Article pubs.acs.org/accounts
Next-Generation Multifunctional Electrochromic Devices Guofa Cai,† Jiangxin Wang,† and Pooi See Lee* School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore CONSPECTUS: The rational design and exploration of electrochromic devices will find a wide range of applications in smart windows for energyefficient buildings, low-power displays, self-dimming rear mirrors for automobiles, electrochromic e-skins, and so on. Electrochromic devices generally consist of multilayer structures with transparent conductors, electrochromic films, ion conductors, and ion storage films. Synthetic strategies and new materials for electrochromic films and transparent conductors, comprehensive electrochemical kinetic analysis, and novel device design are areas of active study worldwide. These are believed to be the key factors that will help to significantly improve the electrochromic performance and extend their application areas. In this Account, we present our strategies to design and fabricate electrochromic devices with high performance and multifunctionality. We first describe the synthetic strategies, in which a porous tungsten oxide (WO3) film with nearly ideal optical modulation and fast switching was prepared by a pulsed electrochemical deposition method. Multiple strategies, such as sol−gel/inkjet printing methods, hydrothermal/inkjet printing methods, and a novel hybrid transparent conductor/electrochromic layer have been developed to prepare high-performance electrochromic films. We then summarize the recent advances in transparent conductors and ion conductor layers, which play critial roles in electrochromic devices. Benefiting from the developments of soft transparent conductive substrates, highly deformable electrochromic devices that are flexible, foldable, stretchable, and wearable have been achieved. These emerging devices have great potential in applications such as soft displays, electrochromic e-skins, deformable electrochromic films, and so on. We finally present a concept of multifunctional smart glass, which can change its color to dynamically adjust the daylight and solar heat input of the building or protect the users’ privacy during the daytime. Energy can also be stored in the smart windows during the daytime simultaneously and be discharged for use in the evening. These results reveal that the electrochromic devices have potential applications in a wide range of areas. We hope that this Account will promote further efforts toward fundamental research on electrochromic materials and the development of new multifunctional electrochromic devices to meet the growing demands for next-generation electronic systems.
1. INTRODUCTION The growing energy and environmental challenges require the development of sustainable and renewable resources. Therefore, materials that are capable of converting and saving energy are highly desirable. Electrochromism refers to the special functionality of materials whose optical properties can be reversibly and persistently changed by the application of a small electric field. These materials have shown a wide range of promising applications, such as smart windows for energyefficient buildings, information displays, self-dimming rear mirrors for automobiles, electronic papers, electrochromic eskins, and so on.1−6 Electrochromism is a promising energysaving technology when used to make smart windows that can dynamically control the amount of solar heat and lighting input of the building as an active response to the changes of interior temperature and light conditions. Usually, electrochromic devices have a multilayer structure consisting of transparent conductors, an electrochromic film, an ion conductor, and an ion storage film (Figure 1). As the core layer of an electrochromic device, the electrochromic layer changes its optical properties and reverts to the original state under © XXXX American Chemical Society
Figure 1. Configuration of a typical electrochromic device.
