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- Sprache: Englisch
Comprehensive reference focusing on features of promising new materials and devices for electrochromic and integrated multifunctional systems
Next-Generation Electrochromic Devices: From Multifunctional Materials to Smart Glasses covers the basic concepts and the potential use of electrolytes, conducting polymers and multifunctional materials for the development of electrochromic (EC) and integrated systems, focusing on the influence of solid-state electrolytes and interface features on the design of new device structures and simplified manufacturing.
The book is divided into three parts. Part I explores the chemistry of the main components of devices with a special focus on the main critical material issues, covering mixed-ion and electron conductors, electrodes, and more. Part II describes EC and multifunctional devices, such as photoelectrochromic smart windows and see-through ECOLED displays, and the main characterization techniques for the study of material properties, interfaces and device performance. Part III comprehends device manufacturing, scale-up procedures, and discusses the main benefits of smart windows in terms of energy savings, visual comfort, and environmental impact, proposing contextually a multitude of pioneering ideas and concepts with a specific insight into emerging devices in the era of Artificial Intelligence (AI), immersive reality and invisible technologies.
Next-Generation Electrochromic Devices includes information on:
- Inorganic and organic electrochromic materials, including graphene, 3D transitional metal oxides, Prussian blue, viologens, conducting polymers, organic mixed ionic and electronic materials, and highly transparent electrodes
- Electrolytes including inorganic, liquid, gel, and solid-state polymers, their ionic conductivity and transport properties
- Thin film deposition methods: chemical deposition through solution processing techniques, sol-gel, Langmuir-Blodgett, electrochemical and physical deposition by means thermal and electron-beam evaporation, sputtering, pulsed laser, and molecular beam epitaxy deposition
- Electrochemical analysis of materials, interface, and device durability
- Organic mixed ionic and electronic conductor materials for innovative and multifunctional optoelectronic systems
- Optical, structural, chemical, and physical methods for the study of electrochromism and material properties including NMR, X-Ray diffraction analysis, XPS, UV-Vis, FTIR, and Raman spectroscopy
- Energy efficiency of EC glazings and their impact on thermal and visual comfort
- Emerging materials for chromogenic systems, smart windows, and new energy devices
- Fully integrated ECOLED see-through displays and multifunctional smart devices for immersive reality and invisible technologies
- Impact of AI and next-generation technologies on social, human, and environmental changes
Next-Generation Electrochromic Devices is an essential reference on the subject for materials scientists, chemists, physicists, as well as architects, electrical and civil engineers. It can be also a source of inspiration for artists, graphic designers, and art workers.
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Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
About the Author
Preface
Acknowledgments
1 Introduction
1.1 Electrochromism: A Brief Note on the History and Recent Evolution
Part I: Materials
2 Electrochromic Materials
2.1 Inorganic Electrochromic Materials
2.2 2D Materials
2.3 Organic Electrochromic Materials
3 Mixed Ionic and Electronic Conductors
3.1 Semiconducting Polymers and Small Molecules
3.2 Structure–Property Relationship and Charge Transport in Disordered Organic Materials
3.3 Potential Impact of Mixed Conductors on the Design of New EC and Multifunctional Devices
4 Electrolytes
4.1 Liquid Electrolytes
4.2 Polymer Electrolytes: From Gel to Solid Polymers
4.3 Inorganic Electrolytes
5 Electrodes
5.1 Transparent and Conducting Oxides
5.2 Carbon-Based Electrode Materials
5.3 Metal Nanowires and Metal Grids
6 Critical Material Issues
Part II: Devices
7 Device Structure: The Key Role of the Interfaces in the Device Design
7.1 Electrochromic Devices
7.2 Electrochromic Multifunctional Devices
8 Thin-Film Processing Technologies
8.1 Chemical Deposition
8.2 Physical Depositions
9 Analysis of Device Performance
9.1 Optical Spectroscopy
9.2 Electrochemical Analysis
9.3 Chemical and Physical Methods for Electrochromism and Analysis of Material Properties
9.4 Characterization of Mixed Ionic and Electronic Conduction Materials
Part III: Scale-Up, Energy and Environment, and Next-Generation Technologies
10 Construction of Smart Windows: From Laboratory to Industry Scale
10.1 Manufacturing Processes: Materials and Deposition Techniques
10.2 Scale-Up Procedures: Electrochromic Windows and Large-Area Photovoltaic Modules
10.3 Laminated Smart Windows and Adhesive Electrochromic Smart Films
11 Energy-Efficient Electrochromic Glazings for Green Buildings
11.1 Energy Demand and Consumption in Buildings: Energy Saving of Electrochromic Glazings
11.2 Effect of Electrochromic Glazings on Visual Comfort: Usable UDI and DGI
12 Emerging and Next-Generation Technologies for Fabrication of Dynamic Tintable Windows
12.1 Smart Photoelectrochromic and Thermochromic Windows: Green Technologies Toward More Sustainable Buildings
12.2 Emerging and Next-Generation Technologies
12.3 The New Era of Artificial Intelligence: Toward Immersive Reality and Invisible Technologies
References
Index
End User License Agreement
List of Tables
Chapter 7
Table 7.1 Summary of quasi-solid and solid PVCDs (1996–2024) displaying the ...
Table 7.2 Different ECOLED device structures and main figures of merit.
Chapter 9
Table 9.1 Key performance parameters of quasi-solid and solid-state EC devic...
List of Illustrations
Chapter 1
Figure 1.1 Chemical structure and pictograms of methyl viologen dichloride, ...
Figure 1.2 Prussian blue pigment used (a) in Van Gogh's Starry Night on the ...
Chapter 2
Figure 2.1 Photograph of solutions, illuminated from underneath, containing ...
Figure 2.2 EC device structure characterized by a standard five-layer config...
Figure 2.3 Periodic table of elements, where the transition metals are highl...
Figure 2.4 (a) CVs of nanoparticles, amorphous and crystalline WO
3
films....
Figure 2.5 (a) Cross-sectional SEM image of highly porous WO
3
films prepared...
Figure 2.6 (a) Scheme of ion diffusion and charge transfer in QD materials i...
Figure 2.7 Surface morphology of MoO
3
films on FTO-coated glass substrates a...
Figure 2.8 (a) Schematic representation of ion intercalation sites in
h
-MoO
3
Figure 2.9 Antireflection (a) and EC (b) properties of TiO
2
NW-based smart g...
Figure 2.10 Dual-band electrochromism: (a) transmittance measurements of Nb-...
Figure 2.11 (a, b) SEM images of NiO films deposited on ITO-coated glass by ...
Figure 2.12 (a) SEM images of printed NiO films with six layers annealed at ...
Figure 2.13 SEM images (top) and corresponding low magnification images (bot...
Figure 2.14 (a) Schematic illustration of the supercapacitor design based on...
Figure 2.15 (a) Graphic representation of 2D WO
3
·2H
2
...
Figure 2.16 (a) 2D WO
3
nanosheets through exfoliation from bulk WS2...
Figure 2.17 (a) Framework of PB and PBA where other transition-metal M(II) i...
