Resonant‐Cavity‐Enhanced Electrochromic Materials and Devices

Abstract

With rapid advances in optoelectronics, electrochromic materials and devices have received tremendous attentions from both industry and academia for their strong potentials in wearable and portable electronics, displays/billboards, adaptive camouflage, tunable optics, and intelligent devices, etc. However, conventional electrochromic materials and devices typically present some serious limitations such as undesirable dull colors, and long switching time, hindering their deeper development. Optical resonators have been proven to be the most powerful platform for providing strong optical confinement and controllable light–matter interactions. They generate locally enhanced electromagnetic near‐fields that can convert small refractive index changes in electrochromic materials into high‐contrast color variations, enabling multicolor or even panchromatic tuning of electrochromic materials. Here, resonant‐cavity‐enhanced electrochromic materials and devices, an advanced and emerging trend in electrochromics, are reviewed. In this review, we will focus on the progress in multicolor electrochromic materials and devices based on different types of optical resonators and their advanced and emerging applications, including multichromatic displays, adaptive visible camouflage, visualized energy storage, and applications of multispectral tunability. Among these topics, principles of optical resonators, related materials/devices and multicolor electrochromic properties are comprehensively discussed and summarized. Finally, the challenges and prospects for resonant‐cavity‐enhanced electrochromic materials and devices are presented. We believe this review will definitely promote the communications among optical physics, electrochemistry, and material science.
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Resonant-Cavity-Enhanced Electrochromic Materials and
Devices
Jian Chen, Ge Song, Shan Cong, and Zhigang Zhao*
With rapid advances in optoelectronics, electrochromic materials and
devices have received tremendous attentions from both industry and
academia for their strong potentials in wearable and portable electronics,
displays/billboards, adaptive camouflage, tunable optics, and intelligent
devices, etc. However, conventional electrochromic materials and devices
typically present some serious limitations such as undesirable dull colors, and
long switching time, hindering their deeper development. Optical resonators
have been proven to be the most powerful platform for providing strong
optical confinement and controllable lightmatter interactions. They generate
locally enhanced electromagnetic near-fields that can convert small refractive
index changes in electrochromic materials into high-contrast color variations,
enabling multicolor or even panchromatic tuning of electrochromic materials.
Here, resonant-cavity-enhanced electrochromic materials and devices, an
advanced and emerging trend in electrochromics, are reviewed. In this review,
w e will focus on the progress in multicolor electrochromic materials and
devices based on different types of optical resonators and their advanced and
emerging applications, including multichromatic displays, adaptive visible
camouflage, visualized energy storage, and applications of multispectral
tunability. Among these topics, principles of optical resonators, related
materials/devices and multicolor electrochromic properties are compre-
hensively discussed and summarized. Finally, the challenges and prospects
for resonant-cavity-enhanced electrochromic materials and devices
are presented.
1. Introduction
Electrochromism is a well-studied phenomenon in which the
optical properties (transmittance, reflectance, or absorption) of
certain materials change reversibly under an electric field.[]
J. Chen, S. Cong, Z. Zhao
School of Nano-Tech and Nano-Bionics
University of Science and Technology of China
Hefei 230026, China
E-mail: zgzhao2011@sinano.ac.cn
J. Chen, G. Song, S. Cong, Z. Zhao
Key Lab of Nanodevices and Applications
Suzhou Institute of Nano-Tech and Nano-Bionics
Chinese Academy of Sciences
Suzhou 215123, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202300179
DOI: 10.1002/adma.202300179
Because of their unique properties, es-
pecially their optical switching behavior
and ultralow power consumption, elec-
trochromic materials and devices have been
developed for various potential applications
such as displays,[, ] smart windows,[–]
wearable and portable electronics,[– ] and
electrically reconfigurable optics,[] since
the pioneering work of Deb in .[]
Electrochromic materials, which serve as
thekeycomponentsinelectrochromicde-
vices, can generally be categorized into two
types according to their material compo-
sition, that is, inorganic and organic ma-
terials. Inorganic electrochromic materials
are at the forefront of real-world appli-
cations and widespread commercialization
because of their high optical contrast, high
cycling lifetime, good thermal and chemi-
cal stability, and good durability.[,] How-
ever, compared with their organic coun-
terparts, inorganic electrochromic mate-
rials exhibit extremely undesirable drab
colors,[– ] which greatly hinders their
development and expansion toward ad-
vanced applications in high-quality dis-
plays, sophisticated optical devices, etc.
A common example is tungsten triox-
ide (WO), with the appearance in one-
fold blue color with altered lightness un-
der dierent applied potentials.[, ] Most
organic electrochromic materials are organic small molecules or
conducting polymers; they have the unique advantages of rich
color tunability, fast response, and easy processing.[] To ob -
tain organic electrochromic systems with an extremely broad
color gamut over the visible spectral region, multicolored elec-
trochromic devices of red, green, blue (RGB), and black have
been constructed by using dierent leuco dyes or various func-
tional end-group modifications.[, ] Although these RGB-to-
black multicolored electrochromic devices exhibit potential for
full-color tunability, the use of hybrid systems of various organic
electrochromic materials in these devices greatly increases the
complexity of device fabrication and integration and the cost of
commercial devices. As a result, single organic electrochromic
materials are still plagued with problems of poor full-color tun-
ability. In addition, some organic electrochromic materials re-
quire relatively high electrochemical potentials to produce sharp
color contrast, which degrade these materials and result in poor
cycling stability and a short lifetime during color switching.[– ]
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By contrast, both inorganic and organic electrochromic de-
vices tend to exhibit a major weakness in practical use, namely,
long switching times. The switching time can typically be ex-
pressed as 𝜏∝L/D,whereDis the ionic diusivity, and Lis the
layer thickness.[, ] To generate suciently high color contrast,
relatively thick (hundreds of nanometers) electrochromic layers
are needed.[] These thick electrochromic layers result in long
switching times (on the order of seconds or even longer), which
limits their usage to some advanced applications such as video-
speed displays, and ultrafast reconfigurable optics.
Optical resonators, which can produce a strong optical field
that enhances light–matter interactions,[] represent a building
block of modern photonics and have resulted in the realization of
numerous transformative optical devices with desirable function-
alities such as lasers,[] color filters,[–] optical sensors,[ ,] ul-
trasensitive spectroscopes,[] nonlinear optical isolators,[] and
light buering and storage devices.[] Many types of optical res-
onators have been introduced into electrochromic materials and
devices in recent years, including plasmonic resonators,[, ] Mie
resonators,[, ] Fabry–Perot (FP) cavities,[, ] photonic crystal
(PhC) cavities,[, ] andhybridcavities.
[, ] The introduction of
these resonators is a promising strategy to address the challenges
of poor full-color tunability, poor cycling stability, long response
times, and limited applications. The improvement can be under-
stood in terms of the following four points. ) Electrochromic
materials with beautiful and distinct colors over the entire visi-
ble spectral range can be obtained by configuring the appropri-
ate optical resonators. These optically resonant configurations
provide enhanced electromagnetic near fields at distinct reso-
nant wavelengths, which magnify the eect of the refractive in-
dex change and dielectric variation of electrochromic materials
under dierent voltages. The reason is that the resonance condi-
tion depends sensitively on the interaction between the confined
electromagnetic near field and local dielectric environment.[]
) When an optical resonator is used, even a very thin elec-
trochromic layer has useful color properties comparable to those
of the electrochromic material alone, and the response time is
greatly reduced. ) Remarkably, an electrochromic device com-
bined with an optical resonator has an excellent cycling lifetime
because high color contrast can be obtained even under low ap-
plied voltages, and ultrafast switching performance requires only
very brief voltage pulses.[] ) Because of the enhanced light–
matter interactions, high quality factor (Q-factor), and ultrasmall
volume, optical resonators also oer a promising platform for
the development of applications such as multichromatic displays,
adaptive visible camouflage, visualized energy storage, and appli-
cations of multispectral tunability.
In recent years, optical resonators have been applied to en-
hance multicolored electrochromic performance in a signifi-
cantly increasing number of studies. However, most reviews
have emphasized the key role of advanced plasmonic and dielec-
tric materials, as well as basic chromogenic structures in static
and dynamic structural color generation.[,– ] Although elec-
trochromic materials are mentioned, there is a no comprehensive
review of the use of optical resonator enhancement in multicol-
ored electrochromic materials and devices. In addition, the basic
physical theory and mechanism of various optical resonators for
electrochromic applications are seldom covered. In this review,
we focus on recent advances in multicolored electrochromic ma-
Figure 1. Resonant-cavity-enhanced electrochromic materials and devices
based on different types of optical resonators, including plasmonic res-
onators, Mie resonators, FP cavities, PhC cavities, and hybrid cavities, as
well as their extended applications.
terials and devices based on dierent types of optical resonators
such as plasmonic resonators, Mie resonators, FP cavities, PhC
cavities, and hybrid cavities (Figure 1). Next, recent trends in their
integration into novel applications toward multichromatic dis-
plays, adaptive visible camouflage, visualized energy storage, and
applications of multispectral tunability are introduced (Figure ).
In addition, we briefly outline basic concepts of conventional elec-
trochromic materials and devices, as well as the principles of
optical resonators with dierent cavity configurations for struc-
tural color generation and electrochromic modulation. When ap-
propriate, other multicolored electrochromic technologies based
on these optical resonators without electrochromic materials are
also covered, for example, liquid crystals (LCs), electrochemical
deposition, and the migration of silver atoms. In conclusion, the
outlook and critical challenges for multicolored electrochromism
are discussed.
2. Conventional Electrochromic Materials and
Devices
Electrochromism has been at the forefront of smart materials re-
search since , and its fundamental concepts have been ex-
tensively investigated and reviewed.[,,– ] In this section, we
briefly introduce typical electrochromic materials, device archi-
tectures, and key performance indices.
2.1. Typical Electrochromic Materials
Most typical electrochromic materials can be classified as inor-
ganic or organic. Inorganic electrochromic materials mainly in-
clude WO,[,, ] nickel oxide (NiO),[, ] manganese dioxide
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Figure 2. Conventional structures of electrochromic devices. a) Single-electrode structure. b) Layered structure. c) All-in-one structure. d) Lateral struc-
ture.
(MnO),[, ] vanadium oxides (e.g., VO,V
O,andV
O),[– ]
titanium dioxide (TiO),[,] iron hexacyanoferrate (Prussian
blue),[] and D MXenes (e.g., TiCTx,Ti
CTx,Ti
CNTx,
Nb.C, Ti.Nb. CTx,andTi
AlC).[,– ] They have excellent
optical contrast and photostability but exhibit low electrical con-
ductivity, poor switching speed, and limited color tunability.
There are two main types of organic electrochromic materials:
organic small molecules (e.g., viologen and its derivatives),[, ]
and conducting polymers [e.g., polypyrrole, polythiophene, and
polyaniline (PANI)].[– ] Organic small molecules are charac-
terized by their rich color palette, high color purity, and good
color tunability, but they exhibit undesired thermal diusion
and poor cycling stability in devices. Organic conducting poly-
mers that are easily fabricated by solution processing are at-
tractive due to their electrical conductivity. However, substan-
tial shortcomings, such as poor color saturation, still limit
their application in colorful displays. In addition, some emerg-
ing organic electrochromic materials such as metal–organic
frameworks,[– ] covalent organic frameworks (COFs),[–] and
hydrogen-based organic frameworks[] are developed to im-
prove electrochromic performance due to their high porosity,
tunable structure, and ease of modification. Their large spe-
cific surface area facilitates adequate contact with the electrolyte
for stable and rapid switching.[,, ] For example, Yu et al.
fabricated a donating–accepting type-conjugated D COF film
with N,N,Nʺ,Nʺ-tetrakis(p-aminophenyl)-p-benzenediamine as
donor, and ,,-benzothiadiazole-,-dicarboxaldehyde (BTDD)
as acceptor.[] The as-fabricated electrochromic COF films exhib-
ited fast response speeds (. s for coloring and . s for bleach-
ing), but their color variations were still similar to those of con-
ventional NiO materials from opaque black to transparent. Over-
all, several pressing problems with conventional electrochromic
materials that must be addressed, such as poor color tunability,
low response speed, poor cycling stability, and limited applica-
tions.
2.2. Electrochromic Device Architectures
Electrochromic devices for use as basic working units for practi-
cal applications of electrochromic materials have been designed
and fabricated since the pioneering work of Deb in .[]
While many current studies and applications are based on single-
electrode electrochromic structures which are tested in an elec-
trochemical cell set with a three-electrode system (Figure 2a),
the fabrication of whole devices is still essential to real-world
applications. A conventional electrochromic device is typically
manufactured as a five-layered structure with an electrochromic
layer, an ion storage layer, an electrolyte layer, and two conduc-
tive electrodes (Figure b–d). Among these functional layers,
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the electrochromic layer is the key component of a device; its
candidate materials are discussed in detail in Section .. The
ion storage layer is composed of active materials for charge bal-
ance to match the reduction (oxidation) of the electrochromic
layer, which is typically made of NiO, iridium dioxide, or cerium
dioxide.[– ] The electrolyte layer is used for ion transport, in-
cluding that by solution, gel, and solid electrolytes.[,– ] The
two conductive electrodes provide electron and charge transfer;
in conventional electrochromic devices, they are most commonly
made of indium tin oxide (ITO) or fluorine-doped tin dioxide.
Depending on their mode of operation, electrochromic devices
can be classified into transmission-type devices, which control
the spectral variability of the transmitted light, and reflection-
type devices, which regulate the reflection spectrum. Addition-
ally, two device architectures, that is, layered and lateral struc-
tures, are designed for actual applications.[ ] In a layered de-
sign, the electrochromic layer and a conducting electrode can be
regarded as a working electrode, and the ion storage layer and
other conducting electrode can be considered as a counter elec-
trode; the electrolyte is sandwiched between the working elec-
trode and counter electrode (Figure b). This design is suitable
for conventional electrochromic devices for some applications
(e.g., smart windows); it enables the maximum active color area,
excellent ionic diusion, and a fast response. However, because
the design is layered, the colors generated by the electrochromic
layer will suer from the color eects of the ion storage layer
in reflective display devices, resulting in low brightness and
poor chromaticity. It is remarkable that when the electrochromic
material in the layered structure is mixed with the electrolyte,
the resulting electrochromic mixture (one layer containing dis-
solved electrochromic materials and electrolytes) is sandwiched
between the two conducting electrodes in a facile configura-
tion (Figure c).[, ] This all-in-one design, while limited to
electrochromic materials that are soluble in electrolytes (e.g.,
viologens),[, ] provides a high level of coloration and simpli-
fies the manufacturing process, making them much attractive
from an industrial point of view. In a lateral design, the work-
ing electrode and counter electrode are arranged in parallel, and
the electrolyte is coated on top of both electrodes (Figure d). Al-
though lateral designs have longer response times than layered
designs owing to the extended ionic diusion path, a wider range
of materials, shapes, and placement positions can be used for the
ion storage layer, ensuring the diversity of the display colors and
integration of the device.[, ] However, these designs still re-
quire that the observer looks through the electrolyte layer, which
degrades the color quality and reflectance in real devices. There-
fore, new device architectures, for example, a reversed design in
which the electrolyte and ion storage layer can be placed behind
the electrochromic layer, are urgently needed.[]
2.3. Key Performance Indices
In recent decades, a number of commonly used key performance
indices, covering optical modulation and the contrast ratio (CR),
response time, coloration eciency (CE), and cycling stability,
have been developed to quickly and accurately evaluate the perfor-
mance of electrochromic materials and devices for applications of
interest. Here, we give a brief overview of these key performance
indices.
2.3.1. Optical Modulation and Contrast Ratio
Optical modulation (also known as optical contrast) is the most
basic performance index. It is the dierence in the optical trans-
mittance (∆T) or reflectance (∆R) of electrochromic materials
and devices at a specific wavelength before and after color switch-
ing. It is usually defined as
ΔR=Rbleached −Rcolored or ΔR=Rbleached −Rcolored ()
where Tbleached and Rbleached are the transmittance and reflectance
in the bleached state, respectively, and Tcolored and Rcolored are the
transmittance and reflectance in the colored state, respectively.
In addition, the CR indicates the degree of color variation of
electrochromic materials and devices between bleaching and col-
oration, which is defined as follows
CR =Tbleached
Tcolored
or CR =Rbleached
Rcolored
()
Both parameters are widely used to quantify the color-
switching ability or color contrast of an electrochromic material
or device under a given applied voltage. High optical modulation
and CR values are generally expected to indicate better perfor-
mance.
2.3.2. Response Time
Response time is the most important performance index for eval-
uating the response capability of electrochromic materials and
devices. It is defined as the time required for the entire opti-
cal modulation at a specific wavelength to change by % dur-
ing coloration or bleaching. It can be divided into the coloration
and the bleaching times. Note that the wavelength in the visi-
ble spectral range where the optical modulation is maximal is
usually chosen to be the specific wavelength at which the opti-
cal modulation and response time are calculated. The response
time of a device is significantly aected by a number of fac-
tors, such as the ionic conductivity of the electrolyte, insertion
ion species, thickness and morphology of the electrochromic
layer, diusion rate of ions in the electrochromic layer, magni-
tude of the applied voltage, substrate sheet resistance, and device
size. For example, Shao et al. developed an all-solid-state elec-
trochromic device based on proton transmission with ultrafast re-
sponses (coloration to % in . s and bleaching to % in . s)
by employing a poly(,-ethylenedioxythiophene):polystyrene
sulfonate (PEDOT:PSS) film as the H+source.[] In addition,
improving the morphology of the materials has been proposed
as another ecient strategy to enhance the dynamics of the
electrochromic layer response and reduce the response time
by obtaining a nanostructured material with numerous active
sites and shorter ion diusion distances. Following this strat-
egy, Kim et al. prepared an amorphous mesoporous (diameters of
– nm) WOfilm for electrochromic devices by evaporation-
induced self-assembly, achieving ultrafast responses (. s for col-
oration and . s for bleaching).[] The response times of these
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electrochromic devices (within seconds) are sucient for smart
windows or antiglare rearview mirrors; however, they do not meet
the video refresh frequency requirements of electrochromic dis-
plays. Because the human eye perceives images at a refresh rate
of ∼ Hz as a motion picture, and flickering disappears entirely
at ≈ Hz, a response time of no more than  ms is necessary for
electrochromic devices. However, it is generally dicult to obtain
using conventional electrochromic materials and devices. Optical
resonators can be exploited as a unique approach to significantly
enhance the response speed.[,, ]
2.3.3. Coloration Efficiency
The CE is the change in optical density (ΔOD) at a characteristic
wavelength for a given injected charge per unit area, which is
given by the following equation
CE =ΔOD
Q=log (CR)
Q()
where Qrepresents the injected charge density (C cm−). It is a
practical index proposed by researchers to evaluate the energy ef-
ficiency of electrochromic materials and devices. Electrochromic
materials and devices with higher CE values are generally capa-
ble of higher optical modulation while consuming fewer charges
during color switching. These electrochromic materials and de-
vices are clearly more desirable for some applications, such as
smart windows and displays, because of their higher energy e-
ciency.
