Comprehensive Investigation of the Effect of Annealing on Electrochromic Properties of WO₃ Films

Abstract

Tungsten trioxide (WO₃) is the most widely studied cathodic electrochromic (EC) material, serving as the core component of energy-efficient smart windows, displays, and optical modulation devices. Post-deposition annealing, as a critical post-processing technique, precisely regulates the microstructure, crystallinity, oxygen vacancy concentration, and electronic structure of WO₃ thin films, thereby directly determining their EC performance. This review summarizes the research progress of annealing effects on WO₃ films, focusing on the synergistic regulation of annealing temperature, atmosphere, and dwell time. It elaborates on the fundamental EC mechanisms of amorphous and crystalline WO₃, including polaron hopping and free-electron Drude behavior, and analyzes the influence of different deposition methods (magnetron sputtering, sol–gel, electrodeposition, etc.) on the annealing response of films. The optimal annealing windows for balancing optical modulation, coloration efficiency, switching speed, and cycling stability are clarified: moderate temperatures (200–350 °C) and inert/air atmospheres yield mixed amorphous–nanocrystalline structures with optimal oxygen vacancy content. Current challenges such as the inherent contrast–stability trade-off and thermal budget limitations of flexible substrates are discussed, and future directions including spatially resolved annealing, interface co-design, and machine learning-assisted optimization are prospected. This work provides a theoretical reference and process guidance for the development of high-performance WO₃-based EC devices.

1. Introduction

Driven by the depletion of fossil fuel reserves and the growing severity of climate change, the global energy crisis has deepened, creating an urgent demand for energy-efficient building technologies [1,2]. The building sector accounts for approximately 30%–40% of global energy consumption, of which a significant fraction is consumed by heating, ventilation, and air conditioning (HVAC) systems and indoor lighting [3]. Smart windows based on electrochromic (EC) technology offer a compelling solution by dynamically regulating solar radiation and visible light transmission to reduce this energy burden [4,5]. Advanced dual-band EC smart windows have demonstrated energy savings of 20%–27% in office buildings compared with conventional glazing [6], propelling the global EC materials market to an estimated USD 2.3 billion in 2025, with a projected compound annual growth rate of 10.5% through 2034 [7].

Electrochromism is defined as the reversible and persistent change in the optical properties of a material, including transmittance, reflectance, and absorptance, upon the application of an external electric field [8,9]. Since Deb first reported reversible coloration in tungsten oxide thin films in the 1960s [10], EC materials have found applications in smart windows [4,5] anti-glare rearview mirrors [11], low-power reflective displays [12], and camouflage systems [13]. Among the numerous candidates investigated—spanning inorganic transition metal oxides and organic conducting polymers—tungsten trioxide (WO₃) stands as the benchmark cathodic EC material, dominating over 80% of smart window research [14]. Its prominence stems from high optical modulation, large coloration efficiency, broad visible-to-near-infrared spectral tunability, robust electrochemical reversibility, and relatively low fabrication cost [15,16]. The underlying EC mechanism and key performance metrics are detailed in Section 2.

The morphology of the as-synthesized WO₃ powder was further characterized via scanning electron microscopy (SEM), with representative images displayed in Figure 1a,b. The WO₃ particles feature a hierarchical structure assembled from two-dimensional nanosheets, with a typical lateral dimension of approximately 1 μm.

The rich polymorphism and structure–property relationships of WO₃ itself form the foundational layer upon which annealing exerts its effects. Beyond the common monoclinic phase, unique structural variants such as the two-dimensional octahedral molecular sieve h′-WO₃ offer distinct ion transport channels that directly influence electrochromic kinetics and efficiency [17]. The response of any WO₃ phase to thermal treatment is governed by its fundamental lattice dynamics and the energy landscape for phase transformations [18,19]. Ultimately, annealing serves as a powerful tool to selectively modify this structural landscape, simultaneously tailoring the crystallinity, morphological features, and optical properties of the film, as evidenced by comprehensive investigations linking specific annealing conditions to multifaceted property evolution [20].

A central challenge in WO₃-based EC devices is the inherent trade-off between performance and durability [13]. Amorphous WO₃ films generally deliver higher optical modulation, greater coloration efficiency, and faster switching, whereas crystalline films exhibit markedly superior long-term cycling stability [21,22,23]. Resolving this dichotomy is essential for the practical deployment of EC smart windows.

Post-deposition thermal annealing is arguably the most direct lever for navigating this trade-off, as it enables precise control over the crystallinity, grain morphology, surface topology, and defect chemistry of WO₃ films [24,25]. Annealing temperature governs the amorphous-to-crystalline phase transition, grain growth kinetics, and film densification [20]. Au et al. [25] showed that the Li⁺ diffusion coefficient in sol-gel WO₃ films peaked at 250 °C—an intermediate regime where partial crystallization enhanced ion transport without forfeiting the benefits of the amorphous phase. The annealing atmosphere is equally consequential. Yu et al. [21] found that WO₃₋ᵧ films annealed in Ar versus O₂ at the same temperature attained comparable crystallinity yet diverged sharply in oxygen vacancy concentration: Ar-annealed films, richer in vacancies, exhibited superior optical modulation and coloration efficiency, while O₂-annealed films showed markedly improved cycling stability. This result underscores the dual role of oxygen vacancies—beneficial for EC activity at moderate levels, yet detrimental to structural integrity when excessive [22]. Moreover, the annealing response is not universal; films prepared by magnetron sputtering [21], sol-gel processing [24,25], electrodeposition [26,27], hydrothermal synthesis [28], spray deposition [29] and pulsed laser deposition [30] possess distinct as-deposited microstructures that fundamentally determine their subsequent thermal evolution.

Existing reviews have tended to discuss annealing temperature, atmosphere, and deposition route separately rather than within a coupled framework. The present review therefore seeks to place annealing studies reported for different WO₃ film systems on a more comparable basis. A brief comparison with representative reviews on WO₃ electrochromic films is provided in

Table 1. Comparison of representative reviews on WO₃ electrochromic films.

Ref.	Main Focus	Annealing as Central Theme	Key Distinction from the Present Review
[13]	Amorphous WO3 for electrochromic devices: structure, optimization strategies, and applications	No	Focuses on amorphous WO3 and optimization/application strategies rather than annealing-dependent comparison across WO3 film systems
[14]	WO3 thin films prepared by different deposition techniques and their electrochromic properties	No	Focuses mainly on deposition methods; annealing is not treated as the main organizing variable
[15]	Research progress on WO3 thin films, including preparation methods, morphology control, doping, and device integration [30]	No	Broad WO3 thin-film progress review; annealing appears as one of several processing factors rather than the central analytical framework
[16]	Structure, properties, deposition techniques, and applications of WO3 thin films for electrochromic devices	No	General review of WO3 thin films and applications; does not treat annealing temperature, dwell time, and atmosphere as a coupled theme

.

Existing reviews have primarily focused on the overall electrochromic performance of WO₃ films [14,16], comparisons between amorphous and crystalline materials [13], or analyses of deposition techniques [15]. By contrast, annealing has less often been treated as a central analytical theme linking structural evolution, defect chemistry, and device performance. Against this background, the present work adopts a critical narrative review approach and proposes a three-parameter framework for interpreting and comparing the existing literature, in which annealing temperature, dwell time, and atmosphere are considered as coupled process variables. On this basis, the review re-examines the structure–property relationships of WO₃ films prepared by different deposition routes, the trade-off between high optical contrast and long-term durability, and the practical issues associated with device implementation, including thermal budget, standardized testing, and stability assessment.

This article is intended as a critical narrative review rather than a formal systematic review or meta-analysis. The relevant literature was identified mainly through Web of Science, Google Scholar, and publisher databases using keywords such as “WO₃”, “tungsten trioxide”, “electrochromic”, “annealing”, “heat treatment”, “oxygen vacancy”, “crystallization”, and “thin film”. The discussion focuses on studies addressing annealing temperature, dwell time, annealing atmosphere, crystallinity evolution, optical modulation, and coloration efficiency, while early representative studies are included where necessary to clarify the development of mechanistic understanding. Because substantial differences exist among studies in terms of deposition method, film thickness, substrate, electrolyte, inserted ion species, and testing protocol, this review does not attempt to define universally optimal annealing conditions. Instead, it aims to identify comparable trends, key trade-offs, and issues that remain to be clarified.

2. Fundamentals of Electrochromism in WO₃

2.1. Crystal Structure of WO₃

Tungsten trioxide (WO₃) is a versatile transition-metal oxide whose electrochromic properties are intimately linked to its crystal structure [31,32]. At the most fundamental level, WO₃ adopts a perovskite-derived ReO₃-type framework in which corner-sharing WO₆ octahedra form a three-dimensional network [16,33]. In an ideal cubic perovskite (ABO₃), the A-site cations occupy the large cuboctahedral cavities; in stoichiometric WO₃, these sites remain vacant, resulting in an open framework that readily accommodates small cations (H⁺, Li⁺, Na⁺) during electrochromic ion insertion [32,34]. The corner-sharing connectivity of the WO₆ octahedra creates a network of interconnected channels that serve as the primary pathways for ion diffusion and electrontransport, directly governing coloration efficiency, switching kinetics, and optical modulation [31,35].

WO₃ exhibits pronounced polymorphism, with at least five thermodynamically stable or metastable phases that undergo a well-defined temperature-driven phase-transition sequence [36,37]. Starting from the lowest temperature, the sequence is: ε-WO₃ → δ-WO₃ → γ-WO₃ → β-WO₃ → α-WO₃ [38,39]. As schematically illustrated in Figure 2, these transitions involve progressive tilting and distortion of the WO₆ octahedra while preserving the corner-sharing topology [37,38]. The room-temperature γ-WO₃ phase is the most commonly encountered in bulk and thin-film samples; its moderate octahedral distortion yields a relatively open structure that balances ionic accessibility with structural stability [40]. At elevated temperatures, the orthorhombic β and tetragonal α phases exhibit higher symmetry and larger unit-cell volumes, which in principle facilitate faster ion diffusion but are rarely retained in electrochromic devices operated near ambient conditions [36,39].

