Revealing Enhanced Optical Modulation and Coloration Efficiency in Nanogranular WO₃ Thin Films Through Precursor Concentration Modifications

Abstract

Electrochromic (EC) materials allow for dynamic tuning of optical properties via an applied electric field, presenting great potential in energy-efficient technologies, such as smart windows for effective light and temperature regulation. The precise control of precursor concentration has proven to be a powerful approach in tailoring the physicochemical properties of semiconducting metal oxides. In this study, we employed a one-step electrodeposition technique to fabricate tungsten oxide (WO₃) thin films, systematically exploring how varying precursor concentrations influence the material’s characteristics. X-ray diffraction analysis revealed significant changes in diffraction patterns, reflecting subtle structural modifications due to concentration variations. Additionally, scanning electron microscopy revealed significant changes in the microstructure, showing a progression from small nanogranules to larger agglomerations within the film matrix. The W-25 mM thin film delivered exceptional EC performance, efficiently accommodating lithium ions while showcasing superior EC properties. The optimized electrode, denoted as W-25 mM, showcased exceptional EC metrics, featuring the highest optical modulation at 82.66%, outstanding reversibility at 99%, and a notably high coloring efficiency of 83.01 cm²/C. These findings emphasize the importance of precursor concentration optimization in enhancing the EC properties of WO₃ thin films, contributing to the advancement of high-performance, energy-efficient materials.

1. Introduction

A large share of global primary energy estimated at 30% to 40% is dedicated to buildings, which utilize this energy for heating, cooling, lighting, and appliances. The proportion of energy consumed in buildings is increasing, particularly in more affluent countries [1,2,3]. Recent statistics from the United States illustrate this rise, with energy consumption for buildings climbing from 34% in 1980 to 41% in 2010. Much of this energy is derived from fossil fuels such as coal, oil, and gas, resulting in carbon dioxide emissions that contribute to global warming and rising sea levels, with the potential for significant secondary effects, including social unrest [4,5,6].

The buildings sector consumes large amounts of energy but has great potential for energy savings, which often go unrealized due to hindered by outdated construction practices. Windows and glass facades, or glazings, are major weak points in energy efficiency, allowing excess thermal and solar energy to enter, increasing cooling demands [7,8,9]. Green technologies can improve glazing performance by optimizing solar energy management and enhancing thermal insulation. Chromogenic materials can, including photochromic, thermochromic, and electrochromic materials, regulate light and heat transmission in glazings, adapting to external factors. Photochromic materials respond to UV light, thermochromic materials react to temperature changes, and EC materials adjust their properties when exposed to electrical voltage, enabling dynamic control of energy efficiency [10].

Electrochromic materials can be classified into three main groups: inorganic compounds, single molecular systems, and organic polymers. A significant amount of research has been dedicated to transition metal oxide thin films, such as tungsten oxide, vanadium oxide, molybdenum oxide, iridium oxide, titanium oxide, and nickel oxide [3,11]. WO₃ has emerged as one of the most promising EC materials due to its exceptional optical modulation capabilities, stability, and fast response times. WO₃ exhibits electrochromism through a reversible redox process, wherein ions such as protons (H⁺) or lithium ions (Li⁺) are intercalated into its crystal structure. This ion insertion changes the oxidation state of tungsten from W⁶⁺ to W⁵⁺, leading to a visible color change from transparent to blue. The ability of WO₃ to reversibly control its transparency makes it particularly useful in energy-saving applications, such as smart windows, which can regulate light and heat transmission, significantly reducing energy consumption for heating and cooling systems in buildings [1]. The EC process in WO₃ is driven by the following reaction:

WO₃ + xM⁺ + xe⁻→M_xWO₃

(1)

where M⁺ is the intercalating ion (e.g., H⁺ or Li⁺), and x represents the extent of ion insertion, which governs the degree of coloration. A range of methods has been employed to synthesize WO₃ thin films, including thermal evaporation, spray pyrolysis, sol-gel synthesis, sputtering, hydrothermal techniques, and electrodeposition [12,13,14]. While each method has its advantages, electrodeposition stands out for several reasons. Unlike other techniques, electrodeposition can produce thin films at room temperature over large surface areas, ensuring excellent uniformity and precise control over film thickness. Additionally, it is a cost-effective process, requiring relatively low equipment investment. These benefits make electrodeposition an attractive option, particularly for applications that demand scalability and control over material properties, such as EC devices [15,16,17].