2.3.4. Cycling Stability
Cycling stability is a critical index for evaluating the lifetime of
electrochromic materials and devices; it characterizes whether
their optical modulation remains stable over a certain number
of coloring/bleaching cycles. To meet the requirements of real-
world applications, the cycling stability should be at least –
cycles with little performance deterioration; this requirement is
the major bottleneck for the further development and widespread
commercialization of electrochromic materials and devices. The
cycling stability of an electrochromic device can be aected by var-
ious factors, including the stability of the electrochromic layer,
counter electrode material, electrolyte properties, magnitude of
the applied voltage, and device dimensions. For example, rel-
atively thick (hundreds of nanometers) electrochromic layers
are necessary to achieve suciently high color contrast in elec-
trochromic devices. However, these thick electrochromic films
adhere poorly to the conductive substrate, and the active ma-
terials in these films tend to peel o during cycling, signifi-
cantly degrading the cycling stability.[,] In addition, exces-
sive switching voltages in electrochromic devices degrade elec-
trochromic materials because of structural changes or chemical
side reactions, resulting in poor cycling stability and a short life-
time during color switching. Designs using optical resonators are
expected to have better cycling stability. An electrochromic de-
vice combined with an optical resonator can achieve high color
contrast even with ultrathin electrochromic layers or at low ap-
plied voltages and has an excellent cycling lifetime because of its
ultrafast switching performance, which requires only very brief
voltage pulses. In Section ., we discuss such devices and give
examples.
3. Resonant-Cavity-Enhanced Electrochromic
Materials and Devices
Conventional electrochromic materials and devices suer from
numerous diculties, such as poor full-color tunability, long re-
sponse times, and poor cycling stability; they cannot yet meet
real-world requirements. To address these issues and promote
development, multicolored electrochromic materials and devices
have been developed, where the performance is enhanced by dif-
ferent types of optical resonators. These fascinating investiga-
tions represent the state of the art in this field and have significant
implications for follow-up studies. To highlight the importance of
optical resonators for dynamic color enhancement, we organize
this review in terms of the types of optical resonators, rather than
the performance indices of electrochromic materials and devices.
A brief summary of the characteristics, key metrics, advantages,
and disadvantages of dierent optical resonators is provided in
Table 1.
3.1. Principles of Optical Resonators
Optical resonators have emerged as an eective tool oering
strongly confined electromagnetic fields in ultrasmall volumes
and enhanced light–matter interactions. Many types of optical
resonators with dierent resonance mechanisms have been ap-
plied in structural color generation, including plasmonic res-
onators, Mie resonators, FP cavities, PhC cavities, and hybrid cav-
ities. In this section, we briefly introduce the fundamental prin-
ciples of dierent optical resonators and present their key func-
tionalities and tunability potential for better multicolored elec-
trochromic performance.
3.1.1. Plasmonic Resonators
Plasmonic colors are structural colors arising from resonant in-
teractions between light and metal nanostructures (e.g., metal
nanospheres). They were applied as early as the fourth century,
in the Roman Lycurgus Cup, to create vivid coloration.[,]
Surface plasmon resonance (SPR) is the electromagnetic-field-
induced collective oscillation of free electrons at the metal–
dielectric interface. These free-electron oscillations can be con-
fined on an isolated spherical nanoparticle in a metallic plas-
monic array structure (localized SPR, LSPR) or propagate along
a planar interface between the metal and dielectric in a periodic
plasmonic array (a surface plasmon polariton, SPP). LSPR typ-
ically occurs in a homogeneous spherical metallic nanoparticle
(Figure 3a), whose diameter is smaller than the wavelength of
the incident light (i.e., – nm).[ ] In this case, the optical
extinction cross-section 𝜎ext of the metal nanoparticle associated
with the LSPR spectrum includes the scattering cross-section
𝜎sca and absorption cross-section 𝜎abs, which can be expressed
as follows[ ]
𝜎sca =𝜋
kr
𝜀m−𝜀d
𝜀m+𝜀d

()
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Tabl e 1 . A brief summary of different optical resonators.
Optical
resonator
Characteristic Key metrics Advantages Disadvantages
Plasmonic Hot spot 1) Geometrical size
2) Array periodicity
3) Composition
4) Refractive index of the surround-
ings
1) Good electrical conductivity
2) Strong local electric field en-
hancement
3) Multiple degrees of freedom
for color generation and
modulation
4) Easy to integrate with differ-
ent electrochromic materials
1) High Ohmic losses
2) Limited color gamut
3) Susceptible to oxidation and wear
Mie High-refractive-index dielectric
materials
1) Geometrical size
2) Refractive index of the dielectric
materials
3) Refractive index of the surround-
ings
1) Low optical losses
2) Good CMOS compatibility
3) Strong magnetic resonances
4) Large color gamut
1) Poor electrical conductivity
2) Refractive index limitation
FP Simplest configuration 1) Thickness of the dielectric layer
2) Refractive index of the dielectric
layer
1) Easy to integrate with differ-
ent electrochromic materials
2) Easy to optimize thickness
3) Easy large-scale fabrication
4) Good color tunability
5) Good design flexibility
1) Interference effect
2) Angle-dependent
3) Limited spatial resolution
PhC Periodic arrangement of the
dielectric materials
1) PBG
2) Refractive index of the building
materials
3) Lattice spacing
4) Stack architecture
5) Refractive index of the surround-
ings
1) Can be directly fabricated by
electrochromic materials
2) Easy to tune the PBG
1) Relative weak electric field en-
hancement
2) Too thick film layer
Hybrid Hybridization or coupling 1) Cavity type
2) Refractive index of the building
materials
3) Refractive index of the surround-
ings
1) Shares the advantages of dif-
ferent types of optical res-
onators
2) High freedom of combina-
tion different types of optical
resonators
1) Relatively complicated manufac-
turing
2) Requires careful optimization of
parameters
𝜎abs =𝜋krIm 𝜀m−𝜀d
𝜀m+𝜀d()
where kis the wavevector of light, ris the radius of the nanopar-
ticle, ɛmis the complex dielectric function of the metal, and
ɛdis the dielectric constant of the surrounding medium. As
presented in Equations () and (), 𝜎sca and 𝜎abs are maxi-
mized when |ɛm+ɛd| is a minimum, resulting in remark-
able scattering and absorption peaks in the LSPR spectrum of
the nanoparticle (Figure b). As a result, plasmonic coloration
based on LSPR is determined mainly by the geometrical size
(r) and composition of the metal nanoparticle (ɛm), as well as
the dielectric function of the surrounding environment (ɛd). All
three parameters can be tuned in response to an electrochromic
stimulus.
In addition to isolated spherical metallic nanoparticle, another
implementation of LSPR has been reported. It is based on a
metal–insulator–metal stack array, where the thickness of the in-
sulator is much smaller than the wavelength, and has been en-
abled by the tremendous development of micro- and nanofabrica-
tion technologies.[, ] In this multilayer stacked configuration,
the collective oscillation of free electrons (i.e., LSPR) occurs in the
topmost metallic nanostructures and is analogous to that of an in-
dividual metallic nanoparticle. In addition to LSPR, a gap mode,
which is called the gap plasmon resonance (GPR), can be gener-
ated by strong near-field coupling because the topmost metallic
nanostructure is optically close to the bottom metallic film.[] In
this case, the topmost metallic nanostructure can be regarded as
a dipole. The dipole can induce an image dipole with an opposite
charge distribution on the bottom metallic film. The oscillation in
opposite directions of these two dipoles produces a magnetic res-
onance in the multilayer stacked configuration, which typically
results in strong optical field enhancement in the gap because
of strong localization of the light energy within the ultrathin di-
electric spacer.[] Note that the geometrical size of the topmost
metallic nanostructure and the distance between it and the bot-
tom metallic film (i.e., the thickness of the dielectric spacer) in
these plasmonic resonators based on GPR can be carefully con-
trolled to generate vibrant structural colors. In addition, the gap
plasmon resonator gives rise to strong optical field coupling in
the gap, resulting in high sensitivity to the refractive index of the
dielectric.[] Thus, an additional degree of freedom can be pro-
vided for dynamic color tuning by using an electrochromic stim-
ulus to alter the refractive index of the dielectric.
The enhancement of multiple colors can also be observed
in plasmonic arrays (e.g., hole arrays); it is explained by the
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Figure 3. Principles of optical resonators. a) Schematic diagram of LSPR generated by spherical metallic nanoparticles. b) Characteristic absorption
spectrum of plasmonic nanoparticles as a function of wavelength. c) Schematic of the lowest-order Mie resonance (MD mode) of a dielectric nanosphere.
The electric field of the incident light induces a circular displacement current inside the dielectric nanoparticle, which excites the MD mode. d) Scattering
spectrum of Mie resonator with MD resonance as a function of wavelength. e) Schematic view of FP cavity in which an optically transparent dielectric
layer is sandwiched between two facing parallel metal mirrors. f) Reflection spectrum of an asymmetric FP cavity as a function of wavelength. g)
Schematic diagram of 1D PhC cavity. h) Typical reflection spectrum of 1D PhC cavity as a function of wavelength. i) Schematic illustration of two coupled
damped resonators with a driving force f1applied to one of them. Two resonators with very different damping rates are coupled to produce narrow and
broad spectral lines, resulting in Fano resonance. j) Reflection spectrum of Fano resonance as a function of wavelength, which exhibits a characteristic
asymmetric spectral profile with a sharp change between a dip and a peak.
excitation of a SPP at the interface between a metal and a dielec-
tric material.[, ] The SPP dispersion relation can be written
as[, ]
kSPP =k𝜀m𝜀d
𝜀m+𝜀d
()
where kSPP and kare the wavevectors of the SPP wave and free-
space light, respectively; ɛmand ɛdare the dielectric functions
of the metal and dielectric, respectively. To excite the SPP, addi-
tional momentum must be transferred to compensate for the mo-
mentum mismatch between the free-space light and SPP wave,
because the momentum of the free-space light (ℏk) is smaller
than that of the SPP (ℏkSPP) at the same wavelength.[ ] For ex-
ample, for a D plasmonic array, a SPP can be excited when the
following resonant condition is fulfilled[,, ]
kSPP =ksin𝜃±iGx±jGy()
where 𝜃is the incident angle, Gxand Gyare the Bragg vectors for
the two periods of the array, and iand jare integers that indicate
the order of the SPP mode for Gxand Gy, respectively.
From Equations ()–(), we can see that the plasmon res-
onances are modulated by manipulating the geometrical pa-
rameters of the metal nanostructures, periods of the plasmonic
arrays, and the dielectric functions of the metal nanostruc-
tures themselves and the surrounding media. However, tun-
able on-demand plasmon resonant modes are still elusive. Af-
ter the design and fabrication of plasmonic nanostructures,
they are usually unalterable and remain in their configura-
tions with fixed optical properties. This challenge can be over-
come by using a plasmonic nanostructure filled with an elec-
trochromic material that acts as a surrounding medium. Ac-
tive plasmonic colors can be obtained by varying the dielectric
functions (or refractive indices) of this medium under an elec-
trochromic stimulus, because plasmon resonances are sensitive
to the dielectric function (or refractive index) of the surrounding
medium.
3.1.2. Mie Resonators
All-dielectric nanostructures made of high-refractive-index semi-
conducting materials such as silicon (Si),[, ] germanium
(Ge),[, ] tellurium (Te),[] and TiO,[ ,] have been found
to support Mie resonances and are typically called Mie resonators.
Unlike plasmonic resonators based on metallic nanoparticles,
which suer from high energy dissipation during structural
color generation because of the inherent Ohmic losses of the
metal itself over the visible spectral range, Mie resonators are
expected to be better candidates for high-quality structural color
generation because of their strong magnetic responses with low
losses.[, ] According to Mie theory,[,] the extinction cross-
section 𝜎ext, scattering cross-section 𝜎sca , and absorption cross-
section 𝜎abs of a subwavelength high-index dielectric spherical
particle can be expressed as
𝜎ext =
x
∞

m=
(m+)Re {am+bm}()
𝜎sca =
x
∞

m=
(m+)am+bm()
𝜎abs =𝜎ext −𝜎sca ()
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where xis a size parameter, which is defined as x=𝜋nr/𝜆,
where 𝜆is the wavelength of light; ris the radius of the dielec-
tric nanoparticle; and nis the refractive index of the medium
surrounding the dielectric nanoparticle. In addition, amand bm
are the multipolar Mie scattering coecients; they are typically
related to the electric and magnetic responses of the dielectric
nanoparticle, respectively. For example, aand bcorrespond to
the electric and magnetic dipolar modes, respectively.[] For a
dielectric nanosphere with positive permittivity illuminated by
a plane wave, electric and magnetic dipole (MD) resonances
of comparable strength are excited. The MD resonance occurs
when the diameter of the dielectric nanosphere is approximately
𝜆/nd,wherendis its refractive index.[ ] It results from cou-
pling between the incident light and a circular displacement cur-
rent produced by the confined electric field within the dielectric
nanosphere (Figure c). This type of strong MD resonance can be
observed in the Mie-type scattering spectrum (Figure d). These
MD resonances can occur in the visible spectral range when the
diameter of the dielectric nanosphere is ≈– nm;[ ] they
make the dominant contribution to the generation of brilliant
structural colors.[ ] In addition to the electric dipole and MD
resonances, dielectric nanospheres with larger dimensions can
excite higher-order modes, that is, magnetic quadrupole and elec-
tric quadrupole resonances.[,, ] These fundamental mul-
tipoles in Mie resonators can produce high-quality-factor reso-
nances that result in high-purity and high-resolution structural
colors.[, ]
Similar to that of plasmonic resonators, all-dielectric Mie
resonant coloration depends primarily on the geometrical
size (r) and refractive index (nd) of the high-index dielec-
tric nanoparticle, as well as the refractive index of the sur-
rounding environment (n), according to Equations ()–(). As
a result, dynamical Mie resonant coloration can be activated
by tuning one of these parameters under an electrochromic
stimulus.
3.1.3. Fabry–Perot Cavities
The simplest configuration of a FP cavity is an optically transpar-
ent dielectric layer sandwiched between two facing parallel metal
mirrors (Figure e). Constructive and destructive interferences
occur at a certain wavelength as a result of repeated light reflec-
tion by the confining surfaces, which contributes to a reflection
valley in the reflectivity spectrum of an opaque metallic substrate
(Figure f), or a transmission peak in the transmittance spectrum
of an optically thin bottom metallic layer.[,] FP cavities have
been extensively investigated for use as color filters that selec-
tively reflect or transmit a certain portion of visible light,[,– ]
owing to their many advantages such as structural simplicity, sig-
nificant design flexibility, and excellent color tunability. In addi-
tion, they do not require time-consuming and costly micro- and
nanofabrication techniques such as e-beam lithography (EBL),
nanoimprint lithography, or focused ion beam (FIB) processing.
The thickness (d) of the dielectric layer in the FP cavity can be ad-
justed to integer multiples of half wavelengths for constructive
interference to produce distinct structural colors with dierent
resonant wavelengths.[, ] Specifically, the enhanced reflection
and transmission behaviors of light incident in a FP cavity can be
described as follows[, ]
R=Rm⋅(−cos 𝛿)
+R
m−Rmcos 𝛿()
T=T
m
+R
m−Rmcos 𝛿()
where Rand Tare the total reflection and transmission of the
FP cavity, respectively; Rmis the reflectivity of the metal mirror,
Tmis the transmissivity at the metal–dielectric interface, and 𝛿is
the phase dierence between reflections. Equations () and ()
show that the reflection Rand transmission Tof the FP cavity
depend on the phase dierence 𝛿
𝛿=𝜋
𝜆
⋅n⋅d⋅cos𝜃()
where 𝜆is the wavelength of the light; nand dare the refrac-
tive index and thickness of the dielectric layer in the cavity, re-
spectively; and 𝜃is the internal angle of incidence of light in the
dielectric layer. Reflection minima (dips) for an opaque metallic
substrate or transmission maxima (peaks) for an optically thin
bottom metallic layer occur at the resonant wavelength 𝜆mwhen
the phase dierence 𝛿is an even multiple of 𝜋, that is
𝛿=m𝜋=𝜋
𝜆
⋅n⋅d⋅cos𝜃()
where mis an integer and represents the resonance mode num-
bers. For a simple case with normally incident light in the cavity,
Equation () is modified as
𝜆m=n⋅d
m()
which reveals that the refractive index nand thickness dof the
dielectric layer in the cavity are the most important parameters
for generating various structural colors.
Note that a FP cavity can provide strong electric field enhance-
ment inside the cavity. When an electrochromic material is placed
in a FP cavity as a structural dielectric material, the already
minute changes in the refractive index of the electrochromic ma-
terial under dierent applied voltages are sensitively captured
by the enhanced FP resonance eect, resulting in marked color
changes. In addition to the refractive index changes, the thick-
ness of the electrochromic layer also enables the direct control
of the coloration.[ ] However, once the electrochromic layer is
physically fabricated, it is dicult to alter its thickness. An eec-
tive pathway to continuously tune the color over a wide spectral
range by varying the eective thicknesses during electrochromic
operation must be developed.
3.1.4. Photonic Crystal Cavities
PhCs are made of periodic high- and low-refractive-index dielec-
tric materials.[, ] A photonic bandgap (PBG),[, ] that is,
the constructive reflection of light of specific wavelengths, can
be generated by the alternation of the refractive index, which
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produces brilliant structural colors. PhCs are generally classi-
fied as D, D, or D according to their periodic arrangement
in space. For simplicity, D PhCs (Figure g), which are called
Bragg stacks, Bragg reflectors, or Bragg mirrors, are used to re-
veal the eects of the parameters of the crystal structures on the
photonic properties. For a simple case with normal incidence and
D PhCs of nonabsorbing materials, the peak position of the re-
flection spectrum (i.e., the Bragg wavelength, 𝜆B), the reflectivity
(R), and the bandwidth (∆𝜔, expressed in terms of the frequency)
are given by the following equations[– ]
m𝜆B=nLdL+nHdH()
R=
n−nsnL
nHN
n+nsnL
nHN

()
Δ𝜔=
𝜋
⋅𝜔B⋅sin−nH−nL
nH+nL()
where mis the diraction order; nLand nHare the refractive in-
dices of the low- and high-refractive-index dielectric layers, re-
spectively; dLand dHare the thicknesses of the low- and high-
refractive-index dielectric layers, respectively; nand nsare the
refractive indices of the surrounding medium and substrate, re-
spectively; Nis the number of bilayers; and 𝜔B=𝜋c/𝜆B,where
cis the speed of light in vacuum. The reflection spectra of D
PhCs typically fall in the spectral region shown in Figure h,
where the PBG is detected as the marked peak. Dissimilar re-
flecting regions can be tailored with remarkable accuracy to
generate various structural colors by tuning the lattice spacing,
layer refractive index, stack architecture, and properties of the
surroundings.
As in FP cavities, electrochromic material layers can also be
used as building blocks for the construction of D PhC cavi-
ties. An applied electrical stimulus can change the refractive in-
dex of these layers, and a corresponding change in the struc-
tural color can be expected. For D PhC cavities, the following
two possibilities are available. ) Colloidal crystals such as self-
assembled polystyrene (PS) or silica nanospheres,[,– ] can
be embedded in the electrochromic materials. The as-formed
composite opal structure will exhibit tunable color when an elec-
trochromic stimulus is applied. ) Electrochromic films with
inverse-opal structures can be retained by removing the colloidal
crystal templates; they also produce brilliant electrochromic col-
ors with potential tunability. As a result, multicolor enhance-
ment can be obtained in PhC cavities using electrochromic active
materials.