Figure 2.18 Different redox states of viologen.
Figure 2.19 (a) Electrodeposition of rGO-PV composite starting from GO and c...
Figure 2.20 (a) Synthetic route of MPV and DPV. (b) Schematic diagrams of de...
Figure 2.21 (a) Structures of polythiophene derivatives used for completing ...
Figure 2.22 (a) Family of EDOT-, ProDOT-, and AcDOT-based ECPs prepared usin...
Figure 2.23 (a) Redox states of PANI.(b) Redox switching between oxidati...
Figure 2.24 (a) (A) Photographs of flexible PANI films prepared by combining...
Chapter 3
Figure 3.1 (a) Different operando characterization techniques reveal the str...
Figure 3.2 (a) Schematic representation of (a) polymer chains and (b) how a ...
Figure 3.3 Microstructure of (a) a semicrystalline polymer film, (b) disorde...
Chapter 4
Figure 4.1 (a) Scheme of the highly conductive hybrid SPE based on amine-ter...
Figure 4.2 (a) Formation of gel electrolytes based on MOP. (b) UV/Vis absorp...
Figure 4.3 (a) The individual layers of solid-state EC devices are successiv...
Figure 4.4 Cartoon of the ionic motion of a lithium-ion in a PEO-host.
Figure 4.5 (a, b) Different possible paths of Li
+
transport in PEO-passi...
Figure 4.6 (a) Impedance spectra and the corresponding equivalent circuit fo...
Chapter 5
Figure 5.1 (a) CNT films with different layers (1L, 2L, 4L, and 10L) on PET ...
Figure 5.2 (a) Ag NWs ink in ethanol solvent (left) and deposited on a plast...
Chapter 7
Figure 7.1 (a) Stratigraphic layout and (b) schematic representation of a ge...
Figure 7.2 Thermal curing process of the VBV moiety. (a) Scheme of the propo...
Figure 7.3 Device structure of (a) ECD-A, and (b) ECD-B. (c) Illustration an...
Figure 7.4 Schematic diagram of device type for integrated EC devices: CCM, ...
Figure 7.5 (a) PECD structured with TiO
2
and WO
3
on separate surfaces: color...
Figure 7.6 (a) Schematics of PECD and PVCD. (b) Transmittance spectra of PEC...
Figure 7.7 (a) Operating mechanism of a solid-state prototype PV–EC device. ...
Figure 7.8 Working principles at closed and open circuit conditions of the P...
Figure 7.9 (a) Device structure of two-electrode light-driven EC device. Ele...
Figure 7.10 (a) Layout of PECD, (b) transmittance modulation of PECD, powere...
Figure 7.11 (a) Application fields for colorful, highly transparent, and dec...
Figure 7.12 (a) Sketched architecture of the quasi-solid polymeric PECD in i...
Figure 7.13 (a) Schematic cross-sectional structure of the EC/PV laminated v...
Figure 7.14 (a) SEM of a microstructured ST perovskite device (b). Photograp...
Figure 7.15 (a) PVCD: schematic representation of EC and three PV cells on t...
Figure 7.16 (a) Transmittance modulation of the simplified single-glass mult...
Figure 7.17 (a) Schematic view of the vertically integrated ECOLED device st...
Figure 7.18 (a) Schematic representation of EC/OLED device structure. (b)
J–
...
Figure 7.19 (a) Photographs of the display device when biased at −3.6 V (lef...
Figure 7.20 (a) Image of neutral and reduced Oct
2
TTz
2+
/1 mM PC in room l...
Figure 7.21 (a) Preparation of the WO
3
polycrystalline film and assembly of ...
Figure 7.22 (a) Scheme of functioning mechanisms for multifunctional self-po...
Chapter 9
Figure 9.1 (a) CV of amorphous, crystalline, and nanoparticle WO
3
films, (b)...
Figure 9.2 (a, b) CV curves at scan rates ranging from 2 to 15 mV s
−1
....
Figure 9.3 A simplified representation of Randles circuit and its constituen...
Figure 9.4 EIS plot of (a) ECD-A and (b) ECD-B for different RH % and consta...
Figure 9.5 CV for WO
3
films (300 nm thick) immersed in LiClO
4
–PC (sweep rate...
Figure 9.6 In situ optical transmittance at 550 nm upon varied electrochemic...
Figure 9.7 (a) Current and (b) charge density as a function of the number of...
Figure 9.8 (a) W 4f HAXPES spectra after binding energy correction for diffe...
Figure 9.9 XPS depth profiles of WO
3
films (300-nm thick) deposited on glass...
Figure 9.10 Analysis of XPS depth profile for the all-solid-state single-gla...
Figure 9.11 Measurements of (a) water self-diffusion coefficients (D), and (...
Figure 9.12 Structure and nanostructure of the h′-WO
3
framework. (a) SEM and...
Figure 9.13 (a) Scheme of the device with indications of drift length,
ℓ,
...
Figure 9.14 1D Normalized intensity profiles as a function of distance X fro...
Chapter 10
Figure 10.1 (a) Schematic view of the web-coated device based on complementa...
Figure 10.2 Process temperature and vacuum conditions of each representative...
Figure 10.3 Commonly used printing and coating methods in the EC device fabr...
Figure 10.4 (a) Scheme illustrating the influence of the coating techniques ...
Figure 10.5 (a) Structures and photography of CMY representative EC polymers...
Figure 10.6 (a) Scheme representing device assembly characterized by an elec...
Figure 10.7 Photographs of EC device with an active area of 2.5 × 2.25 cm
2
i...
Figure 10.8 (a) AFM measurements: comparison of one-step and two-step deposi...
Figure 10.9 (a) Schematic illustration of vacuum thermal evaporation process...
Figure 10.10 High-quality ECP-magenta film printed with slot-die and microgr...
Figure 10.11 (a) SEM images: (A) rectangular TiO
2
colloidal crackle pattern;...
Figure 10.12 (a) Bleached and fully colored states of the scaled-up device a...
Figure 10.13 (a) Transmission spectra and photographs of large-area HV/EDTA ...
Figure 10.14 (a) Schematic representation and photographs of large-area proc...
Figure 10.15 Examples of interior (left panel) and exterior (right panel) vi...
Figure 10.16 (a) Number of published results from 2013 to 2018 categorized b...
Figure 10.17 The R2R coatings were performed with (a) R2R-coated perovskite ...
Figure 10.18 (a) Illuminated IV curves of doctor-bladed (DB) and slot-die co...
Chapter 11
Figure 11.1 (a) Blackbody spectra for the indicated temperatures (vertical s...
Chapter 12
Figure 12.1 (a) From left to right: (A, B) photograph and schemes of the T-p...
Figure 12.2 Smart glass public Tokyo toilets: the glass remains transparent ...