In addition to the perovskite-derived polymorphs, two hexagonal variants have attracted considerable attention for electrochromic applications because of their distinctive tunnel structures [41,42]. The classical hexagonal phase, h-WO₃ (space group P6/mmm), consists of WO₆ octahedra arranged in (WO₆)₆ and (WO₆)₃ rings that stack along the c-axis to form one-dimensional hexagonal and trigonal tunnels [42,43]. These open channels provide low-energy pathways for cation insertion, conferring h-WO₃ with superior ion-storage capacity and rapid switching kinetics compared with the more compact monoclinic phases [44]. A more recently discovered polymorph, the novel hexagonal h′-WO₃, further expands this family [17]. Built from (WO₆)₆ “wheels” that generate large (WO₆)₆ tunnels, h′-WO₃ additionally contains (WO₆)₄ and (WO₆)₃ secondary tunnels interconnected by corner-sharing octahedra [17]. This multi-tunnel architecture yields an exceptionally open framework with enhanced microporosity, making h′-WO₃ particularly promising for ultrafast electrochromic devices [17,45]. Both hexagonal phases can be stabilized in thin films or nanostructures, and their tunnel dimensions are highly tunable through synthesis conditions [46].

When prepared as thin films via common techniques such as sputtering, sol-gel processing, or electrodeposition, tungsten trioxide (WO₃) often forms in an amorphous state (a-WO₃) [47,48]. Despite the absence of long-range translational order, a-WO₃ maintains well-defined short-range order at the atomic scale. Its structure is dominated by distorted [WO₆] octahedra interconnected through corner-sharing W–O–W linkages, while a minor proportion of edge-sharing configurations is also present [49,50]. This disordered yet locally coherent network results in a broad distribution of W–O bond lengths and O–W–O bond angles, yielding a more open and defect-rich structure compared to its crystalline counterparts [51]. The short-range order in a-WO₃ is commonly characterized by techniques such as Raman spectroscopy, which exhibits broad bands in the region of ~600–850 cm⁻¹ corresponding to W–O stretching vibrations [40,50], and X-ray absorption fine structure (XAFS) analysis, which indicates a coordination number of tungsten close to six but with pronounced static disorder [49,52]. This structural flexibility endows a-WO₃ with exceptionally high optical modulation and coloration efficiency, making it particularly attractive for applications such as electrochromic devices [47,53]. However, the metastable and disordered network generally leads to inferior long-term cycling stability relative to crystalline WO₃, necessitating a performance–stability trade-off in practical implementations [48,54,55].

2.2. Electrochromic Mechanism

2.2.1. Double-Injection/Extraction Model

WO₃ is governed by the simultaneous injection of cations and electrons, which can be universally described by the well-established double-injection or extraction model, as shown in Figure 3 [10,56]:

$$\mathrm{W}{\mathrm{O}}_3+\mathrm{x}{\mathrm{M}}^{+}+{\mathrm{x}\mathrm{e}}^{-}\leftrightarrows {\mathrm{M}}_{\mathrm{X}}{\mathrm{W}\mathrm{O}}_3$$

(1)

2.2.2. Coloration Mechanism

While the double-injection model provides a macroscopic description of the electrochromic process, the microscopic origin of optical absorption in colored MxWO₃ differs fundamentally between amorphous and crystalline films, constituting one of the most debated topics in electrochromic research.

In amorphous WO₃ films, the predominant coloration mechanism is attributed to small polaron hopping, as first proposed by Schirmer et al. Upon electron injection, the inserted electron becomes localized at a W⁶⁺ site, reducing it to ${\mathrm{W}}^{5+}$ and polarizing the surrounding lattice to form a small polaron—a quasiparticle consisting of the localized electron coupled with its associated lattice distortion. Optical absorption arises when incident photons provide sufficient energy for the polaron to hop from one tungsten site to an adjacent one. The classical intervalence charge transfer model describes this process as [56,57]:

$${\mathrm{W}}_{\mathrm{A}}^{5+}+{\mathrm{W}}_{\mathrm{B}}^{6+}+\mathrm{h}\mathsf{\nu }\to {\mathrm{W}}_{\mathrm{A}}^{6+}+{\mathrm{W}}_{\mathrm{B}}^{5+}$$

(2)

This transition gives rise to a broad absorption band centered in the near-infrared to red region of the visible spectrum, which accounts for the characteristic deep blue coloration [58,59,60]. The absorption coefficient α is directly proportional to the product of the concentrations of ${\mathrm{W}}^{5+}$ and ${\mathrm{W}}^{6+}$ sites, and the absorption peak position depends on the polaron binding energy and the phonon energy involved in the hopping process [61,62,63].

Furthermore, at higher degrees of intercalation, a secondary absorption mechanism may emerge through bipolaron hopping between ${\mathrm{W}}^{4+}$ and ${\mathrm{W}}^{6+}$ sites [58]:

$${\mathrm{W}}_{\mathrm{A}}^{4+}+{\mathrm{W}}_{\mathrm{B}}^{6+}+\mathrm{h}\mathsf{\nu }\to {\mathrm{W}}_{\mathrm{A}}^{6+}+{\mathrm{W}}_{\mathrm{B}}^{4+}$$

(3)

This bipolaron transition contributes primarily to absorption in the shorter-wavelength region, explaining the evolution from blue to nearly opaque coloration at high x values [58]. Notably, Lee et al. [64] proposed an alternative model in which the dominant small polaron transition occurs between charge-induced W⁵⁺ states and pre-existing ${\mathrm{W}}^{4+}$ states in oxygen-deficient amorphous films, rather than between ${\mathrm{W}}^{5+}$ and ${\mathrm{W}}^{6+}$ as in the classical IVCT model. This model successfully explains why oxygen-deficient amorphous films exhibit higher coloration efficiency while remaining transparent in the as-deposited state [64].

In well-crystallized WO₃ films, like the monoclinic (γ-WO₃) phase, the coloration mechanism transitions from localized polaron hopping to delocalized free-electron behavior, which can be described by the Drude model [22,57]. In this regime, the injected electrons are not localized at individual tungsten sites but instead occupy delocalized states within the conduction band, behaving as a free-electron gas. The resulting optical absorption is dominated by free-carrier scattering with ionized impurities and phonons, leading to a reflectance modulation that is most pronounced in the near-infrared region rather than the visible spectrum [22,57,65].

The distinction between small-polaron absorption in amorphous WO₃ and Drude-like free-carrier behavior in more crystalline WO₃ provides a useful conceptual framework, but the actual coloration mechanism is often more complex than a simple binary transition. In many WO₃ film systems, these two contributions may coexist, and their relative importance can vary with crystallinity, defect density, intercalation level, inserted ion species, and detailed phase composition. In partially crystallized or nanocrystalline films, localized polaron hopping may still contribute significantly in disordered regions or near grain boundaries, while more delocalized carrier behavior may emerge in ordered domains. Accordingly, the optical response of WO₃ should be discussed as the result of a variable balance between localized and delocalized electronic processes, rather than as a strict substitution of one mechanism by the other.

Kamal et al. [22] systematically compared the optical behavior of amorphous and crystalline WO₃ films upon proton insertion, demonstrating that crystalline films exhibit significant NIR reflectance modulation reaching ~25%, consistent with Drude-like metallic behavior, whereas amorphous films display a broad visible-range absorption characteristic of polaron transitions. The absorption peak in crystalline films is observed to shift to lower energies (~0.72 eV) upon crystallization above 400 °C, compared to ~1.38 eV in amorphous films [64], reflecting the fundamental transition from localized to delocalized electronic states.

Notably, the polaron and free-electron mechanisms are not mutually exclusive and can coexist within the same material system. Zimmer et al. [66] investigated the coloration mechanism of NaxWO₃ thin films and demonstrated that the relative contributions of polaron absorption and Drude-type free-electron behavior are strongly governed by the degree of crystallinity. Specifically, the polaron mechanism dominates in films with low crystallinity, whereas the Drude mechanism plays an increasingly prominent role as crystallite size increases and long-range order is enhanced.

2.2.3. Ion Transport Dynamics

The chemical diffusion coefficient of the intercalating species is widely regarded as a key kinetic parameter, as it directly governs the ion insertion rate and thereby dictates the overall switching speed of electrochromic devices [67]. For WO₃ films, D values are typically measured by electrochemical techniques including electrochemical impedance spectroscopy, the galvanostatic intermittent titration technique, and the potentiostatic intermittent titration technique. Amorphous WO₃ films exhibit a wide range of diffusion coefficient (D) values, as lithium-ion diffusion is strongly dependent on the microstructure, porosity, thickness, and intercalation degree x of the film [68]. Proton diffusion is generally faster than Li⁺ diffusion due to the smaller ionic radius and lower activation energy for hopping [57,67].

The chemical diffusion coefficient is not an intrinsic constant; instead, it varies strongly with the Li⁺ intercalation stoichiometry x. At low Li⁺ insertion levels, D is relatively high, as ample vacant lattice sites are available for ion migration. With increasing x, site saturation and repulsive Coulombic interactions between intercalated Li⁺ ions lead to a gradual decrease in D. This concentration-dependent diffusion behavior can be well described by the lattice gas model [60,67]. Vuillemin and Bohnke [67] developed a comprehensive kinetic model for amorphous WO₃ films in both acid and lithium electrolytes, revealing a two-step intercalation mechanism with distinct diffusion regimes corresponding to surface and bulk insertion processes.

The activation energy for ion intercalation into the WO₃ lattice is another critical kinetic parameter, reflecting the energy barrier that ions must overcome during migration through the W–O framework. For monoclinic crystalline WO₃, first-principles calculations have revealed that the migration barrier for Li⁺ is highly anisotropic, ranging from 0.2 to 1.2 eV depending on the crystallographic direction, with the most favorable pathways typically along the channels formed by corner-sharing WO₆ octahedra [69]. In amorphous WO₃, the disordered structure creates a distribution of migration barriers rather than well-defined channels, which results in a lower average activation energy and accounts for the generally higher diffusion coefficient and faster switching kinetics observed in amorphous films compared to their crystalline counterparts [59,60].