3.1.5. Hybrid Cavities
Dierent types of optical resonators have unique advantages and
disadvantages. Hybrid cavities obtained by combining various
types of cavities exhibit the merits of each type of cavity. Vari-
ous hybrid cavities have been reported,[– ] and used to gen-
erate vibrant colors with high brightness and contrast. A reso-
nance can usually be described as a harmonic oscillator under a
driving force.[ ] Therefore, to examine the physics of hybrid cav-
ities, a simple model of two coupled mechanical oscillators with a
driving force can be used. It is described by the following matrix
equation[, ]
𝜔−𝜔−i𝛾g
g𝜔−𝜔−i𝛾x
x=if
f()
where 𝜔and 𝜔are the resonant frequencies, 𝛾and 𝛾are the
damping coecients, xand xare the oscillator amplitudes, f
and fare external forces with driving frequency 𝜔,andgis a
coupling coecient that describes the interaction between the
oscillators. Various resonant eects may occur in such hybrid
systems; a very special example is Fano resonance, which occurs
when only one of the oscillators is driven by a harmonic force
(i.e., f≠, f=; Figure i). In this weak coupling regime, the
two oscillators have distinctly dierent damping rates (𝛾>> 𝛾),
which results in the narrow (weakly damped) and broad (strongly
damped) spectral lines required for Fano coupling. The coupling
parameter gis smaller than the larger damping rate 𝛾, which sat-
isfies 𝛾>> g>> 𝛾. As a result of destructive interference, the
obtained Fano resonance spectra exhibit a characteristic asym-
metric spectral profile with a sharp change between a dip and a
peak (Figure j). The amplitude of driven oscillator  in the spec-
tral vicinity of the resonance of oscillator  is given by
x(Ω)≈f

𝛾

𝜔−𝜔+𝛾

⋅
(q+Ω
)
+Ω

where Ω=𝜔−𝜔+g
𝛾⋅𝜔−𝜔
+q⋅
𝛾+q
g()
denotes the dimensionless frequency, and qis the Fano param-
eter, which describes the degree of asymmetry of the Fano line
shape. For simplicity, Equation () can be normalized as well-
known Fano formula[]
𝜎(E)=D(q+Ω
(E))
+Ω
(E)()
where 𝜎(E) is the Fano spectrum, Eis the energy, Dis the am-
plitude of the Fano resonance, qis the Fano parameter, and
Ω(E)=(E−E)/Г,whereEis the resonance energy, and Гis
the resonance width.
Interest in the study of Fano resonators for structural color-
ing originates mainly from the sharp spectral curves of such res-
onators with high Q-factors.[, ] This attractive feature under-
pins the production of excellent colors with high purity and high
brightness. In addition, sharp Fano resonances require only a
small spectral shift to produce high contrast, which can enhance
the field intensity and light–matter interaction, and thus signif-
icantly improve the sensitivity of the resonator to environmen-
tal changes. As a result, Fano resonators combined with elec-
trochromic materials are expected to be excellent candidates for
substantially enhanced multicolor electrochromic performance.
As in other optical resonators, two approaches can be used to
achieve dynamic color generation. ) Electrochromic materials
can be used as building blocks for constructing Fano resonators.
) Electrochromic materials can be used as environmental media
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surrounding passive Fano resonators. Among other factors, the
intensity and resonance wavelength of Fano resonance excitation
determine the perceived color, which can be eciently tuned by
electrochromism-induced complex refractive index modulations
of a functional Fano material itself or of the functional medium
surrounding a passive Fano resonator.
3.2. Structural Colors from Optical Resonators
Optical resonators can manipulate light by various optical res-
onances to produce structural colors. These colors do not fade
over time because of the stability of the optical resonators, and
they can be continuously tuned simply by changing the dimen-
sions and components of the functional materials used as build-
ing blocks in the optical resonators. As long ago as the fourth cen-
tury AD, the Romans used optical resonators to create brilliant
and vivid coloration. They blended gold nanoparticles, typically
– nm in size, into glass materials to produce the exquisite Ly-
curgus Cup.[ ] Because of the LSPR eect of gold nanoparticles,
the cup appears green when viewed in reflected light and red in
transmitted light, that is, when light illuminates it from the in-
side. Moreover, some stained-glass windows, for example, those
in Chartres Cathedral,[ ] exhibit various and vivid colors in day-
light (Figure 4a), owing to plasmonic resonators composed of
gold (Au), silver (Ag), or copper (Cu) nanoparticles embedded in
the glass. With advances in nanofabrication technologies, novel
optical structures such as metallic metasurfaces can be produced
for the generation of higher-quality colors. The morphological
characteristics of these plasmonic nanostructures can be well
controlled with high precision to obtain the structural colors with
high resolution (at the optical diraction limit).[ ] In addition to
developing a predesign process, Zhu et al. reported an approach
to realize high-resolution plasmonic color patterns by the laser
postprocessing of nanoimprinted plasmonic metasurfaces.[ ]
As shown in Figure b, the metasurfaces were fabricated using
abundant and recyclable aluminum (Al) as the plasmonic ma-
terial, and the surface morphology was subsequently modified
by laser heating. Because dierent surface morphologies sup-
port dierent plasmonic resonances, dierent colors covering
all visible wavelengths can be produced. Using this technique,
they also realized full-color laser-printed patterns with an un-
precedented resolution of ≈  dots in.−(DPI) over large
areas.
However, because of the intrinsic Ohmic losses of metals such
as Au, Ag, and Al in the visible spectral range, plasmonic res-
onators cannot easily create very distinct color impressions and
exhibit a limited color gamut. An alternative is to use Mie res-
onators composed of high-index dielectric nanostructures with
low optical losses and excellent complementary metal oxide semi-
conductor (CMOS) compatibility.[,] The use of such Mie
resonators for color generation can further improve the color
vibrancy and reduce the cost. For example, Sun et al. fabri-
cated TiOmetasurfaces as Mie resonators (Figure c), and
subsequently produced high-brightness (% reflectance), high-
saturation [ full width at half-maximum (FWHM) of ≈ nm],
and high-resolution (  DPI) structural colors.[ ] These
high-quality colors originated physically from the electric dipole
and MD resonances.[ ] The precise control of the geometrical
sizes of TiOblocks on metasurfaces resulted in colors that were
widely distributed in the Commission Internationale de l´E-
clairage (CIE) chromaticity diagram beyond the standard RGB
(sRGB) color gamut.
Various vibrant colors can also be obtained using FP cavities.
For instance, Kim et al. prepared a grain-boundary-free, ultra-
flat single-crystal Cu thin film using atomic sputtering epitaxy,
and obtained a uniform, controllable CuO layer from the single-
crystal Cu thin film by controlled heating.[] They ultimately
fabricated a FP cavity consisting of CuO/Cu thin films. The
CuO layer thickness was precisely controlled to obtain repre-
sentative vivid colors covering the entire visible spectral range
(Figure d). However, a conventional FP cavity commonly ex-
hibits very sharp and narrowband absorption with a broadband
nonresonant reflection as background,[] which results in low
color brightness and poor color purity in reflection. To signifi-
cantly improve the purity or saturation of the desired reflected
colors, an alternative approach is to use a broadband absorbent
material such as tungsten (W), Ge, or nickel as the top reflec-
tor of a FP cavity.[,] Additionally, FP cavities are typically
sensitive to the incident angle of light, where the resonance
shifts toward shorter wavelengths with increasing angle of inci-
dence. This angle-dependent color eect is a critical drawback
for many applications such as displays and color printing. It can
be addressed by adopting materials with high refractive index,
introducing additional phase-compensating layers, or exploit-
ing strong interference eects in ultrathin absorbing dielectric-
based nanocavities.[,,,, ] In particular, Kats et al. pro-
posed a new type of FP cavity consisting of ultrathin Ge films
with thicknesses between  and  nm on Au substrates.[]
Because of the large optical attenuation in the highly absorb-
ing Ge films at visible frequencies,[ ] the resulting nontriv-
ial interface phase shifts enabled strong resonant behavior in
the ultrathin FP cavity, which significantly increased the angu-
lar independence of various generated colors. As a result, the
viewing angle of the Ge/Au cavity was increased to °.More-
over, although the colors generated using conventional FP cav-
ities are produced by much simpler fabrication processes that
do not require time-consuming and costly nanofabrication pro-
cesses such as FIB etching and EBL, their spatial resolution
is inherently limited owing to constructive interference mecha-
nisms. Choi et al. developed a room-temperature, ambient, and
low-cost polymer-assisted photochemical metal deposition (PPD)
technique and used it to print ultrathin (≈ nm), smooth Ag
films as the top reflector in a FP cavity for generating vivid and
saturated colors.[ ] Crucially, the PPD technology could print
complex patterns in various colors without substrate limitations
and with high spatial resolution (down to . μm), which is
comparable with that of current colorant-based color printing
methods.
Inspired by coloration strategies in nature, researchers have at-
tempted to generate vivid and brilliant structural colors by adopt-
ing PhC cavities.[,,– ] Among PhC cavities, D PhC cavi-
ties are currently the most interesting and have been extensively
investigated.[– ] Although their planar structure exhibits a
simple optical response, they represent a platform for under-
standing deep physical concepts. For example, Ito et al. devel-
oped an organized stress microfibrillation process in glassy poly-
mer films to create D PhC cavity structures for structural color
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Figure 4. Structural colors from different optical resonators. a) Stained-glass window in Chartres Cathedral, France. The vivid plasmonic colors were
generated by embedded metallic nanoparticles. Reproduced under the terms of the CC-BY license.[172 ] Copyright 2021, The Authors, Published by
Wiley-VCH. b) Plasmonic colors obtained by the laser postprocessing of plasmonic metasurfaces. Ultrahigh-resolution color patterns can be laser-
written onto prefabricated, uniformly colored Al-based plasmonic metasurfaces. Reproduced with permission.[174 ] Copyright 2015, Springer Nature.
c) High-brightness, high-saturation, high-resolution, and wide-gamut structural colors from TiO2-based Mie resonators. Reproduced with
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generation.[ ] They used optical standing-wave technology to
fabricate a periodic stress field within polymer thin films such as
PS, polycarbonate, poly(methyl methacrylate) (PMMA), and poly-
sulfone. When the thin films were exposed to a weak solvent,
these residual stresses were released to create alternating layer
structures of cavity and microfibril-filled polymers (Figure e).
These multilayered porous structures were identified as D PhC
cavities,[ ] and exhibited various high-brightness colors across
the full visible spectrum. This organized stress microfibrillation
process is expected to become a low-cost, large-scale color print-
ing process that generates colorful images (e.g., the Mona Lisa
painting) with a resolution of up to   DPI (Figure e). Sim-
ilarly, Miller et al. adopted an optical standing wave technology
to sculpt D PhC cavity structures in an interesting and prac-
tical manner, and the structures produced reflected structural
colors.[ ] More specifically, they created the desired structural
colors and sophisticated colorful patterns in commercially avail-
able photoresponsive elastomer sheets simply using a standard
light projector. The resulting structural colors were aligned with
the spectral distribution of the image created by a standard light-
projection device, and the image area and resolution of the col-
ored patterns was determined from the size and resolution of
the projected image. As a result, this method is highly attractive
for generating large-area, high-resolution, low-cost, and scalable
structural colors.
However, a single optical resonator does not produce per-
fect structural colors. Thus, hybrid cavities have been ex-
tensively investigated to generate the desired structural col-
ors by combining the advantages of dierent types of optical
resonators.[,,, ] For instance, the coupling of FP and dis-
ordered plasmonic cavities enabled a transition from broadband
absorption to tunable reflection and generated a broad range of
structural colors and even black, which is hard to produce us-
ing conventional optical resonators.[ ] The Chinese watercolor
painting The Peony Flower by Baishi Qi was printed perfectly us-
ing this technology (Figure f). Additionally, the disordered plas-
monic system in the hybrid cavity proposed above is material-
independent, and no sophisticated and time-consuming lithog-
raphy process is needed. Subsequently, Mao et al. also used a
similar hybrid cavity to realize a highly counterintuitive system
in which the hybrid cavity was composed of dierent plasmonic
materials (e.g., Ag, Au, Pd, and platinum (Pt)), which exhibited
almost identical optical responses that produced structural col-
ors with indiscernible dierences.[ ] Moreover, although all-
dielectric Mie resonators can generate structural colors with high
brightness, these structural colors appear under dark-field illumi-
nation, which limits their practical applications.[, ] To obta i n
bright-field structural colors, Li et al. created a hybrid cavity by
coupling a Mie resonator and a FP cavity.[ ] Such hybrid cav-
ities can potentially provide strong backscattering enhancement
in Mie resonators based on Si nanodisks. High-brightness bright-
field structural colors were obtained with diraction-limited res-
olution. To highlight the feasibility of colorful pattern genera-
tion, a painting of a lotus with vivid colors and a deep subwave-
length pixel-level resolution of   DPI was reproduced per-
fectly under bright-field illumination (Figure g). Furthermore,
ElKabbash et al. developed a Fano resonator by hybridizing of two
FP cavities (Figure h), which overcame the major problem of
poor color purity in a single FP cavity.[] The obtained reflected
spectra exhibited a very sharp asymmetrical Fano peak, which re-
sulted in ultrahigh-saturation structural colors with a FWHM of
≈ nm. Remarkably, the Fano resonator also exhibited a unique
optical phenomenon that allowed it to simultaneously transmit
and reflect the same color while the thickness of the bottom Ag
layer was reduced from  to  nm (Figure h), which was im-
possible in a single FP cavity.
3.3. Resonant-Cavity-Enhanced Multicolor Electrochromic Tuning
Conventional, electrochromic materials and devices have unde-
sirable drab colors. Many eorts have been made to cover a wider
color gamut, for example, by material design,[] the develop-
ment of a new mechanism,[] the use of color overlays,[,] and
mix-and-match strategies.[, ] However, the fabrication com-
plexity and resulting low-quality colors of the electrochromic ma-
terials and devices still challenge researchers and limit advanced
applications. Here, inspired by the various brilliant structural col-
ors obtained using optical resonators (Section .), we describe a
fascinating approach, the combination of electrochromic materi-
als with optical resonators, which has excellent potential for over-
coming these limitations. In contrast to other approaches, thin
layers, or rather tiny volumes of electrochromic layers, are su-
cient to obtain brilliant and saturated coloration in this promising
scheme, which develops an innovative field worth exploring. In
the following subsections, we describe in detail advances toward
the realization of dynamically tunable colors in electrochromic
materials and devices enhanced by dierent types of optical res-
onators, including plasmonic resonators, Mie resonators, FP cav-
ities, PhC cavities, and hybrid cavities. When appropriate, other
multicolored electrochromic technologies based on these opti-
cal resonators without electrochromic materials, for example, the
use of LCs, electrochemical deposition, and the migration of Ag
atoms, are also mentioned.
permission.[133 ] Copyright 2017, American Chemical Society. d) Structural colors of FP cavities consisting of Cu2O/Cu thin films. Various vivid col-
ors were realized by precisely controlling the thickness of the Cu2O layer. Reproduced under the terms of the CC-BY license.[147] Copyright 2021, The
Authors, Published by Wiley-VCH. e) Structural colors of 1D PhC cavities consisting of multilayered porous structures generated by organized stress
microfibrillation. High-resolution colorful images (e.g., the Mona Lisa painting) were produced in polymer films. Reproduced with permission.[187 ] Copy-
right 2019, The Authors, Published by Springer Nature. f) Coupling between FP and disordered plasmonic cavities produced a broad range of structural
colors and even black. The Chinese watercolor painting The Peony Flower by Baishi Qi was printed perfectly by this hybrid cavity. Reproduced under
the terms of the CC-BY license.[190 ] Copyright 2020, The Authors, Published by Springer Nature. g) Bright-field structural colors obtained from a hybrid
cavity coupled with a Si-nanodisk-based Mie resonator and SiO2/Si FP cavity. Because of strong backscattering enhancement in the hybrid cavity, colorful
high-luminance and high-resolution patterns (e.g., lotus painting) are available even under bright-field illumination. Reproduced with permission.[165 ]
Copyright 2021, American Chemical Society. h) Ultrahigh-saturation structural colors from a Fano resonator generated by coupling two FP cavities. The
resulting Fano-resonant multilayer films exhibit unique optical properties in their translucent state, enabling them to simultaneously transmit and reflect
the same color. Reproduced with permission.[167] Copyright 2021, Springer Nature.
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3.3.1. Plasmonic-Resonant-Enhanced Electrochromism
Because of their excellent electrical conductivity, plasmonic res-
onators have become the preferred approach to multicolored elec-
trochromic technology. In Section .., the coloration mecha-
nism was discussed in detail; a broad range of colors can be dy-
namically generated under an external electric field, either by
varying the sizes of the metallic nanostructures or by chang-
ing the dielectric properties of the metallic nanostructures them-
selves or the surrounding media. First, rich colors can be gener-
ated by the straightforward tuning of the intrinsic dielectric prop-
erties of the metallic nanostructures without any additional func-
tional materials. For instance, the electron density of a metal de-
termines its plasmonic frequency, and thus the plasmonic reso-
nance frequency of metal nanostructures. However, it is typically
dicult to tune the electron density of metals because of eec-
tive Debye screening. Kim et al. recently reported an unconven-
tional electrochromic behavior that may solve this problem.[ ]
They fabricated an electrochromic device filled with a solution
electrolyte of acetonitrile containing .  tetrabutylammonium
hexafluorophosphate, which was sandwiched between two facing
electrodes: a blank ITO electrode as the counter electrode and an
ITO electrode modified with Au nanocubes as the electrochromic
electrode. When a low voltage (−. to . V) was applied, the
peak of the scattering spectrum of the device shifted only from
 to  nm without any significant electrochromic behavior,
which was attributed to the virtually constant free carrier con-
centration in the Au nanocubes. By contrast, at highly negative
potentials beyond the threshold (−. to −. V), the peak of the
scattering spectrum exhibited a noticeably faster blueshift from
 to  nm, with a significant color change from yellow to
green. This unexpected electrochromic behavior beyond the con-
ventional understanding was attributed to a change in the Fermi
level, which increased the free electron generation rate by in-
creasing the potential.
In addition to the intrinsic electronic properties, the size
of the metallic nanostructure also determines the perceived
color.[,] For example, Chu and co-workers applied electro-
chemical deposition in multicolor electrochromic technology and
achieved continuous color tuning from red to blue by reversibly
electrodepositing Ag shells surrounding Au nanoparticles.[]
They fabricated a plasmonic electrode with Au nanodome arrays
by etching with an anodized aluminum oxide template and sub-
sequent thermal evaporation of Au; they then packaged it into an
electrochromic device composed of a dimethyl-sulfoxide-based
gel electrolyte containing Ag+ions. When a voltage of . V
was applied to the device, a Ag shell of tunable thickness was
grown on the Au nanoparticles, causing the plasmonic reflec-
tion peak to be blueshifted from  to  nm. As a result,
the reflected color of the device could be continuously tuned
between red, orange, green, and blue over the full visible re-
gion (Figure 5a). Note that these color changes occurred within
 s. As an interesting demonstration, a mechanical chameleon
containing two color sensors was constructed; it automatically
changed color to match the background for active camouflage
(Figure a).
