Electrochromic devices based on surface-modified nanocrystalline TiO2 thin-film electrodes

Abstract

Nanocrystalline TiO2 thin film electrodes on conductive glass were modified with monolayers of different electrochromic compounds (mono-, di- and trimeric N,N'-dialkyl- ordiphenyl-4,4'-bipyridinium salts) equipped with TiO2 anchoring groups (An = benzoate, salicylate, phosphonate). The synthesis of these compounds is reported. Different approaches have been studied to increase the surface concentration Gamma(CS) of electrochemically active coloring centers (CS) on TiO2. The electrodes were checked coulometrically and spectroelectrochemically under potentiostatic conditions in MeCN/TEAP. Gamma(CS) of mono- and oligomeric viologens was shown to depend on the ratio (CS/An) of CS to anchoring groups (An). A cone-shaped trimeric arborol-type viologen was prepared with the intention to fill out the space above the convex surface of the nanoparticles particularly well. Preliminary results of a new type of TiO2 solid-phase supported synthesis of the viologens is reported. Electrochromic devices including filters and displays have been prepared. The filter devices (12-100 cm(2)) consist generally of OTE/TiO2-poly-viologen/glutaronitrile-LiN(SO2CF3)(2) spacer/Prussian Blue/OTE and exhibit optical density changes up to 2 (transparent to blue or yellowish to green and red-brown (at higher potential)) at switching times in the range of 1-3 s. Even higher optical density changes (at slower switching times) were achieved with systems such as OTE/TiO2-polyviologen/glutaronitrile-LiN(SO2CF3)(2) spacer/Prussian Blue-TiO2/OTE. The display devices prepared include reflective displays with two to four separately addressable segments ((OTE/TiO2 (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/molten salt spacer/Zn) or (OTE/TiO2 (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/glutaronitrile-LiN(SO2CF3)(2) spacer/Prussian Blue/OTE), as well as transparent systems with up to four addressable segments such as: OTE/TiO2 (both structured)-poly-viologen/glutaronitrile-LiN(SO2CF3)(2) spacer/Prussian Blue/OTE. (C) 1999 Elsevier Science B.V. All rights reserved.