Temperature-dependent electrochromic cycling performance of solution-processed WO₃ films

Hoon Kang^a, Gyung Hyun Kim^a, Sungho Kang^a, Tae Hoon Park^a, Kwanchul Kim^a, Hwan Kyu Kim*^b and Yekyung Kim*^a
aAdvanced Institute of Convergence Technology, Seoul National University, Suwon-si, Gyeonggi-do 16229, Republic of Korea. E-mail: yekyung@snu.ac.kr
^bDepartment of Advanced Materials Chemistry, Korea University, Sejong 30019, Republic of Korea. E-mail: hkk777@korea.ac.kr

First published on 7th August 2025

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Introduction

Electrochromic materials demonstrate reversible optical changes when electrically stimulated through electrochemical redox reactions. They have attracted considerable research attention for applications such as energy-efficient smart windows for buildings, automobiles, and airplanes, as well as for low-power displays and adaptive optical devices.^1,2 Electrochromic materials are categorized into organic and inorganic types; the former includes viologens, poly(3,4-ethylenedioxythiophene), and polyaniline, whereas the latter includes WO₃, MoO₃, V₂O₅, and NiO_x.^2–4 Among these electrochromic materials, WO₃ is the most extensively studied^5–14 because of its high coloration efficiency (CE), wide optical modulation, and excellent chemical stability. The chemical states of WO₃ reversibly change via the insertion or extraction of small cations (e.g., H⁺ and Li⁺), resulting in a color change under an applied electric field. The electrochromic reaction of WO₃ can be described by

WO3 (transparent) + xM+ + xe− ↔ MxWO3 (deep blue)	(1)

A reduction reaction occurs when a negative voltage is applied, turning the film blue. Conversely, applying a positive voltage induces oxidation, which restores the film to its transparent state. This reversible reaction in WO₃ is driven by electron transfer between the non-equilibrium states of W⁵⁺ and W⁶⁺. In amorphous WO₃, this behavior is attributed to localized electrons causing polaronic transitions, whereas in crystalline WO₃, it follows the Drude model.^15–20 Structural variations in WO₃ lead to differences in electron transport mechanisms, which influence the mobility of electrons and ions and ultimately affect the electrochromic performance. Thermal treatment is essential for optimizing the properties of WO₃ thin films by adjusting their crystallinity and microstructure. Untreated WO₃ thin films often suffer from structural instability, which limits their performance over extended cycling. In contrast, WO₃ thin films that have been properly annealed demonstrate enhanced electrochemical stability due to reduced defects and improved amorphous structure. This leads to better electrochromic performance. For example, the transition from amorphous to crystalline WO₃ alters ion diffusion pathways and electrochemical reactivity, which impacts the electrochromic performance. Amorphous WO₃ provides high ion mobility and fast response times; however, it suffers from challenges in terms of maintaining structural stability during long-term operation. Conversely, crystalline WO₃ offers better structural stability but restricts ion mobility because of its dense and orderly lattice structure. The electrochromic performance of WO₃ thin films is significantly affected by its crystalline properties and by its structural, chemical, and morphological properties, which are highly dependent on the fabrication process. Despite the extensive studies on WO₃, the relationship between thermal treatment conditions, structural changes, and electrochromic performance remains a subject of ongoing investigation.

Key challenges, such as ensuring reversibility and long-term electrochemical stability, need to be addressed to enable the widespread adoption of electrochromic devices across various applications. However, many studies focused on initial electrochromic performance, with less emphasis on long-term cycling stability critical for practical applications.

In this study, WO₃ thin films were fabricated via spin-coating followed by thermal treatment at various temperatures, and the influence of their characteristics on the long-term stability of electrochromic performance was investigated. Although a simple and cost-effective deposition technique, spin-coating generally yields lower film uniformity and somewhat inferior electrochemical performance compared to methods such as sputtering. Nevertheless, spin-coating remains an attractive option owing to its versatility and ease of process optimization. Careful tuning of fabrication parameters such as precursor composition, coating speed, and post-treatment conditions can significantly improve the performance of spin-coated films, offering wide potential for applications. Through comprehensive analysis of the thermal, structural, chemical, and electrochemical properties of the prepared films, we identified optimal thermal treatment conditions for balancing ion mobility, structural stability, and long-term cycling performance. Moreover, the operational stability of the electrochromic WO₃ layer was carefully examined for all films prepared at various annealing temperatures from low to high. The long-term operational behavior of WO₃ was studied in terms of its crystallinity, chemical composition, and structural characteristics. By comparing the WO₃ films, this study provides insights and foundational understanding for enhancing the long-term stability of these systems in future research.

