Electrochemical properties and electrochromic device applications of polycarbazole derivatives

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Electrochromic Smart Materials

Fabrication and Applications

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Smart Materials

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Hans-Jo¨rg Schneider, Saarland University, Germany Mohsen Shahinpoor, University of Maine, USA

Titles in this series:

1: Janus Particle Synthesis, Self-Assembly and Applications 2: Smart Materials for Drug Delivery: Volume 1

3: Smart Materials for Drug Delivery: Volume 2 4: Materials Design Inspired by Nature

5: Responsive Photonic Nanostructures: Smart Nanoscale Optical Materials

6: Magnetorheology: Advances and Applications

7: Functional Nanometer-Sized Clusters of Transition Metals: Synthesis, Properties and Applications

8: Mechanochromic Fluorescent Materials: Phenomena, Materials and Applications

9: Cell Surface Engineering: Fabrication of Functional Nanoshells 10: Biointerfaces: Where Material Meets Biology

11: Semiconductor Nanowires: From Next-Generation Electronics to Sustainable Energy

12: Supramolecular Materials for Opto-Electronics 13: Photocured Materials

14: Chemoresponsive Materials: Stimulation by Chemical and Biological Signals

15: Functional Metallosupramolecular Materials

16: Bio-Synthetic Hybrid Materials and Bionanoparticles: A Biological Chemical Approach Towards Material Science

17: Ionic Polymer Metal Composites (IPMCs): Smart Multi-Functional Materials and Artificial Muscles Volume 1

18: Ionic Polymer Metal Composites (IPMCs): Smart Multi-Functional Materials and Artificial Muscles Volume 2

19: Conducting Polymers: Bioinspired Intelligent Materials and Devices 20: Smart Materials for Advanced Environmental Applications

21: Self-cleaning Coatings: Structure, Fabrication and Application 22: Functional Polymer Composites with Nanoclays

23: Bioactive Glasses: Fundamentals, Technology and Applications 24: Smart Materials for Tissue Engineering: Fundamental Principles 25: Smart Materials for Tissue Engineering: Applications

26: Magnetic Nanomaterials: Applications in Catalysis and Life Sciences 27: Biobased Smart Polyurethane Nanocomposites: From Synthesis to

Applications

28: Inorganic Two-dimensional Nanomaterials: Fundamental Understanding, Characterizations and Energy Applications

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29: Ionic Liquid Devices 30: Polymerized Ionic Liquids

31: Nanogels for Biomedical Applications

32: Reactive Inkjet Printing: A Chemical Synthesis Tool

33: Electrochromic Smart Materials: Fabrication and Applications

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Published on 17 December 2018 on https://pubs.rsc.org | doi:10.1039/9781788016667-FP001

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Electrochromic Smart Materials

Fabrication and Applications

Edited by

Jian Wei Xu

Institute of Materials Research and Engineering A*STAR, Singapore Email: jw-xu@imre.a-star.edu.sg

Ming Hui Chua

Institute of Materials Research and Engineering A*STAR, Singapore Email: chua_ming_hui@imre.a-star.edu.sg

and

Kwok Wei Shah

Institute of Materials Research and Engineering A*STAR, Singapore Email: bdgskw@nus.edu.sg

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Smart Materials No. 33 Print ISBN: 978-1-78801-143-3 PDF ISBN: 978-1-78801-666-7 EPUB ISBN: 978-1-78801-682-7 Print ISSN: 2046-0066 Electronic ISSN: 2046-0074

A catalogue record for this book is available from the British Library rThe Royal Society of Chemistry 2019

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Preface

Rapid development in the field of optoelectronics over the past decades has brought about game-changing technologies and innovations that see great commercial values and benefits to our daily lives. Electrochromism is one of the key domains in optoelectronics. Since Deb S. K. reported the first electrochromic device in 1969, which demonstrated the reversible colour change of tungsten oxide thin films under the application of an external electrical bias, electrochromism research including electrochromic materials, fabrication techniques and architectures and components of electrochromic devices flourished. This has lead to the development of a large number of practical applications such as smart windows, smart eye-wear glasses, electronic paper, electronic displays and eye-wearable electronics, some of which have been commercialized. This book covers the major topics related to the phenomenon of electrochromism – fundamentals and principles, classes of materials, device fabrication and various applications. This book also provides a comprehensive overview of this field, which can serve not only as an introduction to readers new to this area but also as a resource providing detailed, in-depth knowledge and insights to the seasoned audience.