However, both of these electrochromic technologies without
active electrochromic materials are not suitable for real-world
applications such as smart windows and displays because of
their poor color bistability, poor cycling stability (< cycles),
long response time (several seconds), and easy degradation.
Therefore, a combination of electrochromic materials and plas-
monic resonators is considered to be more promising option.
Following this approach, Xu et al. demonstrated high-contrast,
fast monochromatic, and full-color electrochromic switching us-
ing plasmonic nanoslit arrays functionalized with two dier-
ent electrochromic polymers PANI and poly(,-dimethyl-,-
propylenedioxythiophene) (PolyProDOT-Me).[] The fabricated
Au and Al metallic nanoslit arrays provide the tight spatial con-
finement and high local field intensity associated with SPPs,
enabling the use of extremely thin conductive polymers as ac-
tive materials to achieve high optical contrast and thus ultrafast
switching. However, because of the high SPP propagation losses
of Au, the electrochromic electrode had a limited color gamut.
Another Al nanoslit electrode was considered to be a good can-
didate for full-color tunability, and distinct vibrant colors cov-
ering the entire visible spectrum were generated by tuning the
period of the nanoslit array (Figure b). When voltages of +.
and −. V were applied, the plasmonic electrochromic electrode
with dierent colors could actively switch between the colored
state (color on) and dark state (color o), exhibiting excellent color
bistability (Figure b). Even when a  nm thick PolyProDOT-
Meelectrochromic layer was used, a high color contrast of
–% was still obtained in the proposed plasmonic-resonator-
enhanced electrochromic electrode, which significantly reduced
the switching time to  ms. This short switching time can in
principle satisfy the requirements of video displays if further op-
timizations are performed.
Although a plasmonic resonator with nanoslit arrays is a
good option for enhancing the multicolor performance of elec-
trochromic materials, it requires extremely expensive lithogra-
phy techniques and is dicult to scale. Peng et al. oered a
practical solution in .[] They fabricated an electrochromic
electrode without using lithography techniques by anchoring a
Au nanoparticle encapsulated by a PANI shell on a planar Au
mirror. Interestingly, highly confined electromagnetic hotspots
appeared in the PANI shell between the Au nanoparticle and
Au mirror (Figure c), which amplify the eect of the refrac-
tive index changes of PANI under dierent voltages because the
resonance condition depends sensitively on the interaction be-
tween the confined and enhanced electromagnetic field and the
local dielectric environment. When the voltage was swept from
−. to . V at a rate of  mV s−, the scattering peak shifted
from  to  nm, and the color changed from vivid red to
vivid green (Figure c), because of the refractive index change
of PANI, ∆n=.. A helpful feature similar to that reported in
the pioneering works of Xu et al. contributed to this result.[]
That is, a plasmonic electrochromic electrode composed of an
extremely thin PANI shell (e.g.,  nm) still exhibited excellent
color-switching performance with high optical and color contrast
(>%), high refresh rates (> Hz), good bistability (> min),
and ultralow power consumption ( fJ per pixel), in part because
the highly confined electromagnetic field was enhanced by the
plasmonic resonator. Remarkably, the proposed electrochromic
electrode not only demonstrated continuous color tuning on the
single-nanoparticle level, providing active plasmonic pixels with
the smallest area reported to date, but also has the potential to be
scaled up to centimeter-scale films (Figure c). This plasmonic
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Figure 5. Plasmonic-resonator-enhanced electrochromic materials and devices. a) Plasmonic color tuning from red to blue covering the full visible re-
gion by the reversible electrodeposition of Ag shells on Au cores. A mechanical plasmonic chameleon composed of these electrochromic devices was
fabricated to obtain adaptive camouflage by the integration of sensing and spectral analysis. Reproduced with permission.[196 ] Copyright 2016, Amer-
ican Chemical Society. b) Panchromatic plasmonic electrochromic electrodes using Al nanoslit arrays functionalized with the electrochromic material
PolyProDOT-Me2. The plasmonic electrochromic electrode can be electrically switched between a colored (on) state and a dark (off) state because of
strong broadband optical absorption in oxidized PolyProDOT-Me2. Reproduced under the terms of the CC-BY license.[29] Copyright 2016, The Authors,
Published by Springer Nature. c) Plasmonic electrochromic electrodes based on individual PANI-covered Au nanoparticles on a Au mirror. Various
plasmonic colors were obtained as the refractive index of the PANI shell changed during the electrochromic process. Reproduced with permission.[50]
Copyright 2019, American Association for the Advancement of Science. d) All-solid-state electrochromic devices enhanced by gap plasmonic resonators
composed of electrochromic material WO3. Significant color tuning from blue to purple that was realized as the refractive index of LixWO3was varied
under different voltages. Reproduced with permission.[12] Copyright 2019, American Chemical Society.
electrochromic electrode based on electrochromic nanoparticle-
on-mirror constructs was further optimized recently by Peng
et al.[ ] Using aerosol jet printing, they deposited colloidal Au
nanoparticles coated with a  nm thick PANI shell on an ultra-
thin metallized polyethylene terephthalate (PET) substrate and
obtained large-area (tens of centimeters), flexible, and pattern-
able plasmonic electrodes. These advanced electrochromic elec-
trodes exhibited electrically tunable vivid color dynamics, sug-
gesting that they might be suitable for flexible and wearable ap-
plications.
However, these plasmonic electrochromic electrodes are typ-
ically studied in an electrochemical cell with a three-electrode
system,[,, ] and the observed color properties, such as color
gamut, applied voltages, response time, and cycling ability, are
dicult to directly replicate on a device for actual applications.
Therefore, it is essential to study the dynamically tunable colors
of a plasmonic device. In , Li et al. fabricated an all-solid-
state electrochromic device with a lateral architecture for excel-
lent color visibility. It consisted of an electrochromic electrode
based on an Al/LixWO/Al gap plasmonic resonator, a counter
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electrode of Li.FePO, and an optically transparent solid poly-
mer electrolyte (Figure d).[] The gap plasmonic resonator en-
hanced light–matter interactions in the gap between an Al mir-
ror and Al nanorod; consequently, the thickness of the LixWO
spacer layer can be dramatically reduced to as low as  nm, en-
abling significant dynamic tuning of its resonant properties and
the resulting colors. When voltages of . and −. V were ap-
plied, the refractive index of LixWOdecreased from . to . in
the visible spectral range. Because of the strong electromagnetic
field confinement and enhancement inside the gap, the GPR and
the associated plasmonic color were sensitive to changes in the
refractive index of the LixWOlayer. Therefore, as the refractive
index of LixWOchanged, the plasmon resonance wavelength
was blueshifted by  nm in the visible range, with significant
color tuning from blue to purple (Figure d). Moreover, the ul-
trathin LixWOlayer not only avoided strong optical absorption
in the device, enabling virtually complete retention of the plas-
monic resonance intensity, but also reduced the response time
of the all-solid-state device to  s.
In short, plasmonic resonators provide multiple degrees of
freedom for the realization of multicolor electrochromism. Vari-
ous beautiful colors can be obtained by the electrically straightfor-
ward adjustment of the intrinsic properties of the metallic nanos-
tructure in the plasmonic resonator, such as the size, shape,
and dielectric index. However, the durability and stability of the
colors decrease over multiple cycles of electrochromic opera-
tion because of the oxidation and wear of the Ag/Au. More-
over, this approach is limited by poor bistablility and long re-
sponse time, which are unsuitable for some applications, for ex-
ample, energy-ecient display technologies. As a result, elec-
trochromic materials and plasmonic resonators perfectly com-
plement each other and are considered to represent a more
promising approach. Plasmonic resonators can provide vibrant
colors; however, these colors are static and unalterable. Although
electrochromic materials alone exhibit undesired single colors,
their optical properties can switch rapidly and reversibly in re-
sponse to an electric field. Subsequently, excellent multicolor
electrochromic properties, such as high contrast, a large color
gamut, fast response speed, excellent bistability, long lifetime,
low power consumption, and suitability for mass production,
have been demonstrated by combining electrochromic materials
with plasmonic resonators. However, materials with good overall
performance are rare. For example, performance indices of some
electrochromic nanoparticle-on-mirror structures are compara-
ble to or even exceed those of other plasmonic electrochromic
electrodes.[] However, their color chromaticity is considerably
limited by the inherent confinement of the plasmonic resonance
frequencies accessible to Au nanoparticles. Despite these impres-
sive advances, several challenges remain that require further re-
search, for example, eorts to widen the color gamut, further im-
prove the response times, or simultaneously optimize all the per-
formance parameters of actual devices.
3.3.2. Mie-Resonant-Enhanced Electrochromism
Compared to plasmonic resonators, Mie resonators, which have
no intrinsic losses, are a more promising alternative for imple-
menting high-performance multicolor electrochromic technolo-
gies with properties such as broad color gamut and high reflec-
tion eciency. The colors obtained using Mie resonators can be
tuned by manipulating properties such as the refractive index,
size, and geometry, in analogy to the adaptability of plasmonic
resonators. For example, varying the relative heights of Mie res-
onators on a metallic mirror can significantly aect their scat-
tering eects. Using this concept, Holsteen et al. demonstrated
tunable structural color from a single Si nanowire (Si NW) across
the entire visible spectrum by simply modifying the distance be-
tween the Si NW and an Al mirror (Figure 6a).[ ] However, it is
dicult to actively electrically modulate the color changes result-
ing from such geometrical changes. Inspired by reconfigurable
nanomechanical photonic metamaterials,[ ] they developed a
nanoelectromechanical system platform to actively alter the po-
sition of the Si NW on the mirror by applying dierent voltages;
they tuned the Mie scattering peak from  to  nm, produc-
ing a vibrant color change from red to green (Figure a).
An alternative strategy for Mie-resonance-related multicolor
electrochromic technology is to change the polarization state of
light incident onto or scattered from anisotropic Mie resonators
by the active electrical tuning of LCs. For example, Rho and co-
workers demonstrated electrically tunable colors with dierent
shades of gray by employing an array of Mie resonators com-
bined with LCs.[] The Mie resonators, which were composed
of high-refractive-index, low-loss hydrogenated amorphous Si (a-
Si:H), produced bright and saturated spectral colors in reflection
because of their strong Mie scattering properties. The resulting
colors were further enhanced using the quasi-guided-mode reso-
nances of the periodic array. Because anisotropic Mie resonators
with dierent geometries were used, the optical response de-
pended on the linear polarization of incident light, enabling di-
rect light modulation by employing electrically tunable LC cells,
and thus providing a linear transition between the bright and
dark states of the obtained colors (Figure b). Because of this
novel approach, not only were bright, saturated RGB colors pro-
duced, but also black and white pixels, along with all combina-
tions of states in between, were obtained, making it possible to
obtain a full color gamut. Notably, the proposed continuous gray-
level modulation is indispensable for the implementation of pho-
torealistic displays. We systematically discuss its use in this con-
text in Section .. In addition, the time required for the colors to
switch between dierent states was improved to on the order of
milliseconds owing to ultrafast electrical modulation of the LCs.
However, the thickness of a LC cell is generally on the order of mi-
crometers, which severely degrades the color visibility and limits
the miniaturization and integration of the entire electrochromic
device.
Thus, Mie resonators are theoretically more appropriate for
enhancing the multicolor properties of electrochromic materi-
als, as only a very thin electrochromic layer is required to pro-
duce significant color changes, and they are fully capable of meet-
ing the miniaturization requirements of real-world applications.
However, they have rarely been studied. The reason is twofold.
First, the refractive index of most electrochromic materials, es-
pecially organic electrochromic materials, is typically smaller
than ..[,– ] Dielectric nanoparticles composed of these
low-refractive-index electrochromic materials typically do not ex-
hibit strong Mie resonant scattering in the visible region be-
cause of the poor light confinement arising from the small
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Figure 6. Mie-resonator-enhanced electrochromic materials and devices. a) Color tuning of a single Si NW from red to green using Mie resonance
was realized by electrically modulating the distance between the Si NW and an Al mirror. Reproduced with permission.[199] Copyright 2017, American
Association for the Advancement of Science. b) Electrically tunable colors with different gray levels were obtained using an array of a-Si:H-based Mie
resonators in combination with LCs. Reproduced under the terms of the CC-BY license.[43] Copyright 2022, The Authors, Published by Springer Nature.
index contrast between the electrochromic material and air. Some
electrochromic materials with refractive indices greater than .,
such as WOand TiO, can be used as building materials to
fabricate Mie resonators with high brightness and a wide color
gamut. However, as the refractive index decreases below . dur-
ing further electrochromic processes, their Mie resonant scatter-
ing properties may be suppressed. However, the substrate has a
dramatic eect on the Mie resonance properties.[ ] Because of
the conductivity requirements of electrochromic processes, ITO-
coated glass slides are usually employed as substrates to sup-
port the electrochromic material. However, the refractive index
of ITO is ≈.,[ ] which strongly aects the Mie scattering prop-
erties of the electrochromic materials. Consequently, the use of
Mie resonators to enhance the multicolor performance of elec-
trochromic materials has become a major scientific problem that
urgently requires solution. Guo and co-workers demonstrated
that metal-dressed low-index SiOcan produce enhanced Mie
resonance modes.[ ] This remarkable work is expected to in-
spire further studies of Mie-resonator-enhanced multicolor elec-
trochromic materials. Lu et al. recently reported the implemen-
tation of chemical proton doping and dedoping to switch the Mie
resonance and obtain significant color changes on low-refractive-
index PANI nanospheres because of the large variation in the real
part of the dielectric function of PANI in the visible range.[]
This result is clearly a step toward to the expected behavior.
In summary, Mie resonators can be employed to obtain high-
brightness, wide-gamut multicolor electrochromics if several fac-
tors are taken into account, such as the choice of electrochromic
material (e.g., WO, PANI) and the fabrication of a low-refractive-
index conductive substrate,[, ] as well as the global design of
the resonant cavity structure.
3.3.3. Fabry–Perot-Cavity-Enhanced Electrochromism
FP cavities can be perfectly integrated with conventional elec-
trochromic devices by replacing the transparent conductive layer
in these devices with a thin layer of metal. The thin metallic
layer serves as both a reflector and a current collector; that is,
it not only provides the good electrical conductivity necessary for
charge transfer during the electrochromic process, but can also
be combined with the electrochromic dielectric layer to create
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Figure 7. FP-cavity-enhanced electrochromic materials and devices. a) FP-cavity-type electrochromic devices composed of metallic W and electrochromic
material WO3. Rich color modulation was achieved by controlling the thickness of the WO3layer and subsequent electrochromic operation. Reproduced
under the terms of the CC-BY license.[44] Copyright 2020, The Authors, Published by Springer Nature. b) High-brightness multicolored electrochromic
electrodes based on a FP cavity consisting of an ITO/Cu multilayered reflector and WO3. Reproduced with permission.[211] Copyright 2022, Elsevier. c)
High-purity multicolored electrochromic electrodes based on multilayered W/WO3FP cavities. A green electrode provided rich color tuning from green to
cyan, blue, and dark violet at different applied voltages, with large Q-factors (>4.5). Reproduced with permission.[213 ] Copyright 2021, American Chemical
Society.d) FP-cavity-type electrochromic devices filled with the electrochromic material TiO2. For example, the electrochromic device consisting of 100 nm
thick TiO2presented significant color tuning from yellow to cyan, with spectral modulation of up to 114 nm. Reproduced with permission.[30] Copyright
2022, American Chemical Society. e) Electrochromic electrodes based on FP cavities were fabricated by employing a special electrochromic material,
pT34bT, as a dielectric layer sandwiched between a 50 nm thick Al mirror and a thin translucent metallic bilayer of 5 nm thick Cr and 7 nm thick Au. The
electrode was panchromatically tuned mainly by electrically altering the thickness of pT34bT in the FP cavity. Reproduced with permission.[215] Copyright
2021, Wiley-VCH. f) FP-cavity-based all-solid-state inorganic electrochromic devices composed of an ITO (45 nm)/Fe2O3(100 nm)/Ag (140 nm)/TiW
(70 nm) four-layer film stack. Wide-gamut color tuning was realized by electrically controlling the migration of Ag atoms to increase the thickness of the
Fe2O3layer and reconfigure the Ag layer. Reproduced with permission.[216] Copyright 2021, Springer Nature.
an asymmetric FP cavity for enhanced multicolor performance.
For example, our group proposed and demonstrated a novel elec-
trochromic device based on FP cavities, which enable rich and
subtle color tuning of WOmaterials.[] The FP cavity was com-
posed of a thin layer of metallic W with a thickness of  nm
topped by a WOlayer (Figure 7a). It provided strong interference
resonances with up to % reflectance modulation, inducing the
selective absorption/reflection of visible light, which generated
specific colors. In addition, a wide range of brilliant colors across
the entire visible spectrum was produced before voltages were
applied by adjusting the thickness of the WOlayer (Figure a).
When a voltage of −. V was applied for the insertion of Li+into
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the WOlayer, its refractive index decreased continuously from
. to . at a wavelength of  nm. As a result, the FP res-
onance peak of a device with a  nm thick WOfilm shifted
from  to  nm, and the color changed from red (. V) to
yellow (−. V) and green (−. V). This very large color modu-
lation range ( nm) is considerably beyond the reach of con-
ventional electrochromic materials. In addition to rich color tun-
ing, the device produced subtle color modulation. For example,
at scanned voltages ranging from . to −. V, a blue sample
exhibited color variation from steel blue to sky blue, royal blue,
cerulean blue, ocean blue, and, finally, slate blue, which is highly
attractive for sensing applications.[] The color saturation was fur-
ther improved by adding an ultrathin metallic layer (e.g., Ag)
on the active WOlayer. As a demonstration, images of butter-
flies with dierent colors were fabricated, as shown in Figure a,
and they were switched between dierent color states within a
few seconds. Remarkably, the entire fabrication process was rel-
atively straightforward; it did not require time-consuming and
expensive nanofabrication techniques, and it was fully compat-
ible with standard commercial electrochromic processes. How-
ever, the intrinsic optical loss of the metallic W layer substantially
suppressed the reflectivity of the proposed electrochromic device
with a FP cavity to less than %, resulting in muted colors and
a dull appearance. Therefore, our group subsequently presented
an improved FP-cavity-based electrochromic device.[ ] In this
work, an ITO/Cu multilayered reflector was employed to replace
the W reflector; the reflectivity of the FP cavity was improved
from ≈% to over % (Figure b). As a result, the modified
device exhibited high color brightness (between % and %)
during the electrochromic process. Note that even in the highly
absorbing state when Li+was intercalated into the WOlayer, the
reflectance remained quite high (≈%).