The interplay between film crystallinity and ion transport kinetics has profound implications for the optimization of electrochromic performance: amorphous films offer faster kinetics and larger optical modulation but are susceptible to ion trapping and cycling degradation, while crystalline films provide more stable and reversible ionic pathways at the expense of reduced switching speed [58,59]. This trade-off is a core theme in the annealing optimization process discussed in the subsequent sections of this article.

The distinct electrochromic behaviors of amorphous and crystalline WO₃ can be traced back to differences in their local structure, defect landscape, and ion-transport pathways, rather than being an intrinsic ranking of “better” versus “worse” materials. In amorphous WO₃, the disordered W–O–W network and larger free volume provide abundant open channels for small cations, together with a high density of localized W⁵⁺ polaron sites that couple strongly to visible-light absorption. This combination typically yields large optical modulation and short coloration/bleaching times, but the same disordered network also contains a broad distribution of shallow, deep, and irreversible ion-trapping sites, which accumulate during prolonged cycling and lead to gradual capacity fading and residual absorption in the bleached state. In contrast, crystalline WO₃ (particularly the monoclinic γ-phase and hexagonal tunnel structures) offers well-defined ion-transport channels and a mechanically robust skeleton that resists structural rearrangement over repeated insertion/extraction. However, the more delocalized electronic states in crystalline WO₃ shift part of the optical response toward Drude-like free-carrier behavior in the near-infrared while reducing the polaron-dominated visible absorption and often slowing ionic diffusion through denser grains and grain boundaries. From this perspective, the widely observed contrast–stability trade-off in WO₃ is not a fixed material limitation, but a direct consequence of how disorder, porosity, polaron density, and framework rigidity are simultaneously modulated by processing—especially by annealing.

2.3. Key Performance Parameters

The parameters optical modulation amplitude, coloration efficiency, switching response time, cycling stability, and memory effect collectively determine the practical applicability of the material in device-level applications.

2.3.1. Optical Modulation Amplitude

Optical modulation amplitude (ΔT) is defined as the difference in optical transmittance between the bleached state ${(\mathrm{T}}_{\mathrm{b}})$ and the colored state ${(\mathrm{T}}_{\mathrm{c}})$ at a specified wavelength:

$$\Delta \mathrm{T}={\mathrm{T}}_{\mathrm{b}}-{\mathrm{T}}_{\mathrm{c}}$$

(4)

This parameter quantifies the optical contrast of electrochromic films and serves as a key intuitive indicator of their optical modulation performance. For WO₃ films, ΔT is typically quantified at 550 nm, which matches the peak sensitivity of human vision, as well as near-infrared wavelengths of relevance to solar energy management [14,57].

Reported ΔT values for WO₃ films span a wide range, strongly depending on film thickness, morphology, crystallinity, and measurement conditions. State-of-the-art WO₃ films with optimized nanostructures have demonstrated ΔT values exceeding 80% in the visible region: for instance, ZnO-doped WO₃ films achieved ΔT = 87.9% in the 400–700 nm range [70], hexagonal-phase WO₃ microflower arrays reached 86.8% at 610 nm [71], and Mo-doped WO₃ films attained 84.5% [72]. At NIR wavelengths, even higher modulation amplitudes have been reported, such as 86.8% at 1200 nm for Ti-doped WO₃ [73].

2.3.2. Switching Response Time

Switching response time refers to the time required for the electrochromic film to transition between its bleached and colored states, typically defined as the time to achieve 90% of the full optical modulation. The coloration time (${\mathrm{t}}_{\mathrm{c}}$) and bleaching time (${\mathrm{t}}_{\mathrm{b}}$) are often asymmetric, with bleaching generally being faster than coloration due to the different kinetic barriers involved in ion insertion versus extraction [14,74].

The switching speed is fundamentally governed by the ionic diffusion kinetics within the WO₃ film and can be approximated by the relationship:

$$\mathrm{t}\approx {\displaystyle \frac{d^2}{D}}$$

(5)

where d is the film thickness, and D is the chemical diffusion coefficient of the intercalating ion [57,74]. This relationship highlights two primary strategies for enhancing switching speed: (1) reducing film thickness and (2) increasing the ionic diffusion coefficient through microstructural optimization [74].

The diffusion coefficient in WO₃ is highly sensitive to the film’s crystalline state: amorphous films generally exhibit higher D values than fully crystallized monoclinic films, leading to faster switching kinetics in amorphous or amorphous/nanocrystalline mixed-phase films [59,75].

2.3.3. Cycling Stability and Durability

Cycling stability, defined as the ability of the electrochromic film to maintain its optical modulation and charge capacity over thousands of consecutive coloration cycles, is a decisive parameter for practical device longevity [14,57].

The primary degradation mechanism in WO₃ films during extended cycling is ion trapping—the irreversible accumulation of intercalated ions that cannot be fully extracted during the bleaching step. Zhang et al. identified three distinct types of ion traps in amorphous WO₃ films: (1) shallow traps, arising from the transformation of amorphous lithium tungsten bronze to orthorhombic Li₂WO₄, which consumes WO₆ octahedra and suppresses polaron hopping between W⁵⁺ and W⁶⁺ sites, degrading only the colored state; (2) deep traps, associated with structural reconfigurations and the formation of W⁴⁺ sites, which degrade both colored and bleached states; and (3) irreversible traps, consisting of non-decomposable coupled W⁴⁺–Li₂WO₄ phases, which permanently reduce the short-wavelength transmittance [58].

Crystalline WO₃ films generally exhibit superior long-term cycling stability compared to amorphous films, attributed to the more stable and well-defined ion diffusion pathways within the ordered lattice, which resist structural rearrangement during repeated intercalation/deintercalation [57,76]. For example, denser WO₃ films with controlled crystallinity have demonstrated degradation of only ~14% in colored-state transmittance after 3000 cycles, compared to ~30% degradation for porous amorphous films after 1000 cycles [58]. Through composite strategies, exceptional cycling durability has been reported: GO-Bi-WO₃ films maintained less than 13.6% optical loss after 10,200 cycles [77], and potentiostatically pretreated WO₃ films achieved less than 10% optical loss after 100,000 cycles [76].

2.3.4. Coloration Efficiency

Coloration efficiency is a critical figure of merit that quantifies the optical change produced per unit of intercalated charge, defined as:

$$\mathrm{C}\mathrm{E}={\displaystyle \frac{\mathsf{\Delta }\mathrm{O}\mathrm{D}}{\mathrm{Q}}}={\displaystyle \frac{\mathrm{l}\mathrm{n}\left({\mathrm{T}}_{\mathrm{b}}/{\mathrm{T}}_{\mathrm{c}}\right)}{\mathrm{Q}}}$$

(6)

ΔOD is the change in optical density, and Q is the inserted charge density. A high CE value indicates that a large optical modulation can be achieved with a small amount of charge insertion, which is desirable for minimizing energy consumption and extending device lifetime [14,57].

2.3.5. Memory Effect

The memory effect describes the ability of an electrochromic film to retain its colored or bleached state after the external voltage is removed [78,79]. This bistable property is essential for energy-efficient operation, as it eliminates the need for continuous power supply to maintain a desired optical state.

In 2003, Nah et al. [79] demonstrated that the application of a protective Ta₂O₅ surface layer on WO₃ films significantly enhanced the memory effect by suppressing the dissolution and back-diffusion of intercalated protons, maintaining a stable colored state over extended open-circuit periods.

In 2023, Faceira et al. [78] systematically investigated the origin of the memory effect in sputtered WO₃ films, defining it as the retention of the colored state for cathodic electrochromic materials under open-circuit conditions. They identified that the memory effect is predominantly governed by the film’s morphology and microstructure rather than composition alone: denser films with lower porosity exhibit superior open-circuit retention because they limit the diffusion of intercalated ions back to the electrolyte/film interface [78].

3. WO₃ Film Deposition Techniques and Their Annealing-Dependent Structure–Property Evolution

The electrochromic performance of WO₃ is not determined by post-treatment alone, but is also conditioned by the initial microstructural blueprint established during deposition. Different physical and chemical fabrication routes generate distinct film densities, porosities, hydration levels, and local stoichiometries, which in turn define how the film subsequently responds to annealing. Therefore, the focus of this section is not the annealing outcome itself, but the deposition-dependent starting state.

3.1. Chemical Solution-Based Deposition Routes

Solution-based fabrication routes, including sol–gel, electrodeposition, spray pyrolysis, and hydrothermal/solvothermal synthesis, produce films with as-deposited characteristics that differ fundamentally from those of PVD-fabricated films. Solution-derived films generally contain residual impurities such as solvent, hydroxyl groups, organic ligands, and structural water. They also exhibit an inherently more porous, granular microstructure, and their as-deposited phase is typically hydrated tungsten oxide instead of stoichiometric amorphous WO₃ [59,80]. Consequently, annealing plays a dual role in solution-derived films: it drives off volatiles and condenses the W–O–W network, and then promotes crystallization at higher temperatures.

The sol-gel route is attractive for its simplicity, low cost, and compatibility with dip-coating, spin-coating, and spraying on large-area substrates [59]. sol-gel WO₃ films are typically deposited as amorphous WO₃·nH₂O with high porosity, and their electrochromic performance is strongly dependent on the subsequent annealing temperature.

In 2020, Au et al. performed a systematic post-annealing study on sol-gel WO₃ films from 27 °C to 450 °C and found that the favorable temperature was around 250 °C: at this point, most structural water had been expelled and the W–O–W connectivity was well established, but the film had not yet crystallized, yielding the maximum Li⁺ diffusion coefficient and a large optical modulation of ~62% [25].

Above 300 °C, crystallization reduced pore volume and ion accessibility, causing CE and ΔT to decline. Deepa et al. compared amorphous and nanocrystalline sol-gel WO₃ films and reported that amorphous films exhibited superior visible-range modulation and higher CE, whereas their nanocrystalline counterparts showed better NIR reflectance modulation consistent with the free-electron Drude mechanism [59]. These findings collectively establish a design rule for solution-derived WO₃: annealing should be sufficient to remove solvents and improve mechanical robustness but not so aggressive as to collapse the porous network that facilitates ion transport.