alternating potential modulation. Many inorganic and organic materials can be used as electrochromic layers, such as transition metal oxides (WO3 is typically used), Prussian blue, viologens, conducting polymers, etc. Because of the poor Received: April 12, 2016
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modulation via inkjet printing. Inkjet printing is an emerging film preparation technology with many advantages such as low cost, high resolution, efficient material usage, and applicability to a variety of large-scale substrates. In addition, it has the capacity to deposit electrochromic materials on a specific location and control the film thickness by variation of the number of printed layers. Multilayer and continuous WO318 and NiO19 films without aggregation were successfully prepared by the inkjet printing method. Large optical modulation was achieved by the multilayer WO3 (72% at 633 nm) and NiO (64.2% at 550 nm) films. The cyclic stability of electrochromic films is one of the important parameters in practical applications, which determines the lifespan of the electrochromic device. Improving the stability of electrochromic films has been the subject of intensive study recently.20−23 We have discovered that a selfassembled polymer of linear polyethylenimine (LPEI) on ITO can stabilize a variety of electrochromic films. The electrochemically deposited Ti-doped vanadium oxide films sustain over 1500 cycles in a three-electrode system and sustain an optical modulation of more than 90% after 600 cycles in a twoelectrode system, a remarkable improvement compared with that without LPEI modification.24,25 The LPEI modification of ITO also enhances the stability of other electrochromic films, such as electrochemically deposited WO3 films and hydrothermally synthesized V2O5 with a nanobelt−membrane hybrid structure.24,26 It is widely accepted that the electrochromic phenomena of transition metal oxides is attributed to injection/extraction of electrons and cations.27−31 As an example, in the extensively investigated electrochromic materials of WO3, the reversible electrochromic phenomenon is attributed to the following reaction:32−35
durability of organic electrochromic materials under UV exposure outdoors, organic electrochromic devices are typically suitable for indoor applications, while the inorganic materials are usually used for outdoor smart windows. The ion storage layer with or without electrochromism is needed to balance the charges shuttled from the active electrochromic layer through the electrolyte. NiO, CeO2, and IrO2 are promising ion storage materials. The center layer of the ion conductor is a transparent electrolyte in the liquid or solid state. Commonly used transparent conductors as the top and bottom electrodes are indium tin oxide (ITO)-coated glass and fluorine-doped tin oxide (FTO)-coated glass. Electrochromic materials, devices, and their applications have been well reviewed by experts in the field, especially by Granqvist,7,8 Mortimer,9,10 and Deb.11 Major progress has been achieved in the theoretical and practical aspects of electrochromic devices. Still, electrochromic devices with more functionality and novel features are being sought to extend their application areas. For example, flexible, foldable, stretchable, and wearable electrochromic devices have been developed to meet the escalating demands of portable and wearable electronics. Meanwhile, it would be highly attractive to integrate both energy storage and electrochromism into one device, for example, a smart window with the capability of indicating its energy storage. In this Account, we present our recent research progress in developing different strategies to prepare and characterize state-of-the-art electrochromic devices with high performance and multiple functionalities.
2. STRATEGIES FOR HIGH-PERFORMANCE ELECTROCHROMIC LAYERS The ideal optical modulation of an electrochromic material is 100%, that is, the material can be fully opaque in the colored state and fully transparent in the bleached state under alternating potentials. The preferred transmittance of the window can be readily tuned on the basis of the demands. Till now, only a few research groups have reported optical modulation of electrochromic materials over 80%,12−16 let alone 100%. We recently fabricated porous WO3 films with ultrahigh transmittance modulation by a novel, facile, and economical pulsed electrochemical deposition method with a time interval of 1.1 s between pulses in the deposition solution containing Na2WO4·2H2O.17 The WO3 film prepared by the pulsed electrochemical deposition method with a short time interval exhibits a porous and nanoscale interconnecting network structure. The reaction can be described as follows: 2WO4 2 − + 4H 2O2 → W2O112 − + 2OH− + 3H 2O
WO3 + x M+ + x e− ↔ MxWO3
where the cation M+ can be either H+ or an alkali metal ion such as Li+, Na+, or K+. The rate-determining steps of ion intercalation and extraction are under diffusion control and are limited to a very thin surface layer of the electrochromic materials, and the kinetics and magnitude of ion insertion and the electrochromic performance are strongly dependent on the microstructure of the material. A deeper understanding of the local electrochemical properties will be beneficial to establish relationships between the structural design and electrochemical activity. Scanning electrochemical microscopy (SECM) has been successfully applied to studies of the localized charge transport process and electrochemical reactions in our recent research.36,37 The relationships between the structure and electrochemical activity of the electrochemically deposited WO3 films were recently explored by our group via SECM.17 The propagation of the charged species in the porous WO3 film was found to be much faster than that in the compact WO3 film in the given time domain, and this correlated to its microcosmic porous structure and higher surface activity, which in turn account for its better electrochromic performance.