Srivastava et al. developed WO₃ thin films for (EC) electrochromic windows using potentiostatic electrodeposition from acetylated peroxytungstic acid (PTA). Despite being X-ray amorphous after annealing at 100 °C, transmission electron microscopy (TEM) revealed a nanostructured film consisting of both amorphous and hexagonal microcrystals. The films demonstrated fast bleaching kinetics, with a bleaching time of (8 s), high transmission modulation (68%), and stable electrochemical properties stability after 10,000 cycles. This demonstrates the significant influence of a microstructure’s effect on EC performance [18]. In another study, Zhang et al. developed a dual-phase laminated film of amorphous WO₃ and crystalline WO₃ on ITO-coated glass using an electrodeposition-assisted sol-gel method on indium tin oxide (ITO)-coated glass. This film showed superior performance compared to individual amorphous WO₃ and crystalline WO₃ films, achieving a larger optical modulation range (70.6% at 633 nm) and a higher coloring efficiency of 53.6 cm²/C. The dual-phase film structure also demonstrated improved cyclic stability and faster response times, due to the synergistic effect between the two phases and the rougher surface, which affects and enhanced surface contact [19].

Yamanaka et al. successfully electrodeposited WO₃ films directly from PTA, synthesized by dissolving tungsten or tungsten carbide powder in an aqueous hydrogen peroxide solution. These films showed coloration efficiency and response rates similar to vacuum-evaporated films [20]. Meanwhile, Deepa et al. reported the growth of mesoporous tungsten oxide films via electrodeposition with sodium dodecyl sulfate, achieving enhanced electrochromic properties, including higher coloration efficiency and faster switching kinetics than previously reported, compared to earlier mesoporous tungsten oxide films [21].

The EC performance of WO₃ is heavily influenced by its structural and chemical properties, which are, in turn, affected by the synthesis method and the concentration of precursors used. Changes in precursor concentration during synthesis can lead to significant differences in ion diffusion rates, coloration efficiency, and durability—all of which are key factors in optimizing WO₃ performance for EC applications. Given the importance of optimizing EC materials for energy-efficient technologies, this study aims to explore how varying precursor concentrations impact the EC properties of WO₃ films. By systematically adjusting precursor concentrations during the electrodeposition process, this research seeks to determine how these changes affect optical modulation, ion intercalation kinetics, and cycling stability.

2. Experimental Section

2.1. Reagents and Materials

All chemicals, such as sodium tungstate dihydrates (Na₂WO₄·2H₂O, 99.99%, Sigma–Aldrich, Seoul, Republic of Korea), hydrogen peroxide (H₂O₂, 30%, Sigma–Aldrich), and nitric acid (HNO₃, Sigma–Aldrich), were of analytical grade and used without further purification. The electrolyte solution consisted of lithium perchlorate (LiClO₄, 99.99%, Sigma–Aldrich, Seoul, Republic of Korea) dissolved in propylene carbonate (PC, 99.7%, Sigma–Aldrich). Fluorine-doped tin oxide (FTO) glass substrates were provided by MTI Co. Ltd., Seoul, Republic of Korea. Prior to deposition, the FTO glass was thoroughly cleaned using ultrasonication in ethanol, acetone, and deionized (DI) water for 15 min each.

2.2. Synthesis of WO₃ Thin Films

This work builds upon prior findings, introducing targeted modifications in WO₃ sol synthesis by varying precursor concentrations [16]. For each preparation, 100 mL of Na₂WO₄·2H₂O solution at concentrations of 20 mM, 25 mM, and 30 mM was separately mixed with double-distilled water (DDW) and stirred for 15 min. The addition of 0.5 mL of 30% hydrogen peroxide (H₂O₂) to each solution produced a deep yellow color, signaling the formation of peroxo-tungstic acid (PTA). Nitric acid (HNO₃) was then slowly introduced while keeping the solution at 45 °C, after which the PTA solution was cooled to room temperature. The prepared PTA solutions, each with a distinct Na₂WO₄ concentration (20 mM, 25 mM, and 30 mM), were used for electrodepositing WO₃ thin films under a ±1 V potential. To assess how varying the precursor concentration influences the structural, compositional, and EC characteristics of the WO₃ thin films, 20 electrodeposition cycles were conducted for each solution. The films were identified as W-20 mM, W-25 mM, and W-30 mM, corresponding to the precursor concentration. This systematic variation in precursor concentration provided a comprehensive understanding of its effects on the properties of the electrodeposited WO₃ films.