Spray pyrolysis produces WO₃ films by spraying a tungsten precursor solution onto a heated substrate. The substrate temperature during deposition effectively acts as an in situ anneal, so the as-deposited films may already be partially crystalline. Regragui et al. demonstrated that spray-pyrolyzed WO₃ films deposited below 300 °C were amorphous but crystallized to monoclinic WO₃ upon additional air-annealing above 350 °C [81].

In 2007, Patil et al. used a pulsed spray technique to fabricate fibrous, reticulated WO₃ films with a high surface area and reported CE values of ~40–50 cm² C⁻¹ and relatively fast switching, attributable to the open morphology that promotes electrolyte penetration [82].

In 2013, Manceriu et al. further improved the microstructure by introducing surfactant-assisted spray pyrolysis on FTO substrates, obtaining porous films with well-controlled grain sizes [83]. A practical caveat for spray-pyrolyzed films is the risk of chlorine contamination when WCl₆ is used as the precursor, which can introduce trap states and degrade cycling stability; post-annealing in air helps to volatilize residual Cl but must be balanced against the concurrent crystallization [81].

Hydrothermal routes grow WO₃ nanostructures directly on substrates from aqueous solutions at moderate temperatures and autogenous pressures [83,84]. The resulting morphologies are often highly anisotropic and crystalline as-grown, which means that the role of post-synthesis annealing differs from that in other solution methods.

In 2010, Jiao et al. prepared hydrothermally grown WO₃ nanostructured films and showed that the as-synthesized films already exhibited a good electrochromic response; a brief calcination at 300 °C served mainly to improve substrate adhesion and remove residual organics without significantly altering the crystalline phase [83].

In 2025, Wu et al. recently confirmed this trend: hydrothermal WO₃ nanorod films annealed at 200, 300, and 400 °C showed negligible differences in electrochromic properties, indicating that the as-grown crystalline nanorods are already thermally “mature” and relatively insensitive to further annealing [84]. This stands in marked contrast to sol-gel or electrodeposited films, where a difference of 50–100 °C in annealing temperature can dramatically alter performance. However, hydrothermal nanostructures suffer from poor cycling stability because the loosely interconnected nanorod morphology is mechanically fragile, and annealing alone cannot remedy this structural weakness [85].

3.2. Physical Vapor Deposition (PVD) Routes

Physical vapor deposition (PVD) is one of the most established routes for preparing WO₃ electrochromic films because it enables relatively precise control over film thickness, composition, adhesion, and large-area uniformity. In general, PVD-derived WO₃ films are denser and chemically simpler than wet-chemical films, and their initial structure is strongly influenced by deposition energy, oxygen partial pressure, and substrate temperature. Consequently, their annealing response is often highly sensitive. Moderate heat treatment can improve short-range order, relieve internal stress, and adjust oxygen stoichiometry, whereas excessive annealing may induce full crystallization and grain coarsening, thereby reducing ionic accessibility. Accordingly, annealing in PVD-derived WO₃ films should be viewed primarily as a structural tuning step rather than as a simple activation process.

Magnetron sputtering, electron-beam evaporation, pulsed laser deposition, and thermal evaporation share a common feature in that they generally produce WO₃ films with relatively high density, low porosity, and strong adhesion to the substrate. When deposited near room temperature, these films are usually X-ray amorphous, and, in sputtered and evaporated systems, their stoichiometry is highly sensitive to the oxygen partial pressure during growth [86]. As a result, the effective annealing window is often narrow. Moderate heating may improve short-range order and reduce sub-stoichiometric defects, whereas excessive heating promotes crystallization and grain growth, which can decrease ion-accessible surface area and slow switching kinetics.

Magnetron sputtering is the most widely used PVD technique for WO₃ electrochromic films because of its excellent thickness uniformity, good reproducibility over large areas, and the ability to tune film stoichiometry through the gas-flow ratio and target power [14,80]. As-deposited films prepared at room temperature are typically amorphous and nearly stoichiometric, while increasing the oxygen flow during reactive DC sputtering progressively suppresses oxygen deficiency and sub-gap absorption, although at the expense of deposition rate [86]. Upon post-deposition annealing, sputtered WO₃ films generally show a clear structural evolution: mild heating (60–200 °C) mainly relieves residual stress and slightly improves short-range order without visible crystallization, whereas temperatures above 300–350 °C promote the nucleation of monoclinic γ-WO₃ crystallites [87]. Additional support for the importance of deposition-defined microstructure is provided by the GLAD magnetron sputtering study of Rydosz et al., in which WO₃ films annealed at 400 °C for 4 h in air crystallized into monoclinic WO₃, while the tilt-angle-controlled density varied from 6.48 to 5.13 g cm⁻³ and the refractive index at 633 nm decreased to 1.89, indicating that the final annealing response remained strongly dependent on the deposition-induced columnar geometry and porosity [88]. Using rapid thermal annealing in N₂ atmosphere, Usha and Lee showed that 300 °C yielded the highest H⁺ diffusion coefficient and the largest optical modulation in RF-sputtered WO₃, which was attributed to an amorphous/nanocrystalline mixed-phase microstructure that balances ion accessibility and structural integrity [87]. This concept was further demonstrated by Chu et al., who engineered a biphasic WO₃ bilayer consisting of 148 nm amorphous WO₃ and 115 nm crystalline WO₃ by two-step sputtering and controlled annealing at 400 °C, achieving ΔT = 76.57% at 630 nm and a cycling retention of 45.02% after 600 cycles, substantially exceeding the performance of either single-phase counterpart [89,90].

Electron-beam evaporation deposits WO₃ by irradiating a WO₃ or WO₃/W source with a focused electron beam under high vacuum. The deposited films are typically amorphous and exhibit optical band gaps in the range of 3.2–3.4 eV [91]. Sivakumar et al. investigated e-beam-evaporated WO₃ films annealed at 200–300 °C and observed a gradual transition from an amorphous state to a monoclinic single-phase texture with needle-like crystallites [91]. Annealing-induced densification improved substrate adhesion and shifted the optical absorption edge to longer wavelengths; however, the processing window for electrochromic optimization remained limited, and deposition at a substrate temperature of 200 °C followed by post-annealing at 300 °C gave the best combination of transmittance modulation and switching kinetics [91]. Compared with sputtered films, e-beam-evaporated WO₃ often shows slightly lower packing density and a more columnar growth morphology.

Pulsed laser deposition (PLD) is particularly useful for mechanistic studies because laser fluence, background gas pressure, and substrate temperature can be independently adjusted to control crystallinity during growth. Rougier et al. showed that WO₃ films deposited by excimer-laser ablation at room temperature under low O₂ pressure were amorphous, blue-tinted, and conductive, indicating a highly sub-stoichiometric state, whereas deposition at 400 °C under 10⁻¹ mbar O₂ produced crystalline, nearly stoichiometric films with an optical band gap of 2.8 eV [30]. Water incorporated into low-temperature PLD films was found to facilitate proton transport and enhance the initial electrochromic response, whereas annealing above 250 °C removed this structural water and reduced switching speed. This result indicates that the hydration level of as-deposited PLD films can significantly influence their subsequent annealing response.

Resistive thermal evaporation is the simplest PVD route, in which WO₃ powder is directly sublimated onto the substrate [14]. The as-deposited films are typically amorphous. Usta et al. systematically annealed thermally evaporated WO₃ films from 200 °C to 800 °C and found that the amorphous state persisted up to approximately 320 °C, whereas at ≥330 °C, a sharp amorphous-to-monoclinic transformation occurred, accompanied by a marked increase in grain size and surface roughness [92]. From an electrochromic perspective, this abrupt crystallization threshold effectively separates thermally evaporated films into a low-temperature regime, in which annealing improves performance while preserving the amorphous matrix, and a high-temperature regime, in which the film rapidly becomes crystalline and exhibits the Drude-like near-infrared reflectance modulation discussed in Section 2.2, at the expense of visible-range switching contrast [92].

WO₃-based heterostructures prepared by reactive sputtering provide an additional example of the role of interface design. In TiO₂/WO₃ and WO₃/TiO₂ bilayers deposited by DC reactive magnetron sputtering and annealed at 400 °C for 4 h, monoclinic WO₃ remained the dominant phase regardless of layer order, and the bilayers showed an average optical transmittance of approximately 58% together with improved mechanical stability relative to single-layer WO₃ [93]. Although this study was performed in a photocatalytic rather than an electrochromic context, it still supports the broader view that reactive sputtering can be used to construct structurally robust WO₃-based multilayers with well-defined interfaces.

Figure 4 schematically illustrates the operating principles of magnetron sputtering, electron-beam evaporation, pulsed laser deposition, and thermal evaporation, as well as the distinct initial film states that govern their subsequent annealing response.

3.3. Deposition Method–Annealing Correlation

The preceding discussion reveals a consistent pattern: the annealing response of a WO₃ film is governed less by the annealing conditions themselves than by the initial microstructure that the deposition route creates.

Table 2. Deposition method–annealing response summary for WO₃ electrochromic films.

Deposition Route	Optimal Annealing Range	Annealing Effect	Electrochromic/Optical Indicators	Key Ref.
Magnetron sputtering	60–300 °C	Tune short-range order; avoid grain coarsening	EC performance improved after RTA; suitable for smart-window modulation	[87,88]
E-beam/thermal evaporation	200–330 °C	Improve adhesion; crystallization onset at ~330 °C	Tvis ≈ 60%–80%; Eg ~3.06/3.00 → 2.80/2.72 eV after annealing	[91,92]
PLD	Substrate-dependent [30]	Expel structural water; adjust stoichiometry	Eg ~3.2 eV (RT) → ~2.8 eV (400 °C substrate)	[30]
Sol–gel	250 °C	Remove solvent + condense W–O–W; pore collapse if too high	Amorphous or lightly crystallized films usually show better EC reversibility than overly dense nanocrystalline films	[25,59]
Electrodeposition	60–100 °C	Dehydration; over-annealing reduces CE	ΔT = 65.9%; CE = 64.1 cm2 C−1 @ 638 nm; CENIR = 73.3 cm2 C−1; Eg 3.40 → 3.31 eV	[88,96]
Spray pyrolysis	300–400 °C	Volatilize Cl residues; refine composition	CE = 34 cm2 C−1 @ 630 nm; Eg ≈ 3.1 eV; broad EC response over 250–2500 nm	[80,82]
Hydrothermal	200–300 °C	Mild annealing mainly improves adhesion/stability; microstructure and thickness often dominate over annealing itself	For nanorod films: ΔT = 51.1%; tc/tb = 7.3/3.8 s; CE = 41.8–44.8 cm2 C−1; best when thickness ≤ 290 nm	[84,85]

summarizes these trends. It should be noted that atomic layer deposition and printing-based techniques are also being explored for WO₃ electrochromic films [94,95]; however, systematic annealing–performance correlations for these methods remain scarce and are therefore not discussed in the present review.