(1)
W2O112 − + (2 + 3x)H+ + 3x e− → 2HxWO3 +
(2 + x) 8−x H 2O + O2 2 4
HxWO3 → WO3 + x H+ + x e−
(4)
(2) (3)
Because of its unique porous structure, the WO3 film exhibits nearly ideal optical modulation, which reaches about 97.7% at 633 nm, and short switching times of 6 and 2.7 s for the coloration and bleaching processes, respectively. High coloration efficiency (118.3 cm2/C), and excellent cycling stability are also achieved by porous WO3 on ITO-coated glass. Besides the electrochemically deposited porous WO3 films, we also prepared electrochromic films with large optical
3. TRANSPARENT CONDUCTIVE LAYER With the escalating demands of stretchable and wearable displays, electrochromic e-skins, and highly deformable electrochromic films, the corresponding technologies are envisioned to be developed in soft electrochromic devices. The commonly used rigid ITO- and FTO-coated glass cannot be applied in B
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Accounts of Chemical Research these innovative electrochromic devices because of their rigid and brittle nature. In order to realize these innovative concepts, flexible, foldable, and stretchable conductors with high electrical conductivity and transparency are crucial and inevitably necessary. We have recently demonstrated that electrodeposited WO3 on a flexible ITO-coated poly(ethylene terephthalate) (PET) substrate can achieve excellent electrochromic performance with a large optical modulation of 96.6% and short colored/bleached switching time of 14/6.6 s.17 In order to avoid the brittle nature of the ITO and FTO materials, a variety of flexible transparent electrodes have been investigated as ITO and FTO substitutes, such as carbon nanotubes (CNTs),38 graphene,39,40 conducting polymers,41 metal nanowires,42,43 and metal grids.44 Among these transparent electrodes, the metal-based ones are of particular interest because of their high conductivity and high transparency. We have fabricated flexible silver grid electrodes by self-assembling and sintering of Ag nanoparticles into periodic uniform squares on a PET substrate.18 The Ag grid electrodes show a high transmittance of 82 ± 3% in the spectral range of 350−900 nm and high conductivity (sheet resistance < 5 Ω/sq), as shown in Figure 2a. The qualitative results of an electrochromic test after
Figure 3. (a) Schematic illustration of the structure of the silver grid/ PEDOT:PSS hybrid film. (b) Transmittance spectra of WO3 in the colored (−0.7 V) and bleached (0.1 V) states over the wavelength range from 300 to 900 nm. (c, d) Optical modulation changes of the WO3 on silver grid/PEDOT:PSS hybrid film subjected to repeated (c) compressive bending and (d) tensile bending with a curvature radius of 20 mm. Reproduced with permission from ref 45. Copyright 2016 Wiley-VCH.
has also been improved by Cui’s group via full encapsulation of the nanowires under large-area and monolayer graphene films.46 The obvious disadvantage of PET substrates is that they cannot endure large bending angles. We recently developed nanopaper to replace PET as the device substrate. The nanopaper made of nanocellulose possesses obvious advantages such as a ubiquitous source, biocompatibility, recyclability, and a degradable nature.47 We adopted a novel transfer method to achieve conductive and transparent nanopaper with percolating silver nanowire (AgNW) on the surface of the nanocellulose. The foldability of the nanopaper electrode was demonstrated by repeated folding tests under ±180°, as shown in Figure 4d. The nanopaper electrode still maintained its high conductivity below 1 Ω/sq even after 200 cycles of folding test. Even after creases are formed after the folding, the AgNW can still adhere strongly to the nanopaper (Figure 4b,c). The extremely high flexibility of the AgNWs enables the nanopaper to be bent at large angles. The long nanowire structure is able to maintain the interconnection under large mechanical deformations, as reflected in a circuit connection experiment that lights up light-emitting diode (LED) bulbs before and after crumpling of the nanopaper (Figure 4e,f). The transparent nanopaper is helpful in enhancing the coloration efficiency and cycling stability when used in electrochromic devices. The excellent electrochromic performance of the nanopaper makes it a promising material for next-generation reflective e-papers or displays. Apart from flexible and foldable electrochromic devices, stretchable and wearable electrochromic devices have potential applications in future smart clothes, implantable displays, and eskins. Stretchable and wearable electrochromic devices based on AgNW elastic conductors were fabricated in our previous report.48 In order to fabricate the stretchable electrochromic device, the AgNW networks were embedded in a polydimethylsiloxane (PDMS) elastomer matrix as stretchable conductors, as shown in Figure 5. The device consisted of a AgNW/PDMS elastic conductor and electrochemically deposited WO3 active layer. The embedded AgNW percolating
Figure 2. (a) Transmittance spectra (with PET as the reference) of silver grids that remain conductive when bent. (b) Electrochromic cell before and after application of the negative potential. Reproduced with permission from ref 18. Copyright 2014 Royal Society of Chemistry.