Experimental

Fabrication of electrochromic WO₃

Electrochromic WO₃ thin films were fabricated by spin-coating a precursor solution followed by thermal treatment (see Fig. S1). The precursor solution was prepared by dissolving 10 wt% tungsten(VI) chloride (WCl₆, Sigma-Aldrich, ≥99.9%) in isopropyl alcohol (2-propanol, IPA; Sigma-Aldrich, 99.5%), followed by stirring at room temperature (RT) for over 12 h. The indium tin oxide (ITO) coated glass substrate (15 Ω, Wooyang GMS) was cleaned using an ultrasonic bath with deionized water, ethanol, and acetone for 15 min, followed by surface treatment in the UV-ozone cleaner for 1 h. The prepared precursor solution was spin-coated onto the substrate with an acceleration time of 5 s and a rotation speed of 3000 rpm for 45 s. The coated films were thermally treated at 80, 190, 250, 300, 350, and 400 °C for 1 h each to form the WO₃ layer. Films prepared at different temperatures were designated as WO80, WO190, WO250, WO300, WO350, and WO400, respectively.

Physical and chemical characterizations of WO₃ thin films

The thermal properties of WO₃ thin films were analyzed through DSC (DSC-250, TA Instruments) and TGA (TGA-8000, PerkinElmer). For the analysis, the precursor solution was prepared by drying at 50 °C to completely evaporate the solvent, which yields a powder for thermal measurements. Measurements for both DSC and TGA were conducted in the range from 30 to 400 °C at a heating rate of 5 °C min⁻¹. The surface and cross-sectional morphologies of the prepared WO₃ films were examined using FIB-SEM (Helios 5 UC, Thermo Fisher Scientific). The crystalline structure of WO₃ films was characterized using XRD (D8 Discover, Bruker) over a 2θ range of 5–90°. Further, the chemical composition of electrochromic thin films was analyzed using XPS (AXIS Supra+, Kratos Analytical).

Electrochemical and electrochromic performance analysis

The electrochemical properties of the fabricated WO₃ thin films were analyzed using a potentiostat/galvanostat (Multi Autolab/M204, Metrohm Autolab) with a three-electrode system. The WO₃ films prepared in this study were used as the working electrode, while a platinum wire and Ag/AgCl electrode served as counter and reference electrodes, respectively. The electrolyte used in this study was 0.1 M LiClO₄ dissolved in propylene carbonate. Cyclic voltammetry (CV) was conducted to evaluate the redox behavior of WO₃ within a voltage range of −0.8 V to +1.0 V at scan rates of 100, 50, and 10 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was performed to determine the interfacial resistance between the WO₃ thin films and the electrolyte. EIS measurements were conducted over a frequency range of 10⁵ Hz to 1 Hz at an applied potential of 0 V. The electrochromic performance of WO₃ was assessed by applying a voltage through a potentiostat/galvanostat and measuring the transmittance using a UV-vis spectrophotometer (OPTIZEN™ POP, KLAB). Transmittance spectra were recorded for the as-deposited, colored (−0.8 V for 60 s), and bleached states (+1.0 V for 60 s) over a wavelength range of 200–800 nm. The cycling experiment was performed by alternating between the colored (−0.8 V) and bleached states (+1.0 V), with each state maintained for 60 s. This process was repeated for 500 cycles with continuous transmittance monitoring at 550 nm. These measurements provided insights into the durability and long-term electrochromic performance of the films.

