Color Control in €-Conjugated Organic Polymers for Use in Electrochromic

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Chem. Rev. 2010, 110, 268320

Color Control in -Conjugated Organic Polymers for Use in Electrochromic DevicesPierre M. Beaujuge* and John R. Reynolds*The George and Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science and Engineering, Department of Chemistry, University of Florida, Gainesville, Florida 32611 Received April 8, 2009

Contents1. Introduction 2. Toolbox of Polymer Electrochromism 2.1. Electrochromic Contrast 2.2. Coloration Efciency 2.3. Colorimetric Analysis 2.4. Reectance Analysis 2.5. Switching Rate 2.6. Stability 2.7. Optical Memory 3. Color Control in -Conjugated Polymers 3.1. Multicolored EC Polymers 3.1.1. Homopolymers and Alternating Copolymers 3.1.2. Random Copolymers 3.2. Cathodically Coloring Polymers 3.2.1. Poly(dioxythiophene)s and Analogues 3.2.2. Donor-Acceptor Systems 3.3. Anodically Coloring Polymers 3.3.1. Poly(3,4-dioxypyrrole)s: From Cathodically to Anodically Coloring ECPs 3.3.2. Polyuorenes and Analogues 4. Polymer Electrochromic Devices 4.1. Absorption/Transmission ECDs 4.1.1. Dual EC Material Approach 4.1.2. Color Control in Dual EC Cells and Devices 4.1.3. Interpenetrating Polymer Network-Based ECDs 4.2. Reective ECDs 4.2.1. Extending Electrochromism into the IR 4.2.2. Spectral Modulation and Device Optimizations 4.3. Patterning Electrodes for PECDs 4.3.1. Microporous Electrodes via Metal-Vapor Deposition 4.3.2. Screen Printing 4.3.3. Line Patterning 4.4. Advances and Perspectives in PECDs 4.4.1. Use of Ionic Liquids in PECDs 4.4.2. EC Blends, Laminates, and Composites 4.4.3. EC Nanotubes 4.4.4. EC Nanobers 4.4.5. Strictly All-Polymer ECDs 4.4.6. Advances and Perspectives in e-Paper Devices 4.4.7. First Inkjet-Printed ECD 268 270 270 270 271 271 271 272 272 272 272 272 284 286 286 293 302 302 305 306 307 307 308 308 308 308 309 309 310 310 311 311 311 311 312 312 312 312 313 5. 6. 7. 8.

4.4.8. Photoelectrochromic Devices (PhotoECDs) 4.4.9. Light-Pen Input Device Conclusions and Outlook Glossary Acknowledgments References

313 313 314 314 315 315

1. IntroductionIn an emerging era of exible, rollable, or foldable highperformance electronic displays, the perspective of manufacturing low-cost functional materials that can be easily processed over large areas along with being mechanically tolerant has become a sine qua non to commercial viability. In parallel, the idea of designing multifunctional semiconducting materials that can be integrated in various operating systems by inducing simple changes in device architectures has triggered new challenges for materials scientists who are now trying to answer the following: How can organic electronics reach or exceed the performance of their inorganicbased counterparts for a specic technology? Should they complement existing technologies in target applications, or should they seek to replace them in the most conventional areas of use? Can they remain cost-effective regardless of the number of manufacturing steps? In portable applications where lightness, thickness, and low-power consumption are further desired, organic electronics that can be precisely printed, stamped, sprayed, dropcast, or spin-coated into predened patterns offer a competitive alternative to their conventional inorganic homologues. However, solution-processability, solid-state ordering, efcient electroluminescence, stable color-switching, and high charge-carrier mobility, for instance, are a variety of interconnected factors difcult to match up for the synthetic chemist, when not a priori exclusive. In this context, -conjugated organic polymers combining mechanical exibility, and ease in band-gap/color-tuning via structural control, along with the potential for low-cost scalability and processing, are attractive in the fast-growing area of plastic electronics. For instance, in considering the most established applications, polymeric semiconductors are expected to nd widespread application in thin lm transistors (OTFTs),1,2 photovoltaic cells,3,4 radiofrequency identication (RFID) tags, sensors,5 memories,6,7 and lightemitting diodes (OLEDs).8 Of all the possible applications, light-emitting and nonemissive electrochromic devices (ECDs) require precise control of the colors displayed in terms of their hue, saturation, intensity, and/or their brightness. Besides the requirements for aesthetically pleasing color