Chapters 1–2 of this book give a brief introduction of the concept of electrochromism, and touch on some of the useful applications of electro-chromic materials; the fundamental principles of electroelectro-chromic materials and devices with a focus on device components and fabrication techniques as well as crucial device parameters are described. Many classes of com-pounds were found to exhibit electrochromism. In the past decades, the focus of electrochromic materials study has shifted from inorganic metal– oxides and metal-complexes, to organic p-conjugated functional molecules and polymers. Owing to their good processability, flexibility and low cost, conjugated polymers constitute an attractive class of materials for

Smart Materials No. 33

Electrochromic Smart Materials: Fabrication and Applications Edited by Jian Wei Xu, Ming Hui Chua and Kwok Wei Shah rThe Royal Society of Chemistry 2019

Published by the Royal Society of Chemistry, www.rsc.org

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electrochromic applications. Therefore, the recent advance of a variety of conjugated polymers including their synthesis approaches, electrochromic properties, etc. is highlighted in a different capacity in Chapters 3–11. In particular, Chapters 3–4 review numerous classes of conjugated polymers and their different synthesis approaches with a specific focus on donor– acceptor (DA) type conjugated polymer synthesis strategy that is effective to fine tune electrochromic properties of corresponding conjugated polymers. Chapter 8 expands its scope of DA type conjugated polymers to unique DA type polymer structures starting from some interesting cruciform, star-shaped and dendritic monomers. Recently, electrochromic materials that additionally exhibit electrofluoresecence properties have been studied from the point of view of both fundamentals and applications, and thus Chapters 5, 7 and 9 summarize the latest advances in these types of electrofluorochromic materials. Electroactive polymers derived from high-performance building blocks including carbazole, arylamine, etc. are discussed and they are presented in detail in Chapters 10 and 11, respectively.

In addition, some of the traditional electrochromic materials including viologen (Chapter 12), metallo-supramolecular polymers (Chapter 13) and metal oxides with an emphasis of their corresponding nanostructures (Chapter 14) are discussed. Electrochromic processes such as metal elec-trodeposition that is able to control and synthesize materials with diﬀerent nanostructures for better electrochromic performance is also highlighted (Chapter 15). Finally, this book concludes an important application of electrochromism with Chapter 16, showcasing a major use in smart windows and fenestration technology. The evolution of these smart windows provides a timely push towards greener buildings through more sustainable energy-saving solutions in this rapidly developing world.

Through this book, it is envisaged that readers should not only under-stand the fundamentals of electrochromism, from materials diversity, mechanism to device fabrication, but more importantly, will appreciate the holistic and multi-disciplinary nature of this field, and many innovative solutions and technologies it has brought to our daily lives. Whether it is the self-dimming smart windows that passengers are sitting next to on a Boeing 787 Dreamliner plane or the auto-dimming anti-glare smart mirrors in the car for increased safety, the importance of electrochromic technology is here to stay and will continue to expand its role in our daily lives.

Finally, the editors wish to express their immense appreciation to all authors for their eﬀorts in contributing to this book.

Jian Wei Xu, Ming Hui Chua, and Kwok Wei Shah Republic of Singapore

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Introduction to

Electrochromism

MING HUI CHUA, TAO TANG, KOK HAW ONG, WEI TENG NEO

AND JIAN WEI XU*

Institute of Materials Research and Engineering (IMRE), the Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634

*Email: jw-xu@imre.a-star.edu.sg

1.1 General Introduction

Chromic materials are materials which exhibit a reversible colour change in response to an external stimulus such as temperature (thermochromism)1 and light (photochromism).2 The source of the colour changes is the variation in absorption spectra of the materials across the UV–visible–near-infrared (NIR) region. Besides the above-mentioned stimuli, oxidation and reduction of certain substance upon application of an electrical bias can also lead to distinct photo-optical and colour changes. This phenomenon is known as ‘‘electrochromism’’.3–6 Electrochromic (EC) materials generally exhibit colour changes between two coloured states or between a coloured state and a bleached state. Materials that reveal coloured hues in their oxi-dised or reduced states are referred to as anodically colouring or cathodically colouring respectively.10–12Several EC materials that exist in multiple redox states reveal the unique ability to switch between several coloured states. This is known as polyelectrochromism.7–9EC materials are highly applicable in smart windows and optical display technology. Furthermore, as the region of optical changes can be extended beyond the UV–visible region into the