Spectrally narrowband absorption and broadband nonreso-
nant reflection in a single FP cavity result in reflective structural
coloration with inferior purity,[] which makes FP cavities un-
suitable for a wide range of applications such as reflective dis-
plays and color filters. To address this problem, our group in-
troduced a multilayered FP cavity structure that enabled the dy-
namic alternation of multiple colors with high contrast and high
color purity (Figure c).[] The proposed multilayered FP cavi-
ties provided stronger confinement of the incident light at spe-
cific optical wavelengths compared to individual FP cavities, re-
sulting in sharp reflectance spectral profiles that allowed the re-
alization of colorful devices with high color saturation and Q-
factor values. For example, the green device exhibited rich color
modulation varying from green to cyan, blue, and dark violet
(Figure c), with narrow FWHM values (< nm) and large
Q-factors (>.) under applied voltages of  to −. V. Notably,
in addition to achieving high-purity colors, the device based on
the multilayered FP cavity configuration exhibited a wider color
gamut (at least % more than that in a previously reported work)
during the electrochromic process.[]
In addition to WOmaterials, other electrochromic materi-
als, such as Si and TiO,[,] have been employed as building
blocks to construct FP cavities for structural color tuning and
thus enhance their multicolor properties. TiOis a popular elec-
trochromic material with a high refractive index and low absorp-
tion coecient in the visible region, especially in the blue region,
and is known to exhibit larger refractive index modulation (∆n),
better cycle life, and less volume expansion than WOmaterials
during ion insertion. For instance, Eaves-Rathert et al. fabricated
a FP cavity structure by employing anatase TiOas the dielec-
tric and a titanium (Ti) metal backplane as a reflector.[] The re-
fractive index was significantly modulated (from . to . at
 nm) with the complete transformation of anatase TiOto or-
thorhombic Li.TiO(LTO) when a potentiostatic voltage of −V
was applied. As a result, the reflection peak exhibited a signifi-
cant blueshift of  nm, changing the color from yellow to cyan
(Figure d).
However, to the best of our knowledge, the color tuning of any
individual FP cavity using these approaches did not cover the en-
tire visible spectral region because of the limited refractive index
variation of most electrochromic materials. Therefore, to obtain
full-color tuning, it is necessary to use three subpixels, that is,
subtractive (cyan, magenta, and yellow) or additive (red, green,
and blue) color mixing. However, the use of these hybrid systems
increases the device fabrication complexity and further reduces
the reflection eciency of the entire device. Achieving full color
tuning across the entire visible spectrum with high brightness
in a device with a single FP cavity has proven to be a dicult
challenge. A FP cavity composed of a special electrochromic ma-
terial that provides good actuation properties in terms of thick-
ness variation can help to overcome these limitations. Jonsson
and co-workers recently demonstrated a FP cavity with electri-
cally tunable cavity volume variation by using an electrochromic
conducting polymer, poly-thieno[,-b]thiophene (pTbT) as a
dielectric layer sandwiched between a  nm thick Al mirror and
a thin translucent metallic bilayer of  nm thick chromium (Cr)
and  nm thick Au (Figure e).[] Here, pTbT was chosen for
the production of vibrant and high-brightness structural colors
because of its low bandgap and low visible absorption. The thick-
ness of the pTbT layer was varied to obtain cavities with reso-
nances throughout the entire visible spectrum, resulting in a vari-
ety of colors including violet, blue, green, yellow, orange, and red
(Figure e), demonstrating the possibility of achieving full color
by controlling the pTbT thickness under electrochromic stim-
ulation. The authors patterned the top Al film into pixelated areas
that enabled Na+transport from the electrolyte, a .  aqueous
solution of sodium dodecylbenzene sulfonate, to pTbT. When
a cyclic voltage between −and+. V was applied at  mV
s−, the cavity resonance of a green sample with an initial thick-
ness of  nm was tuned so that the reflection peak remained
rather high (>%) and shifted from  to  nm (Figure e),
changing the reflected color from blue to cyan, green, orange, and
red. This remarkable color modulation was attributed mainly to
the significant thickness variation; small changes in the refrac-
tive index probably contributed as well. Further approaches, in-
cluding stepwise electrochemical control and the fabrication of
nanoholes in the Al layer, were employed to obtain wider color
tuning with a cavity resonance spanning over  nm in the vis-
ible spectrum because of sucient ion transport (Figure e).
In addition, Yan et al. proposed a FP-cavity-based all-solid-
state inorganic electrochromic device to obtain wide-gamut color
tuning.[ ] The as-fabricated device was composed of an ITO
( nm)/FeO( nm)/Ag ( nm)/TiW ( nm) four-layer
film stack (Figure f). The ITO and TiW films are conductive lay-
ers, and the functional layers include Ag and amorphous FeO
as a reflector and dielectric spacer, respectively, to generate vivid
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structural colors. Interestingly, under an electric field, the amor-
phous FeOallowed Ag+ions to enter and pass through it, in-
creasing the thickness of the FeOlayer and reconfiguring the
Ag layer (Figure f). Consequently, the reflection spectrum of the
device changed dramatically when a positive bias of  V was ap-
plied, where the maximum intensity of the red peak at  nm
decreased significantly and continuously, whereas that of the blue
peak at  nm increased. Thus, dierent colors were produced
at dierent bias times, and the color changed from the initial or-
ange color to purple, blue, and green at bias times of , , and
 s, respectively (Figure f). This electrochromic process was
completely reversed when a negative bias of −. V was applied.
Remarkably, colorful displays with high-resolution nanopixel pat-
terns were realized by conductive atomic force microscopy, which
has potential for large-scale coloration applications.
Overall, FP cavities are considered a promising system for
enhancing multicolor electrochromic materials and devices be-
cause of their ease of fabrication and resonance control. The FP
cavity can provide enhanced electromagnetic fields in the elec-
trochromic layer so that only a very thin electrochromic layer
(– nm) is required to produce significant colors, and the
cavity resonances and resulting colors are sensitive to changes
in refractive index and thickness. Rich, high-brightness, high-
saturation, and high-resolution colors have been electrochromi-
cally tuned by configurating dierent FP cavity architectures of
electrochromic materials. Furthermore, thickness control is the
most straightforward means of obtaining full color, compared to
the limited tuning available by varying the refractive indices of
electrochromic materials; however, it is limited by its long col-
oration times, uneven chromaticity, poor cycling lifetime, and dif-
ficulty in scaling to macroscopic dimensions. Although polymers
are easier to control electrically in terms of thickness,[– ] they
tend to deteriorate under a high driving voltage and exhibit poor
cycling performance. Therefore, all-inorganic devices are more
attractive for practical applications. To date, there have been only
limited investigations in this direction. More research is needed
to explore the possibility of using inorganic electrochromic mate-
rials in the FP cavity configuration to obtain full-color tunability
over large areas with high switching speeds and uniform chro-
maticity.
3.3.4. Photonic-Crystal-Cavity-Enhanced Electrochromism
PhC cavities are also commonly used as optical cavities to en-
hance the multicolor performance of electrochromic materials.
Early research on combining PhCs with electrochromic mate-
rials focused on the tunability of PBGs rather than on color
modulation,[ ] and the PBGs showed very little variation. The
first demonstration of significant tuning of the structural color
of PhC cavities using electrochromic materials, in , was an
electrochromic Bragg mirror consisting of a high-refractive-index
WOand low-refractive-index NiO, the reflected color of which
was electrically tuned by reversible refractive index variation.[]
When a switched voltage of ±. V was applied, the orange elec-
trochromic Bragg mirror turned dark green; subsequently, the
color cycled between dark green and yellow. Similarly, Xiao et al.
fabricated a monomaterial electrochromic Bragg mirror based on
WOto obtain the large and reversible modulation of reflected
colors.[] The refractive index of the WOlayers can be manip-
ulated by changing the deposition angle because WOfilms pre-
pared at dierent grazing angles have dierent porosities, where
the WOfilm deposited at °(WO-°) had a high refractive
index of . at  nm, whereas that deposited at °(WO-°)
had a low refractive index of .. These alternating high- and
low-refractive index WOlayers on ITO conductive substrates
easily form an electrochromic Bragg mirror that yields colorful,
highly luminous structural colors (Figure 8a). When the prepared
electrochromic Bragg mirrors were subjected to a switched volt-
age of ±. V, the reflection peaks shifted remarkably; conse-
quently, the resulting bright colors were further modulated ow-
ing to the significant change in the refractive index of the WO
layers (Figure a). For example, the blue Bragg mirror changed
from royal blue to light slate blue, whereas the yellow-green and
red mirrors became green and orange, respectively (Figure a).
In addition to D Bragg mirrors, D PhC cavities have been
used for colorful electrochromism. For example, Kuno et al. pre-
pared a D composite opal structure composed of PANI@PMMA
core–shell nanoparticles, which exhibited tunable color when
an electrochromic stimulus was applied.[ ] For example, a
yellowish-green film became dull orange at . V and grayish-
blue at . V. However, the resulting colors were extremely dull,
even with a maximum reflectance of less than %, making them
highly unsuitable for practical applications other than camou-
flage. Qu et al. demonstrated another possible use of D PhC
cavities for unique color tuning, namely, the use of inverse-opal
electrochromic NiO films (Figure b).[] By simply controlling
the diameter of PS spheres, NiO films of dierent colors, for ex-
ample, purple and orange, were obtained. The purple or orange
inverse-opal electrochromic NiO films immersed in   LiClO
in a propylene carbonate solution changed to green or brown,
respectively, because the D PhC cavities are sensitive to the re-
fractive index of the surrounding medium. Subsequently, when
a voltage from −. to . V was applied, the resonance peaks in
the reflection spectrum shifted very little, and the color switched
only between light green and dark green, or only between light
brown and brown, owing to intensity variations (Figure b), prob-
ably because of the rather small change in refractive index (n)
and the large change in the extinction coecient (k) of the NiO
film. Thus, an electrochromic device with tunable reflections of
dierent colors was implemented that was capable of controlled
brightness and the production of dierent shades of gray, which
are useful for vivid and realistic displays.
PhC cavities do allow the colorization of monochromatic elec-
trochromic materials, and the resulting colors can be continu-
ously and reversibly tuned in response to an electric field. How-
ever, a critical limitation of the techniques described above for
multicolor electrochromics based on PhC cavities is that the color
modulation is considerably smaller than that of other optical res-
onators. The reason is most likely the relatively weak cavity en-
hancement of the PhC cavity, which cannot convert the finite
refractive index modulation of the electrochromic material into
significant spectral shifts. The small spectral modulations may
be sucient for precision optics; however, the resulting color
variations are negligible for macroscopic applications. Another
limitation is the long response time. PhC cavities require elec-
trochromic films of sucient thickness, usually on the order
of micrometers, as building blocks to obtain significant colors,
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Figure 8. PhC-cavity-enhanced electrochromic materials and devices. a) Monomaterial electrochromic Bragg mirrors of WO3. When different voltages
were applied, the refractive index of WO3changed, resulting in remarkable multicolor modulation of the Bragg mirror. Reproduced with permission.[47]
Copyright 2017, Wiley-VCH. b) 3D PhC-cavity-based electrochromic electrode composed of inverse-opal electrochromic NiO films, which exhibited
modulation of the luminance or grayscale at different voltages. Reproduced with permission.[222 ] Copyright 2021, AIP Publishing.
which greatly increases the ion diusion distance, and thus the
the switching time. Nevertheless, the Bragg mirror is worthy of
further in-depth investigation as an optical resonator to improve
the multicolor properties of electrochromic materials. We antic-
ipate that the above issues may be addressed by one or both
of the following approaches. ) The dierence in refractive in-
dex of the two electrochromic materials used to construct the
Bragg mirror (e.g., TiOand PANI) should be as large as pos-
sible to reduce the bandwidth and number of periods while en-
suring high-brightness colors. Thus, the color saturation can be
improved, and the overall film thickness can be reduced. Note
that the reflection spectrum will exhibit a sharp profile because
of the narrow bandwidth, which is sensitive to the refractive in-
dex of the electrochromic material. Consequently, a narrow re-
flection band shift can be obtained by applying an electric field,
which results in a significant color change. In addition, the thin
electrochromic layer increases the color switching rate. ) A hy-
brid cavity concept with integrated Bragg mirrors and FP cavi-
ties can be developed for multicolor enhancement.[ ] Bragg mir-
rors with ultrahigh reflectivity are used instead of the metal re-
flector in the FP cavity and can provide strong optical feedback
that amplifies subtle changes inside the FP cavity. These cavi-
ties are then combined with electrochromic materials in antici-
pation of achieving tunable colors with high brightness and high
saturation.
3.3.5. Hybrid-Cavity-Enhanced Electrochromism
Single cavities have drawbacks such as poorly tunable color
chromaticity, narrow viewing angles, and long response times.
However, hybrid cavities have shown great promise for mul-
ticolor electrochromics because of their versatility and high
performance. The most popular type of hybrid cavity is the
plasmonic–FP hybrid cavity.[,,,,] For instance, Hopmann
and Elezzabi reported a hybrid cavity by coupling a plasmonic
resonator and a FP nanocavity to obtain high chromaticity and
highly stable color modulation of WOmaterials.[] The hybrid
cavity was composed of a  nm thick Au mirror, a  nm
thick WOelectrochromic layer, and a  nm thick Au nanohole
array. The Au nanohole array greatly improved the color chro-
maticity because it allowed coupling to localized surface plas-
mons and provided strong plasmonic extinction. As a result, the
reflection spectrum of the hybrid cavity had a distinct single peak
in the wavelength range of – nm, with a maximum peak-
to-valley fluctuation of up to %, as the Au nanohole array ex-
hibited significant plasmonic absorption above  nm and be-
low  nm (Figure 9a). This behavior diered from that of a
single FP nanocavity without the Au nanohole array, which had
a broadband reflection spectrum with high reflectance across a
wide spectral range between  and  nm. An electrochromic
device was assembled by employing the designed hybrid cavity
as a working electrode, in which the Au nanohole array can also
serve as a transport path for the insertion of Li+ions into the
electrochromic WOlayer in the nanocavity. When the device
was continuously lithiated from  to  mC cm−by applying a
voltage of − V, the resonance peak of the reflection spectrum
shifted from  to  nm as the refractive index of WOde-
creased dramatically, from . to . at  nm, resulting in
significant color tuning within a few seconds, from red to or-
ange, yellow, and green (Figure a). The colors were exception-
ally vivid and bright, highly saturated, and more impressive than
that generated from the single FP cavity. Notably, because of the
scalability of the colloidal lithography process and the compact
periodic alignment of the resulting Au nanoholes, this device
had the potential to provide large-area, high-resolution displays.
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Figure 9. Hybrid-cavity-enhanced electrochromic materials and devices. a) Electrochromic devices based on a hybrid cavity realized by coupling a
plasmonic resonator (Au nanohole arrays) and a FP nanocavity (WO3/Au). High chromaticity and highly stable color modulation of WO3materials
were achieved in the hybrid-cavity-based electrochromic devices. Reproduced with permission.[49] Copyright 2020, American Chemical Society. b) Elec-
trochromic electrodes consisting of the electrochromic polymer PProDOTMe2grown on a hybrid cavity structure realized by coupling a plasmonic
resonator (Au nanoprotrusions) and a FP cavity (Al2O3/Al). The electrochromic electrode enabled high color contrast (>50%) and ultrafast switch-
ing speed (on the order of milliseconds) because of the Au nanoscale protrusions and thin PProDOTMe2. Reproduced under the terms of the CC-BY
license.[28] Copyright 2021, The Authors, Published by Wiley-VCH. c) Electrochromic electrodes based on a hybrid cavity implemented by coupling two
FP cavities (WO3/Ag and Ag/ITO/Ag). When a voltage of ±0.8 V was applied, reversible switching between Fano and FP resonances was achieved,
resulting in significant spectral modulation accompanied by high-quality tunable color. Reproduced with permission.[224] Copyright 2022, Wiley-VCH.
Alternatively, Au nanoprotrusions can also be used as a plas-
monic resonator by coupling with the FP cavity to enhance the
multicolor electrochromic properties. Dahlin and co-workers re-
cently fabricated Au nanoprotrusions by uniformly coating a 
nm thick Au film on  nm PS spheres (Figure b),[] which
provided strong plasmonic eects at resonance wavelengths.
When they were coupled to an AlO/Al FP cavity, the plas-
monic absorption was significantly enhanced, causing the hy-
brid cavity to exhibit distinct colors with high reflectivity and
excellent chromaticity. Crucially, in contrast to the planar struc-
ture, the Au nanoscale protrusions not only enhanced the color
quality, but also provided a large active area and a high posi-
tive surface curvature, which increased the mobility of the ions
and further improved the switched speeds (Figure b). The au-
thors also emphasized that the ion drift motion was nonnegli-
gible during ion transport and contributed to the high switch-
ing speed. In addition, a device was assembled for dynamic
color tuning where the electrochromic polymer dimethylpropy-
lenedioxythiophene (PProDOTMe) was grown on the hybrid
cavity. Relatively high color contrast was obtained with only a
very thin PProDOTMelayer because of the enhanced elec-
tromagnetic fields provided by the hybrid cavity. According to
the equation 𝜏∝L/D, this thin electrochromic polymer also
increased the switching speed. As a result, ultrafast switch-
ing, and even video-speed switching (– ms), of reflected
colors was achieved with high contrast (as large as % re-
flectance modulation) (Figure b). This ultrafast electrochromic
operation subjected the electrochromic polymer to very short
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voltage pulses, thus preventing its degradation and greatly
improving the cycle lifetime to tens of millions of cycles. Finally,
the device operated with ultralow power consumption during
video-speed switching (<mWcm
−) and had negligible power
consumption (<μWcm
−) in bistable mode because of the
unique bistable properties of electrochromics.
In addition to coupling a plasmonic resonator and a FP
cavity, our group proposed and demonstrated a hybrid cavity by
coupling two FP cavities (WO/Ag and Ag/ITO/Ag resonators)
for multicolor electrochromics, which originated from reversible
switching between Fano and FP resonances.[] The hybrid
cavity for a blue sample consisted of a five-layer structure of WO
( nm)/ITO ( nm)/Ag ( nm)/ITO ( nm)/Ag ( nm),
where the  nm thick ITO film was a protective layer that made
no optical contribution. In the absence of lithiation, the hybrid
cavity was simply a superposition of two FP cavities; it exhibited
a typical FP cavity reflection spectrum with a clear resonance
dip at the resonance wavelength of  nm (Figure c). When a
voltage of −. V was applied, the WOwas fully lithiated, and
the WO/Ag FP cavity became a broadband absorber cavity (i.e., a
LixWO/Ag resonator). When the resulting broadband absorber
was weakly coupled to the bottom narrowband absorber (the
Ag/ITO/Ag resonator), a sharp asymmetric Fano peak at  nm
appeared in the reflection spectrum (Figure c), suggesting that
the Fano resonance was generated in the hybrid cavity. As a
result, this remarkable spectral modulation not only produced
unique photonic modulation, but also resulted in exciting
color changes from blue to magenta. Richer color modulation
was achieved by simply adjusting the thickness of the ITO
sandwiched between two Ag layers. Moreover, a unique optical
property appeared that cannot be obtained using the single FP
cavity; the fully lithiated hybrid cavity reflected and transmitted
the same color in the semitransmissive state, whereas without
lithiation it exhibited complementary colors similar to those of
the single FP cavity (Figure c). Interestingly, the color modula-
tion mechanism was unprecedented and may result only from
the extinction coecient change (∆k) of the WOfilm rather than
the refractive index change (∆n). This behavior diers greatly
from that of the conventional single cavity, where the active color
tuning depended on ∆n, and the intensity depended on ∆k.