Guide
Cover
Table of Contents
Title Page
Copyright
About the Author
Preface
Acknowledgments
Begin Reading
References
Index
End User License Agreement
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Next-Generation Electrochromic Devices
From Multifunctional Materials to Smart Glasses
Pierluigi Cossari
Author
Dr. Pierluigi CossariNational Research CouncilSapienza UniversityPiazzale Aldo Moro 5Rome 00185Italy
Cover Image: © Vertyr/Shutterstock
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All rights reserved (including those of translation into other languages, text and data mining and training of artificial technologies or similar technologies). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978-3-527-34925-8ePDF ISBN: 978-3-527-83256-9ePub ISBN: 978-3-527-83257-6oBook ISBN: 978-3-527-83258-3
About the Author
Dr. Pierlugi Cossari, PhD, is a researcher at the Italian National Research Council, Institute of Nanotechnology (CNR-Nanotec), Rome. After obtaining his MS in Chemistry from Sapienza University, Rome, in 2005, he worked at CNR for about 4 years, focusing on the study of the structure, morphology, and chemico-physical properties of nanomaterials and polymers by spectroscopic, electrochemical, and AFM analysis. He earned his PhD in Nanotechnology from the University of Genoa (Genoa, Italy) jointly with Marburg University (Marburg, Germany) under the supervision of Professor C. Nicolini (2010–2013), acquiring experience on the synthesis, optical spectroscopy, and electrochemistry of semiconducting polymers, especially polyanilines, their derivatives, and carbon-based nanocomposites, as well as on the technological development and fabrication of chemical sensors for volatile organic compounds. He then joined the Center for Advanced Devices and Materials at CNR-Nanotec and the University of Salento (Lecce, Italy) as a postdoctoral researcher under the supervision of Professor G. Gigli (2014–2021), working on the synthesis, fabrication of thin films, spectroscopic and electrochemical analysis of electrochromic materials (organic and inorganic), polymer electrolytes, as well as the design and engineering of electrochromic and multifunctional electrochromic–electroluminescent, photoelectrochromic, and photovoltachromic devices. His current research focuses on the study of conducting polymers and organic mixed electronic and ionic conducting materials for the development of innovative optoelectronic, bioelectronic devices, and multifunctional smart windows. Since 2013, he has published numerous articles in different prestigious peer-reviewed journals, participated in the writing of divulgation essays on electrochromism and emerging devices, and authored a patent for the invention of a multifunctional solid-state device for solar control, photovoltaic conversion, and artificial lighting.
Preface
Over the past decades, significant advances have been made in the development of electrochromic (EC) materials and devices with the concrete promise of being applied to smart windows and new and fascinating energy-saving electrochromic multifunctional technologies. In literature, electrochromic materials and devices have been well-reviewed in numerous articles and influential books by experts in the field, especially addressing the main concepts associated with the fundamentals, material science, manufacturing procedures, energy, and environmental issues. More relevant evolution has been achieved in both theoretical and functional aspects of the chromogenic materials and device engineering and, to date, advanced multifunctional systems with new features would result extremely promising for new application areas. The possibility to effectively combine multiple functions, by integrating electrochromism with photovoltaic, lighting (OLED), or energy storage into one device, would indeed be strongly attractive for further progress of smart windows, chromogenic glasses, and see-through display technologies. Ultrathin flexible EC devices, capable of being processed on plastics or fabrics, are also particularly interesting and will be the object of intensive research in the near future in order to meet the growing demands of flexible, stretchable, and wearable electronics. Several other applications and prototypes developed in these last years, such as EC batteries, adaptive camouflage devices, and thermochromic solar cell windows, confirm the extraordinary versatility of these systems.
In this book, the fundamental principles of electrochromism and all the consolidated results related to the EC materials and devices are largely treated, with a focus on the results and technological advancements achieved at laboratories of Italian National Research Council (CNR) in the last few years. A huge attempt was made to provide a comprehensive framework of the solid-state EC materials and devices, contextualizing our earlier results with respect to the state of the art. Part I concerns the basic component materials of the device and the main critical material issues, covering both inorganic and organic classes of the EC materials, electrolytes, mixed ion and electron conductors, and electrodes. The second part (Part II) describes EC and multifunctional devices, including photoelectrochromic and photovoltachromic smart windows, EC electroluminescent displays, EC energy storage devices, and EC transparent batteries, highlighting their different device structure and operation mechanisms, thin-film material processing, and the more common characterization techniques for the study of device performance and material properties. Part III instead concerns device manufacturing and scale-up procedures, from laboratory to industrial scale, as well as the energy-saving, visual comfort, and environmental impact issues of the EC glazings. This last part provides a wide insight into new materials and device structures, proposing some pioneering ideas, original insights, and new concepts for the realization of next-generation EC devices with multiple functionalities for dynamic tintable windows and advanced optoelectronic devices, including thermochromic and EC batteries. Finally, the book closes with some interesting food for thought on how artificial intelligence and machine learning instruments can affect the research products and technological development in this field have an impact on society and human being, with the aim of providing a more realistic prediction of how these smart technologies and multifunctional devices could further evolve in the next future.
Rome, Italy Dr. Pierluigi Cossari
Acknowledgments
I wish to express my appreciation and esteem to all the colleagues, friends, and the technical staff of the Center for Advanced Materials and Devices of the CNR-Nanotec, who have worked with me over these past 10 years in the fascinating research field of materials and devices for electrochromism and optoelectronics. My special thanks are due to all the scientists and thinkers, both past and present, who have contributed with their original works to make this field extremely interesting, stimulating a critical reflection and careful discussion not only on the advancements and significant results achieved but also on the main scientific, technological, and social challenges. I would also like to thank Dr. Lifen Yang (Commissioning Editor, Wiley-VCH), who commissioned the book, and Dr. Gudrun Walter (Editorial Director, Wiley-VCH), and also express my gratitude to the production team at Wiley-VCH for their professionalism, constant support, and care.
1Introduction
1.1 Electrochromism: A Brief Note on the History and Recent Evolution
Electrochromism can be defined as the reversible color change of some specific classes of chemical compounds, both inorganic and organic, occurring upon their electrochemical reduction and oxidation. This phenomenon was reported primarily in 1815 by the prominent Swedish chemist Jons Jacob Berzelius, who observed a color change in heated WO3 when exposed to a hydrogen flux. As further outlined by other influential authors, in 1824, the German chemist Friedrich Wohler, famous for being the first to isolate the chemical elements of beryllium and yttrium in their pure metallic forms and for synthesizing the organic compound urea from inorganic reagents, also known as Wohler synthesis, was fascinated by the beauty of WO3 and its impressive color change upon reaction with sodium which resulted in a gold-like color, likely due to the formation of sodium tungsten bronze [1]. Several other acute observations and notes on the chromism of WO3 and its electrochemical behavior have been continuously reported over the years. In this context, the term electrochromism was coined in 1961 by J. R. Platt, who observed a color change in some conjugated organic dyes upon application of a strong electric field [2]. However, it was only in 1969, with the seminal paper of Satyen K. Deb on the electrochromic (EC) coloration of WO3 thin films, that electrochromism began to follow the first steps toward a more systematic investigation taking into account the technological aspects and the applications [3]. Since then, numerous leading scientists have reported significant works on the basic fundaments of electrochromism, contributing to provide a more comprehensive knowledge of the EC phenomena and the electrochemical processes and to the continuous progress of new materials and devices.