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NIR, the thermal infrared and even the microwave region, these EC materials are potentially useful in defence related applications.13

Many diﬀerent classes of compounds were reported to exhibit EC prop-erties: (i) transition metal oxides (WO3 and TiO2),14–23 metal coordination

complexes (CoFe(CN)6and Prussian Blue),24–28organic molecular dyes (e.g.

viologen)29–33and organic conducting polymers (e.g. polythiophenes, poly-anilines and poly(3,4-ethylenedioxythiophene (PEDOT))).34–44 Amongst these, organic EC materials possess advantages such as intense colouration, ease of structural modification, good processability, low cost and good film-forming ability. On the other hand, their inorganic counterparts were re-ported to exhibit good chemical and electrochemical stabilities as well as a wide range of working temperatures.45 In addition, organic/inorganic na-nocomposites were also developed to combine the advantages of both or-ganic and inoror-ganic EC materials. Such hybrid materials could be prepared from the use of either only EC-active organic or inorganic materials or both. In the EC nanocomposites, much attention has been paid to modifying the interfacial interactions between organic and inorganic parts because such interactions are vital for structure strength, mass transport, electron conduction and EC performance.46–48

1.2 History of Electrochromism

The first EC device was documented by Deb in 1969, where he demonstrated the controlled and reversible changing of colour with the use of tungsten trioxide (WO3).49,50 Since then, many classes of EC materials and

corres-ponding devices have been reported, which include metal oxides, viologens and conjugated polymers. Due to their facile colour changes in the visible region, EC materials were highly sought after and employed for optical display applications. Early research in the US, Soviet Union, Japan and Europe on EC materials were motivated by their potential applications in information displays. There were intense research eﬀorts during the first half of the 1970s at several large international companies such as IBM,51,52 Zenith Radio,53,54 the American Cyanamid Corporation55 and RCA in the US56–60 as well as Canon in Japan,61 Brown Boveri in Switzerland62 and Philips in the Netherlands.63Through the years, electrochromism continues to receive wide attention in the area of fundamental research. In the mid-1980s, interest in EC materials was boosted again given the potential ap-plication in fenestration technology, which was deemed as a way to achieve better energy-eﬃciency in buildings. The newly conceived ‘‘smart’’ window technology could vary the transmittance of light and solar energy, leading to energy savings and indoor comfort.64–67Moving on, breakthroughs in device engineering and manufacturing techniques allow for electrochromism to move beyond traditional applications such as smart windows and optical displays into emerging applications such as wearable electronics and defence-related technologies.

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1.3 Mechanism of Electrochromism and EC Devices

EC materials undergo colour (and sometimes, fluorescence) changes upon the application of an electric field. Generally, the mechanism of EC activities involves the electrochemical oxidation and/or reduction of EC materials, resulting in changes in the optical band-gap, which is thus reflected in colour changes observed. In most cases, a constant supply of electric current is required to sustain a certain colour associated with an electro-oxidised or -reduced state. There are, however, some materials that require almost zero-current consumption to maintain a certain colour state, which is known as the ‘‘memory eﬀect’’. The detailed mechanism of electrochromism will be discussed in subsequent chapters.

For real-life applications, EC materials have to be incorporated into functional EC devices. Typically, EC materials exist as thin films within the EC devices, allowing them to be in close contact with electrodes and elec-trolytes for electric current to flow through the devices. An EC thin-film device normally adopts a multi-layered structure as shown in Figure 1.1, which can be used to tailor the optical properties of a device on applying a voltage, and revert to the original state when the polarity of the voltage is reversed. Having good electrical contact between layers is required to ensure good stability and EC performance. As shown in Figure 1.1, a typical EC device has at least five layers: transparent-conducting oxide (TCO) layer/ ion-storage layer (IS)/ion-conducting layer (electrolyte)/EC layer/TCO layer, superimposed on one substrate or be positioned between two substrates in the laminate configuration. In this configuration, the EC layer is coated on one side of ion conductor while an ion-storage layer is located on the other side of ion conductor. The use of ion-storage layer is to obscure the galvanic-cell basis of operation. The conducting layer is mainly responsible for carrying the charge from a power source to the corresponding EC layer.68–72 The ion-conductor, which is made up of small mobile ionic charge carriers, ensures the completion of the circuit by facilitating the transfer of ions

Figure 1.1 The bare bones of an EC device.