the WO3 film was printed on the Ag grids by inkjet printing are shown in Figure 2b. A well-known disadvantage of purely metal-based transparent electrodes is their tendency to oxidize not only in air but also in electrochemical reactions. After coating with one layer of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) on the Ag grid in our work, the Ag grid/PEDOT:PSS hybrid film became completely impervious to oxygen and moisture (Figure 3a).45 The resistance of the hybrid film was 0.62 Ω/sq and remained almost the same with only minute fluctuations of within 0.02 Ω/sq through a monitoring duration of 2 months in air (28 °C, 65% relative humidity). After electrodeposition of the WO3, the hybrid film presented noticeable electrochromism with an optical modulation of up to 81.9% at 633 nm (Figure 3b) and fast switching (2.8/1.9 s for bleaching/coloration, respectively) as well as excellent electrochemical stability. The WO3 on the silver grid/PEDOT:PSS hybrid film sustained a transmittance modulation of about 79.1% after it was subjected to 1000 cycles and maintained a transmittance modulation of about 67.3% even after it was subjected to 2000 cycles. Moreover, the WO3 on silver grid/PEDOT:PSS hybrid film possessed outstanding flexibility and mechanical stability under both compressive and tensile bending tests. As can be seen from Figure 3c, the optical modulation decayed 7.5% after 1200 compressive bending cycles and showed 20% degradation after 800 tensile bending cycles (Figure 3d). The electrochemical stability of flexible transparent electrodes based on silver and copper nanowires C
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Figure 4. (a) Schematic of the folding procedure. (b, c) FESEM images of the nanopaper electrode folded to (b) −180° and (c) +180°. (d) Sheet resistance vs cycle number with 100 cycles to −180° and another 100 cycles to +180° conducted in a consecutive manner. (e, f) Digital images of the foldability test of the AgNW nanopaper electrode connected in a circuit with LED bulbs (e) before crumpling and (f) after crumping. Reproduced with permission from ref 47. Copyright 2015 Wiley-VCH.
structures impart excellent stretchability to the conductors. The as-fabricated stretchable electrochromic devices was mechanically robust and could be stretched, folded, twisted, and crumpled without performance degradation. These unique properties make them very attractive candidates for nextgeneration stretchable and wearable electrochromic display applications. Figure 5d,e shows digital photographs of the colored and bleached devices in the relaxed and stretched (50% strain) states, respectively. The device shows a large reflective modulation at around 350 nm, fast switching (1 and 4 s for the coloration and bleaching processes, respectively), and good cycling stability in the relaxed state. The stretchable device can still maintain its functionality even when stretched up to 50%. Wearable and implantable soft display panels could potentially be implanted on clothes or skin for the display of stationary and even moving texts and images or to instill chameleon changes of skin or textile color for camouflage purposes. Inspired by our previous results, we further fabricated a wearable electrochromic device by implanting AgNW display pixels on a cotton textile substrate (Figure 6a).48 Three addressable display pixels can be controllably colored and bleached individually or in any combination. Moreover, the wearable electrochromic devices are mechanically robust and could maintain their functionalities after various forms of deformation. Our prototypes of stretchable and wearable electrochromic devices pave the way for unprecedented applications such as wearable display panels implanted on clothes and skin. Recently, Bao’s group also demonstrated a chameleon-inspired stretchable e-skin based on stretchable electrochromic devices. The stretchable e-skin is capable of changing colors with tactile-sensing properties.4
Figure 5. (a) Schematic of the stretchable electrochromic device. (b, c) Stretchable devices being twisted and crumpled, showing excellent mechanical robustness. (d, e) Examples of the patterned device in bleached and colored states at 0 and 50% strain, respectively. Reproduced with permission from ref 48. Copyright 2013 American Chemical Society.