10.1021/cr900129a 2010 American Chemical Society Published on Web 01/13/2010

Color Control in -Conjugated Organic Polymers

Chemical Reviews, 2010, Vol. 110, No. 1 269

Pierre M. Beaujuge is a Ph.D. candidate working with Prof. John R. Reynolds at the University of Florida in Gainesville. A native of France, he began his graduate studies at the Graduate School of Chemistry and Physics of Bordeaux (ENSCPB) in France in 2002. As part of his graduate program, he joined in 2004 the research laboratories of Arkema, Inc. in King of Prussia, PA, where he worked under the supervision of Dr. Gary S. Silverman and Dr. Roman Y. Korotkov. In 2005, Pierre moved to the University of Florida (UF). In 2006, while at UF, he received his Diplome dIngenieur from the Graduate School of Chemistry and Physics of Bordeaux. His doctoral research is currently directed toward developing novel -conjugated semiconducting polymers with tunable optical and charge transport properties for optoelectronic applications. He is involved in device fabrication aspects via collaboration with Prof. Franky So at the Materials Science and Engineering department of UF.

In both light-emitting and electrochromic (EC) technologies, changing the composition of the polymers at the molecular level has been the most extensively investigated approach to color-tuning so far. Hence, a variety of synthetic strategies have been described over the years spanning varying the overall planarity of the backbones as a function of steric hindrance, changing the electron-rich and -poor character of the building blocks incorporated in the repeat unit, increasing the conjugation lengths via the use of fused heterocycles able to reduce the polymers bond-length alternation, copolymerizing different monomers randomly and in different feed ratios, and so on. Alternatives have consisted in blending different electroactive components, or creating laminates and composites with other types of chromophores or insulating materials. Combining high contrast ratios, fast response times, and narrow potential windows of application with the perspective for long-term optical stability, processable electrochromic polymers (ECPs) are now impacting the development of both transmissive and reective EC technologies. With ubiquitous target applications encompassing smart window products, e-papers (see Figure 1, Note: prototype is not ECP-based here), optical shutters, transmissive and reective displays (see Figure 2), self-darkening mirror devices, or optical memories, for example, ECPs have been subjected to signicant synthetic effort during the past decade, and soluble systems with new colors, higher contrasts, and improved ambient stability have been successively developed. In parallel, some of the most effective strategies described in the construction of organic electronic-based devices have been applied to polymer ECDs, enhancing their lifetime and performance considerably. Given their relative ease of synthesis in general, ECPs are now only a step away from

John R. Reynolds is a V.T. and Louise Jackson Professor of Chemistry at the University of Florida with expertise in polymer chemistry and serves as an Associate Director for the Center for Macromolecular Science and Engineering. His research interests have involved electrically conducting and electroactive conjugated polymers for over 30 years with work focused on the development of new polymers by manipulating their fundamental organic structure to control their optoelectronic and redox properties. His group has been heavily involved in the areas of visible and infrared electrochromism, light emission from polymer and composite LEDs (both visible and near-infrared), and light-emitting electrochemical cells (LECs). Further work is directed to using organic polymers and oligomers in photovoltaic cells. Reynolds obtained his M.S. (1982) and Ph.D. (1984) degrees from the University of Massachusetts in Polymer Science and Engineering. He has published over 225 peer-reviewed scientic papers, has 7 patents issued and 17 patents pending, and served as coeditor of the Handbook of Conducting Polymers, which was published in 2007. He serves on the editorial boards for the journals ACS Applied Materials & Interfaces, Macromolecular Rapid Communications, Polymers for Advanced Technologies, and Journal of Macromolecular Science, Chemistry.

Figure 1. Plastic Logics e-paper, namely, Take anywhere, read anywhere. Adapted with permission from Plastic Logic, http:// Copyright 2009 Plastic Logic.

patterns, displays, for instance, are expected to include emissive or nonemissive chromophores which can be brought together as pixels or superimposed to recreate several other desirable colors via the principles of color mixing theory.

Figure 2. Electrochromic polymers that can change between a transparent colorless form and an opaque black form offer potential uses in nonemissive displays or signage. Adapted with permission from Krebs, F. C. Nat. Mater. 2008, 7, 766-767. Copyright 2008 Nature Publishing Group.

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being accessible for technological development and transferred to everyday products. In this review, we will rst open the toolbox of polymer electrochromism and outline the basic concepts and wellestablished characterization techniques in this area. This rst section will also refer to a few more recently proposed analytical tools and give