4. Effects of Annealing Temperature

Annealing temperature represents one of the most critical post-deposition parameters that dictate the structural and functional performance of tungsten oxide films. It determines the crystallinity evolution of the film, governing whether it remains amorphous, undergoes partial crystallization, or fully transforms into a thermodynamically stable crystalline phase. It also strongly modulates grain size, porosity, oxygen vacancy concentration, and the density of accessible ion-transport pathways. Nevertheless, no universally optimal annealing temperature exists: the ideal value is determined by the as-deposited microstructure, the target functional properties, and even the selected heating rate and holding duration. This section reviews reported evidence across three distinct temperature regimes.

4.1. Low-Temperature Regime (<200 °C)

Below about 200 °C, WO₃ films generally remain X-ray amorphous regardless of deposition route, and annealing in this range does not usually induce significant nucleation. Instead, the dominant effects are more subtle: removal of physically adsorbed water or residual volatile species, relief of internal stress, and limited short-range atomic rearrangement [88]. Accordingly, the low-temperature regime is best understood as a structural relaxation and volatile-removal stage rather than a crystallization window.

This regime is particularly important for solution-derived WO₃ films. Au et al. annealed sol-gel WO₃ films between 27 and 450 °C and found that, already at 100 °C, most residual solvent had been removed, while the resulting open porous structure produced a ΔT of about 62% [25]. Abareshi and Haratizadeh likewise reported that electrodeposited WO₃ annealed at only 60 °C showed ΔT = 65.9% and CE = 64.1 cm² C⁻¹, with both values decreasing progressively as the annealing temperature was increased [88]. These results suggest that, in hydrated or chemically immature films, mild heating can substantially improve electrochromic performance without altering the amorphous framework.

The response of dense PVD-derived films is generally less pronounced in this temperature range. Usha and Lee applied rapid thermal annealing to RF-sputtered WO₃ and found that at 100 °C, ΔT increased only marginally from 67% in the as-deposited state to 68%, while the diffusion coefficient rose from 1.35 × 10⁻¹¹ to 1.76 × 10⁻¹¹ cm²s⁻¹ [87]. A similar tendency was observed for electron-beam-evaporated films. Madhuri et al. showed that the room-temperature film, representing the lowest thermal budget, exhibited the highest CE of 30.63 cm²C⁻¹ and ΔT of 34% at 550 nm, whereas increasing the substrate temperature above 250 °C reduced CE to 17.20 cm²C⁻¹ [97]. Gupta et al. further demonstrated that, in e-beam WO₃ films, deposition rate could dominate performance more strongly than mild post-treatment: at 1.5 Å s⁻¹, the as-deposited film achieved ΔT = 79% at 550 nm, although CE remained relatively modest at 17.5 cm²C⁻¹ [98]. Taken together, these results indicate that, for dense PVD films deposited below about 200 °C, mild annealing usually provides only limited gains compared with the more substantial improvements seen in solution-derived systems [99].

Consistent with this interpretation, Zhang et al. tracked the band gap of sol-gel WO₃ from 100 to 500 °C and found that it changed only slightly, from 3.62 eV at 100 °C to about 3.55 eV at 200 °C [100]. Thus, the beneficial effects observed in this low-temperature regime are mainly microstructural rather than electronic or crystallographic in origin. In summary, below 200 °C, annealing improves electrochromic performance primarily through volatile removal, stress relaxation, and pore-structure optimization, rather than through phase transformation.

4.2. Moderate-Temperature Regime (200–350 °C)

Between about 200 and 350 °C, many WO₃ thin films begin to undergo the first significant stages of crystallization, although the exact onset remains strongly dependent on the deposition route. In this regime, nanocrystalline domains nucleate within the amorphous matrix, and the resulting partially ordered structure often provides a favorable balance between ion accessibility and structural integrity. For this reason, the intermediate-temperature range is frequently associated with comparatively balanced electrochromic performance.

This trend is clearly observed in solution-derived and electrodeposited WO₃ films. Au et al. identified approximately 250 °C as a favorable annealing temperature for sol-gel WO₃, at which the W–O–W network became sufficiently condensed while the film retained enough porosity for efficient ion insertion, leading to the highest Li⁺ diffusion coefficient [25]. Morankar et al. further showed that this intermediate-temperature window is also highly effective for electrodeposited WO₃: films annealed at 250 °C exhibited strong redox activity, ΔT = 79.35% at 600 nm, CE = 97.91 cm²C⁻¹, and a cathodic diffusion coefficient of 2.61 × 10⁻⁹ cm² s⁻¹, all significantly superior to films treated at 50 or 450 °C [26]. Kang et al. reported a similar trend for solution-processed WO₃ films treated between 80 and 400 °C, showing that films processed at 300 °C displayed the most favorable Li⁺ insertion/extraction kinetics, the highest coloration efficiency, and stable performance over 500 cycles, whereas treatment at and above 350 °C led to reduced stability because ion migration became increasingly restricted in the denser crystalline structure [101].

Related evidence from sputtered and structurally tailored WO₃ films points in the same direction. Usha and Lee showed that rapid thermal annealing at 300 °C produced the highest optical modulation (ΔT = 80%) and the largest Li⁺ diffusion coefficient (3.35 × 10⁻¹¹ cm² s⁻¹) in RF-sputtered WO₃, and attributed this response to the formation of an amorphous/monoclinic mixed-phase heterostructure [87]. Zhao et al. likewise obtained a mixed amorphous/nanocrystalline WO₃ microstructure by controlling the substrate temperature during RF sputtering and reported ΔT = 72.5% at 1000 nm, CE = 80.5 cm² C⁻¹, and fast switching behavior [102]. Although this structure was achieved during deposition rather than by a separate post-annealing step, it still supports the broader conclusion that a partially ordered WO₃ framework can outperform both fully amorphous and fully crystallized endpoints. Kumar et al. similarly showed that a porous WO₃ film prepared by glancing-angle deposition at room temperature achieved ΔT = 85% and CE = 54.5 cm² C⁻¹ at 900 nm, whereas a subsequent 400 °C treatment degraded rather than improved the response [103].

Overall, the onset of crystallization in this moderate-temperature regime is not fixed, but depends strongly on deposition route and thermal dose [104]. For thermally evaporated films, Usta et al. reported an abrupt crystallization threshold near 330 °C [92], whereas partial crystallization in sol-gel WO₃ may begin as low as 250 °C [105]. In sputtered films, by contrast, clear XRD signatures often do not appear until about 300–350 °C [21,87]. Thus, the practical significance of the 200–350 °C regime lies less in defining a universal crystallization threshold than in its frequent ability to promote partial ordering without fully sacrificing ion-transport pathways.

4.3. High-Temperature Regime (>350 °C)

At temperatures above about 350 °C, WO₃ films typically lose most of their amorphous character and progress toward fully crystallized states. Under equilibrium conditions, bulk WO₃ is generally expected to follow the phase sequence γ-monoclinic → β-orthorhombic → α-tetragonal upon heating [37]. Accordingly, within the temperature range relevant to most electrochromic devices, γ-monoclinic WO₃ is often the thermodynamically expected phase, and sputtered or evaporated films annealed at 400–500 °C are commonly reported to adopt a monoclinic structure [104,105]. Thin films, however, do not always follow this equilibrium path. Metastable hexagonal WO₃ has frequently been observed in films derived from hydrated or oxygen-deficient precursors [106]. Ramana et al. reported an amorphous → monoclinic → orthorhombic evolution in PLD films between 30 and 500 °C, while residual structural water in the intermediate stage helped stabilize a hexagonal-like arrangement [37]. Chen et al. likewise obtained hexagonal, tetragonal, and monoclinic phases from thermally oxidized W foils annealed at 400, 500, and 600 °C, respectively, although these were separate end-point samples rather than an in situ continuous phase-transition sequence [106]. These observations indicate that the crystalline phase observed at a given temperature depends not only on the temperature itself, but also on the initial film state and precursor chemistry, including hydration level, oxygen stoichiometry, and residual impurities.

From an electrochromic standpoint, high-temperature crystallization is frequently associated with reduced visible-range performance because ion-insertion sites contract, grain coarsening becomes more pronounced, and porosity decreases. Kim and Park reported an approximately tenfold drop in Li⁺ charge density at 400 °C together with a coloration efficiency of about 24 cm² C⁻¹, while ΔT fell below 3% at 500 °C [105]. Usha and Lee observed a similar deterioration in RF-sputtered WO₃ films, where ΔT decreased from 80% at 300 °C to 60% at 400 °C, accompanied by a drop in the Li⁺ diffusion coefficient from 3.35 × 10⁻¹¹ to 0.81 × 10⁻¹¹ cm²s⁻¹ [87]. In electron-beam-evaporated films, Madhuri et al. likewise found that increasing the substrate temperature above 250 °C reduced the coloration efficiency from 30.6 to 17.2 cm² C⁻¹ [97]. Taken together, these results suggest that, in many WO₃ thin-film systems, complete crystallization tends to weaken optical contrast and ion-transport kinetics even though long-range structural order is improved.