With regard to WO3 and other inorganic EC materials, the renowned book Handbook of Inorganic Electrochromic Material of Claes-Göran Granqvist (1st Edition – March 16, 1995, Elsevier) represents a milestone in the literature of inorganic materials covering electrochromism in metal oxides, material preparation, especially via thin film technologies, characterization methods, electro-optical properties, device design, and performance analysis [1]. Although 30 years have passed since the first publication, it still remains a needful and essential book for a comprehensive and scrupulous knowledge of the fundaments and basic principles of EC inorganic oxides.
Starting with the electrochromism of metal oxides, especially transition metal oxides, such as tungsten, molybdenum, iridium, titanium, manganese, vanadium, nickel, cobalt, niobium, research efforts have also extended to the exploration of the chromism of organic compounds. These include viologens, phenothiazines, and various dyes and pigments such as orthotolidine, anthraquinone, and Prussian blue (PB). Among these, viologen is one of the dyes that have been more extensively studied for electrochromism, despite becoming more popular as paraquat for its herbicidal properties and high toxicity to mammals, including humans (Figure 1.1). Upon exposure, paraquat causes severe inflammation and can potentially lead to severe lung damage with an irreversible pulmonary fibrosis, also known as paraquat lung, with a high rate of mortality ranging between 60% and 90%. Several murders, and especially numerous suicides, have been indeed committed by using this lethal poison, including the case of Isabella Blow (1958–2007), one of the most influential fashion celebrities of all time. Moreover, the use of paraquat has also been associated with the onset of Parkinson's disease in farm workers. Initially produced and marketed with the trade name of Gramoxone in 1962 by Imperial Chemical Industries (Berkshie, England), it was widely used as a herbicide particularly for weed and glass control. However, due to extreme toxicity, paraquat was withdrawn from the market of the European Union in 2007 and is now only used by licensed applicators in the United States. On the other hand, despite its elevated riskiness, it remains one of the most commonly used herbicides worldwide for its effectiveness and its wide availability at low cost. Viologens have also found applications in the EC technology, resulting in one of the most studied classes of EC organic materials. This is due to their ability to operate at extremely low driving voltages, determining high optical contrast, and fast switching dynamics. The electrochemical behavior of 1,1′-dimethyl-4,4′-bipyridinium, methyl viologen (MV), was first reported by Michaelis and Hill in the early 1930s, who observed the violet color of the reduced state [4]. However, it was in the 1980s that viologens were extensively studied for EC applications, exploiting their ability of giving rise to three differently colored oxidation states: dicationic, monocationic radical (violet), and neutral species (red/orange). The fact that the bipyridinium radical of MV is one of the most stable known organic radicals, it allowed the effective preparation of air-stable solids making it suitable for the fabrication of EC devices. However, the low write-erase efficiency, i.e. the percentage of coloration that can be converted back to its original state, of MV in aqueous-based EC devices, along with the high solubility of the dicationic and radical species leads to the poor device durability and operational failure. To address this issue, various strategies have been employed, such as incorporating long alkyl chains on the nitrogen substituents, in order to obtain solid ECs or favoring the interaction between the viologen and an immobilized polymeric surface or electrode. Interestingly, among the various organic materials, viologen systems are particularly promising for industrial applications and, to date, remain the primary organic EC material used commercially.
Figure 1.1 Chemical structure and pictograms of methyl viologen dichloride, more commonly known as the highly toxic and poisonous herbicide paraquat, which is still used in numerous countries for weed and grass control.
Source: USFWS Mountain-Prairie/Flickr/CC BY 2.0.
Several other organic species with EC properties, including pyrazolines, quinones, carbazoles, and phenylene diamines have been proposed in literature as potentially interesting for practical applications. Recently, new small molecules with interesting redox activity and optical properties have been developed, such as thiophene- and furan-based porphyrinoids, triphenylamine derivatives, and tetrathiafulvenes or dibenzofulvene derivatives. In addition to their high optical contrast in the visible range, dibenzofulvene derivatives also exhibit a near-infrared (NIR) electrochromism across a broad spectrum due to optically induced intervalence charge transfer transitions (IVCTs). This makes them particularly promising for further development of devices and smart window technologies.
About PB, it has been widely used by artists and painters from the early eighteenth century to the end of twentieth century for its impressive deep blue color tone and its incredible magnetic and seductive effect. As seen in Figure 1.2, it is the characteristic pigment of Picasso's Blue Period, or the color used by Van Gogh to create a moody, dramatic, midnight blue in many famous works, including “Starry Night,” “Starry Night on Rhone,” and “Terrace of a Café at Night,” PB was also widely adopted by the painters during Baroque and Rococo periods, and it remains today as an important pigment for paints, lacquers, printing inks, and other color uses. An example is the clock faces of the Elizabeth Tower, more commonly known as Big Ben, sits atop the Palace of Westminster (London, England), which was recently restored to its original 1859 color scheme of PB and gold in order to bring the Tower back to the original design and vision by the architects Charles Barry and Augustus Welby Pugin. The first example of electrochromism for PB thin films was reported in 1978 by Vernon D. Neff from Kent State University (Ohio, United States). He deposited a thin film of PB on platinum electrodes using ferric chloride and potassium ferricyanide, demonstrating the reversibility of coloration [5]. In 1982, Japanese chemists K. Itaya and K. Shibayama developed a more reproducible method for the deposition of PB thin films through the electrochemical reduction of ferric-ferricyanide solution [6].
Figure 1.2 Prussian blue pigment used (a) in Van Gogh's Starry Night on the Rhone (1888, Musée d'Orsay, Parigi) and (b) in The Old Guitarist of Pablo Picasso (1903, Art Institute of Chicago).
Source: Musée d'Orsay/Wikimedia Commons/Public Domain, Flickr/Public Domain.
After these pioneering works, extensive research has been carried out on the structure, chemico-physical and electrochemical properties, and electrochromism of PB and its analogs. Significant results have also been reported on energy-storage properties for applications in batteries and supercapacitors [7–9]. PB undergoes three color changes passing from colorless to blue and then to brown, depending on its redox state. The fully oxidized brown state is unstable, and for practical EC applications, only the reversible switching between colorless and blue can be used, which is associated with the insertion or extraction of balancing cations (preferentially K+ or NH4+). Due to its three-dimensional (3D) zeolite-type structure, PB presents highly reversible intercalation/deintercalation of certain cations, excellent redox activity, and fast charge/discharge processes. This makes PB and its derivatives effective alternatives for the manufacturing of innovative battery cathode/anode materials and supercapacitor electrode materials. Additionally, its ability to capture monovalent metallic cations has been used in the pharmaceutical field for the preparation of drugs that can sequester specific toxic heavy metals, particularly thallium (Tl+) and radioactive cesium (134Cs+,137Cs+).