Introduction to Electrochromism 3

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between electrodes.48,76–79 Finally, epoxy and relevant sealants are used to ensure that electrolyte is not leaked during operation.73–75

Optical modulation of EC devices is mainly aﬀected by H1or Li1transport for devices backed by a single glass or a polyester substrate. However, laminated devices show some diﬀerence compared to their counterparts. Particularly, devices using H1transport normally use electrolytes containing polyethylene oxide (PEO), a copolymer of sodium vinylsulfonic acid and 1-vinyl-2-pyrrolidinone and poly-2-acrylamido-2-methyl-propane sulfonic acid.80–84Meanwhile, the counter electrodes are polyaniline, Prussian Blue, or a mixture of two, which lead to a large modulation range of visible light.85–92On the other hand, laminated devices with Li1transport are a bit distinct. In other words, the polymers consist of poly-methyl methacrylate (PMMA) copolymerized with polypyrrole,93–95 propylene carbonate,96–98 silane,99,100polypropylene,101,102 glycidyloxypropyl trimethoxysilane copoly-merized with tetraethylene glycol,103,104 polyethylene glycol methacrylate copolymerized with PEO,103,105 ormolyte or polyvinylidene fluoride.105–109 Those polymers are ion-conducting via adding an optimal Li salt. In add-ition, V2O5, SnO2doped with Mo and Sb, and TiO2with or without additions

of ZrO2 or CeO2are used as the counter electrode in the system with Li1

transport.110–117

1.4 Applications of EC Materials

Over the past decades, EC materials and devices have been widely applied in a number of areas, particularly information displays, variable reflectance mirrors, smart windows and variable emittance surfaces. The principles of the four stated applications are shown in Figure 1.2. EC materials and de-vices can be applied to translucent, transparent or mirror surfaces, and the amount of light absorbed, reflected or passing through can be modulated by controlling electric current passing through the devices. In general, all EC devices can be classified based on their operating mode—transmission or reflection.

Recently, there has been a strong resurgence in the development of EC-based display-oriented devices such as ‘‘electronic paper’’ with a focus on cheap printable EC ‘‘labels’’118–120with excellent viewing properties,37,121–124 ‘‘active’’ authentication devices and sensor platforms with EC-based read-out.125–127 These will be discussed in further detail in the following subsection.

1.4.1 Smart Glass/Windows

One of the most prominent applications of EC technology is for smart glass and windows. Such EC windows can switch reversibly between transparent and opaque states and across diﬀerent degrees of opacity simply by varying the electrical potential applied. As such, the amount of external light, glare and solar radiation (hence heat from outside) entering through the window

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can be modulated easily.130–135 This, in turn, leads to potential energy and cost savings as the reliance of indoor lighting and temperature (e.g. air conditioning) control is reduced. Similarly, indoor privacy can be main-tained at the wish of the user, simply by switching the smart window from transparent to opaque, eﬀectively eliminating the need for shades or cur-tains. Smart windows have been used in buildings, vehicles and even on planes. The market size for EC glass was estimated to be $1.17 billion in 2013 and this is expected to expand to $2.59 billion by 2020, representing a compounded annual growth rate of over 10%.136

The key advantage of EC smart glass is that it requires electric power only during switching. In contrast, alternative technologies such as suspended particle devices and polymer-dispersed liquid crystal devices require the application of continuous power in order to maintain the glass in a trans-parent state.137 Figure 1.3 demonstrates the configuration and mechanism of an EC window. In the configuration, the window functions as an elec-trochemical cell in which two conducting glass panes are separated by an electrolyte material. At an open-circuit voltage, both the working and counter electrodes are transparent, allowing both heat and light to pass through. The EC window thus exists in the ‘‘bright mode’’. The EC window can switch to ‘‘cool mode’’, where heat is blocked while allowing the natural light to pass

Figure 1.2 Principles of four diﬀerent applications of EC devices. Arrows indicate incoming and outgoing electromagnetic radiation; the thickness of the arrows signifies radiation intensity.

