Multi-Color Electrochromic Polymers on Reflective ...

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and John R. Reynolds. Department of Chemistry, Center for Molecular Science and. Engineering, University of Florida, Gainesville, FL 32611. INTRODUCTION.
Multi-Color Electrochromic Polymers on Reflective Devices

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Avni A. Argun, Ali Cirpan, Pierre-Henri Aubert, and John R. Reynolds

Gold Polymer CE

Department of Chemistry, Center for Molecular Science and Engineering, University of Florida, Gainesville, FL 32611.

Gel Electrolyte Porous Substrate

INTRODUCTION Electrochromism is defined as a reversible optical change in a 1 material induced by an external voltage. Among electrochromic (EC) materials, conjugated polymers have received much attention due to their ease of processability, rapid response times, and high optical contrasts. Furthermore, their synthetic versatility allows derivitization of the monomer structure and the control of the optoelectronic properties. Using band gap control, or by adjusting the composition of copolymers and blends, multiple color states in the full spectral range can be obtained in both the neutral and charged forms. Of the conjugated EC polymers, derivatives of poly(thiophene)s, poly(pyrrole)s, and poly(aniline)s are the most studied. Electrochromism in conjugated polymers occurs through changes in the π electronic character accompanied by reversible insertion and extraction (doping) of ions upon oxidation or reduction. Figure 1 shows a two-electron oxidation of a dimethyl substituted poly(3,42 propylenedioxythiophene) (PProDOT-Me2). In its neutral state (left), PProDOT-Me2 is deep blue and shows a semiconducting behavior with an energy gap (Eg). Upon doping, the band structure of the neutral polymer is depleted to generate charged carriers (polarons and bipolarons) with lower energy transitions, which are responsible for increased conductivity and high transmissivity in the visible region.

O

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Deep blue

+2e A-

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S + O

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Highly transmissive

Figure 1. Two electron oxidation process of the conjugated polymer PProDOT-Me2 to produce the quinodial dication.

Active Layer (PProDOT-X2)/Gold Transparent cover

Figure 2. Schematic representation of a reflective ECD. Active polymer layer is electrochemically deposited or spray coated. In one example, a PProDOT-Me2 film was electrochemically deposited as the active layer on a gold-coated porous electrode and a reflective ECD was assembled as shown in Figure 2. When a negative voltage is applied, the surface-active layer has the lowest reflectance in the visible region (558 nm). The device exhibits a reflectance contrast (∆%R) value of 55% in the visible region and 70% in the NIR region. The electrochromic switch is fully reversible and occurs in less than 200 ms. The long-term switching stability of the device was established by continuously stepping the voltage between the two redox states and recording the optical contrast loss. After 180,000 switches, the initial contrast only dropped by 7%. We have also investigated the open circuit memory of PProDOTMe2 using this device platform. The power supply is disconnected after a one second voltage pulse (~1.0 V) is applied and the reflectivity of the device was monitored. The open-circuit percent reflectance change was measured to be 0.4 %/min in the visible region which assures color preservation with only minimal refreshing every 5 minutes. In another example, a dialkoxy substituted soluble PProDOT was spray coated onto a porous gold electrode from a 5mg/mL toluene solution and a reflective ECD was again constructed. Solution processing of polymer films introduces the possibility of constructing large area ECDs and patterned devices. The optical and electrochemical properties of the polymer films showed similar characteristics to the electrochemically deposited films. An unusual electrochromic switching property was observed as the device was switched between –1 V and 0 V. The reflectance contrast at 2000 nm was greater than 70 %, while at 609 nm, the ∆%R was less than 3%. The result is the creation of an ECD that is IR active while undergoing no visible color change. Despite a large contrast in the NIR region, the spectra are quite similar in the visible region.

Recently in the Reynolds group, the synthetic flexibility of alkylenedioxythiophenes have allowed for the synthesis of monomers derivatized with sufficiently long alkyl chains which induce solubility of the polymer in organic solvents. Grignard metathesis coupling was utilized in the synthesis of a soluble and processable dibutyl derivative of poly(3,4-propylenedioxythiophene).3 Highly homogenous polymer films were then formed by spray coating on various electrodes such as ITO or gold.4 Here, we apply both electrochemical and spray coating techniques to deposit electrochromic polymers on porous metallized membranes which are used to construct reflective type polymer electrochromic devices (ECDs).

ACKNOWLEDGEMENTS We gratefully acknowledge funding from the AFOSR (F49620-031-0091) and ARO/MURI program (DAAD 19-99-1-0316) for the funding.

RESULTS AND DISCUSSION Figure 2 schematically shows the layer-by-layer construction of a surface active reflective ECD.5 The EC polymer was deposited onto a metal-coated porous polymer membrane (active layer electrode) and onto a metal-coated polymer film (counter electrode) by electrochemical deposition or by spray coating. The outward facing electroactive polymer on the porous membrane is responsible for the surface reflectivity modulation whereas the counter electrode polymer is hidden and only contributes to balance the electroactive sites.

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REFERENCES Monk, P. M. S.; Mortimer R. J.; Rosseinsky, D. R. Electrochromism: Fundamentals and Applications; VCH: Weinheim, Germany, 1995. Welsh, D. M.; Kumar, A.; Meijer, E. W.; Reynolds, J. R. Adv. Mater. 1999, 11, 1379. Welsh, D. M.; Kloeppner, L. J.; Madrigal, L.; Pinto, M. R.; Thompson, B. C.; Schanze, K. S.; Abboud, K. A.; Powell, D.; Reynolds, J. R. Macromolecules, 2002, 35, 6517. Cirpan, A.; Argun, A. A.; Grenier, C. R. G.; Reeves, B. D.; Reynolds, J. R. J Mater Chem 2003, 13, 2422. a.) Bennet, R. B; Kokonasku,W. E.; Hannan, M. J.; Boxall, L. G.; US Patent # 5,446,577, 1995. b.) Chandrasekhar, P. US Patent # 5,995,273, 1999.

Polymeric Materials: Science & Engineering 2004, 90, 40