In the more recent book Electrochromic Materials and Devices edited by Roger J. Mortimer, David R. Rosseinsky, and Paul M. S. Monk (June 2015, Wiley), all materials and devices for electrochromism are discussed in detail, providing a comprehensive bibliography until 2015 with invited contributions from leading experts of the field [10]. This edited book builds upon their earlier monographs on electrochromism, offering the most complete and accurate collection of key issues and basic concepts concerning materials, devices, and applications [11, 12]. Spanning from an in-depth description and discussions of materials, including metal oxides, PB, viologens, conjugated conducting polymers, transition metal coordination complexes and polymers, organic NIR materials, and metal hydrides, to polymer electrolytes, nanostructured EC materials, and associated systems (gyroid-structured electrodes for EC/supercapacitors, plasmonic EC nanocrystals, etc.), it also covers the aspects related to the applications of EC materials, such as smart windows, fabric EC displays, some exemplifying device case studies (e.g. EC foil, EC glazing in a UK office, and photoelectrochromic devices) alongside environmental impact considerations.
An undoubted and significant contribution to the advancement of the science of organic electronic materials in the EC field was made by John R. Reynolds Research Group at the Georgia Institute of Technology, especially through their focus on electrically conducting and electroactive conjugated polymers. Over the past 40 years, Reynolds and his colleagues have developed new EC polymers by manipulating their optoelectronic and redox features providing deep insights into the charge transport, the electronic structure of conjugated redox-active organic molecules, and the EC mechanism and the electrochemical behavior of organic compounds. A complete treatise on the general concepts and structure–property relationships governing color in conjugated polymers and EC switching control was reported by Anna M. Österholm, D. Eric Shen, and John R. Reynolds in Chapter 6 “Electrochromism in conjugated polymers – strategies for complete and straightforward color control” of the renowned book (last edition) Handbook of Conducting Polymers (edited by Reynolds Terje A. Skotheim and Barry C. Thompson) [13]. This chapter provides an in-depth description of soluble, high molecular weight EC polymers focusing on organic systems that switch between colored and clear states. It also addresses experimental parameters (e.g. electrolyte concentration, cell geometry, film thickness, and substrate resistance) that affect device performance, along with a quantitative comparison of material properties. With specific reference to semi-conjugated polymers, also known as conducting plastics, it is worth highlighting that in the last two decades these materials have found relevant industrial exploitation due to the huge advantage of being manufactured as thin solid films through low-cost solution processing techniques, such as inkjet printing. Beginning with the seminal papers (1974–1978) by A. J. Heeger, A. G MacDiarmid, and H. Shirakawa (Nobel Prize in Chemistry, 2000, for the discovery and development of electrically conductive polymers) on the discovery of electrically conducting organic polymers, these revolutionary materials have been applied in different sectors, especially in the electronic field for electronic devices, display technologies (electroluminescent, polymer light-emitting devices), or functional coatings. For example, doped polyaniline (PANI) was used as electromagnetic shielding of electronic circuits or a corrosion inhibitor, while polypyrrole is employed as an active thin layer in various sensing devices. Poly(dialkylfluorene) derivatives are used as emissive layers in full-color displays. Some of these conducting polymers, such as PANIs and poly(ethylenedioxythiophene) (PEDOT), are also suitable for EC and smart windows applications since they possess a whole range of fascinating colors depending on their redox and protonation forms. Additionally, they can be manufactured in large sheets on different substrates (e.g. glass, plastics) and offer unlimited visual angles. Not least important, the concurrent progress in the development of electrolytes and electrodes materials has played a key role in the continuous evolution of EC devices and smart window technologies, passing from those outdated systems using liquid electrolytes to solid-state ones or by employing highly performing, multifunctional, and durable electrodes.
In this book, the key aspects of materials and devices reported in the literature are largely treated, providing a critical discussion on the main challenges affecting materials, device performance, and durability, and how these problems can be overcome in order to fabricate more stable EC smart windows and multifunctional devices. The latter ones are particularly attractive for next-generation EC technologies, responding to the growing market demand of “invisible,” portable, and wearable electronic devices. Over the past decade, significant advancements have been made in the development of multifunctional EC devices, such as those realized by combining EC cell with electroluminescent (EL) organic materials for innovative see-through displays that function under various lighting conditions or by the full integration of EC and photovoltaic cells into photoelectrochromic and photovoltachromic devices. Special emphasis is given to systems based on all-solid-state configuration, the rational design of nanostructured materials, and interface engineering, as well as new manufacturing strategies, including those based on simplified single-substrate structures and more “green” manufacturing procedures with low environmental impact and capable of reducing production costs and waste disposal.
In summarizing, along with a wide dissertation on the materials, the analysis of device performance, and discussion on manufacturing procedures, the book explores the energy challenges and environmental impacts. It offers numerous sparks and valuable insights on the emerging and next-generation smart glass technologies having as scenario the new era of Artificial Intelligence (AI) and even more technical society. The book is intended to be a useful source not only for master's and doctoral students but also for researchers in various scientific and technological sectors as well as innovators and designers in the field.
Part IMaterials
2Electrochromic Materials
Electrochromic (EC) materials are defined as chemical species that reversibly change color through an electrochemical reduction (gain of electrons) or oxidation (loss of electrons) reaction [14], resulting in a variation of their electronic absorption (UV–Vis) bands. Commonly, these materials switch between a transparent clear, bleached state to a colored one, or more in general, between two or more colored states. When EC materials have more than two redox states, they can display several colors and are defined as polyelectrochromic materials (Figure 2.1).
Electrochemical devices based on these materials are capable of modulating transmittance, reflectance, absorbance, or emittance. In a typical EC device configuration, cathodic and anodic EC materials are deposited onto two transparent and conductive substrates (electrodes), with the electrolyte interposed between these two EC electrodes (Figure 2.2). Alternatively, an ion storage film, acting as a mixed conductor, can be used in place of a complementary anodic EC material. Electrically switching the circuit enables redox reactions, which in turn cause color changes in the EC films.
Several chemical species and compounds belong to these materials, including transition metal oxides, conducting polymers, Prussian blue (PB) systems, viologens, small molecules, metallo phthalocyanines, and two-dimensional (2D) materials.
Hereafter, for a clearer subdivision, these classes of materials are organized into three main categories: inorganic, organic, and nanostructured 2D materials. In the following sections, the main properties of these materials are reviewed, providing insight into the chemico-physical and optoelectronic features, charge transfer (CT) properties, and the chromogenic behavior. A particular focus is placed on the main limiting factors affecting practical device operation and industrial application, such as electrochemical stability, durability, and scalability.
The final section surveys the critical material issues, pointing out their potential use for the effective development and manufacturing of EC devices and integrated multifunctional systems, including photovoltachromic and EC-organic light-emitting diode (OLED) devices.
Figure 2.1 Photograph of solutions, illuminated from underneath, containing (from left to right) ECP-magenta, -orange, -yellow, -green, -cyan, -blue, and -black, proceeding from top to bottom at increasing concentrations.
Source: Reproduced with permission from Dyer et al. [15]/American Chemical Society.
Figure 2.2 EC device structure characterized by a standard five-layer configuration, including two electrodes, the electrolyte, and the EC materials. The red arrows indicate the transport of positive ions from the electrolyte within the EC active material under the action of an electric field (ion intercalation).