Hybrid cavities clearly oer a promising solution to challenges
thatcannotbeovercomeusingasinglecavity.However,theyhave
not been widely studied. Thus, additional types of hybrid cavi-
ties merit investigation and development to address intractable
problems in multicolor modulation, such as limited gamut, low
reflectance and chromaticity, strong angular sensitivity, and dif-
ficulties in the independent control of color and intensity. For
example, by coupling a Bragg mirror and a FP cavity,[] the
modulation of colors with high luminance and purity is the-
oretically possible in this extraordinary hybrid cavity even if a
certain amount of absorption occurs after the desired thin elec-
trochromic layer is colored, because the Bragg mirror has ultra-
high reflectivity and provides a high Q-factor for the FP cavity.
Another possible example is the hybridization of disordered Mie
or plasmonic resonators and FP cavities. Constructive interfer-
ence between disordered Mie or plasmonic resonators and a FP
cavity composed of an electrochromic layer on a metallic reflector
may strongly enhance the scattering eects, resulting in angle-
independent color tuning.
4. Advanced and Emerging Applications
Electrochromic materials and devices are considered promising
light management technologies for various applications such as
smart windows, displays, camouflage, antiglare rearview mir-
rors, and optical filters. However, because of the intrinsic fea-
tures of electrochromic materials, conventional electrochromic
devices exhibit only limited color hues, which dramatically lim-
its their development for higher-order applications and hinders
their commercialization. To address these issues and improve the
performance of multicolor electrochromism, optical resonators
have become a pivotal platform in recent years because of their ul-
trasmall volume, excellent light trapping and manipulation capa-
bilities, and enhanced light–matter interactions. These resonant-
cavity-enhanced electrochromic materials and devices have re-
ceived considerable attention and have shown excellent poten-
tial for use in advanced and emerging multichromatic displays,
adaptive visible camouflage, visualized energy storage, and appli-
cations of multispectral tunability.
4.1. Multichromatic Displays
In contrast to commercially available active light-emitting dis-
plays based on light-emitting diodes (LEDs) or LC display (LCD)
technologies, electrochromic displays have attracted extensive at-
tention from academia and industry as a typical passive nonemis-
sive display and are expected to be a competitive candidate for
next-generation displays because of their extremely low power
consumption and high visibility in bright environments, espe-
cially when used outdoors. However, it has been challenging to
develop electrochromic displays with performance comparable to
that of emissive displays. The reason is that the limited and dull
colors and slow switching speeds (on the order of seconds) of
conventional electrochromic displays prevent vibrant image qual-
ity and fast refresh rates, respectively, and also limit their usage
to applications such as simple electronic labels and outdoor bill-
boards. Resonant-cavity-enhanced electrochromic materials and
devices have recently emerged as a new approach to obtain excel-
lent electrochromic performance, in particular, dynamically rich
and vivid color modulation and fast switching speeds, where mul-
tichromatic displays are an important application in this field. To
achieve competitive display performance and to quickly and ac-
curately evaluate the potential of novel electrochromic technol-
ogy for display applications, a series of key performance indi-
cators of display technologies must be considered,[] such as
brightness, contrast, chromaticity, resolution, angle dependence,
response time, power consumption, and lifetime (Table 2). In ad-
dition, key technical issues such as the need for flexibility, indi-
vidually addressable pixels, gray-scale modulation, and manufac-
turing techniques compatible with mass production must also be
addressed to truly move toward commercialization (Table ).
The brightness of multichromatic displays can be character-
ized in terms of the absolute reflectivity of the device surface. For
example, a recently commercially available electrophoretic dis-
play has a maximum reflectivity of only % and thus exhibited
very low brightness.[ ] Although front-light configurations can
improve the brightness of the display by varying the illumination
conditions, a high absolute reflectivity remains critical for obtain-
ing color images with good visibility. The ideal reflectivity value
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Tabl e 2 . A brief summary of key performance indicators and technical is-
sues of multichromatic displays.
A brief summary of key performance
indicators and technical issues of
multichromatic displays.
Key technical issues
Brightness Flexibility
Contrast
Chromaticity Individually addressable pixels
Resolution
Angle dependence Gray-scale modulation
Response time
Power consumption Manufacturing techniques compatible
with mass production
Lifetime
is close to %, which is potentially obtainable using a strongly
reflective optical resonator. For example, Dahlin and co-workers
fabricated a WOelectrochromic display electrode based on a FP
cavity with a highly reflective  nm Pt mirror, and obtained a
high reflectivity of >% (Figure 10a).[] However, when elec-
trolytes and counter electrodes are introduced to assemble elec-
trochromic devices for real-world applications, the characteris-
tic reflectivity drops sharply to below % (Figure a). To ad-
dress this dicult problem, the authors proposed a reversed de-
vice structure that maintained high reflectivity in real devices
and was independent of the electrolytes, ITO conductive layers,
and counter electrochromic materials (e.g., NiO). Remarkably,
the reflectivity (brightness) remained quite high (≈%) even
in the absorbing lithiated state (Figure a). The contrast of the
novel multichromatic displays can be defined as the dierence
in reflectivity (∆R) between a colored (on) state and an absorb-
ing (o) state, or the shift in resonance wavelength (∆𝜆)be-
tween two color states. For example, the hybrid-cavity-enhanced
electrochromic display proposed by Dahlin and co-workers ex-
hibited RGB-to-black switching at high contrast (% reflectiv-
ity change) (Figure b).[] In addition, the FP-cavity-enhanced
electrochromic display proposed by our group produced dier-
ent color-state switching, from red to yellow and green, with
a very large wavelength shift (∆𝜆= nm) and likewise ex-
hibited high contrast.[] For these multichromatic displays, the
chromaticity, which describes the vibrancy of the colors, is the
most important performance parameter for good image quality.
This fact is reflected in the CIE  diagram, where the highest
purity and most vivid colors have monochromatic coordinates,
such as the standard RGB coordinates, and lie at the edges of
the diagram. Each subset spanned by two or more primary col-
ors is called a gamut and represents the range of available col-
ors. To realize multicolor or even full-color displays, resonant-
cavity-enhanced electrochromic materials and devices must have
the largest possible color gamut. The most straightforward op-
eration is to mix primary colors to produce subpixels, for exam-
ple, by subtractive (cyan, magenta, and yellow) or additive (red,
green, and blue) color mixing methods.[ ] Only the structural
parameters of the proposed optical resonators must be adjusted
to provide the primary colors in the RGB triplet, and combined
electrochromic materials are used to control the color-on or color-
o states, which act synergistically to produce a full-color display.
For example, Dahlin and co-workers fabricated microscale RGB
pixels on the basis of a hybrid cavity constructed by coupling a
plasmonic resonator (Au nanoholes) and a FP cavity (AlO/Ag),
where the electrochromic polymer polypyrrole was used to turn
the colors on and o.[] The authors demonstrated that multiple
secondary colors, such as yellow, purple, cyan, and even neutral
gray, were produced by a combination of RGB pixels (Figure b).
To demonstrate the potential of full color electrochromic display,
a display with metasurface RGB pixels was designed to present
sophisticated patterns, such as a university logo, in dierent
colors. Similarly, Lee et al. reported that the three primary col-
ors (RGB) were obtained by regulating the thickness of a WO
electrochromic layer sandwiched in a FP cavity.[] The authors
used photolithography to produce RGB pixels and subpixels that
perfectly reproduced the colorful image Hyangwonjeong. When
electrical stimulation was applied, the full-color electrochromic
display was turned on and o by switching the optical proper-
ties of WO(Figure c). However, even if an individual pixel
enhanced by an optical resonator can provide % reflectivity,
the maximum reflectivity is reduced to at most % when RGB
subpixels (i.e., multiple micro-electrochromic devices) are used
arranged side by side to create a color image, because each color
can occupy only a maximum of one-third of the total area. Thus,
an alternative strategy may be preferable; that is, some of the
resonant-cavity-enhanced electrochromic materials and devices
mentioned in Section . can provide many dierent colors, or
even full color, to cover a larger color gamut. In particular, to re-
duce the number of subpixels, Zhang et al. demonstrated a re-
flective electrochromic display consisting of a FP cavity (WO/W)
electrode and a sodium-ion-stabilized vanadium oxide electrode,
which achieved  representative color states in the D CIE color
space through the color overlay eect, exhibiting tunable multi-
color features.[ ] Furthermore, although continuous color tun-
ing of resonant-cavity-enhanced electrochromic displays is possi-
ble, the available color gamut is insucient for current displays
because of the lack of independent gray-scale modulation. Re-
searchers have been focusing on this area and using a combina-
tion of optical resonators and electrochromic materials or LCs to
obtain a wide range of luminosity modulation to yield a wider
color gamut for more realistic multichromatic displays.[, ]
Another important performance metric is the resolution,
which depends on the surface density of the pixels. Multiple types
of optical resonators are available to provide the high resolution
required for displays. For example, Dahlin and co-workers used
a hybrid cavity composed of Cu nanohole arrays and an AlO/Al
FP cavity to implement a high-resolution electrochromic display,
with a resolution of up to  DPI based on RGB pixels,[ ] which
is close to the highest resolution (≈ DPI) that can be resolved
by the eye at handheld distances. In addition, the obtained resolu-
tion can be further improved, as its limit is set by optical lithog-
raphy. Higher resolution can also be obtained by using certain
types of optical resonators. For example, plasmonic resonators
can provide a high resolution of up to   DPI.[ ] How-
ever, its commercialization is greatly hindered by the extremely
expensive lithography techniques required for high resolution,
which are dicult to implement on a large scale. Peng et al. pro-
vided an interesting solution to this problem.[, ] As shown in
Figure d, the proposed individual plasmonic resonator based
on a Au nanoparticle/PANI/Au mirror can be considered as a
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Figure 10. Multichromatic displays. a) FP-cavity-based reversed device structure that avoids the effects of electrolytes and counter electrodes to obtain
high brightness. The reflectivity (brightness) of the electrochromic display device remained quite high (≈60%) even in the absorbing lithiated state.
Reproduced under the terms of the CC-BY license.[111 ] Copyright 2021, The Authors, Published by American Chemical Society. b) Microscale RGB pixels
based on a hybrid cavity were realized by coupling a plasmonic resonator (Au nanohole arrays) and a FP cavity (Al2O3/Ag). The electrochromic polymer
polypyrrole was used to turn the colors on and off. Various secondary colors, such as yellow, purple, cyan, and even neutral gray, were produced by
combining RGB pixels to demonstrate a full-color electrochromic display. Reproduced with permission.[48] Copyright 2016, Wiley-VCH. c) RGB subpixels
based on Ag/WO3/Ag FP cavities were realized by controlling the thickness of the WO3electrochromic layer. The colorful image of Hyangwonjeong was
perfectly reproduced using these RGB subpixels, and the color image was turned on and off by switching the optical properties of WO3using different
voltages. Reproduced with permission.[226 ] Copyright 2020, American Chemical Society. d) Plasmonic pixel cell based on a PANI-covered individual Au
nanoparticle on a Au mirror. These pixel cells were aerosol jet printed on a flexible PET substrate to fabricate flexible, multicolor electrochromic displays
with extremely high spatial resolution (<100 nm). Reproduced under the terms of the CC-BY license.[198 ] Copyright 2020, The Authors, Published by Wiley-
VCH. e) High-quality color pixels based on hybrid cavities obtained by coupling gap plasmonic resonators (Al/Al2O3/Al) and disordered Al plasmonic
resonators. These 100 ×100 μm2pixel arrays were integrated into a commercialized LCD to fabricate an electrochromic display that can present a true
video display. Reproduced with permission.[228] Copyright 2020, Proceedings of the National Academy of Sciences.
pixel cell, and an electrochromic display with extremely high spa-
tial resolution (< nm) was fabricated.[ ] Remarkably, the dis-
play was compatible with aerosol jet printing for mass produc-
tion and can be manufactured on flexible substrates [such as
PET, polydimethylsiloxane, polyimide, and stainless steel foil],
making it ideal for displays in flexible and wearable electronics
(Figure d). Although pixelated electrochromic displays can re-
produce color images with excellent chromaticity and high reso-
lution and enable simple color tuning or on/o switching, it is
very dicult to obtain the arbitrarily switching or dynamic ad-
justment of displayed information for truly lifelike representa-
tion, which is required for commercial displays. Therefore, the
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major challenge of achieving selective and local control of micro-
and nanoscale pixels in multichromatic electrochromic displays
must be addressed. Franklin et al. oered a solution that involves
direct integration with an actively addressed conventional LCD
panel.[ ] The authors fabricated a hybrid cavity by coupling a
gap plasmonic resonator (Al/AlO/Al) and a disordered Al plas-
monic resonator to provide high-quality color pixels, which were
subsequently integrated into a commercial LCD to manufacture a
novel type of reflective hybrid display (Figure e). The prepared
displays were imaged in  × μmpixel arrays, depending
on the light modulation of the LCs, where any pixel could be con-
trolled by applying a voltage that displayed color when  V was
applied and turned dark when  V was applied (Figure e). As a
demonstration, an image of a bee was flown across the active area
with a sinusoidal trajectory in time (Figure e), indicating a true
video display. Remarkably, the display provided angle-insensitive
colors as a result of diuse reflection from the disordered plas-
monic resonator, which is a key performance metric for multi-
chromatic displays. Because of the lack of electrochromic mate-
rials and the use of commercially available LC materials, this dis-
play is not truly electrochromic in the strict sense of the word.
However, it motivates the further development of electrochromic
displays in active matrix driving mode using a thin-film transistor
to control active pixels.[ ] In addition to an active matrix, a pas-
sive matrix can be considered as a promising driving strategy for
multichromatic displays if the signal crosstalk problem caused
by unwanted charge transfer can be solved.[ ]
To truly commercialize multichromatic electrochromic dis-
plays, additional performance indicators, including response
time, power consumption, and lifetime, are of the utmost im-
portance. It is well-known that commercialized LC and LED
displays typically oer switching times of a few milliseconds,
which cannot be achieved with conventional electrochromic
displays. By contrast, resonant-cavity-enhanced multichromatic
electrochromic displays can provide ultrafast response speeds
and even video refresh rates of up to  Hz. This approach was
systematically discussed in Section .. The ultrafast switching
performance requires only very short voltage pulses, limiting the
occurrence of unwanted side reactions in electrochromic mate-
rials and leading to a remarkable increase in display lifetime.
Likewise, relatively small voltage is required to obtain high con-
trast, since the enhancement by optical resonators reduces the
risk of polymer degradation or release from the surface, thus sig-
nificantly increasing the cycle lifetime. As a result, it is possible
to achieve ten million stable cycles without contrast loss by run-
ning at video speed (– ms) and using small voltages (from
−. to +. V, vs Au).[] Note that preparing and encapsulat-
ing the device in a zero-humidity environment also contributes
to a longer lifetime. In addition, in the context of carbon neutral-
ity, novel electrochromic displays with low power consumption
are highly promising. For example, the hybrid-cavity-enhanced
color displays proposed by Dahlin and co-workers operated with
ultralow power consumption (<mWcm
−) even during video
speed switching and had negligible power consumption (<μW
cm−) in bistable mode;[] they consumed much less power than
emissive or backlit screens (≈ mW cm−).[ ]
Resonant-cavity-enhanced materials and devices do provide
the key performance parameters required for multichromatic
electrochromic displays. Some of these color displays have ex-
hibited excellent performance according to individual key met-
rics, such as excellent chromaticity, full-color tuning of a single
pixel, or millisecond response times, but did not exhibit accept-
able overall performance. Moreover, the independent control of
arbitrary pixels is urgently needed. However, these addressed pix-
els will require additional functional layers and electrical con-
nections, which will greatly increase the manufacturing com-
plexity and manufacturing costs and decrease the display e-
ciency. Nevertheless, these emerging full-color display concepts
oer tremendous potential for real-world applications.
4.2. Adaptive Visible Camouflage
In nature, many organisms have acquired the remarkable abil-
ity to adaptively change their skin color or physical appearance
to blend in with their surroundings and to protect, warn, and
camouflage themselves against predators. Chameleons, for ex-
ample, can rapidly and sensitively change their body colors in
response to external stimuli through the contraction and expan-
sion of iridophore cells. Importantly, guanine nanocrystals in su-
perficial iridophores can be ordered to form PhCs, which play an
essential role in the dynamic visible camouflage.[ ] In addition,
cephalopods such as octopuses, squids, and cuttlefish are also
known to be masters of natural camouflage. They can actively
sense surrounding colors and alter the appearance of their skin
by the synergistic action of light-reflecting iridophores and pig-
mented chromatophores to obtain on-demand camouflage.[]
Inspired by these natural examples, various emerging advanced
materials, devices, and technologies have been developed to ob-
tain bioadaptive camouflage in the visible spectral region. Elec-
trochromism is among these enabling technologies that address
the issue of color and appearance camouflage. However, conven-
tional electrochromic materials and devices generally suer from
limitations such as limited color tuning range, long response
times, and insucient intelligence, which limit their practical
applications. For example, the electrochromic material PANI is
extensively used for adaptive camouflage, but it can only re-
versibly switch color between dark green and yellow at an ap-
plied external voltage,[ ] limiting its visible camouflage applica-
tions in more complex environments. Resonant-cavity-enhanced
electrochromic materials and devices enable structural coloration
with broadband spectral shifts in a compact space, which is ex-
tremely attractive for futuristic adaptive camouflage technolo-
gies. For instance, motivated by the camouflage of squids,
Bao et al. proposed and demonstrated a FP-cavity-enhanced
electrochromic film consisting of an amorphous Si (a-Si) layer
and a reflective Cu mirror that consecutively changed color in a
broad visible wavelength range from magenta to medium pur-
ple, dark blue, dark cyan, and even neutral gray at dierent ap-
plied voltages (Figure 11a).[] This wide color tuning originated
from simultaneous changes in both the refractive index and film
thickness of the a-Si layer during reversible lithiation and delithi-
ation processes, which dramatically altered the destructive inter-
ference conditions in the FP cavity (Figure a). This marvelous
squid-like ability to interface with complex environments pro-
vided a novel approach to artificial active camouflage. Similarly,
plasmonic resonators can be used to enhance the multicolor na-
ture of electrochromic materials to highly match the colors of
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Figure 11. Adaptive visible camouflage. a) FP-cavity-enhanced electrochromic films composed of an a-Si layer and a reflective Cu mirror exhibited
wide color tuning from magenta to medium purple, dark blue, dark cyan, and even neutral gray at different applied voltages. Similar to the ability of
squid, this rich color modulation can facilitate adaptive camouflage in complex environments. Reproduced with permission.[214 ] Copyright 2018, Wiley-
VCH. b) Au plasmonic resonators were used to enhance the color contrast of the electrochromic material VxO2x+1for active camouflage in different
natural environments and scenarios, such as the ocean (blue), night (gray), green trees (green), the desert (yellow), and autumn (orange). Reproduced
with permission.[192 ] Copyright 2022, American Chemical Society. c) Asymmetric coloration used by the butterfly K. inachus for mimetic camouflage
was realized by employing a FP cavity consisting of an ultrathin (4–8 nm) metallic reflector and the electrochromic material WO3. Reproduced with
permission.[45] Copyright 2021, Wiley-VCH. d) Butterfly shaped electrochromic device with electrolytes containing embedded SiO2colloidal nanocrystals
can perform both active and passive camouflage. Reproduced with permission.[237 ] Copyright 2021, American Chemical Society.
dierent complex scenes. For example, to exploit the strongly
confined optical fields and enhanced light–matter interactions in
Au plasmonic resonators, Wang et al. doped them into the elec-
trochromic material VxOx+to obtain a variety of monochro-
matic colors with high contrast.[ ] As shown in Figure b,
the Au-plasmonic-resonator-enhanced VxOx+electrochromic
device changed color from blue to gray, green, yellow, and orange
as the voltage increased from −. to . V. These distinct colors
were far superior to the indistinguishable colors produced by the
VxOx+material alone and were ideally adapted for active cam-
ouflage in dierent natural environments and scenarios, such as
the ocean (blue), night (gray), green trees (green), the desert (yel-
low), and autumn (orange).