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through with the reduction of voltage to an intermediate level. Finally, at lower electric potentials, the EC window converts to ‘‘dark mode’’, eﬀectively blocking the transmittance of both heat and natural light.

At present, several EC windows are available on the market. Some notable smart window manufacturers include SAGE Electrochromics, Inc., EControl-Glas GmbH & Co. KG, Saint Gobain Sekurit, GENTEX Corporation, and Asahi Glass. One well-known application of EC glass is in the windows of the Boeing 787 Dreamliner (Figure 1.3).138Used in place of conventional window blinds, EC technology has enabled airline passengers to control the opacity of the windows with the push of a button. Cabin crew can also remotely adjust individual windows or those on the entire plane, freeing them the hassle of checking each individual window before take-oﬀ or landing.

Furthermore, energy storage and electrochromism functions can be grated into a single device, as demonstrated by various groups. By inte-grating an EC device with a solar cell,139–142 photovoltaics,143–147 solar cell glazing, or supercapacitor,148,149 a self-powered smart device can be ob-tained (Figure 1.4). For such energy-harvesting smart windows, light energy is converted into electricity when there is strong incident sunlight, which is then stored within the smart window. Concurrently, the colour of the

Figure 1.3 Applications of EC devices. (a) Design of EC window. Reproduced from ref. 128 with permission from Springer Nature, Copyright 2013. (b) Smart switchable window applied in Boeing aircraft produced by SmartTints. (c) Photographs of the EC lens. Reproduced from ref. 129 with permis-sion from American Chemical Society, Copyright 2015. (d) Automatic dimming mirror based on electrochromism produced by Gentexs

. Print-able and flexible EC displays designed by (e) Prelonic Technologiessand (f) Siemenss.

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window darkens. When the stored energy is discharged, the window returns to its original colour. The marriage of EC and photovoltaic technology thus eﬀectively eliminates the need for an external electric supply to operate the smart window.

1.4.2 Car Rear-view Mirrors

EC technology has also been applied in anti-glare, auto-dimming rear-view mirrors for automobiles.150 These mirrors have built-in sensors that can detect glare from the headlights of following vehicles. The built-in sensors of EC auto-dimming rear-view mirrors are usually cameras or photodiobased photodetectors, which send the signal to a microprocessor. The de-tection of strong glare will send a charge through an EC gel, which eﬀectively darkens to reduce the glare and discomfort for the driver, thereby improving road safety. No manual adjustment of the rear-view mirror is thus required by the driver, who can focus on driving and road conditions. One such product is the Gentex mirror, millions of which have been sold since 1974.

1.4.3 EC Displays

EC displays produce colour in a subtractive manner, through interaction with transmitted or reflected light from an external light source. This is in contrast to a cathode ray tube or a light emitting diode display which emits light. Beginning in the 1980s, steady development in EC materials has produced materials that can exhibit colour changes from colourless to various colours (such as red, green and blue, or cyan, magenta and yellow). This has thus opened up the possibility to generate full-colour EC displays using the RGB or CMY colour models.160 Furthermore, they can also be fabricated using printing processes on flexible substrates, making low-cost devices such as e-papers possible. One of the most common forms of EC displays would probably be in digital clocks and watches. A recent ex-ample of a fully-printed active-matrix EC display on a flexible substrate

Figure 1.4 Scheme of an Energy Storage Smart Window, combining a solar cell with an EC device.

Reproduced from ref. 139 with permission from the Royal Society of Chemistry.

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which utilizes carbon nanotube thin-film transistors as the backplane was reported.151 While this display has only 66 pixels, it demonstrates the significant potential of EC displays for delivering low-cost, large area devices on flexible substrates.