Results and discussion

A simple and cost-effective spin-coating method was employed to fabricate a stable WO₃ electrochromic film, followed by thermal treatment (see Fig. S1). The precursor solution was spin-coated onto substrates and then thermally treated at various temperatures to investigate the structural and chemical properties of the thin films and identify conditions that produced the most stable electrochemical performance. The synthesis of WO₃ from WCl₆ and alcohols has been studied over the years. When isopropyl alcohol is used as a solvent, the reaction proceeds as follows^21–26

WCl6 + C3H7OH → W(OC3H7)Cl5 + HCl	(2)

W(OC3H7)Cl5 → WOCl4 + C3H7Cl	(3)

WOCl4 + 2H2O → WO3 + 4HCl	(4)

Among various alcohols, the moderately low vapor pressure and boiling point of IPA lead to stable evaporation, making it more effective for producing uniform thin films than other alcohols such as methanol or ethanol. The aforementioned sequential reactions indicate the conversion of WCl₆ into WO₃ through intermediate compounds involving hydrolysis and oxidation. The precursor solution was prepared by dissolving WCl₆ in IPA, followed by stirring for an extended period, enabling reactions (2) and (3) to proceed naturally. The formation of WO₃ via reaction (4) was driven by the interaction between WOCl₄ and atmospheric moisture during the spin-coating and thermal treatment processes. In this study, ambient moisture was not restricted, and it affected the film formation process.

The effect of thermal treatment on WO₃ formation was indirectly examined through DSC and TGA analyses. Fig. 1 shows the DSC and TGA curves obtained from the WO₃ powder, prepared by drying the precursor solution at 50 °C. Upon drying, a hydrolysis reaction with ambient moisture was completed, facilitating the evaluation of the thermal behavior of WO₃. The DSC curve exhibits broad endothermic peaks from RT to 200 °C, attributed to the evaporation of water, residual solvent, and by-products of the reaction. The first endothermic peak, observed as overlapping peaks at 91.46 and 123.60 °C (onset point of 30 °C and end point of 141 °C), was attributed to the evaporation of the residual solvent, reaction by-products, and water adsorbed on the surface. The second distinct peak at 157.62 °C (onset point of 141 °C and end point of 240 °C) was likely caused by the evaporation of water molecules chemically bonded to WO₃ or incorporated within its structure. Finally, a weak exothermic reaction appeared beyond 350 °C, which can be attributed to the crystallization of WO₃.^21,27–29 This interpretation was supported by the TGA curve, which showed a significant mass loss up to 240 °C. The first endothermic peak region in the DSC curve, where two peaks overlap, corresponded to the range up to 141 °C, during which the TGA curve exhibited a weight loss of 8.38%. In the subsequent range up to 240 °C, which corresponded to the second peak in the DSC curve, an additional weight loss of 5.02% was recorded. This substantial weight loss indicated the release of moisture. Moreover, additional weight loss from 240 to 400 °C was only 2.56%. The weak exothermic response observed in the DSC curve above 350 °C, without a corresponding sharp mass loss, likely indicated WO₃ crystallization. The thermal analysis highlighted the critical role of thermal processing and ambient moisture in the formation and properties of WO₃ thin films. Moreover, the DSC measurements on WO₃ powders dried at different temperatures revealed that peaks observed in Fig. 1 gradually diminished with an increase in drying temperature (Fig. S2). In the DSC curve of WO₃ dried at 80 °C, the portion of the first endothermic peak at 93.60 °C was no longer observed. For powders dried at 300 and 400 °C, both endothermic peaks were nearly absent. These findings suggested that the WO₃ prepared at higher temperatures contained a significantly lower amount of chemically or physically bound water.²⁸