In addition, resonant-cavity-enhanced electrochromic devices
can mimic some creatures with striking features, such as Kallima
inachus,[ ] and Closterocerus coffeellae,[ ] thereby achieving
unique and amazing camouflage properties. For instance, our
group fabricated a FP-cavity-enhanced electrochromic device on
a flexible PET substrate using ultrathin metallic layers such
as W, Ti, Cu, and Ag as translucent reflectors to obtain asym-
metric coloration properties similar to those of the butterfly
K. inachus.[] To demonstrate biological camouflage, an artifi-
cial butterfly based on this electrochromic device was prepared;
its dorsal side was golden yellow, and its ventral side was sky
blue (Figure c). These brilliant colors, which are similar to
those of real butterflies, are thought to attract mates during the
breeding season. When a voltage of − V was applied, the dor-
sal and ventral sides of the artificial butterfly changed to blue
and leaf-like tan, respectively. When the artificial butterfly en-
countered predators and natural enemies, it closed its wings for
camouflage to protect itself, hiding the brightly colored dorsal
side, and revealing only the dull-colored ventral side, and thus
resembling a dried leaf (Figure c). This fascinating Janus col-
oration can be applied to higher-order asymmetric camouflage
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scenarios, for example, to obtain visible camouflage under two
dierent environmental conditions at the same time. In addition,
Mei and co-workers reported a passive camouflage technique.[ ]
The authors fabricated a PhC-cavity-enhanced electrochromic de-
vice by embedding SiOcolloidal nanocrystals in the electrolyte.
The PhC cavity provided highly reflective and vivid structural
colors while exhibiting transmissive optical eects. The device
achieved reversible active color modulation between blue and
transparent on a white background at switching voltages of +.
and −. V (Figure d). Passive structural color changes were
obtained depending on the background colors; for example, it
appeared blue under marble and became colorless and transpar-
ent against a white background (Figure d). As a result, the de-
vice was capable of both active and passive camouflage, which
operated synergistically to provide the potential for camouflage
against a wide range of backgrounds.
Overall, resonant-cavity-enhanced electrochromic materials
and devices have the potential for a wider range of applications
in adaptive visible camouflage than conventional electrochromic
materials and devices. Despite extensive eorts to create multi-
colored electrochromic materials and devices for camouflage, it
is still dicult to achieve perfect camouflage in complex environ-
ments with variable textures. Pixelated structural coloration pro-
vides a solution to this problem.[ ] By controlling small pixels
individually and precisely, it is possible to obtain dierent color
patterns that mimic any background. In addition to coloration
strategies, morphing is an adaptive camouflage strategy found in
nature.[ ] Therefore, for practical camouflage applications, it is
critical to account for the versatile tunability of colors, patterns,
and even physical morphologies. Finally, the color modulation of
resonant-cavity-enhanced electrochromic materials and devices
must be combined with the evaluation of the camouflage eects,
which requires advanced autonomous systems with feedback, for
example, a highly integrated machine vision system,[, ] rather
than separate color-changing devices.
4.3. Visualized Energy Storage
Owing to the international consensus that reaching peak car-
bon and carbon neutrality are currently the main global chal-
lenges, the development of technologies for energy storage, and
energy saving and the associated advanced technologies has at-
tracted the attention of researchers worldwide. Electrochromism
is among the most promising technologies that can be com-
bined with energy storage technologies and contribute to more
ecient energy use because it oers ultralow power consump-
tion and even the partial recovery of consumed electrical en-
ergy. There are two main types of conventional energy storage
systems: supercapacitors and batteries. Remarkably, there are
many similarities between electrochromic devices and super-
capacitors/batteries in terms of electrode materials, device ar-
chitectures, reaction kinetics, and operating mechanisms, mak-
ing it possible to incorporate electrochromism into supercapac-
itors/batteries to build electrochromic energy storage devices,
including electrochromic supercapacitors and batteries.[ ] In-
terestingly, charges can be stored in these electrochromic de-
vices when they change their color. In addition, these devices
can visually and dynamically display their remaining capacity
through changes in color or pattern.[,, ] However, conven-
tional electrochromic energy storage devices have several limi-
tations, such as dull and unattractive color variation; low con-
trast between dierent colors, which results in the imprecise
perception of changes in the stored energy level; and a fun-
damental contradiction between the high coloring eciency
required for electrochromics and the high charge density re-
quired for energy storage,[ ] which greatly reduces their po-
tential for application in intelligent wearable electronic devices.
To address these issues, the emerging concept of resonant-
cavity-enhanced electrochromic materials and devices has been
applied to energy storage. For example, our group proposed
and demonstrated FP-cavity-enhanced electrochromic superca-
pacitors based on WOmaterials, which exhibited exception-
ally versatile color tunability.[] Conventional WO-based elec-
trochromic supercapacitors exhibited only insignificant changes
in transparency on the exterior.[] By contrast, the proposed FP-
cavity-type electrochromic supercapacitor provided rich, subtle
real-time color changes during charging (for example, from yel-
low to yellow-green, beige-green, pea green, grass green, patina
green, blue-green, sapphire blue, steel blue, and dark blue),
demonstrating high recognition and functionality as a color in-
dicator capable of real-time feedback of the internal energy sta-
tus. More importantly, various complex artistic patterns, such as
the colorful fish pattern, were skillfully created for more sophisti-
cated energy storage information transfer (Figure 12a). The mul-
ticolored supercapacitor exhibited excellent electrochemical be-
havior, with a high area capacitance of . mF cm−, high en-
ergy density of . ×−mWh cm−, fast charging/discharging
speed (on the order of seconds), and long cycle life ( cy-
cles). Thus, it is highly suitable for powering LED lights or
other energy-consuming devices (Figure a). However, because
of the strong electrostatic interaction between the embedded
ions and electrochromic thin film, the electrochromic superca-
pacitor has only a limited electrical energy output and cannot
be fully bleached by powering the external electronics alone.
Therefore, an external bias is required to bleach it, which in-
creases the energy consumption. An emerging zinc (Zn)-anode-
based electrochromic battery provides a promising solution to
this problem.[] Our group fabricated a FP-cavity-enhanced elec-
trochromic Zn-ion battery based on manganese oxide (MnO),
where a Zn plate was used as the current collector and anode, a
MnO/Ti bilayer film was used as a cathode, and a   ZnSO
solution was used as the electrolyte (Figure b).[ ] Due to the
refractive index change of MnO, the electrochromic battery ex-
hibited a variety of structural colors such as coral, orange, red,
crimson, and violet during charging/discharging. In addition,
because of the very high energy density of MnO, the elec-
trochromic battery oered a high capacity of  mAh g−at .
Ag
−; thus, it can power a calculagraph for over  h. In con-
trast to electrochromic supercapacitors, when the Zn anode and
MnO/Ti cathode of the electrochromic battery were connected,
the redox potential dierence between the two electrodes spon-
taneously drove the color change, providing excellent energy re-
trieval functionality,[] with a maximum energy retrieval rate of
% in this work. Remarkably, various real-time, vivid, and distin-
guishable colors were obtained during the charge/discharge pro-
cesses in the FP-cavity-enhanced electrochromic battery; these
colors can accurately indicate the stored energy level (Figure b).
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Figure 12. Visualized energy storage. a) FP-cavity-type electrochromic supercapacitors with the colorful fish artistic pattern based on the electrochromic
material WO3exhibited rich, subtle real-time color changes during charging. Reproduced with permission.[244 ] Copyright 2020, American Chemical So-
ciety. b) FP-cavity-type electrochromic Zn-ion batteries based on the electrochromic material Mn2O3. The electrochromic battery provided high capacity
and exhibited various real-time, vivid, and distinguishable colors during charging and discharging, which indicated the exact level of stored energy. Re-
produced with permission.[246 ] Copyright 2021, Wiley-VCH. c) FP-cavity-type electrochromic Li-ion batteries based on the high-specific-capacity material
Si. The electrochromic battery with a complex chameleon pattern exhibited reversible, continuous, and distinguishable colors ranging from magentato
violet, blue, cyan, and green during the charge/discharge processes. An observer can see the remaining battery charge at a glance. Reproduced under
the terms of the CC-BY license.[250 ] Copyright 2022, The Authors, Published by American Association for the Advancement of Science. d) Li-plasmonic-
resonator-based electrochromic batteries realized by the reversible electrodeposition of Li metal nanoparticles. The color of the windmill battery changed
from colorless to an elaborate series of rich colors as the charging capacity increased from 0 to 0.195 μAh during charging, but disappeared completely
after 0.130 μAh of the capacity was discharged during discharging. Reproduced under the terms of the CC-BY license.[251 ] Copyright 2022, The Authors,
Published by Oxford University Press.
The energy density of current electrochromic energy storage
devices diers greatly, by ≈– orders of magnitude, from that
of conventional devices (e.g., Li-ion batteries).[ ] Therefore, it
is a significant challenge to eciently improve the energy den-
sity of electrochromic energy storage devices while ensuring ex-
cellent electrochromic performance. Si, with a high specific ca-
pacity at room temperature (≈ mAh g−),[ ] is among the
most promising anode materials for the next generation of Li-
ion batteries. Yang et al. employed Si as an active electrochromic
and electrochemical material and Ag as a collector and strong re-
flector to create a FP-cavity-enhanced electrochromic electrode,
which was used in a Li-ion battery with Li metal as the counter
electrode (Figure c).[] This electrochromic battery exhibited
excellent characteristics such as an open-circuit voltage of ≈. V
(vs Li/Li+), an ultrahigh capacity of ≈ mAh g−, and a life-
time of at least  cycles even at a high charge rate of C.
More importantly, the FP-cavity-enhanced Li-ion battery exhib-
ited remarkable color tuning because of the dramatic changes
in the refractive index and thickness of the Si layer during the
charge/discharge processes. For example, a cell consisting of a
 nm thick Si film exhibited a broadband reflection dip tun-
ing from  to  nm during discharge from . to . V,
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Tabl e 3 . A brief summary of resonant-cavity-enhanced electrochromic energy storage devices.
Optical resonators Materials Structural color variation Electrochemical energy storage properties References
FP WO3/W From yellow to yellow-green,
beige-green, pea green,
grass green, patina green,
blue-green, sapphire blue,
steel blue, and dark blue
1) Area capacitance: 23.4 mF cm−2
2) Energy density: 1.13 ×10−3mWh cm−2
3) Charging/discharging time: 3.5/4.9 s
4) Lifetime: 3000 cycles
[244]
Mn2O3/Ti From orange to violet 1) Open-circuit voltage: 1.5 V
2) Specific capacity: 283 mAh g−1
3) Energy retrieval rate: 72%
4) Switching time: 30 s
[246]
Si/Ag From magenta to violet, blue,
cyan, and green
1) Open-circuit voltage: 1.5 V
2) Specific capacity: 3000 mAh g−1
3) Lifetime: 400 cycles
[250]
Plasmonic Li From colorless to purple, blue,
yellow and cyan
1) Energy consumption: 0.390 mW cm−2
(dynamical coloration); 0.105 mW cm−2
(static coloration)
2) Lifetime: 45 cycles
3) Energy efficiency: ≈100%
[251]
resulting in continuous, distinguishable colors ranging from ma-
genta to violet, blue, cyan, and green. An observer sees the re-
maining battery charge at a glance. Because the FP cavities are
easy to prepare, complex and brilliant patterns, such as the col-
orful chameleon shown in Figure c, were obtained by control-
ling only the thickness of the Si layer. The gradual discharge of
the fully charged chameleon from . to ., ., and . V
was accompanied by a distinct change in color. When fully dis-
charged, all the initial colors changed to a dark green similar to
the background, indicating that the chameleon had blended into
its surroundings. When fully recharged, the chameleon was fully
restored to its original state.
A similar multicolor energy storage strategy based on
plasmonic-resonator-enhanced electrochromic Li batteries was
reported by Zhu and co-workers.[ ] A panel-type electrochromic
battery was fabricated as shown in Figure d, where LiFePO
was used as the Li source and cathode, and a perforated MgF/W
film was used as the anode template; they were surrounded by a
Li+liquid electrolyte. During charging, Li ions from the LiFePO
migrated toward the anode and were then reduced, nucleated,
and deposited in prepatterned holes in the MgFfilm. The re-
sulting Li metal nanoparticle arrays exhibited strong plasmonic
resonances that created rich and vivid colors. When the battery
ran out of power, these colors were erased, and the materials
were restored to their original state as the Li metal plasma was
completely removed. Notably, Li metal nanoparticles grew with
increasing charge capacity, which modified the reflectance spec-
tral response, resulting in real-time plasmonic coloration. As a
result, a windmill microcell was fabricated to check the battery
storage capacity in real time. The color of the windmill changed
from colorless to a series of rich and elaborate colors as the charg-
ing capacity increased from  to . μAh during charging; the
color was completely erased after . μAh of the capacity was
discharged during discharging (Figure d). More critically, the
electrochromic battery provided structural color tuning with pro-
nounced high spatial resolution (≈μm) because of the advan-
tages of Li-based plasmonic resonators. In addition, the energy
recovery of Li metal reduced the energy consumption of the elec-
trochromic battery to . mW cm−for the active coloration
state and . mW cm−for the static coloration state, which
is close to zero energy consumption and approaches the energy
eciency limit of commercial Li batteries near %.
In general, resonant-cavity-enhanced electrochromic energy
storage devices are an emerging class of smart power devices with
not only excellent electrochemical energy storage properties, but
also an attractive appearance in terms of rich and vivid structural
color tunability (Table 3). These highly vibrant, distinguishable
colors provide precise and intuitive visual feedback on the en-
ergy level of the energy storage device, eliminating the need for
tedious inspections. They can be used in higher-order application
scenarios, providing a potential strategy for health status moni-
toring and the visual troubleshooting of individual electrochem-
ical energy storage units in energy storage modules. Moreover,
pixelated electrochromic energy storage devices have become im-
portant because of the advantages of optical resonators. For ex-
ample, in the work mentioned above,[ ] an electrochromic cell
with the dual functionalities of high-resolution display and high-
capacity energy storage was developed. This dual-function elec-
trochromic device with ultralow power consumption is ideally
suited for future portable, wearable electronic devices, for use as
both a multichromatic display and a power source to power the
display and other electronic components. This possibility merits
extensive research and development.
4.4. Applications of Multispectral Tunability
The visible spectral range (VIS, – nm), as the part of the
electromagnetic spectrum visible to human eyes, is indispens-
able for conveying visible information to humans. Because of its
importance, recent developments in dynamic photonic modula-
tion using optical resonators to manipulate visible light have re-
sulted in the emergence of various visual applications such as
multichromatic displays, adaptive visible camouflage, and visual-
ized energy storage. The near-infrared (NIR, .–. μm) region
is the range closest to the visible spectrum with large variations
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in refractive index or dielectric function in many electrochromic
materials. Thus, it may be suitable for rapidly switched, distinctly
tunable applications with optical resonator enhancement such
as dual-band smart windows, nanoantennas, optical waveguides,
and highly sensitive sensors.[,– ] Metal oxide nanocrystals
(NCs) possess a metal-nanoparticle-like LSPR eect, resulting in
strong absorption in the NIR band, which can be tuned by atomic
doping or electrochemical doping at sucient potentials.[– ]
For example, Lee and co-workers presented a dual-band (VIS
and NIR) electrochromic window based on Ta-doped TiONCs,
where the Ta-doped TiONCs have strong LSPR absorption that
can be used to modulate the NIR transmittance.[] The smart
window device had the capability to operate in three modes.
When the voltage was . V, the device was colorless and trans-
parent, and exhibited high transmittance (>%) in both VIS and
NIR bands, that is, it was in the “bright” mode. When the voltage
dropped to . V, the device was light blue in color with a good VIS
transmittance (.%), and blocked .% of the NIR light due
to the strong LSPR absorption in NIR region, that is, it was in the
“cool” mode. When the voltage further reduced to . V, the de-
vice became completely dark and blocked .% of VIS light and
.% of NIR light, that is, it was in the “dark” mode. In addition
to metal oxide NCs, metallic polymers also exhibit a strong LSPR
eect.[, ] For example, Giessen and co-workers proposed
and demonstrated a plasmonic-resonator-enhanced switchable
nanoantenna based on the electrochromic material PEDOT:PSS
in the NIR spectral range.[ ] When a bias of + V was applied,
the PEDOT:PSS had high carrier density and metallic optical
properties, because the real part of its dielectric function is less
than  in the NIR spectral range. As a result, the electrochromic
nanoantenna displayed a strong plasmonic resonance at ≈. μm
(Figure 13a), suggesting that it was in the on state. However,
when a bias of − V was applied, the PEDOT:PSS became insu-
lated, without a plasmonic eect (Figure a), owing to the large
reduction in carrier density, which increased the real part of the
dielectric function to greater than . Consequently, the nanoan-
tenna was turned o. More importantly, plasmonic-resonator-
enhanced electrochromic nanoantennas can switch between the
on and o states at video-rate frequencies of up to  Hz. As
an interesting demonstration, the electrochromic nanoantenna
was used for the active control of incident beam steering in
the NIR band in a fixed angular range (Figure a), which is
extremely attractive for augmented and virtual reality technolo-
gies using transmission. The plasmonic resonance position of
the nanoantenna can be conveniently tuned for use in dierent
application scenarios by controlling the shape and size of the
electrochromic nanostructure.[,, ] Similarly, WOwas used
to develop an all-solid-state plasmonic-resonator-enhanced elec-
trochromic waveguide for light modulation in the NIR range.[ ]
As shown in Figure b, the electrochromic waveguide was com-
posed of a  nm thick WOlayer and a  nm thick LiNbO
layer sandwiched between two plasmonic Au electrodes. When a
voltage of −. V was applied, the extinction coecient of WO
increased with increasing lithiation, resulting in high plasmonic
loss at the high field at the WO/Au interface. As a result, for
aμm long waveguide, the transmitted light was rapidly sup-
pressed after irradiation with a  nm laser beam, with a high
modulation depth of ≈ dB (Figure b). The plasmonic res-
onator provided high field enhancement in the waveguide, en-
abling the use of an ultrathin WOlayer and thus ensuring a fast
light modulation response (within  s). In addition, Zhou et al.
designed a hybrid cavity based on the electrochromic material
triphenylamine-based polyamide by coupling two plasmonic res-
onators to produce a strong Fano resonance in the NIR spectral
range with ultrahigh refractive index sensitivity.[] When a volt-
age from  to . V was applied, the electrochromic modulator
exhibited a significant color change in the visible range from col-
orless to bright green, and the sharp Fano resonance peak in its
transmission spectrum was redshifted from  to  nm.
This marked spectral shift enabled more sensitive measurement
of small refractive index changes in the surroundings.