3. Material Characterization

The crystal structure of the samples was examined using X-ray diffraction (XRD; PAN Analytical) with Cu-Kα radiation. Field-emission scanning electron microscopy (FE-SEM; S-4800 HITACHI, Ltd., Tokyo, Japan) was employed to analyze surface morphology, elemental composition, and mapping. Additionally, Raman spectroscopy was performed with an XploRA Plus Raman spectroscope (HORIBA Jobin Yvon S.A.S., Lille, France) to investigate the chemical composition of the WO₃ thin films. The chemical states of the elements in the composition were explored using X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Scientific, Oxford, UK). For electrochemical assessments, a three-electrode configuration was implemented with a battery cycler (Biologic Instrument, Tokyo, Japan,-WBCS3000) electrochemical workstation. The electrochromic optical transmittance was measured using a UV–Vis spectrophotometer (Model: S-3100, SCINCO, Seoul, Republic of Korea) connected to an electrochemical workstation (IVIUM Technologies, COMPACTSTAT, Eindhoven, The Netherlands). These measurements were conducted in a 1 M LiClO₄ + PC aqueous electrolyte, where the WO₃ thin films acted as the working electrode, platinum served as the counter electrode, and Ag/AgCl functioned as the reference electrode.

4. Results and Discussion

4.1. X-Ray Diffraction (XRD) Elucidation

XRD analysis was carried out to investigate the structural characteristics of different precursor varied electrodeposited WO₃ thin films, and the results revealed a predominantly amorphous nature across all samples, as illustrated in Figure 1a. Characteristic peaks corresponding to the FTO substrate were observed in every sample, confirming the preservation of the substrate’s crystalline structure during film deposition. The most significant feature in the XRD patterns was a broad hump centered around 25–27° −2θ, indicating the amorphous nature of WO₃. The broad peak in the lower 2θ range suggests the existence of an amorphous layer throughout the samples. While the XRD patterns of all films were consistent in terms of orientation, there were slight variations in the intensity of the hump, possibly due to differences in film thickness, composition, or localized structural variations. Among the different samples, the W-30 mM thin film exhibited a broader and more pronounced hump compared to the W-20 mM film, which could be attributed to its higher precursor concentration [17]. The increased precursor concentration content may induce greater disorder within the amorphous matrix, resulting in a wider hump. The loose microstructure of these amorphous WO₃ films offers several distinct advantages over their crystalline counterparts. One key benefit is the enhanced charge efficiency, which is critical for applications like energy storage and electrochemical devices. Additionally, the amorphous films show broader optical modulation, making them suitable for optoelectronic applications such as EC devices, where light transmission and reflection properties need to be finely tuned. Furthermore, the films’ rapid switching times make them ideal for fast-response applications, offering faster reaction times in dynamic environments. These characteristics make amorphous WO₃ thin films promising candidates for a range of advanced technological applications where material efficiency, optical control, and speed are critical [16].

4.2. Raman Spectroscopy Analysis

Raman spectroscopy is a well-established technique for probing the vibrational modes of molecules and materials, providing crucial insights into their structural and bonding characteristics. Figure 1b illustrate the Raman spectra of electrodeposited WO₃ thin films at various precursor concentrations: W-20 mM, W-25 mM, and W-30 mM. These spectra reveal the influence of precursor concentration on the WO₃ matrix. A prominent peak at 241 cm⁻¹ is associated with the angular vibrations of O-W-O bonds within the WO₃ structure, reflecting the bending motions of oxygen atoms between tungsten atoms and revealing important aspects of the material’s structural arrangement. Furthermore, the spectra display broadened peaks between 680 cm⁻¹ and about 955 cm⁻¹, which represent the stretching vibrations of W = O and W-O bonds, indicative of the characteristic features of amorphous WO₃ [22]. The broadening suggests a disordered structure that results in less distinct vibrational modes compared to crystalline WO₃. Additionally, a supplementary band around 1094 cm⁻¹ is attributed to the FTO substrate [16]. As the precursor concentration increases, both the intensity of this band and a slight peak shift suggest interactions between the WO₃ film and the underlying substrate. This shift could signify subtle changes in the local electronic environment and bonding dynamics resulting from variations in precursor concentration. These spectral features collectively enhance our understanding of how precursor concentration influences the structural and electronic properties of amorphous WO₃ films, which are vital for their optical and electrochemical performance [17].