Figures 1.5 and 1.6 show the structure and performance of an EC display based on ZnO–PEDOT core–shell nanowires. To generate a low power, long term stable and transparent display, the EC material should ideally possess properties such as high contrast, ultrafast switching time, high colouration efficiency, ultrahigh diffusion coefficient and electrochemical stability .152

Another example is a stretchable EC display which employs PEDOT and polyurethane (PU) as the major components. The display performance is shown in Figure 1.7. The composite film works as a free-standing EC film in an electrolyte solution, and can be combined with other stretchable ma-terials such as hydrogel as a support. Moreover, the developed EC film and device are useful as a non-emissive display component in a stretchable wearable device to indicate electrochemical signals.

1.4.4 Wearable Apparel and Devices

EC technology has also been applied to wearable apparel such as eyewear.153 Like transitional lenses, EC lenses for spectacles and sunglasses can be switched between clear and dark states, eﬀectively protecting users from excessive UV radiation and reducing discomfort to the eyes under bright sunlight. The diﬀerence between transitional lenses and EC lenses/glasses, however, lies in the former having an auto-dimming function due to photochromic properties of the lenses, whereas the latter operates on a small electric input and is user-controlled. This means that users may switch to a darkened ‘‘sunglasses’’ mode, for example, in a shaded environment, which auto-dimming transitional glasses are unable to do. Nonetheless, the auto-dimming function of EC lenses can also be enabled using

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photo-sensors and micro-controllers.154 The low operation voltage and en-ergy consumption of EC devices imply that a single battery can power a device for thousands of switches. In addition, the switching kinetics of EC lenses are comparatively faster than photochromic lenses, and the lenses can switch between more than one colour.129,153For instance, Reynolds et al. reported the use of a colour-mixing method to produce EC lenses that can reveal several shades of brown (Figure 1.8).129 In addition, the lenses were fabricated using a combination of inkjet printing and blade-coating, clearly demonstrating how these organic EC polymers blends can be easily trans-latable in a large scale production of EC smart lenses.

Figure 1.6 (a) The UV–vis transmittance of the substrate, ZnO nanowires and EC display in the coloured and de-coloured states. (b) The comparison of change in monochromic transmittance (640 nm) versus time during (left) de-colouring and (right) colouring by potential switch between1.5 and 1.5 V for 1–105 cycle switches. (For an interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Figure 1.7 (a) Colour change upon redox reactions of the PEDOT/PU film with 30 wt % PEDOT content measured by UV–vis absorbances and mean L* values of the CIELAB representation of the digital camera images. (b) Correlation between the absorbance at 570 nm and the L* value of the film. (c) Repeated colour change of the EC film during 100 redox cycles in 25% McIlvaine buﬀer (pH 5). (d) Custom mobile phone app that measures the L* value of the PEDOT/PU film and plots its temporal change in a real-time manner. Measurement of the L* value of the PEDOT/PU film by the app (left), and a screenshot of the app (right). Reproduced from ref. 160 with permission from American Chemistry Society, Copyright 2017.

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Figure 1.8 Example of EC sunglass lenses of multiple shades of brown. (A) Absorption spectra and photographs of the four commercial sunglass lenses, the circles in the photographs indicate the area of the lens from which the spectra were recorded; (B) a*b* plot showing the colour coordinates for the lenses (%), the evaluated ECPs used for colour mixing (K, R¼ 2-ethylhexyl), and the four brown blends (’).

Introduction

to

Electrochromism

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Excitingly, applications of EC technology are not restricted to hard and rigid lenses, displays or glasses. In recent years, EC technology has found its way into smart and innovative fabrics and devices. A comprehensive review by Kline et al. describes the current progress of EC fabric, challenges to be overcome, and potential applications.155 Moretti et al. also described in detail, in a more recent book chapter, how incorporating EC technology into a ‘‘smart’’ textile allows for textile-based wearable devices.159 The appli-cations could include both aesthetic devices such as shoes and carpets, and functional devices such as diagnostic devices that exhibit a colour change when abnormal blood sugar levels are detected. Unlike conventional in-organic EC materials, in-organic EC materials allow for solution processing and even ink-jet printing techniques for device fabrication. As such, EC devices can also be used directly in contact with the skin. In this regard, electronic skin that changes colour in response to strain, akin to human skin, has been demonstrated.156 This was accomplished by integrating a stretchable EC device with a strain sensor (Figure 1.9), resulting in a real-time visualization of strain on the electronic skin resulting from body movements.