The crystalline state of electrochromic WO₃ films was analyzed using XRD, and the diffraction patterns are presented in Fig. 2. The XRD pattern of the ITO substrate reveals several peaks in the 2θ range of 20–60°. A decrease in the intensity of the substrate peaks was observed in all thin film samples, with the formation of WO₃ films attributed to spin-coating and the subsequent thermal treatment. In addition to the substrate peaks, no additional peaks were detected at thermal treatment temperatures up to 300 °C, indicating that films treated below this temperature exhibited no specific crystalline structure and were thus amorphous. Additional distinct peaks were observed for films with thermal treatment at 350 and 400 °C. These peaks signify the crystallization of the amorphous WO₃. The crystalline WO₃ formed at 350 °C and above exhibits an orthorhombic phase,²¹ with diffraction peaks at 2θ values of 23.15, 24.10, 33.85, 49.65, and 55.55°, corresponding to the (001), (200), (220), (400), and (420) planes, respectively (JCPDS No. 20-1324). These results confirm that the spin-coated WO₃ precursor solution forms amorphous WO₃ at thermal treatment temperatures up to 300 °C, whereas crystallization occurs at higher temperatures. This finding aligns with the thermal analysis results shown in Fig. 1, wherein the exothermic reaction starting at 350 °C corresponds to the onset of crystallization.

XPS was performed for further investigation of the oxidation state of the films. The chemical states of W and O on the thin film surface were analyzed. High-resolution XPS spectra of W 4f and O 1s are presented in Fig. 3, with deconvolution performed using Gaussian–Lorentzian fitting. The deconvoluted W 4f spectrum (Fig. 3) reveals the presence of two tungsten species, W⁶⁺ and W⁵⁺. Prominent peaks at 35.89 ± 0.16 and 38.04 ± 0.17 eV correspond to W⁶⁺ 4f_7/2 and W⁶⁺ 4f_5/2, respectively, while subpeaks at 34.81 ± 0.20 and 36.83 ± 0.15 eV correspond to W⁵⁺ 4f_7/2 and W⁵⁺ 4f_5/2, respectively.^31–36 Further, the O 1s spectrum comprises three species of O²⁻, OH⁻, and interstitial H₂O, located at 530.67 ± 0.22, 532.03 ± 0.29, and 533.25 ± 0.09 eV, respectively.^27,30 These results indicate that thin films formed via the wet chemical process are not composed solely of stoichiometric WO₃ but also contain a small amount of W⁵⁺. The presence of W⁵⁺ in WO₃ films is attributed to oxygen vacancies—associated with the presence of OH⁻ species. Previous studies reported that these defective oxygen species enhance electrochromic performance.^27,31 The analysis of the O 1s spectra supports this finding because the peak corresponding to OH⁻ is the most prominent in WO₃ films treated at 300 °C. To further quantify the degree of oxygen deficiency, the W⁵⁺ to W⁶⁺ ratio was estimated from the deconvoluted W 4f spectra. Using this ratio, the stoichiometric coefficient (x) in WO_x was calculated by applying a chemical balance between the oxidation states, considering the total amount of tungsten detected (Table 1). These results indicate that WO300 possesses the highest density of oxygen vacancies among the thermally treated WO_x thin films—excluding WO80, which was annealed at a temperature lower than the dominant moisture evaporation range. The chemical formula of WO300 was determined to be WO_2.93, representing the most reduced state among the samples (excluding WO80).

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The surface morphology and cross-sectional structure of WO₃ electrochromic films were investigated using FIB-SEM to examine the effects of thermal treatment (Fig. 4 and Fig. S3). Fig. 4 presents the surface and cross-sectional images of films treated at three representative temperatures (80, 300, and 400 °C) selected based on the characteristic features identified in the XRD and XPS analyses. Fig. S3 illustrates the changes in surface morphology across all thermal treatment conditions. As indicated in the magnified surface image (Fig. 4a-1), the WO₃ film treated at 80 °C exhibited numerous cracks on its surface, as well as grains with nano-sized voids. These surface cracks and voids decreased significantly with an increase in the thermal treatment temperature—a trend clearly visible in the surface morphology images in Fig. S3. From 300 °C onward, the surface of the films became significantly smoother with a reduction in both cracks and intergranular voids. The cross-sectional images in Fig. 4 (labeled as Fig. 4a-2, 4b-2, and 4c-2) highlight the structural changes in the films with increasing temperature. At 80 °C, WO80 exhibited voids ranging from a few to a few hundred nanometers within its structure. The films showed progressively dense cross-sectional structures with an increase in the thermal treatment temperature, accompanied by a reduction in film thickness from 390 nm at 80 °C to 260 nm at 400 °C. This structural densification correlates with the formation rate of WO₃ and the effect of ambient moisture. At lower thermal treatment temperatures, the slower formation rate of WO₃ promotes granular aggregation, which leads to void formation between grains. At 80 °C, below the boiling point of water, the moisture within the film does not evaporate rapidly, resulting in a porous structure—consistent with the cracks and vacancies observed in the SEM images. In addition, the results of the thermal analysis support this observation as a large endothermic reaction corresponding to water evaporation was observed for the sample dried at 80 °C (Fig. S2). In contrast, films treated at higher temperatures (300 and 400 °C) exhibit smooth surfaces and dense cross-sections because of the rapid evaporation of internal moisture and accelerated formation of WO₃. This is consistent with DSC results, where almost no water evaporation reaction was detected in the samples prepared at 300 and 400 °C (Fig. S2). These conditions suppress granule formation, which leads to a more uniform and compact structure. The differences in morphology and structure are likely governed by the thermal treatment process and its effect on the evaporation dynamics of water and nucleation and growth of WO₃ (Table 2).