High-temperature treatment is not, however, uniformly detrimental. More recent studies show that the initial microstructure can partly preserve or recover electrochromic performance even after crystallizing anneals. Babu et al. deposited porous amorphous e-beam WO₃ at 6–8 °C and, after annealing at 400 °C, still obtained ΔT = 33.8% at 550 nm and CE = 72.6 cm²C⁻¹, which they attributed to retention of the original porous network during crystallization [107]. Gupta et al. further reported that tuning the e-beam deposition rate to 1.5 Ås⁻¹ enabled ΔT ≈ 79% even after annealing, indicating that growth kinetics, rather than annealing temperature alone, can strongly influence the upper performance limit [98]. Annealing time must also be considered together with temperature: Chen et al. found that, at 400 °C, 30 min gave ΔT ≈ 74% with less than ±2% drift over 500 cycles, whereas extending the treatment to 240 min reduced ΔT to 65% because of excessive densification; the same study further showed that crystallization at 300 °C did not begin until after at least 120 min [104]. Finally, high-temperature-crystallized WO₃ films are not necessarily short-lived [108]. Lee et al. reported nanocrystalline WO₃ particles that endured more than 3000 cyclic-voltammetry cycles in 1 M H₂SO₄ with negligible degradation [109], while Brezesinski et al. showed that mesoporous crystalline films calcined at about 550 °C could retain electrochromic performance even at 70 °C [110,111]. What these stable systems share is that crystallization proceeds without completely destroying the nanoscale porous framework. Thus, the widely observed trade-off between optical contrast and structural durability should be interpreted less as a fixed consequence of crystallization itself than as a consequence of how crystallization is achieved and how much ion-accessible nanostructure is preserved.

4.4. Role of Annealing Duration and Thermal Dose

In addition to the annealing temperature itself, the duration of thermal treatment also plays a critical role in determining the final structure and electrochromic response of WO₃ films.

While annealing temperature defines the thermodynamic tendency for structural evolution, dwell time governs the kinetic extent of structural relaxation, crystallization, grain growth, and pore collapse. For this reason, the effect of annealing on WO₃ films should not be discussed in terms of peak temperature alone. Instead, the total thermal dose, determined by the combination of temperature and duration, is often the more physically meaningful descriptor of annealing-induced microstructural evolution.

The importance of dwell time is illustrated by the recent work of Chen et al. [104]. For sputtered WO₃ films annealed at 300 °C, monoclinic WO₃ diffraction peaks did not appear until the annealing time reached approximately 120 min, indicating that the onset of crystallization is controlled not only by temperature but also by the duration of thermal exposure. Under the same temperature condition, films annealed for 30 min remained largely amorphous, whereas those annealed for 240 min became partially crystalline with larger grains [104]. These results show that dwell time directly determines the extent of structural ordering within a given temperature regime.

Annealing duration also has a pronounced effect on electrochromic performance at fixed temperature. Chen et al. [104] further showed that, at 400 °C, a 30 min anneal preserved a high optical modulation of about ΔT ≈ 74% and maintained a drift below ±2% over 500 cycles, whereas extending the dwell time to 240 min reduced ΔT to about 65% because of over-densification and collapse of the pore network. This comparison indicates that prolonged annealing can degrade ion-accessible pathways even when the nominal annealing temperature remains unchanged.

The role of thermal dose is further supported by rapid thermal annealing results. Usha and Lee [87] reported that rapid thermal annealing of RF-sputtered WO₃ in N₂ at 300 °C produced the highest H⁺ diffusion coefficient and the largest optical modulation, with ΔT = 80% and D = 3.35 × 10⁻¹¹ cm² s⁻¹, without triggering full crystallization. Compared with prolonged furnace annealing, such short high-temperature treatment better preserved the mixed amorphous/nanocrystalline character required for both ion accessibility and structural stability [87]. Taken together, these studies indicate that annealing duration should be treated as an independent design variable, and that optimization of WO₃ films requires a route-specific temperature–time window rather than a temperature criterion alone.

5. Effects of Annealing Atmosphere

Section 4 showed that temperature determines the degree of crystallization. But it does not tell the whole story. Two films annealed at the same temperature can behave very differently—if the atmosphere is different. The reason is simple: the gas environment during annealing controls whether oxygen is added to, removed from, or preserved in the WO₃ lattice. In other words, temperature sets the crystallinity; atmosphere sets the oxygen-vacancy concentration. Since oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) directly govern the density of W⁵⁺ polaron sites, the free-carrier density, and the availability of irreversible ion-trapping sites, the choice of atmosphere is as important as the choice of temperature.

5.1. Inert Atmospheres

Upon annealing of WO₃ in an inert atmosphere or under vacuum, no external oxygen source is available to fill pre-existing lattice oxygen vacancies. Oxygen vacancies initially present in the as-deposited film are therefore retained, and thermal energy further drives the migration of lattice oxygen to the film surface, where it desorbs and creates additional vacancy sites. The overall result is a more reduced and oxygen-deficient WO₃ film compared to one subjected to identical thermal treatment in air or O₂. It should be noted that the corresponding defect concentration is typically inferred indirectly from spectroscopic and compositional characterization results, rather than being measured with unambiguous accuracy.

In 2019, Yu et al. provided the clearest experimental demonstration of this effect [21]. They deposited amorphous WO₃₋ᵧ by reactive DC sputtering and annealed one set of films at 350 °C in Ar and another at 350 °C in O₂, both for 2 h. XRD confirmed that both sets reached the same crystallinity level—the partial monoclinic nanocrystalline phase. But XPS revealed strikingly different ${\mathrm{V}}_{\mathrm{O}}$ concentrations: the Ar-annealed films retained significantly more oxygen vacancies. The electrochromic consequences were immediate. The Ar-annealed films showed higher optical modulation and higher coloration efficiency, because the additional W⁵⁺ sites amplified the small-polaron absorption described in Section 2.2. However, their cycling stability was measurably worse than that of the O₂-annealed films. The authors proposed that excess oxygen vacancies serve as irreversible Li⁺ trapping sites: during repeated intercalation/deintercalation cycles, some Li⁺ ions become trapped at these under-coordinated W sites and cannot be fully extracted, leading to gradual capacity fading and optical performance degradation [21].

In 2024, Usha and Lee’s RTA study in N₂ confirmed this trend quantitatively [87]. As the RTA temperature rose from RT to 400 °C under N₂, the O/W atomic ratio measured by XPS decreased steadily from 2.24 to 1.92, and the W⁵⁺ signal grew monotonically. At 300 °C, this vacancy accumulation coincided with the formation of an amorphous/monoclinic heterostructure and produced the highest ΔT and the largest diffusion coefficient in the series. But at 400 °C—still under N₂—the vacancy population continued to increase while ΔT collapsed to 60% [87]. This counter-intuitive result highlights an important principle: there is an optimal ${\mathrm{V}}_{\mathrm{O}}$ concentration. Below it, too-few polaron sites limit coloration. Above it, the lattice becomes so defective that ion-transport channels are disrupted and the dense crystalline structure dominates over the vacancy benefit.

The practical implication is clear. Inert-atmosphere annealing is excellent for maximizing initial EC performance—provided the ${\mathrm{V}}_{\mathrm{O}}$ level does not overshoot.

5.2. Oxidizing Atmospheres

Annealing in air or O₂ is often associated with a less reduced WO₃ surface and improved reversibility, although the resulting cycling stability likely reflects the combined effects of defect chemistry, phase evolution, and interfacial stabilization.

In 2019, Yu et al. reported that O₂-annealed films showed this general behavior, exhibiting lower ΔT and lower CE than their Ar-annealed counterparts, but significantly longer cycling lifetimes [21]. The long-term CV cycling experiment showed that O₂-annealed films maintained optical modulation over substantially more cycles, whereas the Ar-annealed films degraded progressively, which was attributed to increased Li⁺ accumulation and/or trapping in the more reduced film state. These results suggest that increased oxygen deficiency may improve the initial electrochromic response, but often at the expense of cyclic stability [21].

In 2025, Kamath et al. examined the interplay between O₂ partial pressure during DC sputtering and post-deposition annealing temperature [112]. Films deposited at PaO₂ of 0.3–0.5 mTorr remained amorphous up to 200 °C and began to crystallize at 300 °C, with prominent monoclinic peaks at 400 °C. The key finding was that higher PaO₂ during deposition produced more stoichiometric films that responded differently to annealing: they required higher temperatures to crystallize and showed less cracking and more uniform grain growth at elevated temperatures [112]. In other words, the O₂ budget of the film is not determined by annealing alone but by the cumulative oxygen exposure across the entire fabrication history—deposition and post-treatment combined.

Air annealing represents a compromise between strongly oxidizing and inert annealing environments. Owing to the limited oxygen partial pressure in air, the resulting WO₃ films are often less reduced than those annealed in inert atmospheres but more oxygen-deficient than those treated under pure O₂. This feature makes air annealing a practically useful route for simultaneously improving structural stability and retaining sufficient electrochromic activity. Previous studies have shown that appropriate post-annealing temperatures can promote crystallization and enhance the stability of WO₃ films, while atmosphere-controlled annealing further influences the defect state and thereby contributes to the balance between optical modulation and cycling durability. In this context, air annealing may provide a balanced pathway by partially reducing vacancy-related trapping effects without completely eliminating the defect states that contribute to coloration [21,25].

5.3. Reducing Atmospheres

While inert atmospheres are often associated with the retention of a more reduced film state and oxidizing atmospheres with partial reoxidation, reducing atmospheres generally drive WO₃ toward a more strongly oxygen-deficient condition. Hydrogen annealing is commonly interpreted as promoting the removal of lattice oxygen, which can lead to a more oxygen-deficient WO₃ framework together with increased electronic reduction of tungsten species. The resulting film is typically more sub-stoichiometric and often exhibits substantially increased electronic conductivity and optical absorption.

In 2015, Saleem et al. annealed thermally evaporated WO₃ thin films in H₂ at temperatures up to 550 °C for 2 h [113]. XPS revealed that the W⁴⁺ and W⁵⁺ contributions increased substantially from ~8% to much higher values after H₂ treatment [113]. The optical transmittance decreased progressively with hydrogen annealing temperature, consistent with a more reduced film state and a higher density of polaron- and bipolaron-related absorption centers, such that the film became dark even in the nominally bleached state [113]. Electrical resistivity dropped dramatically, consistent with an increase in electronic carrier density in the more reduced film state. At 550 °C, hydrogen was found to be especially aggressive in reducing WO₃, causing pronounced structural changes [113].