1.4.5 Modulation of Microwave and Near-Infrared Radiation

Most applications of EC materials leverage changes within the visible region of the electromagnetic spectrum. However, EC materials may also be used to modulate radiation in the microwave and NIR regions which are not visible to the human eye.38,157All heated bodies emit black body radiation in the infrared region at room temperature (i.e. ‘‘thermal footprint’’), which can be detected via a thermal sensor. The modulation of NIR radiation is thus particularly useful as it can be used on military hardware to alter infrared signatures of vehicles and structures and thereby evade detection by en-emies’ infrared cameras. EC technology can also be embedded into a spacecraft to conserve heat energy when the spacecraft is facing away from the sun, and reflect and emit heat when facing the sun to prevent overheating.158

Figure 1.9 (a) (Top) Schematic illustration of the interactive colour-changeable system of a strain sensor and an EC device on hand skin. (Bottom) Circuit diagram of the integrated system. (b) (Top) Schematic illustration of the strain sensor with the nanocomposite of PVA/MWCNT/PEDOT:PSS on a PDMS substrate. (Right) Transmittance spectrum of the strain sensor in the visible wavelength range from 380 to 780 nm. The inset shows the transparency of the sensor. (Bottom left) Photograph of the skin-attached sensor on a finger joint. (c) (Top) Schematic illustration of the EC devices consisting of a polyaniline nanofiber/electrolyte/V2O5

with an ITO-coated PET film as an electrode. (Bottom) Colour gradient and photograph of the device with colour change from yellow to dark blue upon applied voltage.

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NIR-modulating EC devices may also be used to enhance the visibility of road signs and thermal cameras that are used to aid drivers when there is poor visibility. For example, EC devices based on conductive polymer PEDOT:tosylate have been demonstrated to modify the thermal signature by up to 10 1C (Figure 1.10).44

1.5 Conclusion

In this chapter, a broad overview of electrochromism, EC materials, their mechanisms, device structure and applications have been introduced briefly. More detailed descriptions of each area will be provided in the subsequent chapters. In conclusion, research in EC technologies has achieved significant breakthroughs over the decades. From the materials aspect, many classes of EC materials have been developed, ranging from traditional metal oxides to more recent organic polymers and small mol-ecules. The architectures of EC devices have also evolved from traditional multi-layered rigid devices to more recent flexible and printable EC devices. This thus opens up a myriad of opportunities for practical applications such as smart windows for greener buildings and vehicles, EC displays with lower energy consumption, anti-glare car rear-view mirrors for greater safety and smart eyewear for better UV-radiation protection. More innovations utilizing EC technologies include smart fabrics and textiles, electronic skin as well as devices that can modulate non-visible radiation for defence and safety purposes. Many of these technologies and applications have been com-mercialised and are available on the market. Although the field of EC ma-terials and their devices is still relatively young, we have witnessed its great potential to transform our everyday lives. It is envisaged that the EC tech-nologies will open up almost limitless, unimaginable possibilities in the near future.

Figure 1.10 Vertical EC PEDOT:Tos device in its reduced (left) and oxidized (right) states (referring to the PEDOT:Tos electrode facing towards the thermal camera), giving an eﬀective temperature modulation of 6.31 1C (8.9 to 3.31 1C).

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Electrochemical properties and electrochromic device applications of polycarbazole derivatives

Electrochromic Smart Materials

Smart Materials

Series editors:

Titles in this series:

How to obtain future titles on publication:

For further information please contact:

Electrochromic Smart Materials

Fabrication and Applications

Edited by

Jian Wei Xu

Ming Hui Chua

Kwok Wei Shah

Preface

Contents

CHAPTER 1

Introduction to

Electrochromism

MING HUI CHUA, TAO TANG, KOK HAW ONG, WEI TENG NEO

AND JIAN WEI XU*

1.1

General Introduction

1.2

History of Electrochromism

1.3

Mechanism of Electrochromism and EC Devices

1.4

Applications of EC Materials

1.4.1

Smart Glass/Windows

1.4.2

Car Rear-view Mirrors

1.4.3

EC Displays

1.4.4

Wearable Apparel and Devices

1.4.5

Modulation of Microwave and Near-Infrared Radiation

1.5

Conclusion

References