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The electrochemical redox reactions of the WO₃ films were investigated. Fig. 5a illustrates the CV results for WO₃ films thermally treated at 80, 300, and 400 °C, measured at a scan rate of 50 mV s⁻¹. The CV results for all WO₃ layers are presented in Fig. S4, and the scan rate-dependent behavior of the CV curves is shown in Fig. S5. When a voltage was applied in the negative direction, the WO₃ exhibited a reduction current response caused by the insertion of Li⁺ ions from the electrolyte solution into the WO₃ film. Conversely, the oxidation current response was due to the extraction reaction of Li⁺ ions from the film when a voltage was applied in the positive direction. The CV curves revealed that the oxidation peak potential shifted toward more negative voltages with an increase in the thermal treatment temperature. Furthermore, as the thermal treatment temperature increased, the current density initially rose to its highest value at 300 °C before decreasing for films prepared at 350 and 400 °C. This trend highlights the effect of thermal treatment on the electrochemical performance of the WO₃ films. Amorphous WO₃ structures are known to exhibit higher diffusion coefficients than crystalline structures, which makes them more favorable for electrochromic applications.^32–35 In this study, WO₃ films treated at 350 and 400 °C, where crystallization occurred, consistently showed significantly reduced electrochemical activity. This reduction was attributed to the dense crystalline structure of WO₃, which impeded ion mobility and reduced electrochromic reactions, thereby resulting in lower current density. To confirm this, the diffusion coefficient was calculated from the CV data, and the interfacial resistance between the film and electrolyte was examined using impedance spectroscopy. Fig. S6 shows the diffusion coefficients (D) calculated using the Randles–Ševčík equation. The D values were 7.12 × 10⁻¹⁰ mA cm⁻² for WO80, 1.76 × 10⁻⁹ mA cm⁻² for WO300, and 1.84 × 10⁻¹⁰ mA cm⁻² for WO400, indicating that the amorphous structure of WO300 has the highest ion exchange. The impedance spectra of WO80, WO300, and WO400 are presented in Fig. 5b. Electrochemical circle fitting was performed for the high-frequency region to extract R_s (ohmic resistance) and R_ct (charge transfer resistance) for each film. The R_s values for the films treated at 80, 300, and 400 °C were 93.91, 88.08, and 210.57 Ω, respectively, while the corresponding R_ct values were 25.63, 22.00, and 80.44 Ω. These results clearly demonstrate that the WO₃ film prepared at 300 °C exhibited the lowest values for both ohmic resistance and Li⁺ charge transfer resistance.