4.3. Morphological Study

FESEM analysis was conducted to examine the effect of varying precursor concentration on the morphology of WO₃ nanostructures. Form Figure 2(a1) it was observed that the W-25 mM sample displays uniformly distributed small granules, which, while favorable in terms of size, may not be conducive to the best EC performance. The smaller granules, despite their uniformity, can limit the formation of efficient pathways for electron and ion transport, leading to less effective switching of the material [23]. The W-20 mM sample, (Figure 2(b1), with its well-formed and uniformly deposited nanogranules, presents an optimal balance between surface area and particle connectivity. The uniformity of the nanogranules ensures that the surface area is maximized for enhanced interaction with electrolytes or other active species, which is crucial for efficient electrochemical performance. At the same time, the particle connectivity is maintained at a high level, allowing for effective charge transport across the material. This combination of increased surface area and improved connectivity enhances both the reaction kinetics and the overall conductivity. The W-30 mM sample, (Figure 2(c1)), with its larger and agglomerated granules, suffers from reduced surface area and inefficient transport pathways, which hinder EC switching. The agglomeration of granules negatively impacts the material’s performance by reducing the speed and reversibility of color changes, ultimately compromising the EC efficiency. The thickness of W-20, W-25, and W-30 thin films significantly influences their electrochromic performance. Generally, film thickness influences the EC parameters’ performance including sufficient optical modulation, cyclic stability, charge storage capacity, and switching speed, because of the deposited material volume, uniformity, and diffusion length. Therefore, an optimal film thickness is crucial to balance these competing factors to achieve the best electrochromic performance. Moreover, variations in film thickness can also affect the film’s mechanical flexibility, adhesion to the substrate, and overall device integration, all of which are important considerations for practical applications. Figure 2(a2,b2,c2) shows the cross-sectional views of the W-20 mM, W-25 mM, and W-30 mM thin films, with measured thicknesses of 198 nm, 226 nm, and 250 nm, respectively [16].

4.4. X-Ray Photoelectron Spectroscopy (XPS) Analysis

XPS was employed to evaluate the stoichiometry, surface composition, and chemical states of WO₃ thin films. The survey XPS spectrum for the W-25 mM sample, illustrated in Figure 3a, clearly indicated the presence of tungsten and oxygen, along with trace amounts of carbon. The detected carbon peaks were attributed to incidental contamination from the environment, highlighting the importance of maintaining a controlled atmosphere during the deposition process to minimize such interference. High-resolution core level spectra for the W (4f) and O (1s) core levels are presented in Figure 3b,c. The Voigt curve fitting analysis revealed two distinct peaks in the W4f core level spectrum (Figure 3b). This core level is characterized by a spin-orbit doublet, with binding energies of 35.39 eV for W4f_7/2 and 37.49 eV for W4f_5/2. The energy separation of approximately 2.19 eV between these peaks is indicative of the W6⁺ oxidation state, confirming the successful synthesis of the WO₃ thin film [24]. In Figure 3c, the application of Gaussian fitting to the O (1s) XPS spectrum of the W-25 mM thin film revealed a peak at 530.6 eV, which is attributed to W-O chemical bonding within the WO₃ structure. This peak reflects the strong interactions between tungsten and oxygen atoms, essential for the stability and functionality of the WO₃ thin films. Additionally, a second peak at 532.3 eV corresponds to residual adsorbed species present on the surface of the electrodeposited WO₃ thin films. These species likely include hydroxide groups, water, and C–O bonds, indicating that the surface of the films is hydrated. The presence of such residual species suggests that the films may have been exposed to moisture during synthesis or handling, which could influence their electrochemical properties and overall performance in applications such as sensors or energy storage devices [25]. Thus, understanding the chemical states and surface composition is crucial for optimizing the functional characteristics of WO₃ thin films.

