Fast-Switching, High-Contrast Electrochromic Thin Films Prepared Using Layer-by-Layer Assembly of Charged Species
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Fast-Switching, High-Contrast Electrochromic Thin Films Prepared Using Layer-by-Layer Assembly of Charged Species Jaime C. Grunlan Department of Mechanical Engineering and Polymer Technology Center, Texas A&M University, College Station, TX 77843-3123, U.S.A. ABSTRACT Thin films were prepared by depositing alternating layers of tungstate anion (WO42-) and poly(4vinylpyridine-co-styrene) (PVP-S) onto an electrode from aqueous solutions. These films have very high contrast (CR > 8) relative to equivalent films prepared using poly(ethylene dioxythiophene) (PEDOT), but suffer from slow color change due to poor electrical conductivity. The switching time of the tungstate-based films was decreased by an order of magnitude, from 30 seconds down to three, by adding layers of indium tin oxide (ITO) particles stabilized with poly(diallyldimethylammonium chloride) (PDDA). In this case, a four-layer repeating structure was created (i.e., PVP-S and PDDA-ITO were each deposited every fourth layer). Unlike tungstate, ITO has a high intrinsic conductivity (~ 104 S/cm) that accounts for the dramatic increase in the switching speed. It is only through the nanometer-scale control of film architecture, provided with the layer-by-layer (LbL) deposition process, that switching speed and contrast ratio can be optimized simultaneously. INTRODUCTION A variety of functional films can be produced using the layer-by-layer (LbL), or electrostatic self-assembly (ESA) technique [1-3]. LbL-based thin films are currently being evaluated for a variety of applications that include controlled drug delivery [4], molecular sensing [5], solid battery electrolytes [6], and photovoltaics [7]. Thin films, typically < 1µm thick, are created by alternately exposing a substrate to positively- and negatively-charged molecules or particles, as shown in Figure 1. In this case, steps 1 – 4 are continuously repeated until the desired number of “bilayers” (or cationic-anionic pairs) is achieved. Bilayers are typically 1 – 10nm thick, but they can become much thicker if particulate materials are deposited [8]. Some of the key factors affecting polyelectrolyte thickness include molecular weight [9], charge density [10], temperature [11], deposition time [12], counterion type [10], solution pH [13] concentration [12,14], and ionic strength [12,14]. The ability to control coating thickness down to the nm-level, easily insert variable thin layers without altering the process, economically use raw materials (due to thin nature), self-heal, and process under ambient conditions are some of the key advantages of this deposition technique [1]. A variety of electrochromic systems have already been prepared using the LbL process to make use of these benefits [3,15-18]. The systems with the highest contrast ratios contain inorganics and take several seconds to change color due to low conductivity in one or both states. Tungstate species (WO42-) have been shown to exhibit excellent electrochromic behavior when complexed with polyvinylpyridine (PVP), but suffer from very
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