Additionally, the spectra in the wavelength range of 200–800 nm for the as-deposited, bleached, and colored states are presented in Fig. S7. The bleached and colored states were measured after applying voltages of +1.0 and −0.8 V for 60 s, respectively. All WO₃ films exhibited a significant decrease in transmittance within the visible light range in the colored state. The optical modulation (ΔT) at 550 nm and 630 nm is presented in Fig. S7, with the highest ΔT recorded for WO300. For amorphous WO₃ (treated up to 300 °C), the transmittance spectra returned to their initial states after bleaching the colored film, which indicates high transparency reversibility. However, films treated at 350 and 400 °C (crystalline WO₃) did not fully recover to their initial transmittance levels. This indicates that the Li⁺ ions inserted into WO₃ were not fully extracted due to insufficient reaction time or slow electrochemical reaction kinetics during the oxidation potential application period, indicating that the insertion of Li⁺ ions was easier than their extraction from the crystalline structure of WO₃. In addition, the optical bandgap (E_g) was analyzed using Tauc plot method (Fig. S8) based on the transmittance spectra measured in the range of 200–800 nm (as-deposited spectra shown in Fig. S7). Overall, E_g tended to decrease with increasing annealing temperature; however, the WO₃ film annealed at 300 °C (WO300) exhibited an anomalously high E_g value of 3.79 eV. The formation of E_g is influenced by several factors, including intermolecular distance, grain size, and structural defects such as oxygen vacancies. Generally, a reduction in intermolecular distance and an increase in grain size lead to a narrower bandgap, whereas structural defects—particularly oxygen deficiencies—tend to widen the bandgap. In the WO₃ films studied here, higher annealing temperatures promoted both grain growth and crystallinity, while simultaneously increasing the density of oxygen vacancies. These competing effects act in opposite directions on E_g. As a result of this interplay, WO300 exhibited a distinct increase in E_g, which can be attributed to the higher density of oxygen vacancies at this annealing temperature. These vacancies facilitate the partial reduction of W⁶⁺ to W⁵⁺, raising the Fermi level and widening the energy gap between the valence and conduction bands. In contrast, at temperatures above 300 °C, further crystallization leads to a decrease in the density of localized states and an enhancement of band alignment, resulting in a narrowing of the optical bandgap.^36,37 Although a narrower bandgap is typically favorable for electron and ion transport, thus enhancing electrochemical reactivity, the results of this study suggest that increased crystallinity and reduced defect states may hinder charge mobility. It indicates that excessive structural ordering can negatively impact electrochemical performance.

To evaluate the effect of transparency reversibility and assess the long-term stability of the electrochromic WO₃ films, cyclic stability tests were performed for 500 cycles of coloration (−0.8 V, 60 s) and bleaching (+1.0 V, 60 s). The transmittance during the cycle test was monitored at a wavelength of 550 nm, and the results are shown in Fig. 6 and summarized in Table 3. Differences in the initial transmittance (Fig. S7) and electrochemical properties (Fig. 5 and Fig. S4) were already observed among the WO₃ fabricated under different thermal treatment conditions. Moreover, the cycle tests provided a clear distinction in the performance of the electrochromic WO₃ films. As indicated in Fig. 6, WO300 exhibited the largest ΔT and the most stable transmittance behavior during the cycle test. In contrast, WO80 showed a rapid decrease in its performance with ΔT dropping below 10% after 45 cycles and below 5% after 75 cycles. Similarly, WO190 displayed a decline in ΔT to below 10% after 40 cycles and below 5% after 75 cycles. These results suggest that WO₃ films treated at temperatures below 190 °C initially exhibit a high transmittance contrast; however, their coloration performance decreased significantly with operation. This dramatic performance degradation was correlated with the surface and cross-sectional structures observed in Fig. 4 and Fig. S3. The amorphous and porous structures of low-temperature annealed WO₃ likely cannot withstand the structural stress caused by repeated Li⁺ ion insertion and extraction, thereby leading to rapid deterioration. Among the remaining films, WO300 demonstrated the most stable cycling performance, while WO250, WO350, and WO400 showed low stability, with ΔT decreasing to 13.09%, 19.03%, and 10.15%, respectively, after 500 cycles. However, these films did not exhibit the dramatic decline seen in WO80 and WO190. Additionally, the cycling behavior differed between the films prepared at 250 and 300 °C and those at 350 and 400 °C. The optical modulation of WO250 and WO300 decreased because of the increased transmittance in the colored state, with a minimal reduction in the bleached state. Conversely, WO350 and WO400 showed the opposite trend, with greater reductions in the transmittance of the bleached state while maintaining coloration. This behavior suggests that, in amorphous WO₃, prolonged cycling restricts the insertion of Li⁺ ions into the film. However, in crystalline WO₃ films, the dense structure impedes the mobility of Li⁺ ions, particularly during extraction, which leads to challenges in the bleaching process. In conclusion, the amorphous WO₃ treated at the highest temperature before crystallization exhibited the best electrochromic performance, highlighting its structural advantage in facilitating Li⁺ insertion and extraction.