5. Electrochromic Analysis

A comprehensive analysis of the cyclic voltammetry (CV) characteristics of WO₃ electrodes was performed using a carefully calibrated three-electrode electrochemical system, which provided optimal conditions for collecting accurate and reproducible data. The experimental measurements were carefully executed over a potential range of +1 to −1 V versus an Ag/AgCl reference electrode. A highly conductive 1 M LiClO₄ in PC aqueous electrolyte was utilized, providing a stable environment for a thorough analysis of the electrode’s electrochemical behavior. CV data for the WO₃ samples with varying precursor concentrations (W-20 mM, W-25 mM, and W-30 mM) were meticulously recorded at a scan rate of 10 mV/s, spanning a potential range from +1 V to −1 V. The corresponding profiles, presented in Figure 4a, highlight distinct electrochemical responses tied to each concentration, providing critical insights into their behavior. CV profiles show a reliable pattern, featuring clear and broad redox peaks that reflect significant electrochemical activity and reversibility. A linear correlation is evident between the area enclosed by the CV curves and the sweep rate. This correlation arises from the higher scan rates, which reduce the diffusion distance at the electrode surface and lead to an increase in current density. This trend suggests that the electrochemical processes are more effectively facilitated at elevated sweep rates due to the minimization of mass transport barriers [26]. Figure 4b–d shows that the W-25 mM electrode displayed a significantly higher redox peak current and a more substantial area under the CV curve, highlighting its enhanced electrochemical performance when compared to the W-20 mM and W-30 mM electrodes. The uniformly distributed nanogranules in the W-25 mM electrode form a homogeneous network that offers ample reactive sites, facilitating effective charge diffusion. Conversely, the W-20 mM and W-30 mM thin films demonstrate lower peak currents in their CV profiles, reflecting a decline in electrochemical activity. The reduced performance is linked to the smaller size of the nanogranules and their more agglomerated surface structure, which limit the number of accessible reactive sites. Consequently, the compactness of these structures hinders the interaction between the electrode and the electrolyte, decreasing chemical reactivity and impeding the electrodes overall electrochemical performance matrix leads to a transition to a deep blue hue. The reverse process during oxidation restores the electrode to its original transparent condition. This reduction (coloration) and oxidation (bleaching) behavior showcases the EC effect, represented by the following redox reaction [2,3,16]:

WO₃ + Li⁺ + e⁻ → Li_xWO₃ (Coloration)

(2)

Li_xWO₃ → WO₃ + Li⁺ + e⁻ (Bleaching)

(3)

Following this, a thorough analysis of the charge transfer mechanism was performed using the CV data to enhance understanding of the current response dynamics. As shown in Figure 4e, the plot of peak current density (i_p) versus the square root of the scan rate $\left({\vartheta }^{{\displaystyle \frac{1}{2}}}\right)$ reveals a linear relationship between the redox current and the scan rate. The diffusion coefficient for Li⁺ ions was determined through the Randles–Sevcik Equation (4), utilizing i_p and $\left({\vartheta }^{{\displaystyle \frac{1}{2}}}\right)$ values measured at a scan rate of 10 mV/s [27].

$$D^{{\displaystyle \frac{1}{2}}}={\displaystyle \frac{i_p}{2.69\times {10}^5\times n^{3/2}\times A\times C\times {\vartheta }^{1/2}}}$$

(4)

The equation defines i_p as the peak current, n as the number of electrons involved in the electrochemical reaction, A as the area of the working electrode (in cm²), C as the concentration of the electrolyte, ϑ as the scan rate, and D as the diffusion coefficient (in cm²/s). The resulting cathodic and anodic diffusion coefficient values for the WO₃ electrodes are presented in

Table 1. The resulting cathodic and anodic diffusion coefficient values for the WO₃ electrodes.

Sample Code	Diffusion Coefficient (cm2/s × 10−10)		${\mathit{Q}}_{\mathit{i}}$ (C/cm2)	${\mathit{Q}}_{\mathit{di}}$ (C/cm2)	Reversibility	tc (s)	tb (s)	Tb (%)	Tc (%)	ΔT600nm (%)	ΔOD	Coloration Efficiency (cm2/C)
	Reduction	Oxidation
W-20 mM	14.37	14.3	0.054	0.053	98.01%	14.79	10.51	90.30	12.03	78.27	2.01	74.44
W-25 mM	18.93	7.63	0.053	0.052	99.00%	11.90	5.42	92.96	10.30	82.66	2.20	83.01
W-30 mM	8.4	6.66	0.060	0.059	97.33%	14.52	10.83	86.29	35.09	51.20	0.89	29.66

, which clearly shows that W-25 mM has a higher diffusion coefficient (cathodic 18.93 × 10⁻¹⁰ cm²/s and anodic 7.63 × 10⁻¹⁰ cm²/s) compared to the other reference electrodes. These findings further support the idea that the EC performance of WO₃ can be enhanced by varying the precursor concentrations.