2.1 Inorganic Electrochromic Materials
Inorganic EC materials are represented by some transition metal oxides and can be divided into two different types: cathodic and anodic as indicated in the Periodic Table of Figure 2.3. According to the EC behavior, cathodic materials change color under charge insertion, whereas anodic ones undergo a color transition upon charge extraction [1]. Among the several oxides of transition-state metals, whose unusual properties can be certainly associated with the unique arrangement of outer d-electrons, the most representative are based on W, Mo, V, Ti, Ir, Ni, and Nb. Oxides of W, Mo, Ti, and Nb are cathodic, while oxides of Ir and Ni are anodic. Vanadium as pentoxide (V5+) is of a hybrid nature, exhibiting both cathodic and anodic features, whereas vanadium dioxide (V4+) is anodic [16]. Thin films of these materials can be fabricated using various methods, such as chemical and physical vapor deposition (CVD and PVD), electrophoretic, chemical solution deposition, and electrochemical deposition. It is important to note that the choice of processing techniques should be carefully evaluated to simplify the deposition methods and device manufacturing, thereby reducing production costs and minimizing raw material waste.
Tungsten trioxide (WO3) and nickel oxide (NiO) remain the most extensively investigated and widely used cathodic and anodic EC materials, respectively. A very accurate and comprehensive treatise on their optical and electronic properties, along with their applications in devices, has been published over the years and can be found in numerous book chapters and reviews [3–5, 17–21].
In this regard, the Handbook of Inorganic Electrochromic Materials by Granqvist stands as a milestone in the literature on electrochromism of inorganic oxides [1]. The following sections will introduce the general classification and behavior for this class of materials, highlighting the most relevant and recent results. A specific section will also be dedicated to nanostructured inorganic EC materials due to their growing scientific interest and the large number of research works published in recent years.
Figure 2.3 Periodic table of elements, where the transition metals are highlighted in clear pink color. Red-and pink-shaded boxes refer to the transition metals whose oxides own anodic and cathodic electrochromism, respectively. Vanadium exhibits both cathodic and anodic behavior.
2.1.1 Transition Metal Oxide Cathodic Materials
As evident by literature review, the number of published research articles on WO3 films is very extensive and the most parts of EC devices use WO3 as the active cathode layer for device fabrication. The strucure of WO3 in its amorphous state is composed of clusters of a few WO6 octahedra, arranged in a corner-sharing perovskite-like manner, in which all the metal centers are of the same type, W6+. As thin film it appears in a highly transparent clear state, and upon electro-chemical reduction to W5+, it passes to a blue color due to the formation of W5+ sites [22].
The transition from W6+ to W5+ and the insertion and extraction of ions into WO3 films can be achieved through the following redox reaction (Eq. 2.1):
Although still controversial, the chromogenic mechanism behind this process is thought to involve electrons and protons, or alkali metal ions (H+, Li+, Na+, or K+), being injected into and extracted from the material upon coloration and bleaching, respectively. The optical absorption and the resulting intense blue coloration of the WO3 film are generally believed to be associated with CT and polaron absorption models. In the CT model, upon photon absorption, the electrons inserted are transferred between two neighboring sites, denoted i and j in Eq. 1, with the consequent appearance of the blue coloration. The polaronic model is based on the concept of small polaron formation due to pronounced electron–phonon interactions, which produces a broad band centered at λ = 0.85 μm extending into the luminous part of the spectrum so that WO3 films attain a blue color in transmission [19, 23–25].
A more in-depth analysis of the electronic properties and optical absorption can be found in pioneering studies [26–29] and recent reviews of Deb and Granqvist [16, 30]. Briefly, coloration takes place upon intercalation of Li+ ions into the WO3 structure and the simultaneous insertion of charge-compensating electrons from the external electric circuit, which shifts the position of the Fermi level into the conduction band in the electronic density of states of WO3. Conversely, the bleaching process is associated to the return of the Fermi level to the band gap via electron extraction.
In any case, amorphous WO3 is required for good EC behavior. On the contrary, crystalline WO3 owns a denser and ordered structure than the amorphous and more porous form, which generally has a negative effect on ionic conduction and switching times, limiting its use in practical devices. On the other hand, however, in case of crystalline nanostructures, e.g. nanoparticles, the EC response can significantly enhance due to their increased surface accessibility and shorter diffusion path lengths compared to bulk systems. Different structural states of WO3 film can thus affect the performance and stability of the devices, depending on their specific electronic, structural, and morphology configurations, which in turn reflect on the electronic band transitions and ion diffusion kinetics as well as charge transport and contact area at the electrolyte/EC electrode interface (EEI) [31]. In a 2006 study by Se-Hee Lee et al. [32], the authors highlighted the different charge insertion capabilities of WO3 nanoparticles, amorphous and crystalline forms prepared using electrophoretic deposition process, thermal evaporation, and evaporation, followed by annealing at 400 °C for two hours. By comparing the cyclic voltammograms (CVs) of these films, measured in 1 M·H2SO4, WO3 nanoparticles showed a higher charge-insertion density over the same time period, and faster kinetics (Figure 2.4a). Specifically, total cathodic charge for WO3 nanoparticles was about 32 mC cm–2 mg–1, whereas only about 3 mC cm–2 mg–1 for crystalline, and about 9 mC cm–2 mg–1 for amorphous films. It is worth noting that the majority of the works on WO3 appeared in the literature studied its behavior in liquid electrolytes and three-electrode cell, reporting results and performance on quasi ideal systems. Hereafter, on the contrary, the most recent and significant results on WO3 based solid-state devices will be mainly reviewed focusing on those potentially exploitable for practical applications, and more attractive for further developments into prototypal devices.
In this context, amorphous (α)-WO3 pillar-like nanostructured films was developed by our group at the Institute of Nanotechnology of CNR (CNR-NANOTEC) for fabricating all-solid-state EC devices with a simplified single substrate structure. These nanostructured WO3 films were deposited at room temperature (RT) by thermal evaporation on the surface of a solid polymer electrolyte layer (Nafion), which enabled the structuring of randomly distributed columnar WO3 formations at the EEI. The resulting interface layer significantly reduced the CT barrier and increased the interfacial area between the electrolyte and WO3, causing a notable improvement in overall performance [33]. This behavior is similar to that observed in mesoporous WO3, where the porous structure of WO3 film increased the contact area with the polymer electrolyte matrix, facilitating the formation of a well-connected hybrid structure [34, 35]. Cross-sectional SEM images of devices based on bulk α-WO3 and α-WO3 pillar-like nanostructured films (Figure 2.4b,c) clearly show the different morhology of WO3/electrolyte interfaces, highlighting how different substrates can affect the WO3 growth. In the case of WO3 deposited on ITO-coated glass substrate (150 nm thick), the resulting active layer revealed a very compact and dense structure with homogeneous and regular morphology (Figure 2.4b). On the contrary, WO3 grown directly on polymer (electrolyte) substrate showed a more irregular morphology with low compactness and pillar-like nanostructures (Figure 2.4c).