4. ION CONDUCTOR LAYER The ion conductor between the electrochromic layer and the ion storage layer is an electrolyte and it can be in either liquid, D
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conductivity of 2.5 × 10−5 S/cm at 52% relative humidity.52 The use of LbL assembly has a great advantage in optimization of the interface between the electrolyte and the electrochromic layer. The method can provide continuous formation of the electrolyte and the electrochromic layer, which can enhance the ion exchange during the electrochromic process and improve the flexibility and mechanical robustness.
5. DIVERSE APPLICATIONS OF MULTIFUNCTIONAL ELECTROCHROMIC DEVICES The flexible, foldable, stretchable, and wearable electrochromic devices reviewed above have mainly benefited from the developments of various conductive substrates. Meanwhile, it will be very useful to integrate other functions with electrochromism into one device with attractive applications. Integration of electrochromism and energy-storage functionality into one device is one interesting perspective.53−56 Such electrochromic energy storage smart windows can not only adjust the sunlight and solar heat through the windows of the building by application of a small electric field but also can be used as energy-storage devices at the same time. Moreover, the level of energy stored in the window can be monitored by the color variations. We have developed different materials and substrates as electrodes for the smart window, such as anodic colored NiO and cathodic colored WO3 materials on ITO glass and flexible silver grid/PEDOT substrates.45,57 The charge/ discharge curves and corresponding in situ transmittance were measured to illustrate the multifunctionality of the electrochromic NiO and WO3 films. The color of the NiO films changes from transparent to brown during the charging process, and then the color fades out during the discharging process (Figure 7a). When the NiO film reaches a fully charged state, the color of the film is dark brown. In the reverse process, when the energy is completely consumed, the film changes to transparent. For the cathodic colored WO3 film, the downward lines in the galvanostatic charging and discharging curves correspond to the charging process and the upward lines to the discharging process (Figure 7b). A color change of the WO3 film toward a dark-blue color is observed when the film is charged. During the reverse discharging process, the reduced W ions are oxidized back to the higher valence states, which are accompanied by color loss to form the transparent state. The color change is sufficiently fast that it can still be observed even at a high current density of 10 A/g when the silver grid/ PEDOT:PSS hybrid film is used as the conductive substrate. The proposed multifunctional smart window can change its color to dynamically adjust the sunshine and solar heat input of the building or provide privacy for the users during the daytime. At the same time, energy can be stored in it. When evening rolls around, the window will change to bleached state and release the stored energy to power light bulbs or other electronic devices. In addition, it has the capability to serve as an energy indicator to monitor the energy stored in the window by its color changes (Figure 7c).
Figure 6. (a) Schematic of the electrochromic electrode implanted onto wearable textiles. (b) Example images showing the ability to control the coloration/bleaching of individual display pixels and their mechanical stabilities against deformations such as crumpling. Scale bars for display pixels: 2 mm. Reproduced with permission from ref 48. Copyright 2013 American Chemical Society.
gel, or solid form. We have reviewed polymer electrolytes for electrochromic device applications in detail previously.49 This part of the Account briefly reviews the progress on polymer electrolytes prepared by our group via layer-by-layer (LbL) methods. In order to realize completely flexible and solid electrochromic devices, LbL-assembled polymer electrolytes were successfully fabricated in our previous work.50 The polymer electrolyte film was fabricated from LPEI, poly(ethylene oxide) (PEO), and poly(acrylic acid) (PAA) on ITO/PET and was composed of four interbonding layers per deposition cycle combining hydrogen bonding and electrostatic attraction in the same structure. A high ionic conductivity of 9.1 × 10−4 S/cm was obtained from the electrolyte tetralayer films. When used in the electrochromic layer based on polyaniline (PANI) with dodecylbenzenesulfonic acid, the highest optical modulation of 38% at 670 nm was achieved. Moreover, electrochromic multilayer films obtained by LbL self-assembly of a complex polyelectrolyte were demonstrated in our recent work.51 Compared with the compact [PANI/PAA]n films, [PANI/PAA−PEI]n films exhibited accelerated growth rates and a porous structure leading to improved electrochromic properties. An enhanced optical modulation of 30% at 630 nm and fast switching was achieved by [PANI/PAA−PEI]30 films. Recently, we successfully prepared LbL self-assembled poly(ethylene glycol) (PEG)−α-cyclodextrin (αCD) complex and PAA with supramolecular interactions. It achieves an ionic
6. CONCLUSIONS AND PROSPECTS The development of simple, economical, and scalable methods for preparing high-performance electrochromic materials and transparent conductors is the first step to realize their practical applications. Rational design and scalable synthetic approaches will endow electrochromic devices with integrated multiple functionalities and wider applications. In this Account, we have E
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Figure 7. (a) Galvanostatic charge/discharge profiles at 2 A/g and corresponding in situ optical responses measured at 550 nm for a NiO nanoparticle film on an ITO substrate. Reproduced with permission from ref 57. Copyright 2015 Elsevier Ltd. (b) Galvanostatic charge/discharge profiles at 1 A/g and corresponding in situ optical responses measured at 633 nm for the WO3 on silver grid/PEDOT:PSS hybrid film. (c) Schematic of the electrochromic energy storage smart window during a charging−discharging process. Reproduced with permission from ref 45. Copyright 2016 Wiley-VCH.
