b301403g - Electrochemical bandgaps of substituted...

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Unformatted text preview: Electrochemical bandgaps of substituted polythiophenes { Tomas Johansson,* a Wendimagegn Mammo, b { Mattias Svensson, b Mats R. Andersson b and Olle Ingana s a a Biomolecular and Organic Electronics, IFM, Linko ping University, 581 83 Linko ping, Sweden. E-mail: tojoh@ifm.liu.se b Departments of Organic Chemistry and Polymer Technology, Chalmers University of Technology, 412 83 Go teborg, Sweden Received 5th February 2003, Accepted 7th April 2003 First published as an Advance Article on the web 28th April 2003 The electrochemical bandgaps for different soluble substituted polythiophenes have been measured by cyclic voltammetry. The effect of substituents on the oxidation/reduction potentials is discussed. Bandgaps obtained by cyclic voltammetry have been found to be in general higher than optical bandgaps. Among regioregular polymers substituted with a phenyl group at position 3 of the thiophene ring, examples are found that give very symmetric voltammograms. Rationalization for this behaviour is discussed from a conformational point of view. Introduction The ability to dope conjugated polymers electrochemically is important for several reasons. Many applications of conjugated polymers, such as light emitting electrochemical cells, 1,2 micro actuators, 35 energy storage, 6 photovoltaic 7 and electrochromic devices, 8 and sensors 9,10 are based on electrochemical transitions between doped and neutral states or rely on the stability of a specific doping level. A detailed understanding of the doping process has remained a scientific challenge. Electrochemical doping is complex and consists of several simultaneous and/or consecutive chemical and physical processes like swelling of the polymer, charge transfer between the electrode and the polymer, insertion of compensating ions into the bulk of the polymer, conformational changes of the polymer chain and change of conductivity. The strong development of polymer electronics during the last decade has made cyclic voltammetry, CV, an established method to assess the energy position of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This is an alternative to methods based on surface photoelectron spectroscopy. Cyclic voltamme- try requires that electrochemical potentials are related to the vacuum level, something associated with both fundamental and practical problems. Although frequently used, the methodolo- gical basis for electrochemical band edge determination is not very well established. The relevance and generality of electro- chemical characterisation of conjugated polymers can be estimated by comparison to other methods. This is frequently done by calculating the electrochemical bandgap, E g,EC , and comparing it with the optical bandgap, E g,opt . We have, therefore, undertaken a systematic study of the electrochemistry of a large number of soluble polythiophenes, to be able to compare E g,EC with E g,opt ....
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b301403g - Electrochemical bandgaps of substituted...

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