Thermal radiation typically occurs primarily in the mid-
infrared (MIR, .– μm) region, where it plays a critical role in a
wide range of technologies, including thermal camouflage, imag-
ing, and thermal management. In particular, dynamic thermal
radiation regulation appears to be an emerging area of research
with excellent potential for applications in adaptive thermal cam-
ouflage, the radiative cooling of energy-ecient buildings, and
clothing for personal thermoregulation.[– ] According to the
Stefan–Boltzmann law, the thermal radiation of an object is pro-
portional to the surface emittance (ɛ) and the fourth power of
the temperature (T).[ ] Therefore, it is particularly important
to precisely control the emissivity and thus the thermal radia-
tion, as straightforward temperature control avoids the waste of
a large amount of additional energy. Because the emissivity of
electrochromic materials can be tuned in response to electrical
stimuli, they may be used as smart materials for the active dy-
namic regulation of thermal radiation. However, conventional
electrochromic materials pose challenges, such as little emis-
sivity modulation and a limited modulation band,[– ] and
their emissivity modulation is far inferior to that of phase-change
materials, for example, Ge–Sb–Te alloys and vanadium dioxide
(VO). Optical resonators can solve these problems. For example,
plasmonic resonators based on metal nanoparticles may cause
LSPR eects and high free-electron-induced losses, which may
result in high absorption at visible–infrared (IR) wavelengths.
By contrast, homogeneous metal films (e.g., Ag, and Al films)
with ultrahigh reflectivity (≈%) over the entire IR spectrum
are considered as ideal IR reflectors for suppressing thermal
radiation. Thus, reversible metal electrodeposition, an emerg-
ing electrochromic technique based on plasmonic resonators,
promises to enable the dynamical modulation of thermal radia-
tion. Following this strategy, Li et al. prepared a thermoradiation-
modulated electrochromic device for adaptive thermal camou-
flage that provided a large modulation of emissivity by the
deposition/dissolution of Ag.[ ] The proposed device consisted
of an IR-transparent BaFsubstrate, a plasmonic nanoscopic Pt
film with high IR absorption and partial IR transmission, a gel
electrolyte containing Ag+ions with high IR absorption, and a
conductive ITO counter electrode. When Ag was not electrode-
posited, the device had high IR absorption and thus high emis-
sivity because of the combined eect of the optical and radia-
tive properties of the nanoscopic Pt film and gel electrolyte layer.
After electrodeposition, the Ag electrodeposited on the Pt film
almost completely reflected IR light, bringing the device into a
low-emittance state. As a result, the device exhibited large, uni-
form, and consistent IR tunability, where the emissivity modu-
lation (∆ɛ) was . and . in mid-wave IR (MWIR, – μm)
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Figure 13. Applications of multispectral tunability. a) Plasmonic-resonator-based switchable nanoantennas based on the electrochromic material PE-
DOT:PSS in the NIR spectral range (≈2.2 μm). Reproduced with permission.[254] Copyright 2021, American Association for the Advancement of Science.
b) All-solid-state plasmonic-resonator-based electrochromic waveguides based on the electrochromic material WO3for light modulation at 1550 nm.
Reproduced with permission.[255 ] Copyright 2021, American Chemical Society. c) Thermoradiation-modulated electrochromic devices with plasmonic
nanoscopic Pt films and Cr2O3layers for adaptive visible and thermal camouflage. The devices enabled high emissivity modulation by exploiting the
deposition/dissolution of Ag, where the emissivity modulation (∆ɛ) was 0.77 and 0.71 in the MWIR (3–5 μm) and LWIR (7.5–13 μm), respectively. The
generated Ag layer and top Cr2O3layer formed a FP cavity that produced significant color modulation for the visible visual camouflage. Reproduced
with permission.[263 ] Copyright 2020, The Authors, Published by American Association for the Advancement of Science. d) Plasmonic-resonator-based
electrochromic devices for solar heating and daytime radiative cooling. Reversible Ag electrodeposition was used to simultaneously modulate the ab-
sorption in the solar band (0.3–1.5 μm) and emissivity at MIR wavelengths (2.5–18 μm). Reproduced with permission.[273 ] Copyright 2021, American
Chemical Society.
and long-wave IR (LWIR, .– μm), respectively. More inter-
estingly, chromium oxide (CrO) layers were used between the
BaFsubstrate and nanoscopic Pt film to produce rich and bril-
liant colors by FP resonance to enhance the visible camouflage
with little eect on the IR modulation performance (Figure c).
Note that the Earth’s atmosphere is largely transparent in the
MIR wavelength range of – μm. This wavelength range, which
is typically referred to as the atmospheric transmission win-
dow, is consistent with the blackbody thermal radiation peak
at approximately ambient temperature ( K), and thus en-
ables the thermal management.[ ] Therefore, the device before
electrodeposition may provide additional radiative heat dissipa-
tion. Referring to this work, Hsu and co-workers also proposed a
plasmonic-resonator-based electrochromic device for solar heat-
ing and daytime radiative cooling by controlling reversible Ag
electrodeposition to simultaneously modulate the absorption in
the solar band (.–. μm) and emissivity at MIR wavelengths
(.– μm).[ ] An ultrawideband transparent conductive elec-
trode consisting of a monolayer of graphene, Au microgrid, and
polyethylene membrane with high optical transmittance in the
visible–IR waveband was developed for the device. As shown in
Figure d, when Ag was not electrodeposited, the device simul-
taneously exhibited high reflectivity (low absorptivity, 𝛼=.)
from the Ag back reflector in the solar band and high emissivity
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(ɛ=.) from the electrolyte at MIR wavelengths. Thus, it can be
used for daytime radiative cooling. When Ag with the appropriate
particle size and distribution discontinuity was electrodeposited,
the Ag nanoparticles acted as a plasmonic resonator that exhib-
ited strong plasmonic absorption in the visible region and high
metal reflection at MIR wavelengths. As a result, the device ap-
peared black in the solar band owing to its high absorptivity (𝛼=
.) and exhibited low emissivity (ɛ=.) at MIR wavelengths,
allowing it to operate in solar heating mode.
In addition to color modulation in the visible band, optical
resonators allow for multispectral tunability beyond the visible.
However, because the plasmonic eects are stronger in the IR
region, plasmonic resonators are often used to dynamically mod-
ulate the transmittance/reflectivity/emissivity in the IR or even
THz region. To the best of our knowledge, other resonators, such
as FP and PhC cavities, have not been applied to improve the
performance of electrochromic multispectral modulation. How-
ever, FP and PhC cavities have been widely used for simulta-
neous static thermal radiation management and visible color
decoration.[– ] In addition, a VO-based FP cavity capable
of passive dynamic radiative cooling by thermochromism was
recently proposed.[ ] Therefore, IR electrochromic devices us-
ing other resonators such as FP cavities, PhC cavities, Mie res-
onators, and some hybrid cavities have excellent potential for
applications involving multispectral tunability after the critical
problem of strong IR absorption/emission by the electrolytes is
solved.
5. Summary and Outlook
In this review, we addressed the critical drawbacks of con-
ventional electrochromic materials and devices, such as poor
panchromatic tunability, poor cycling stability, long response
times, and limited applications. We then comprehensively re-
viewed the fundamental mechanisms of optical resonators and
their ability to enhance multicolor electrochromic properties.
We also discussed the important benefits of resonant-cavity-
enhanced electrochromic materials and devices for advanced and
emerging applications such as multichromatic displays, adaptive
visible camouflage, visualized energy storage, and applications of
multispectral tunability. Optical resonators including plasmonic
and Mie resonators and FP, PhC, and hybrid cavities were dis-
cussed and shown to provide strongly confined electromagnetic
fields in ultrasmall volumes with enhanced light–matter interac-
tions, enabling the multicolor or even panchromatic tuning of
electrochromic materials/devices, even when the electrochromic
layer is rather thin. In particular, each type of optical resonator
has unique characteristics and is most suitable for enhancing
certain color performance parameters of electrochromic mate-
rials and devices. For example, plasmonic-resonator-enhanced
electrochromic materials and devices can provide high-resolution
color images with up to   DPI and ultrafast switching
(on the order of milliseconds) between various colors. By con-
trast, FP cavities are simple in structure and easily scalable, mak-
ing them strong candidates for large-area, flexible, and panchro-
matic tuning. Various optical resonators and electrochromic ma-
terials can be engineered to enhance the desired multicolor elec-
trochromic performance for dierent real-world applications.
Consequently, resonant-cavity-enhanced electrochromic materi-
als and devices have tremendous potential to address the inher-
ent drawbacks of conventional electrochromic materials and de-
vices and enable superior characteristics such as excellent chro-
maticity, high-brightness and high-resolution colors, full-color
modulation, video-speed refresh rates, good cycle life, and small
device volume. However, their further development still faces
challenges (Figure 14).
First, existing strategies do not yield acceptable overall mul-
ticolor electrochromic performance. Several resonant-cavity-
enhanced electrochromic materials and devices exhibit excel-
lent performance according to individual key metrics, for ex-
ample, excellent chromaticity, panchromatic tuning, or millisec-
ond response time, but a candidate with high overall perfor-
mance that can be used for commercial applications has not yet
been identified. Most research has investigated the use of op-
tical resonators to obtain multicolor tuning of electrochromic
materials and to improve specific key performance parameters,
such as luminance, saturation, and color gamut. Therefore, it
is too early to assess the potential of these emerging concepts
for real-world applications according to the results obtained to
date. Balancing and integrating the various performance parame-
ters to achieve overall performance improvement is a major chal-
lenge to be overcome in future studies. However, it may be dif-
ficult because of the limitations of a single optical cavity. Hy-
brid cavities may oer solutions that are worthy of further ex-
ploration. In addition, the performance according to some met-
rics needs further improvement. Although some performance
metrics of resonant-cavity-enhanced materials and devices have
been significantly improved, the results are unsatisfactory over-
all. ) Panchromatic modulation has been achieved by electri-
cally altering the thickness of the electrochromic material in a
FP cavity,[–] or by reversible metallic electrodeposition on
a plasmonic resonator.[] However, the obtained devices have
various drawbacks such as poor bistability, inhomogeneous col-
ors, diculties in large-scale preparation, and, more importantly,
too-short cycle lifetimes of at most a few tens of cycles. Materi-
als with large refractive index dierences before and after ion
intercalation or novel optical resonators capable of converting
the limited refractive index variations of existing electrochromic
materials into larger spectral modulations must be identified
and developed as better solutions to the problem of optimizing
panchromatic modulation. ) Metal-based optical cavities, for ex-
ample, plasmonic resonators and FP cavities, have inherent loss
problems that result in poor brightness. The combination of all-
dielectric-material-based Mie resonators and electrochromic ma-
terials merits further exploration and development to enhance
the luminance. ) The angular dependence of displays is a ma-
jor consideration, but it has been neglected almost in all current
studies. The angular insensitivity of the devices can be improved
by resorting to wrinkled or disordered structures.[,– ] )
Some organic electrochromic materials have exhibited ultrafast
response speeds of up to video refresh rates (– Hz) be-
cause of optical resonator enhancement. However, they tend to
degrade easily during cyclic uses or under outdoor operating
conditions. Thus, it is urgently necessary to identify highly sta-
ble optical resonators suitable for practical use with inorganic
electrochromic materials to enhance their switching speed and
shorten their response time. In addition, electrochromic mate-
rials can be doped with metals,[,,– ] and/or designed as
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Figure 14. Summary of challenges and the corresponding solution paths for resonant-cavity-enhanced electrochromic materials and devices.
porous and nanomaterials,[,,,,– ] which is expected to
further enhance the electrochemical reaction rate. In addition to
the role of optical resonators and material designs, some struc-
tural designs also help to improve the response speed, such
as nanoslit arrays,[] nanoscale protrusions,[] nanoparticle-on-
mirror constructs,[] and metasurfaces.[]
Second, optical resonant cavities need further optimization.
) Scalable and large-area manufacturing of resonant cavities
is a big and urgent challenge in this field. However, some ex-
isting simple and proven approaches provide feasible strate-
gies for large-scale fabrication of resonant cavities, such as opti-
cal standing-wave technology,[] roll-to-roll manufacturing,[ ]
inkjet printing technology,[] and electrodeposition.[ ] ) The
stability of the optical resonant cavity is a challenge during
the electrochromic process. In a volumetric point of view,
electrochromic materials undergo repetitive strain during the
electrochemical cycles, which is specifically the volumetric
contraction and expansion during coloration and bleaching,
leading to structural decomposition and eventual degrada-
tion of performance over the cycles. This is a fatal blow
to the stability of the optical resonance. However, porous
and/or nanostructured electrochromic materials can eec-
tively release the stress inside the materials during the
ion insertion and extraction process,[,,,,, ] which
could thus eectively maintain the structural stability of
materials. Accordingly, they can be combined with opti-
cal resonators, which is expected to achieve thousands of
cycles.
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Third, the independent control of color and intensity is a ma-
jor challenge.[ ] The active color tuning of electrochromic ma-
terials in optical cavities involves changing the refractive index,
whereas on/o switching is obtained by varying the extinction co-
ecients to increase and broaden the absorption. For example,
in many of the studies mentioned above, the extinction coe-
cient of an ultrathin electrochromic polymer was changed to ob-
tain on/o switching.[, ] Ideally, the proposed resonant-cavity-
enhanced materials and devices for reflective display applications
should change color over a wide range or even the entire visi-
ble spectrum when a voltage is applied, turn o/become black
when needed, display high contrast against the background, and
remain reversible. For electrochromic materials, however, it is
very dicult to reach this goal. Color tuning of electrochromic
materials is inevitably accompanied by intensity modulation be-
cause the real (n) and imaginary (k) parts of the optical con-
stants are closely linked and cannot be separated, according to
the Kramers–Kronig relations. Consequently, at this stage of re-
search, the reflectance/brightness decreases significantly as the
color changes, which greatly limits practical applications, espe-
cially in displays, and severely aects the luminosity of the dis-
play, resulting in a poor viewing experience. An eective way
to address this challenge is to find an electrochromic material
in which only the refractive index changes while the extinc-
tion coecient remains constant, or to develop an optical cav-
ity that is sensitive to changes in the refractive index but not
the extinction coecient. In addition, independent and contin-
uous intensity (gray-level) modulation is worth exploring to ob-
tain photorealistic displays. A universal nanophotonic pixel con-
sisting of two Schrödinger subpixels and a transmittance filter is
an interesting concept proposed by Rezaei et al. to address this
issue.[ ]
Furthermore, devices must be extensively modified in re-
sponse to their performance in practical applications, and their
device characteristics need to be further improved. Most state-of-
the-art color performance measurements have been performed
in electrochemical cells of three-electrode systems, which are dif-
ficult to graft onto devices. The device configuration is more com-
plex and involves more diverse considerations, such as the eect
of the counter electrode and electrolyte on color chromaticity and
brightness, the eect of applied voltage magnitude on color con-
trast and cycle life, and the eect of device structure, electrolyte
conductivity, and even inserted ion species on response time.
Therefore, future research should focus on the development of
multicolor all-solid-state electrochromic devices,[] the exploita-
tion of new device structures,[, ] and the development of im-
proved electrolytes.[ ]
Finally, resonant-cavity-enhanced electrochromic materials
and devices need to be further cross-integrated with other ad-
vanced technologies to broaden the application scenarios. )
For integration with advanced display technologies, for example,
LCDs,[, ] more research and engineering work is needed be-
fore practical applications become possible. The primary chal-
lenge of achieving selective and localized control of micro- and
nanoscale pixels must be addressed. ) For integration with flex-
ible, wearable electronics, multifunctionality must be incorpo-
rated, where desirable functions include but are not limited to
aesthetic decoration, multicolor sensing, information display,
and energy storage/power. ) Integration with the emerging con-
cepts of building and personal thermal management is also de-
sirable. Although reversible metal electrodeposition is an eec-
tive means that has yielded excellent results, metallic nanoparti-
cles tend to wear out and become degraded by poisoning during
cycling. Resonant-cavity-enhanced electrochromism is prefer-
able. However, to the best of our knowledge, a resonant-cavity-
enhanced electrochromic material or device for the dynamic
modulation of heating and cooling in its true sense has not yet
been realized, and this gap area is worth exploring. ) For integra-
tion with surface-enhanced Raman scattering (SERS) techniques,
the optical cavity can provide a physically enhanced local light
field.[ ] By contrast, electrochromism can chemically enhance
the activity of SERS materials.[ ] The combination of these two
techniques is expected to be a powerful means of further im-
proving the Raman enhancement factor and reducing the detec-
tion limit of SERS techniques. ) Integration with other advanced
technologies, such as photovoltaics and photocatalysis,[– ] is
also necessary.
We anticipate resonant-cavity-enhanced electrochromic mate-
rials and devices with overall superior performance and versa-
tile scalability will be realized. This direction is an advanced
and emerging approach in electrochromics and merits much
eort from researchers in this field. Current progress is still
slightly slow, and these approaches are far from commercializa-
tion. However, with the rapid development of advanced cavity de-
sign, fabrication techniques, and active electrochromic materials,
electrochromic materials and devices with unprecedented color
properties will be used in more important applications in various
cross-disciplinary fields and will contribute greatly to the devel-
opment of the corresponding research fields.
Acknowledgements
The authors are grateful for the financial support from the National Key Re-
search and Development Program of China (Grant No. 2020YFB1505703).
This work was supported by the National Natural Science Foundation of
China (Grant Nos. 51972331, 52172299, and 22175198). Z.G.Z. would
like to acknowledge the support from the External Cooperation Program
of the Chinese Academy of Sciences (Grant No. 121E32KYSB20190008)
and the Suzhou Municipal Science and Technology Bureau (Grant No.
SJC2021006). S.C. would like to acknowledge the support from Suzhou
Industrial Science and Technology Program (Grant No. SYC2022036).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
electrochromic materials and devices, excellent cycling lifetimes, fast re-
sponse times, light–matter interactions, multicolor properties, optical res-
onators
Received: January 6, 2023
Revised: February 26, 2023
Published online:
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Jian Chen received his B.S. degree in Material Physics from the China University of Petroleum in 2017.
He is currently a Ph.D. candidate under the supervision of Prof. Zhigang Zhao at the Suzhou Institute
of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences. His current research interests include
the design of novel electrochromic materials and devices and their applications in energy conversion
and storage, multicolor displays, and reconfigurable photonic devices.
Ge Song obtained her B.S. degree in Chemistry from the China Agricultural University.She obtained
her M.S. degree in Chemistry from Bristol University in 2018. She received her Ph.D. degree from the
Univesity of Science and Technology of China in 2022. Her research interests include the synthesis of
inorganic semiconductors, and their photochemical properties.
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Shan Cong is an Associate Professor at the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese
Academy of Sciences. He received his Ph.D. degree from the Zhejiang University in 2011. His research
interests include the synthesis of low-dimensional nanomaterials, novel electrochromic materials and
devices, surface-enhanced Raman scattering, and semiconductor photocatalysis.
Zhigang Zhao is a Professor of the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese
Academy of Sciences. He received his Ph.D. degree from the Institute of Metal Research, Chinese
Academy of Sciences in 2006 and did postdoctoral research at Dayton University (USA), Osaka Uni-
versity (Japan), and the National Institute of Advanced Industrial Science and Technology (AIST,
Japan) from 2006 to 2011. His research interests are focused on tungsten-based functional materi-
als and relative optoelectronic applications, in particular electrochromic materials and devices for
over 10 years.
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