lighting. Moreover, it provides the capability to monitor the energy stored in the window by its color changes. In conjunction with the above successes, some challenges still need to be tackled in the electrochromic devices. For instance, electrochemically stable and reliable conductors with good transparency and stretchability for soft electrochromic devices still need to be developed. The trade-off between large optical modulation and high charge density in the energy storage smart window also needs to be carefully addressed. Suitable electrolyte systems that enable robust cycling and fast switching are also needed. Therefore, significant efforts are required to develop high-performance electrochromic devices and integrate them into multifunctional systems. We believe that new materials, rational design, and advanced synthetic strategies will provide solutions for these challenges. With the enabling technologies well-established in the soft and multifunctional electrochromic devices, a revolution may take place in the way we live with these energy-saving and interactive smart systems.
presented our strategies to achieve high-performance electrochromic films and multifunctional electrochromic devices. Porous WO3 films with nearly ideal optical modulation were fabricated by a pulsed electrochemical deposition method. Sol− gel/inkjet printing methods, hydrothermal/inkjet printing methods, and a passivation polymeric layer between transparent conductor/electrochromic layers have been developed to prepare electrochromic films. Benefiting from the developments of soft conductive substrates, different kinds of flexible, foldable, stretchable, and wearable electrochromic devices have been developed. The concept of an electrochromic energy storage smart window has been demonstrated. The smart window can dynamically adjust the daylight and solar heat input of the building or protect privacy during the daytime. Meanwhile, energy can also be stored in the smart window. The stored energy can be released to power light bulbs or other electronic devices as the night ensues. The window changes to the transparent state and maximizes the input of outdoor F
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
G.F.C. and J.X.W. contributed equally.
Funding
We acknowledge funding support from the NRF Competitive Research Programme NRF-CRP13-2014-02. Part of the work was also supported by A*Star-MND Green Building Joint Grant 1321760013 and Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE) that is supported by the National Research Foundation, Prime Minister’s Office, Singapore. Notes
The authors declare no competing financial interest. Biographies Guofa Cai received his B.S., M.S., and Ph.D degrees in the School of Materials Science and Engineering from Henan Polytechnic University, Wuhan University of Technology, and Zhejiang University, P. R. China, respectively. Currently he is a research fellow under the supervision of Prof. Pooi See Lee in the School of Materials Science and Engineering at Nanyang Technological University (NTU), Singapore. His research interests mainly focus on nanomaterials for electrochromic and energy storage applications. Jiangxin Wang is a Ph.D. student in the School of Materials Science and Engineering at NTU. He obtained his B.S. degree in the School of Physical Electronics at the University of Electronic Science and Technology of China (UESTC) in 2010. His current research interests focus on deformable optoelectronic devices. Pooi See Lee is a Professor in the School of Materials Science and Engineering at NTU. She obtained her B.Sc. (Hons) and Ph.D. at the National University of Singapore. Her research work focuses on the theme of electrochemical- and electrical-inspired devices based on nanostructures and nanocomposites for applications in electrochromics, energy storage, electrical memory devices, and nanowire transistors or sensors. She serves as the Associate Chair in the School of Materials Science and Engineering at NTU.
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