Polymer Preprints 2004, 45(1), ELECTROCHROMIC POLYMERS FOR PATTERNED DEVICES Avni A. Argun, Mathieu Berard, Pierre-Henri Aubert, and John R. Reynolds Department of Chemistry Center for Macromolecular Science and Engineering University of Florida, Gainesville, FL 32611. Introduction Research in polymer-based electrochromic devices (ECDs) shows their potential for a variety of useful applications such as multicolored displays and switchable mirrors. Much recent attention has been paid to new polymeric electrochromic materials that exhibit high contrasts, sub-second switching times, and long-lived switching properties. The range of colors obtained spans the entire visible spectrum and also extends through the near and mid-infrared regions. This is due to the ability to synthesize a wide variety of polymers with varied degrees of electron-rich character and conjugation. Poly(3,4- alkylenedioxythiophene)s (PXDOTs) and their derivatives that are developed in the Reynolds’ group exhibit promising electrochromic properties due to their electron rich nature which give them especially low redox switching potentials. Figure 1 shows the chemical structures of three of these polymers that were utilized in ECDs. An emerging aspect of ECD construction that our group has pursued is the metallization of a surface via patterning methods. This is useful since it allows the combination of at least two polymers at both large (centimeter) and small (micrometer) scales that can display a set of colors on a surface. It also allows creating highly contrasted surfaces by multiple colors or optically matched surfaces by color mixing. Moreover, metallization to form contact electrodes may be performed on ionic-permeable materials in order to develop reflective/absorptive surfaces with especially rapid switching rates. Here we report several methods to pattern conducting polymers to build ECDs and we demonstrate some examples of multi-color displays, which are made possible through patterning of electrode surfaces. S O O n S n O O PEDOT PProDOT-Me 2 S O O n PProDOT Figure 1. Chemical Structures of Dioxythiophene Based Electrochromic Polymers Used to Build Patterned Ecds. Results and Discussion Patterning Methods. Two types of patterning methods were employed to structure electrodes. The first type involves electroless deposition of nickel and gold metals on plastic or paper substrates using the line patterning method first described by MacDiarmid et al. 1 The second type is by metal vapor deposition of several metals on porous membranes through a predefined shutter mask. These metallized membranes can then be coated by electrochromic polymers and used in surface-active reflective ECDs as outward facing electrodes. 2,3 Here we report the construction and optoelectronic characterization of reflective ECDs using the latter patterning method. These devices utilize conjugated polymers as electrochromic layers. Reflective ECDs. An ion track etched polycarbonate membrane was used as the porous substrate and the membrane was covered with gold through a mask using a high vacuum metal vapor deposition process. Electrochromic polymers were then deposited onto these electrodes electrochemically or by spray coating. Following a layer-by-layer configuration as shown in Figure 2, the electrochromic polymers were paired to a polymeric counter-electrode in order to assemble reflective ECDs. The counter electrode was first placed as the bottom layer, with the polymer coated side facing up. A thin layer of gel electrolyte was homogenously applied. The patterned membrane was then placed on the top, the front side facing up. When a negative voltage (e.g. – 1.2V) is applied to a pixel comprising the PProDOT-Me 2 polymer as the active layer, the polymer is in its neutral state and it appears blue as this polymer is cathodically coloring. When a positive voltage (e.g. +1.2V) is applied, the polymer switches to its bleached state, therefore showing the gold layer beneath the polymer layer (Figure 2). The device shows high reflectance contrasts of ∆%R = 63% in the visible region and 74% in the NIR region. The device can be switched over 100,000 times with less than 15% contrast loss. Gel electrolyte Red. State WE Ox. State CE Red. State CE Ox. State WE A- Electrochemical oxidation of the top polymer layer Figure 2. Assembly and Operation of a Reflective ECD Comprising Conducting Polymers as the Electroactive Layers. In order to demonstrate the use of patterning in devices, a numeric display ECD was designed and assembled. Seven electrically independent gold pixels were produced on a porous membrane and a device was assembled according to Figure 2. Pixels are independently addressed and the high color contrast was achieved because of the difference in absorptivity of the gold surface and the electrochromic polymer layer. A photograph of the device displaying the number “8” is shown in Figure 3 (left). Spectroelectrochemistry. A full set of reflectance spectra for a PProDOT-Me 2 pixel was obtained as a function of applied voltage as shown in Figure 3 (right). PProDOT-Me 2 is purple-blue in the neutral state (-1.2V) and has low reflectance in the visible region (λ min = 630 nm) due to the strong π-π* absorption. Partial oxidation results in decrease of reflectance in the near IR region due to new absorption bands forming at the expense of the π-π* absorption. Upon complete electrochemical oxidation, the π-π* transition is fully depleted and the reflectance in the visible region increases resulting in the bleaching of the polymer layer. Lower energy transitions that occur in the near-IR region are attributed to the absorption due to charge carriers generated along the polymer backbone. 200 400 600 800 1000 1200 1400 1600 1800 2000 0 20 40 60 80 100 +1.2V -1.2V -1.2V +1.2V +1.2V %R Wavelengh (nm) -1.2V Figure 3. Left: A Photograph of A Reflective ECD Display Showing the Number “8”. Right: Reflectance Spectroelectrochemical Series of a Pprodot- Me 2 Pixel as a Function of Applied Voltage. Voltage Increments are 0.2V. Acknowledgements. We gratefully thank funding from the AFOSR (F49620-03-1-0091) and the ARO/MURI program (DAAD 19-99-1-0316). References (1) Hohnholz D.; MacDiarmid A.G. Synth. Met. 2001, 121, 1327. (2) Cirpan, A.; Argun, A. A.; Grenier, C. R. G.; Reeves, B. D.; Reynolds, J. R. J Mater Chem 2003, 13, 2422. (3) a.) Bennet, R. 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