykwinska a , W. Domagala a , B. Pilawa b , M. Lapkowski a,b,∗,1 5 a Faculty of Chemistry, Silesian University of Technology, 9 M. Strzody Street, 44-101 Gliwice, Poland 6 b Institute of Coal Chemistry, Polish Academy of Sciences, 5 Sownskiego Street, 44-121 Gliwice, Poland 7 8 Abstract 9 Electrochemical doping and dedoping processes in poly(3,4-ethylenedioxythiophene)—PEDOT have been studied by in situ ESR spec- troelectrochemistry with an aim to investigate the behaviour of charge carrying species, generated and annihilated in these processes, with changing potential. As an extension of earlier studies, we investigated the behaviour of PEDOT at high oxidation potentials, beyond the limit of electrochemical stability of the polymer located at ca. 1.6V. Past this limit, a sudden and irreversible drop in the concentration of spins together with narrowing of the ESR signal is observed. Changes in the spectroscopic response of the film are irreversible as evidenced by the course of the subsequent reduction process, which by no means resembles the reduction process of the polymer recorded within the potential window in which the polymer is stable. Supplemented by results of electrochemical studies, it is concluded that direct overoxidation of the polymer chain most probably leads to a decrease in the conjugation length of the polymer’s delocalised -bond, through cross-linking of the polymer chains. Consequently, the remaining spins become trapped in isolated packets where they behave more like oligomer type radicals. 10 11 12 13 14 15 16 17 18 © 2004 Published by Elsevier Ltd. 19 Keywords: Charge carriers; PEDOT; ESR spectroscopy; Overoxidation; Polymer films 20 21 1. Introduction 1 Since the development of poly(3,4-ethylenedioxyth- 2 iophene)—PEDOT in the late 1980s by scientists at Bayer 3 A.G. [1], this polymer to date received an ever-accelerating 4 scientific interest of scientists throughout the world. Today 5 PEDOT is being investigated in a many research fields start- 6 ing from basic polymer science [2] through material science 7 [3], electrochemistry [4], electronics [5] and optoelectronics 8 [6–8], photovoltaics [9–11], corrosion protection [12], novel 9 cell systems [13] down to biosensors [14] as one of future 10 prospects. The reasons for such widespread interest are its 11 much sought for properties like high conductivity, interest- 12 ing electrical and spectrochemical properties associated with 13 its low band gap, electrochromic and antistatic properties 14 and good stability all of which paved this polymer way into 15 numerous important applications [15,16]. These properties 16 ∗ Corresponding author. Tel.: +48 32 237 1743; fax: +48 32 237 1509. E-mail address: lapkowsk@zeus.polsl.gliwice.pl (M. Lapkowski). 1 An ISE member. originate from the high regularity of the polymer backbone, 17 which are purely – ′ coupled, thanks to the blocking of 18 unfavourable  positions by the cyclic 3,4-dioxy substituent. 19 The two oxygen atoms bonded directly to the thiophene back- 20 bone enrich the conjugated -bond with electrons, lowering 21 the oxidation potential of the polymer, while the cyclic char- 22 acter of the ethylenedioxy substituent curbs the disorder that 23 otherwise linear side groups could introduce into the polymer 24 structure. A review of the state of development of PEDOT’s 25 chemistry and contemporary applications together with a rich 26 list of references has been given by Groenendaal et al. [3]. 27 The electronic properties of a conductive polymer can be 28 altered through doping. Nowadays it is widely accepted that 29 depending upon the doping level two different charge carry- 30 ing species can prevail i.e., polarons and bipolarons both of 31 which can propagate the electrical current along the polymer 32 chain [17–19]. Polarons carry a magnetic moment and can 33 therefore be observed and distinguished from diamagnetic 34 bipolarons by means of ESR spectroscopy. Their spectro- 35 scopic response is sensitive however to the chemical envi- 36 ronment they reside in and since bipolarons constitute a part 37 1 0013-4686/$ – see front matter © 2004 Published by Elsevier Ltd. 2 doi:10.1016/j.electacta.2004.10.026