In this investigation, the impact of precursor concentrations on the EC performance of WO₃ thin films is explored through in situ transmittance spectra taken in both colored and bleached states. Figure 5a–c illustrates the in situ transmittance spectra of the electrodes, which were recorded over a wavelength range of 300 to 1100 nm with a potential bias of ±1 V, and the associated transmittance results are summarized in Table 1. The thin films demonstrate significant transparency in their bleached state and transition to a colored state when −1 V is applied. This coloration is driven by polaron absorption resulting from the insertion of ions and electrons [28]. Figure 5b reveals that the W-25 mM sample achieves an exceptional optical modulation (ΔT) of 82.66% at 600 nm, with the bleached state (T_b%) at 92.96% and the colored state (T_c%) at 10.30%. This performance notably exceeds the optical modulations of W-20 (ΔT = 78.27%) and W-30 (ΔT = 51.20%). The remarkable optical modulation observed in W-25 mM can be attributed to its uniform nanogranular surface structure, which provides a shorter ion diffusion path and improves the interaction between the electrode and the electrolyte. This morphology facilitates more efficient ion insertion and extraction, resulting in more pronounced coloration and bleaching effects. Figure 5c illustrates the photos of both the bleached and colored states of the W-25 thin film, showcasing its improved EC performance. Conversely, the W-20 and W-30 samples exhibited reduced optical modulation efficiency, attributed to their less optimal surface structures [22].

The intercalation and deintercalation behavior of Li⁺ ions in the WO₃ electrodes over time was quantitatively investigated through chronocoulometry (CC) analysis. Figure 6a–c displays the resulting CC plots for three different compositions: W-20, W-25, and W-30, each evaluated in an electrolyte composed of 1 M LiClO₄ dissolved in PC. During the process of cathodic polarization, an influx of charges occurs within the electrode material, triggering the reduction of tungsten ions from W⁶⁺ to W⁵⁺. This reduction process leads to the material transitioning to a colored state, as is typical in EC films. Subsequently, applying a reverse potential causes the deintercalation of Li⁺ ions from the structure, reversing the initial reaction. This results in the oxidation of the tungsten back to its higher oxidation state of W⁶⁺, accompanied by the film reverting to its original, bleached state. A crucial factor in assessing the performance of EC materials is their reversibility, which is indicative of how effectively the material can transition between its colored and bleached states across multiple cycles. This reversibility is primarily determined by the charge recovery ratio, which is the ratio of the amount of charge extracted during deintercalation (Qdi) compared to the charge inserted during intercalation (Qi). The EC reversibility is calculated using Formula (5) [29,30].

$$Reversibility={\displaystyle \frac{Q_{di}}{Q_i}}\times 100$$

(5)

The values of Q_i and Q_di, along with the corresponding calculated reversibility percentages, are presented in Table 1. Among the tested electrodes, the W-25 mM electrode exhibited outstanding EC performance, achieving a high reversibility of 99%. This exceptional performance can be attributed to the optimized surface morphology of the W-25 electrode, which significantly enhances its EC properties. In contrast, the W-20 mM and W-30 mM electrodes demonstrated lower reversibility, with values significantly below that of the W-25 mM electrode. This indicates that these electrodes are less capable of fully returning to their original, bleached state during the reverse process, which is crucial for effective EC applications. The diminished reversibility suggests that the structural or compositional characteristics of the W-20 mM and W-30 mM electrodes may hinder efficient deintercalation of Li⁺ ions. This inefficiency in ion removal not only limits their ability to revert to the bleached state but also adversely affects their overall EC efficiency, making them less suitable for applications requiring rapid and reversible color changes. Thus, the findings underscore the importance of optimizing precursor concentration and surface morphology to achieve superior EC performance in WO₃ thin films [16,31].