From an electrochromic standpoint, H₂ annealing is a double-edged sword. A high degree of oxygen deficiency may increase coloration depth, but it also tends to darken the bleached state, reduce bleached-state transmittance, and promote irreversible Li⁺ trapping or accumulation. In practice, H₂ annealing is almost never used for electrochromic applications. Its primary role is in gas sensing and in fundamental studies of defect physics [113,114]. Nevertheless, H₂ annealing data remain instructive because they illustrate the strongly reduced end of the WO₃ defect spectrum and help clarify how oxygen deficiency may be related to optical and electronic behavior.

5.4. The Oxygen-Vacancy Dilemma and Summarize

The data reviewed in Section 5.1, Section 5.2 and Section 5.3 point to a non-monotonic role of oxygen vacancies in annealed WO₃ films, and this non-monotonicity is central to understanding the contrast–stability trade-off. A moderate concentration of vacancy-related states can be genuinely beneficial: it increases the density of W⁵⁺-associated polaron sites, facilitates electron hopping, and often enhances both the initial optical modulation and the coloration efficiency. Once the vacancy concentration exceeds a certain range, however, the same defect chemistry becomes detrimental. Excessive oxygen deficiency promotes residual absorption in the bleached state, creates irreversible ion-trapping sites, distorts the W–O framework, and accelerates capacity decay under long-term cycling. In this sense, oxygen vacancies behave as functional defects only within a limited regime, and as structural and electrochemical liabilities beyond it. This is what we refer to as the oxygen-vacancy dilemma of WO₃ electrochromics.

From a functional standpoint, the key point is that the vacancy level maximizing the initial optical response is not necessarily the vacancy level that maximizes reversibility and cycling stability. Inert or weakly reducing annealing atmospheres tend to preserve or enrich oxygen vacancies and therefore favor high initial ΔT and fast switching, whereas oxidizing post-treatments typically lower the vacancy concentration and yield somewhat smaller optical contrast but substantially better long-term durability [21]. Rapid thermal annealing in N₂ produces a similar effect at moderate temperatures, generating a controlled vacancy-rich but structurally consolidated state that combines relatively high modulation with improved stability [87]. At the opposite extreme, heavy hydrogen annealing drives WO₃ into a strongly sub-stoichiometric regime, in which excessive W⁵⁺/W⁴⁺ formation collapses transparency and undermines electrochromic reversibility, illustrating the harmful side of the vacancy spectrum [113].

The picture is further complicated by the theoretical work of Di Valentin et al. [115]. Using first-principles hybrid-functional calculations on γ-monoclinic WO₃, they showed that oxygen vacancies are intrinsically anisotropic: their electronic structure, ground-state energies, and optical excitations depend on which W–O–W chain direction the vacancy occupies. Different vacancy configurations therefore contribute differently to optical absorption and electronic conductivity. An important implication is that the total vacancy concentration alone is an incomplete descriptor of electrochromic behavior. Two films with apparently similar XPS-derived W⁵⁺/O 1s signatures may still exhibit different optical modulation, charge-transport characteristics, and cycling stability if their preferred orientations, grain textures, or vacancy distributions differ.

This observation also highlights a broader limitation in the current literature. Oxygen-vacancy concentration is often inferred from simplified indicators such as W⁵⁺ content or the deconvolution of the O 1s XPS peak, yet these quantities do not uniquely determine the functional role of vacancies, because surface hydroxylation, residual hydration, and local non-stoichiometry can produce overlapping spectral features. Correlations between “vacancy concentration” and electrochromic performance should therefore be interpreted comparatively within a given processing family, rather than as universal one-to-one relationships across deposition routes.

Although the anisotropic role of oxygen vacancies has not yet been systematically exploited experimentally, it suggests a promising design direction. Rather than treating annealing atmosphere only as a tool for globally increasing or decreasing the total vacancy concentration, future studies may explore texture-controlled or spatially resolved annealing strategies that regulate where vacancy-rich regions are formed and along which crystallographic directions they are stabilized. Such an approach could, in principle, preserve the optical and electronic benefits of vacancy-assisted polaron absorption while suppressing the ion-trapping pathways responsible for long-term degradation, offering a defect-chemistry route to partially resolve the contrast–stability trade-off discussed in Section 6.

Accordingly,

Table 3. Effect of annealing atmosphere on oxygen-vacancy level and EC properties of WO₃ films.

Atmosphere	Effect on ${\mathbf{V}}_{\mathbf{O}}$	EC Performance	Cycling Stability	Typical Use Case	Key Ref.
Ar	Preserves/increases	High ΔT, high CE	Poor	Maximum initial contrast	[21]
N2	Preserves/increases	High ΔT	Not reported separately	RTA processing	[87]
O2	Heals	Lower ΔT, lower CE	Significantly improved	Long-lifetime devices	[21]
Air	Partially heals	Moderate ΔT	Good	Industrial furnace annealing	[104]
H2	Aggressively creates	Very high absorption; dark bleached state	Very poor	Gas sensing; fundamental studies	[113]

summarizes the relationships between annealing atmosphere and electrochromic properties in WO₃ films.

For a smart window that must survive 10+ years and >10,000 cycles, O₂ or air annealing at moderate temperature is the safer path. For a display or sensor that prioritises maximum initial contrast and accepts a shorter lifetime, Ar or N₂ annealing at the optimal temperature will deliver higher performance. And for any application, the ${\mathrm{V}}_{\mathrm{O}}$ level should be treated as a design variable that is deliberately tuned through the combined selection of atmosphere, temperature, and time.

6. Discussion on the Optimal Annealing Window and Discussion on the Optimal Annealing Window and Annealing-Guided Design for WO₃ Electrochromic Devices

6.1. Multi-Parameter Annealing Framework and Structure–Property Evolution

Based on the above comparison, the central issue is not the identification of a single universally optimal annealing temperature, but rather how annealing can be used to decouple optical activity from structural durability in WO₃ electrochromic films. The analyses in Section 3, Section 4 and Section 5 suggest that the commonly reported trade-off between high optical contrast/fast switching and long-term cycling stability should not be viewed as an intrinsic limitation of WO₃ itself, but rather as a consequence of imposing a spatially uniform thermal treatment on a material system whose different regions often require different structural functions. In general, amorphous or weakly crystallized regions favor rapid ion insertion/extraction and large visible-range modulation, whereas more crystalline regions provide a mechanically robust framework and more stable transport pathways during repeated cycling. When a single annealing protocol is applied uniformly across the entire film, these distinct functional requirements are forced into the same microstructure, so improvement in one metric often occurs at the expense of another.

The temperature ranges discussed in this review are better regarded as comparative structural regimes than as fixed performance windows, because the annealing response of WO₃ films depends strongly on deposition route, film thickness, substrate, initial hydration state, thermal dose, electrolyte composition, inserted ion species, and testing protocol. As a result, similar nominal annealing temperatures may correspond to different structural and electrochemical outcomes in different WO₃ film systems.

Although annealing temperature and atmosphere are often discussed separately, their effects on the electrochromic behavior of WO₃ films are intrinsically coupled through the simultaneous regulation of crystallinity, defect chemistry, and ion-transport pathways, while dwell time determines the extent of these transformations. Therefore, the electrochromic performance of WO₃ films should be interpreted from a combined processing–microstructure–property perspective rather than by any single annealing parameter. The complex interplay among these variables is summarized schematically in Figure 5, which provides a qualitative overview of how annealing temperature, atmosphere, and dwell time may jointly shape the representative performance regimes of WO₃ films, without implying universal phase or performance boundaries.

Within this framework, annealing is best conceptualized as a multi-parameter process rather than a single-dimensional variable. Temperature establishes the overall thermodynamic driving force for atomic ordering and crystallization, dwell time dictates the kinetic progression of structural relaxation and grain growth, and atmosphere regulates the redox state and degree of oxygen deficiency within the film. Annealing protocols specified exclusively by peak temperature thus provide an incomplete description of the thermal process.

Accumulating recent evidence lends consistent support to this three-parameter framework. For instance, Chen et al. demonstrated that at a fixed annealing temperature of 400 °C, a 30 min thermal treatment maintained a high optical modulation amplitude (ΔT ≈ 74%), whereas prolonging the dwell time to 240 min at the same temperature triggered pore network collapse and significant performance degradation [104]. Usha and Lee further decoupled temperature and atmosphere effects via rapid thermal annealing under N₂ atmosphere, achieving a ΔT of 80% and a chemical diffusion coefficient D of 3.35 × 10⁻¹¹ cm²·s⁻¹ at 300 °C without inducing complete crystallization [87]. These results collectively indicate that the structural and functional response of WO₃ films to annealing must be evaluated within a coupled processing parameter space defined by temperature, atmosphere, and dwell time, rather than referenced to a single nominal annealing temperature alone.

This coupled framework implies that there is no universally optimal annealing condition for all WO₃ films. Instead, the useful annealing window should be selected in a route-dependent manner according to the deposition method, thermal budget, target microstructure, and device function. In general, a moderate processing window is the most favorable for balancing optical modulation, switching kinetics, and stability, whereas insufficient annealing results in poor activation, and excessive annealing leads to over-crystallization or densification. For dense PVD-derived films, annealing mainly adjusts short-range order, residual stress, and defect distribution, whereas for solution-derived or electrodeposited films, it more strongly governs network condensation and the removal of residual species. Hydrothermal WO₃ nanostructures, by contrast, often require only mild post-treatment to improve adhesion and robustness.

To facilitate a more systematic comparison,

Table 4. Representative literature data on annealed WO3 electrochromic thin films.