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The key metrics of electrochromic performance, such as response time and CE, were evaluated through the cycle test. Response time refers to the time required for a 90% change in ΔT during the transition from the bleached state to the colored state (coloration time, t_c) or from the colored state to the bleached state (bleaching time, t_b).³⁸ Fig. 7a and b show transmittance changes recorded during the 10th and 500th cycles, respectively, while the initial and final response times are summarized in Table 3. For films annealed at 80 and 190 °C, the ΔT values after 500 cycles were reduced to 0.22 and 2.09%, respectively, making the calculation of response times unreliable because of the drastic decrease in transmittance. The WO₃ film annealed at 250 °C exhibited a slight change in response time after 500 cycles, despite a 68.47% reduction in transmittance. This suggests that the amount of Li⁺ ions participating in the electrochromic reaction decreased while the insertion/extraction speed of Li⁺ ions remained largely unchanged. The WO₃ film fabricated at 300 °C maintained consistent response times and exhibited the smallest reduction of transmittance between the 10th and 500th cycles, indicating high cycling stability. WO₃ films with crystalline structures formed at 350 and 400 °C exhibited changes in response time between the initial and 500th cycles. Specifically, the coloration time decreased after 500 cycles, whereas the bleaching time increased; this trend was more pronounced in WO400 compared to WO350. These results indicate that repeated cycling facilitates Li⁺ insertion into the crystalline structure, thereby making it easier over time. However, the extraction of Li⁺ ions appears to be hindered by the dense crystalline structure, which leads to an increase in bleaching time. This observed variation in response time aligns with the findings in Fig. 6e and f, which demonstrate differences in transmittance behavior. The results highlight that, while crystalline WO₃ films may exhibit improved Li⁺ insertion kinetics over the cycle test, the dense structure imposes significant limitations on ion extraction, thereby adversely affecting the bleaching process and overall performance.

CE, a key indicator of electrochromic performance, represents the amount of coloration achieved per unit charge injected into the material. It is defined as the ratio of the change in optical density to the charge density.

OD = log(tb/tc),	(5)

CE = ΔOD/Δ(Q/A),	(6)

where OD, Q/A, t_b, and t_c represent the optical density, charge density, transmittance in the bleached state, and transmittance in the colored state, respectively.^38–40 The relationship between OD and charge density for the films is plotted in Fig. 8 using data obtained during the initial stages of coloration under an applied voltage of −0.8 V. The slope of the OD vs. charge density plot represents the CE value, and the calculated CE values for each film are listed in Table 3. As shown in Fig. 8 and Table 3, the CE values showed a maximum value of 217.39 cm² C⁻¹ for WO300. WO300, which demonstrates the highest CE, efficiently utilizes a small amount of charge to achieve significant coloration. The superior electrochromic performance of the film is attributed to its optimized amorphous structure and enhanced ion mobility.

To further investigate the long-term stability of the electrochromic thin film, long-term tests of up to 3000 cycles were performed. The preliminary results from 500 cycles showed that the color change occurred within 20 s (−0.8 V) and 10 s (+1.0 V) after voltage application. Therefore, the long-term stability test (3000 cycles) was performed for +1.0 and −0.8 V using 10 and 20 s durations, respectively. The transmittance degradation of WO300 was measured to be −23.21% at 500 cycles, −21.07% at 1000 cycles, and −36.79% at 3000 cycles, as shown in Fig. 9.

Among the WO₃ electrochromic layers thermally treated at various temperatures, WO300 exhibited the highest and most stable electrochromic performance, demonstrating the lowest degradation rate during the cycle test compared to that of other films. This superior performance was attributed to the structural properties of WO₃, which were significantly influenced by the thermal treatment temperature.