Figure 2.4 (a) CVs of nanoparticles, amorphous and crystalline WO3 films.
Source: Lee et al. [32]/with permission of John Wiley & Sons.
(b) Cross sectional SEM images of bulk α-WO3 film (300 nm) deposited on commercial ITO (150 nm thick), and (c) α-WO3 pillar-like nanostructured film deposited at RT on polymer electrolyte film of Nafion.
Source: Cossari et al. [33]/with permission of John Wiley & Sons.
The formation of these columnar-like nanostructures are consistent with those of other works where amorphous (α)-WO3 films were prepared via vacuum processes using not ordered or controlled growth conditions more specifically, these studies highlighted that the density, microstructure, and morphology of the deposited films can be affected by varying the parameters used during growth, such as the incidence, type of substrate, and the applied power [1].
Regarding device performance, our solid-state device showed a very good optical contrast (ΔT = 70% at 3 V) and fast kinetics (about 10 s), which were similar to those observed in solid-state devices using nanostructured WO3 [36–39]. EC film based on 2D WO3 nanosheets or WO3 nanorod array with a hexagonal core and amorphous shell enabled, indeed, optical modulations of about 65%, response times ranging approximately between 10 and 20 s, and switching voltages of ±3 V [36, 37].
With regard to solid-state crystalline WO3 several nanostructures, including nanowires, nanorods, nanosheets, or nanoparticles, have been reported over the last decade, showing, in some cases, improved performance, cyclic stability, and a finer control of EC response compared to pure crystalline WO3 and amorphous films. Among these, one-dimensional (1D) nanostructures resulted particularly interesting for fabricating thin-film EC electrodes, as they can facilitate the directional transport of electrons and ions through independent pathways with a more efficient charge transport through the electrode. They also possess a high surface-to-volume ratio and tailorable exposed surfaces, which increase the contact area with the electrolyte layer and the density of states for redox reactions. Another advantage lies in their reduced rigidity at high aspect ratios and/or at low thickness, which allow them to better sustain the intense strain and large volume changes (structural) induced by ion intercalation during coloration. Crystalline 1D WO3 in the form of nanowires or nanorods were easily synthesized using solvothermal or hydrothermal methods, and the corresponding array films were prepared by various techniques, including solution casting (drop casting and spin coating), electrophoretic deposition (EPD), thermal evaporation, or chemical vapor deposition (CVD) [40, 41]. One of the main drawbacks is represented by the low adhesion and the poor contact between the WO3 nanowires and the transparent conductive substrate, which can negatively affect the EC response and cyclic stability. To overcome this limitation, Zhang et al. fabricated well-aligned (hexagonal) WO3 nanowire array films that were in good contact with both the substrate and the electrolyte by growing the nanowires vertically onto the substrate [41]. This was achieved by using a template-free hydrothermal method by adding ammonium sulfate as a capping agent (Figure 2.5a). When compared to micro-brick structured (orthorhombic) WO3 films synthesized without the addition of ammonium sulfate (Figure 2.5b), the highly porous nanowire films showed a significant increase in optical modulation, switching times, and coloration efficiency. Li et al. fabricated instead hexagonal WO3 nanowires directly onto a bare fluorine-doped tin oxide (FTO) glass using the low-cost solvothermal method without the need for a WO3 seed layer (Figure 2.5c), and reported higher transmittance values for the bleached state (about 90%), superior optical modulation (ΔT = 60 %), coloration efficiency (63 cm2 C–1), and CV stability compared to WO3 films synthesized with a seed layer [42].
Figure 2.5 (a) Cross-sectional SEM image of highly porous WO3 films prepared with ammonium sulfate, and (b) a plan view of WO3 films prepared without ammonium sulfate.
Source: Zhang et al. [41]/with permission of Royal Society of Chemistry.
(c) Schematic representation, SEM image, and transmittance spectra of bleaching and coloring states of WO3 nanowire films deposited on FTO glass via solvothermal process.
Source: Li et al. [42]/with permission of Royal Society of Chemistry.
(d) XRD patterns and crystal structure (inset) of m-WO3−x NWs. (e) Surface and cross-sectional (inset) SEM images of the active films. (f) Optical transmittance spectra of a dual-band EC device using Al(ClO4)3/PC as a liquid electrolyte.
Source: (d, f) Zhang et al. [43]/with permission of Royal Society of Chemistry, (e) Zhang et al. [43]/with permission of Royal Society of Chemistry.
(g) SEM cross section and HRTEM image (inset) of the EC film based on aligned hexagonal/amorphous WO3 core/shell nanorod arrays (h@aWNRAs).
Source: Huo et al. [37]/with permission of Royal Society of Chemistry.
More recent advances in synthetic techniques, especially in colloidal wet-chemistry routes, have led to the advanced engineering of WO3 nanostructures in the form of solution freestanding inorganic nanocrystals (NCs), enabling controlled crystal structure and tunable geometric parameters [44]. Particularly attractive is the ability to engineer dual-band EC devices using films of WO3 NCs that independently control the transmittance of near-infrared (NIR) and visible (Vis) light. In this regard, Zhang et al. developed monoclinic WO3 nanowires (m-WO3–x NWs) as clearly shown in XRD patterns crystal structure in the inset along with SEM images of the active films at the surface cross section in the inset (Figure 2.5d,e). An effective and independent control of NIR and Vis light transmittance was achieved by intercalation/deintercalation of Al3+ cations (Figure 2.5f) within the active film, circumventing the conventional use of monovalent ions (e.g. H+, Li+, and Na+) [43]. The improved switching time and cyclic stability were ascribed to the efficient diffusion of Al3+ in m-WO3−x, which was similar to that of Li+, and to a smaller extent Al3+ intercalation/deintercalation.
Interestingly, by combining the properties of amorphous and crystalline WO3 through the synthesis of bifunctional aligned hexagonal/amorphous WO3 core/shell nanorod arrays (h@aWNRAs), where the core is constituted of hexagonal WO3 nanorods and the shell consists of amorphous WO3–x (Figure 2.5g), Huo et al. obtained a marked improvement of EC and pseudocapacitive performance compared to pure WO3 nanorod arrays [37]. The EC response of the optimized h@aWNRAs was indeed characterized by a high optical contrast (about 70%), fast switching times, and a coloration efficiency of 101 cm2 C–1, whereas pseudocapacitive properties are defined by a high specific capacitance (885.8 F g−1 at 1 A g−1), an enhanced rate capability, good cycling efficiency (91.8%), and good specific capacitance retention (57.8% after 2000 cycles).
In line with these results, a few years ago, Giannuzzi et al. provided an in-depth and accurate discussion on the electrochemical process and charge-storage mechanism, governing the EC behavior of specific WO3 active films based on 1D shape controlled m-WO3–x nanostructures having different space-filled solid structures, namely, colloidal solid nanorods (SNRs) and carved nanorods (CNRs) [45]. It was pointed out that the shape affects both Li+ accessibility to the WO3 lattice and its adsorption on the surface. CNR-based WO3 electrode showed, indeed, a more marked pseudocapacitive effect than SNRs, whereas on the contrary, the latter one stored a superior total amount of charge than CNRs. The different features of these two shape-controlled m-WO3−x nanostructured films were ascribed to the bulk- and surface-structure effects rather than variations in the surface-to-volume ratio.
