Dependence on the parameters of the equivalent electrical circuit model with the thickness of viologen-based electrochro
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Dependence on the parameters of the equivalent electrical circuit model with the thickness of viologen-based electrochromic mixture on glass substrate devices David Barrios1, Ricardo Vergaz1, Juan Carlos Torres1, José-Manuel Sánchez-Pena1, Ana Viñuales2, M. Salsamendi2 1 Grupo de Displays y Aplicaciones Fotónicas, Dept. Tecnología Electrónica, Universidad Carlos III de Madrid, C/Butarque, 15, E28911, Leganés, Madrid, Spain 2 Center for Electrochemical Technologies (CIDETEC), Dept. Nuevos Materiales, Parque Tecnológico de San Sebastián - Paseo Miramón, 196, E-20009, San Sebastián, Spain ABSTRACT Several viologen electrochromic devices with different thicknesses on glass substrates were constructed, using a mixture of 4,4’-bipyridine and 1-bromoethane. The thickness of each device was fixed using a thermoplastic spacer. The devices were electrochemically tested with optical and impedance analysis. The range of the transmittance change is highly dependent on thickness. The electrical behavior of the material and the physical and chemical characteristics are derived from the proposed electrical equivalent circuit model. A simple Randles circuit including a Warburg diffusion impedance element, a charge transfer resistance and a double layer capacitive element is proposed for the fittings process. Variations on thickness of internal layer of devices lead to use a short or an open circuit Warburg element. A threshold potential, from which the device is colored, indicates the charge diffusion effects. INTRODUCTION Viologens are bipyridine salts traditionally used as pesticides or redox indicators. Electrochromism, defined as the electrochemical generation of color in accompaniment with an electron transfer (or redox) reaction, appears usually in conducting polymers, WO3, TiO2, Prussian Blue and other materials, such as viologens. By using the latter, the automatically selfdarkening car rear-view mirrors developed by Gentex Corporation has been until date the greatest production electrochromic (EC) application. Operation mode by transmission, instead of by reflection, is used to control visible solar radiation and heating inside buildings with smart EC windows. Passive data displays (i.e. non emissive and requiring external illumination), such as electronic paper, and thermal protection in spatial technology are other possible EC applications. Viologens are representative of organic EC materials. Dicationic form of purchased or synthesized viologen is colorless or has a feeble color. By contrast, one electron reduction of the dication forms the radical cation, highly colored because of an intense intramolecular optical charge transfer. Red-black, olive green or blue colors are obtained depending on if the substituents interact with the bipyridine core [1]. For instance, radical cations containing short alkyl chains are blue (blue-purple in concentrated solution). Bleaching occurs at short or open circuit by homogeneous electron transfer in the bulk of the solution. All plastic flexible viologen EC devices changing to blue an
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