This study highlighted the importance of optimizing thermal treatment conditions to enhance the electrochromic performance of WO₃ layers. Proper thermal treatment can prevent excessive crystallization, facilitating Li⁺ ion mobility within the amorphous WO₃ structure. In addition, structural stability in the film contributes to long-term operational durability. These factors collectively enable improved electrochromic performance and cycling stability. This study underscores the necessity of evaluating cycling stability over hundreds of cycles to fully determine the durability of electrochromic WO₃ films. Performance assessments based solely on initial coloration and bleaching cycles or early electrochromic evaluations are insufficient to determine the long-term stability of films. Therefore, comprehensive cycling tests are critical to ensuring the practical applicability of WO₃-based electrochromic devices. Furthermore, while spin-coating is a simple and cost-effective deposition technique, its electrochemical performance may be somewhat inferior compared to sputtering owing to lower film uniformity. Nevertheless, in this study, we systematically analyzed the electrochemical properties of spin-coated thin films at various annealing temperatures, providing valuable insights into their electrochromic durability. Although many studies have reported the electrochemical properties of various deposition processes, including spin-coating, relatively few have conducted long-term stability tests (Table 4). Therefore, our findings contribute to a broader understanding of the electrochromic durability of spin-coated films, while supporting the exploration of other deposition techniques to further improve film performance.

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Conclusion

This study investigated the effect of thermal treatment on the structural and electrochromic properties of WO₃ thin films fabricated via spin-coating. FIB-SEM analysis showed that films treated at relatively low temperatures (80–250 °C) exhibited cracked and porous structures, while the surface morphologies gradually became smoother in films treated at 300 °C and above. The XRD and XPS analyses confirmed that WO₃ transitioned to a crystalline structure in WO350 and WO400, whereas films treated below 300 °C remained amorphous. Electrochemical characterization and electrochromic performance analysis demonstrated that the film treated at 300 °C exhibited the highest electrochromic performance and was characterized by minimal transmittance degradation, consistent response times (t_b of 11 s and t_c of 3 s), and the highest CE (217.39 cm² C⁻¹). These properties were attributed to the amorphous structure, which optimally supported Li⁺ ion mobility and structural stability. However, amorphous WO₃ treated below 190 °C exhibited a drastic decrease in cyclic performance because of structural defects caused by atmospheric moisture. Mechanically robust films that do not exhibit bulk voids are required to achieve reliable performance. The cycling test results further revealed distinct differences in the transmittance behavior between the amorphous and crystalline WO₃ films. Amorphous WO₃ layers prepared at 250 and 300 °C maintained their bleached state transmittance, while the transmittance of the colored state increased. In contrast, crystalline WO₃ films fabricated at 350 and 400 °C exhibited the opposite trend, with reduced bleached-state transmittance and stable colored-state transmittance—attributed to differences in Li⁺ ion mobility between the amorphous and crystalline structures of WO₃. These findings underscore the importance of balancing structural stability and ion mobility through controlled thermal treatment to achieve durable, high-performance electrochromic devices. Future studies could focus on enhancing the stability and electrochromic efficiency of WO₃ films, such as optimizing deposition parameters and incorporating dopants to fine-tune their electronic and ionic properties. This approach could contribute to the development of more robust and efficient electrochromic devices with long-term operational stability.

Author contributions

Hoon Kang: investigation, data curation and writing – original draft. Gyung Hyun Kim: resources. Sungho Kang and Tae Hoon Park: supervision and writing – review & editing. Kwanchul Kim: funding acquisition and supervision. Hwan Kyu Kim: supervision and writing – review & editing. Yekyung Kim: conceptualization, funding acquisition, supervision, writing – review & editing, and project administration.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

The authors confirm that all relevant data of this study are available within the article and its SI. Data will be available upon reasonable request to the corresponding author.

The SI includes detailed data on the thermal, morphological, electrochemical, and electrochromic properties of the studied WO₃ layer in this study. See DOI: https://doi.org/10.1039/d5nr02842f.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (No. 2021R1F1A1060062 and 2022R1F1A1062840). This work was supported by the Technology Innovation Program (No. 20021828) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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