Among the various promising nanostructures used in electrochromism, WO3 quantum dots (QDs) have emerged as an effective alternative [46, 47]. Cong et al. reported the EC properties of a nonstoichiometric WO3 (WO3−x) QDs with an average crystalline size of 1.6 nm [46], which were synthesized via a simple colloid process using tungsten aryloxide as the precursor. The active film was finally stabilized with a surface layer of octylamine ligand (Figure 2.6a). Significantly, the exchange of the insulating aliphatic amine with short-chained pyridine imparted hydrophilicity and conductivity to the active electrode, allowing for a better aqueous electrolyte infiltration. The resulting devices have a high optical contrast (85% at 633 nm), very fast switching speed (coloration/bleaching time within 1 s), and high coloration efficiency (154 cm2 C–1). This impressive response was attributed to the size quantization of the QDs and the use of pyridine, which shortened the ion diffusion path length and facilitate the mass/CT at the EEI leading to enhanced mass/energy utility, especially at high current density. Low-resolution transmission electron microscopy (TEM) images revealed monodispersed crystals with nearly spherical shapes and an average diameter of 1.6 nm (Figure 2.6b), whereas high-resolution TEM images showed clear fringes in each individual nanocrystal, highlighting the single-crystalline nature of the QDs (Figure 2.6c). However, one of the main constraints of these systems is the very low cyclic stability (< 50 cycles) due to the water solubility of pyridine and the corrosive nature of aqueous electrolyte. In a recent work, this stability issue was addressed by combining WO3 QDs with an Al3+ electrolyte [47]. In this case, the QDs were prepared without organic wrapping, using a simple decomposition process of tungsten acid in ethylene glycol (EG) at 180 °C followed by dialysis to remove the EG, resulting in a WO3 QD isopropanol/water solution suitable for air-brush spraying coating deposition. Therefore, in addition to the very good EC response of WO3 QD films (in 0.2 M Al(ClO4)3/PC electrolyte) with an optical contrast of 94.1% at 633 nm and fast switching times (13.6 and 10 s for coloring and bleaching, respectively), they own an exceptional stability characterized by an ultralong cycle life of 10 000 cycles with only 10% optical contrast loss and 20 000 cycles without degradation at 633 nm. This significant improvement performance was attributed to the ultrasmall size of the WO3 QDs in all three dimensions and the absence of an organic overlayer, which greatly shortened the diffusion paths of intercalation ions and lowered the interface barrier, enhancing charge transport, electron-transfer kinetics, and high reaction rates. On the other hand, the improved stability (cyclability) was mainly associated to the decrease in the extent of lattice expansions for the smaller ionic radius of Al3+ and the ability to support multielectron redox reactions.
Figure 2.6 (a) Scheme of ion diffusion and charge transfer in QD materials in comparison with blocking in bulk materials (left), and the replacement of pristine aliphatic amine ligand formed during QDs growth with conductive pyridine (right). (b) Low-magnification and (c) high-resolution TEM images of as-prepared WO3 QDs.
Source: Cong et al. [46]/with permission of John Wiley & Sons.
Doping WO3 with other TMO materials, such as titanium dioxide (TiO2) or molybdenum trioxide (MoO3), and especially with other metals, is one of the most effective methods to enhance the WO3 performance [48]. In general, mixed TMOs present changes in the morphology due to structural distortion, which in turn facilitates ion migration through the lattice by shortening the ion diffusion length. For example, Zhou et al. studied the influence of Ni doping on the nanostructuring of WO3 films, which were deposited directly onto ITO-coated glass substrates using a seed-free hydrothermal method [49], and showed how different Ni concentration increased crystal defects (decreasing of lattice spacing) of WO3 films with significant changes in their morphology. A Ni content of 1.5% in WO3 induced the formation of vertically aligned nanorods, randomly and densely arrayed, which an improvement of the EC response if compared to undoped WO3, 0.5% Ni-WO3, and 2.5% Ni-WO3. As a result, EC devices based on these mixed oxides exhibited an impressive optical modulation (over 85% from 600 to 900 nm), good coloration efficiency (60.5 cm2 C–1 at 600 nm), and a cycling stability of about 5500 cycles. In a similar way, WO3 doped with other metals, such as niobium (Nb) [50], Mo [51], iron (Fe) [52], Ti [53], and phosphorous (P) [54], largely affects the EC performance of WO3. These induce structural changes with lattice distortions caused by difference in ionic radius variations, structural mobility and vacancy formation. Notably, doping of highly mesoporous WO3 with amorphous niobium oxide (NbOx) gives rise to a mesostructured dual-band EC composite. WOx–NbOx nanocomposite films prepared via polymer templating exhibit superior modulation of Vis and NIR transmittance with fast switching speeds and good durability (at least 2000 switching cycles) [55].
Another interesting cathodic material to be applied for electrochromism is molybdenum oxide (MoO3), which has potential advantages in terms of price and weight than the more widely investigated WO3. However, with respect to its “elder” brother, it suffers from a lower coloration efficiency and performance, making it less attractive for practical applications. Both amorphous and crystalline MoO3 films present electrochromism and like other TMO materials they can be prepared by several techniques, including CVD, electron beam evaporation, sol–gel, sputtering, or spray pyrolysis [56–58]. In its crystalline form, MO3 can exists in three phases: orthorhombic α-MoO3, monoclinic β-MoO3, and hexagonal h-MoO3. The more stable is the orthorhombic one and it is characterized by zigzag chains and a unique 2D layered structure, consisting of distorted MoO6 octahedra with shared edges and corners. The presence of open channels in this structure facilitates both the ion intercalation and deintercalation within the EC film, eventually enhancing the EC response. Dhanasankar et al. prepared MoO3 films via sol–gel dip coating, investigating the influence of various annealing temperatures (ranging from 200 to 350 °C), on the structure and surface morphology [57], as revealed by SEM and XRD analysis of Figure 2.7a–e.
With regard to the optical properties and the EC behavior, films annealed at 250 °C exhibited the best response with a maximum anodic diffusion coefficient of 9.61 × 10−11 cm2 s–1 and optical contrast of about 60% (ΔT = 58.4%). In contrast, MoO3 films annealed at higher temperatures (300°C and 350°C) showed lower diffusion coefficients and decreased performance due to the formation of compact crystalline (α-orthorhombic) nanorod-like structures. These findings were corroborated by previous studies such as that of Patil et al., who observed that crystalline MoO3 films with rod-like nanostructures, prepared by electrodeposition with cathodic reduction of molybdate from aqueous alkaline bath and thermal annealing at 450 °C, showed an optical contrast of 46% and an efficiency of 34 cm2 C–1 at 630 nm [58]. In another work, Lei et al. studied hexagonal MoO3 (h-MoO3) nanobelts obtained by a simple hydrothermal route