Coloration efficiency (CE) is a fundamental parameter in the evaluation of EC materials, serving as a measure of how proficiently a material alters its optical density (ΔOD) relative to the charge density applied. This metric is vital for optimizing both the energy efficiency and visual performance of EC devices. The mathematical expression (6) used to calculate CE underscores its importance in ensuring the device operates with maximum effectiveness, balancing minimal energy consumption with desired optical shifts [10].

$$CE={\displaystyle \frac{\Delta OD}{\left(\raisebox{1ex}{$Q_i$}\!\left/ \!\raisebox{-1ex}{$A$}\right.\right)}}$$

(6)

This relationship connects the variation in optical density to the charge inserted per unit area of the electrode (Q_i/A) at a specific wavelength. The optical density shift is directly influenced by the amount of charge inserted, and it is derived from the transmittance values in the bleached and colored states, as expressed by the following Equation (7).

$$\Delta OD=ln\left({\displaystyle \frac{T_b}{T_c}}\right)$$

(7)

The transmittance values of the bleached (T_b) and colored (T_c) states at a visible wavelength (nm) were utilized to calculate ΔOD and CE for WO₃ thin films, as shown in Table 1. Notably, the calculations revealed that the W-25 mM sample achieved the highest CE among all precursor concentration varied WO₃ thin films, reaching 83.01 cm²/C at 600 nm. This improvement in coloring efficiency highlights the potential for enhanced reaction kinetics and energy storage by varying precursor concentration [2].

The pronounced optical modulation and superior CE can be attributed to the nanogranules uniformity, which increases the active surface area available for charge transfer. A higher CE suggests that a smaller amount of charge is sufficient to induce a significant shift in transmittance. In contrast, the reduced CE values observed in the W-20 mM and W-30 mM samples are likely due to the higher charge insertion necessary for coloration, resulting in smaller ΔOD variations [32].

The ability of EC films to quickly transition between colored and bleached states is essential for their performance in applications like smart windows and displays. Coloration time measures the duration required to switch to a colored state, whereas bleaching time measures how long it takes to return to a clear state. Achieving shorter coloration and bleaching times enhances the usability of EC films by allowing rapid adjustments to their optical properties. The in situ transmittance for WO₃ thin films was plotted in Figure 6d–f to determine the switching times over 40 s at 600 nm. The coloration (T_c) and bleaching (T_b) times, defined as the time required to reach 95% of maximum optical contrast, are key factors in the device’s efficiency. The fast coloration kinetics are driven by the excellent conductivity of tungsten bronze (Li_xWO₃), while the rapid decrease in current during bleaching is due to the material’s transition from conductor to insulator. These results indicate that ion extraction occurs more rapidly than ion insertion, with bleaching consistently outperforming coloring due to the higher conductivity of Li_xWO₃ during the bleaching process. Table 1 highlights the strong performance of the W-25 mM electrode, which exhibited a significantly faster coloration time of 11.90 s and a similarly short bleaching time of 5.42 s [33,34,35].

For smart window applications, ensuring the long-term stability of EC cycling is essential to maintaining reliable performance. This stability allows the window to consistently change color while maintaining its efficiency in controlling light and heat transmission. Researchers evaluated the EC retention of thin films using a UV–Vis spectrometer combined with an electrochemical cyclic tester within a three-electrode setup. Figure 7a–c illustrates the in situ transmittance stability during cycling for WO₃ thin films. Each cycle, lasting 40 s at 600 nm, involved alternating between colored (−1 V) and bleached (1 V) states over 2000 s. The W-25 mM film demonstrated excellent stability, with a minor 5.15% decrease in ΔT after 5000 s. This performance can be attributed to its well-structured surface morphology, which ensures stability, minimizes ion trapping, and offers strong substrate adhesion. In contrast, the W-20 mM and W-30 mM films exhibited 9.62% and 10.84% optical degradation, respectively, after 2000 s, attributed to significant ion entrapment that limited access to active sites, reducing transmittance [34].

6. Conclusions

A straightforward, efficient, and cost-effective electrodeposition technique has been employed to synthesize tungsten oxide thin films with varying precursor concentrations. The resulting WO₃ films exhibited an amorphous structure and a nanogranular morphology, positioning them as promising candidates for the large-scale, economical production of electrochromic (EC) devices. Raman spectral analysis of the WO₃ thin films, provided valuable insights into the redox dynamics of tungsten species, showing changes in valence states. Among the different compositions tested (W-20 mM, W-25 mM, and W-30 mM), the W-25 mM electrode displayed the most remarkable EC performance, achieving the highest optical modulation (82.66%), excellent reversibility (99.00%), and outstanding coloring efficiency (83.01 cm²/C). These enhanced properties are attributed to the large active surface area and the plentiful grain boundaries in the electrode, which promote shorter diffusion pathways and improved electrolyte penetration within the film structure. The exceptional performance of the W-25 mM electrode highlights its significant potential to advance EC technology, paving the way for scalable and cost-effective device production.