Ref.	Deposition	Anneal T	Dwell Time	Atmosphere	Thickness	Crystal Phase	ΔT (%)/λ (nm)	CE	Switching Time	Cycling Stability
[21]	DC sputtering	350	2 h	Ar vs. O2	/	Amorphous-based	87 (as-deposited), 75 (Ar), 51 (O2)/650	40.5 (as-deposited), 36.6 (Ar), 27.2 (O2)	/	Ar poor; O2 improved
[26]	Electrodeposition	50/250/450	/	/	~180/~240/~320 nm	Monoclinic	79.35/600	97.91	9.8/7.5	2.82% ΔT loss @ 5000 cycles
[87]	RF sputtering	100/200/300/400	10 min	N2	/	Mixed amorphous/monoclinic	80/550	/	/	/
[90]	Sputtering bilayer	400	2 h	Air	148 nm (a) + 115 nm (c)	Biphasic	76.57/630	>50	Sub-5 s	45.02% @ 600 cycles
[80]	Spray pyrolysis	500	1 h	Air	~150 nm	Monoclinic	~75 across 250–2500 nm	/	/	Reaction becomes reproducible after 5 cycles
[85]	Hydrothermal	200–400	2 h	/	290/560/990 nm	Crystalline nanorods	51.1/633; 43.8/633; 35.1/633	41.8/44.4/44.8	7.3/3.8; 7.7/6.5; 7.7/6.5	Final/initial ΔT ratio after 150 cycles = 0.33/0.26/0.34
[104]	Sputtering	300 vs. 400	10–240 min	Air	~800 nm	Crystallized at 400	74/550	/	6.7/6.9 s	<1% variation in ΔT after 500 CV cycles
[113]	Thermal evaporation	350/450/550	2 h	H2	305 ± 5 nm	As-deposited amorphous	/	/	/	/

summarizes representative literature data on annealed WO₃ electrochromic thin films, including deposition route, annealing conditions, structural characteristics, electrochromic metrics, and cycling durability.

6.2. Device-Oriented Design: Interface Engineering, Scalability, and Data-Guided Optimization

While the preceding discussion frames annealing as a coupled multi-parameter factor governing the structure–property evolution of WO₃ films, its practical implications can only be fully assessed at the device level. Accordingly, the validity of any annealing strategy should be evaluated not only at the single-film level, but also within the context of a complete electrochromic device configuration.

A core insight from this device-level perspective is that the widely reported contrast–stability trade-off is not an intrinsic limitation of WO₃ as a material. Instead, it frequently arises from the conventional practice of applying a spatially uniform thermal treatment to films in which different regions serve distinct structural and functional roles. This limitation can be effectively mitigated by adopting biphasic or heterostructured film architectures. For example, Chu et al. fabricated a crystalline WO₃ base layer to act as a rigid structural backbone, paired with an amorphous capping layer to enable rapid interfacial ion transport, achieving markedly improved optical modulation retention relative to single-phase reference films [90]. Similarly, a phase-interface-engineered WO₃ heterostructure reduced the charge-transfer resistance by 60% and increased CE by ~20% [116]; biphasic nanocrystals dispersed in an amorphous WO₃ matrix combined high electrochromic activity with improved structural robustness [117]. More broadly, such heterostructured designs corroborate the notion that crystalline WO₃ domains provide structural robustness, while amorphous or weakly crystallized regions maintain fast electrochromic reaction kinetics [118].

At the same time, long-term operational durability is governed not only by bulk film properties, but also by interfacial degradation processes, including Li⁺ trapping, hydrolysis, and side reactions with trace water in the electrolyte. Annealing alone therefore cannot fully address the long-term stability challenge. Its performance benefits must be paired with targeted interface engineering strategies. On the surface passivation front, ultrathin ALD-grown Al₂O₃ overlayers have been shown to suppress water infiltration and preserve near-infrared (NIR) modulation over prolonged cycling [119], while Ta₂O₅ passivation layers deliver a comparable stabilizing effect without compromising optical performance [120]. On the electrolyte engineering front, hybrid-cation electrolyte systems can spontaneously form protective solid-electrolyte interphases that significantly prolong cycling lifespan and retain optical modulation capacity [121]. These findings collectively indicate that annealing and interface engineering should be approached as an integrated co-design challenge, rather than independent optimization tasks.

This same design principle extends to practical device deployment. Device-level variables—including electrolyte composition, counter-electrode configuration, intercalating ion species, encapsulation quality, optical haze, large-area uniformity, humidity sensitivity, UV aging resistance, and cycling test protocols—all influence the measured electrochromic performance of annealed WO₃ films. Therefore, comparisons between different annealing strategies should not be based solely on isolated film-level metrics such as ΔT or coloration efficiency, but must also account for device operational lifetime, fabrication reproducibility, and application-specific operating constraints [122].

This consideration is particularly critical for smart window applications, where the performance target is no longer simply large optical modulation at a single wavelength, but coordinated regulation of visible and near-infrared transmittance under realistic operating conditions. Dual-band electrochromic architectures [123,124] demonstrate that crystalline WO₃ components play a dominant role in NIR modulation, whereas amorphous regions are more effective at supporting visible-range optical switching. Meanwhile, the scaling up to large-area and flexible devices imposes stringent thermal budget constraints. A 1170 cm² flexible electrochromic device incorporating a WO₃-based ion-storage layer has already been demonstrated [125], but the thermal tolerance of polymeric substrates is typically far lower than that of rigid glass substrates. This limitation of low-thermal-budget processing approaches—including rapid thermal annealing [87], photonic curing, and in situ Joule annealing via the ITO electrode at ~250 °C for 10 min [126]—has attracted increasing attention. In parallel, the criteria used to evaluate performance are also shifting. Field tests and whole-building simulations increasingly adopt solar heat gain coefficient (SHGC) and annual energy consumption as primary benchmarks [127], and these metrics may alter the ranking of annealing strategies relative to laboratory-scale ΔT measured at a single wavelength.

Finally, although the annealing parameter space is finite, it is sufficiently large to benefit substantially from data-driven optimization. Faceira et al. combined sputtering experiments with machine learning (ML) to predict the electrochromic properties of WO₃ films as a function of processing parameters [128], and this approach was subsequently extended to ammonium metatungstate-based precursor systems [129]. More broadly, high-throughput computational–experimental frameworks for electrochemical materials are also undergoing steady development [130]. Nevertheless, currently available datasets remain limited in scale and are often biased toward individual laboratory protocols, while inconsistencies in voltage window, electrolyte formulation, and cycling duty cycle can propagate directly into model training and degrade prediction reliability. As a result, ML is unlikely to replace physics-guided experimental investigations in the near future. A more practical role for ML is the assisted screening of viable annealing windows for a given deposition route, followed by targeted experimental validation. This hybrid research strategy aligns well with the multi-parameter optimization framework established at the start of this section.

Overall, future optimization of WO₃ annealing strategies should move beyond maximizing performance in isolated single films toward a holistic co-design framework that integrates deposition route, processing efficiency, substrate thermal tolerance, interface durability, and target device functionality. In this context, annealing is best conceptualized not as a terminal post-deposition finishing step, but as a central coupling hub that connects material design, processing strategy, and real-world electrochromic device deployment.

7. Summary and Outlook

Annealing should not be regarded as a secondary post-treatment in WO₃ electrochromic films, but as a central design variable that links deposition-derived microstructure, defect chemistry, and device-level performance. The evidence reviewed here shows that annealing is not adequately defined by temperature alone. Rather, temperature, dwell time, and atmosphere act as coupled variables that jointly determine crystallinity, porosity, and oxygen deficiency and, ultimately, optical modulation (ΔT), coloration efficiency (CE), Li⁺ diffusion coefficient, switching kinetics, and cycling stability. Accordingly, annealing conditions reported only in terms of peak temperature are insufficient for meaningful comparison across studies.

A key conclusion of this review is that the widely reported contrast–stability trade-off in WO₃ films should not be treated as an intrinsic limitation of the material itself. Instead, it largely reflects the application of spatially uniform thermal treatment to films whose different regions often require different structural functions. In general, amorphous or weakly crystallized WO₃ favors rapid ion insertion/extraction and high optical modulation, whereas more crystalline WO₃ provides greater structural and chemical robustness. This explains why biphasic and phase-interface architectures can substantially relax the trade-off, allowing high ΔT to coexist with improved cycling durability [90,104].

The review also indicates that long-term durability remains the most urgent unresolved challenge for practical WO₃ electrochromic devices. Performance degradation is governed not only by the bulk film, but also by interfacial processes such as Li⁺ trapping, hydrolysis, and reactions with trace water in the electrolyte. In this sense, annealing alone cannot solve the stability problem. Its benefits can only be fully realized when combined with interface passivation and electrolyte engineering, as illustrated by recent progress using protective layers and hybrid-cation electrolytes [119,121]. Future comparisons of annealing strategies should therefore move beyond film-level metrics and adopt device-relevant criteria, including lifetime, large-area uniformity, thermal budget, substrate compatibility, and energy cost.

Another major limitation in the current literature is the lack of standardized, route-sensitive comparisons. The effective annealing window depends strongly on the deposition method, film thickness, substrate, initial hydration state, electrolyte, inserted ion species, and testing protocol. Therefore, broadly stated “optimal” annealing windows should be treated with caution. The more useful goal is to define application-specific processing windows for each deposition route, particularly for smart windows, flexible devices, and dual-band visible/NIR systems [125,127].

Looking ahead, the most promising direction is the transition from uniform whole-film annealing to spatially resolved, device-coupled, and data-guided annealing strategies. Recent studies on amorphous/crystalline heterostructures [90], hybrid-cation electrolyte systems [121], and machine-learning-assisted prediction of electrochromic behavior from processing parameters [128] all suggest that the contrast–stability trade-off, durability ceiling, and multidimensional optimization problem can be alleviated when annealing is treated as part of a film–interface–device co-design framework rather than as an isolated post-processing step. However, progress in this direction will depend equally on better mechanistic characterization and more standardized reporting of annealing parameters and electrochromic metrics. Without such comparability, even advanced optimization approaches will remain constrained by fragmented and non-transferable datasets.

In summary, the future of annealed WO₃ electrochromic films lies not in identifying a single best annealing temperature, but in establishing route-dependent and application-oriented processing windows that integrate annealing with deposition, interface design, and device architecture. Reframing annealing in this way shifts the field from empirical parameter tuning toward process–structure–device co-design, which is likely to be essential for translating WO₃ electrochromic materials from laboratory demonstrations to scalable and durable practical applications.