Effect of Oxygen Flow Rate, Post-annealing Temperature, and Different Electrolyte Concentrations on WO₃ Thin Films Deposited by DC Magnetron Sputtering For Electrochromic Applications

Abstract

In this work, tungsten oxide (WO₃) films were deposited at room temperature and annealed for 2 h at 400°C. The electrochromic and electrochemical properties were studied for two different electrolytes. The films were deposited at different oxygen flow rates of 2, 4, and 6 standard cubic centimeters per minute (SCCM). X-ray diffraction analysis revealed structural characterization of amorphous and crystalline phases. UV-visible spectroscopy optical transmittance revealed 91% transmittance, and energy-dispersive x-ray spectroscopy (EDS) analysis revealed the absence of impurities and the presence of W and O. An electrochemical analyzer was used to characterize the deposited and annealed WO₃ films immersed in the two different electrolyte solutions (H₂SO₄ and LiClO₄ with oxygen flow rates ranging from 2 SCCM to 6 SCCM). It was found that the H₂SO₄ electrolyte of an annealed WO₃ thin film at 2 SCCM demonstrated high coloring efficiency of 50.18 cm²/C, and the LiClO₄ electrolyte of an annealed WO₃ thin film at 4 SCCM demonstrated high coloring efficiency of 20.06 cm²/C.

Introduction

With the continuing rapid depletion of fossil fuel materials, renewable energy sources are increasingly used for electrochemical capacitors.^1,2 Up to 30–40% of the primary energy consumed worldwide is utilized for heating, cooling, ventilation, and electrical appliances in buildings.^3,4,5 Therefore, the use of materials in buildings that can convert and store energy from renewable sources is beneficial. By applying a small electric field, electrochromic substances can change their optical characteristics such as transmittance, reflection, and absorption in a reversible manner. This property makes them extremely appealing for a variety of potential applications, including large-area information displays, antiglare automobile mirrors, and energy-efficient smart windows.^6,7

Numerous studies have been conducted on tungsten trioxide (WO₃), which has been named one of the most interesting inorganic electrochromic compounds.^8,9 An electrochromic material should have 100% optical modulation, meaning that it should be totally opaque while colored and fully transparent when bleached. The coloration efficiency (CE) and kinetics will depend on the morphological, structural, and compositional characteristics, and the suitability for use in a variety of devices, including solar cells,^10,11 smart windows,^12,13,14,15 gas sensors,¹⁶ and photodetectors.¹⁷ The electrochemical equation below¹⁸ controls the electrochromism reaction in WO₃:

$$ {\text{WO}}_{3} \left( {{\text{colorless}}} \right) + x{\text{M}}^{ + } + xe^{ - } \leftrightarrow {\text{M}}x{\text{WO}}_{3} \left( {\text{dark blue}} \right) $$

(1)

where x is the ion concentration, and M represents K⁺, H⁺, Na⁺_, or Li⁺ ions. Various deposition techniques have been used for WO₃ films including sol–gel,¹⁹ hot-filament-assisted synthesis,²⁰ sputtering,^{21,22,23,24,25,26,27,28,29,30,31,32,33} electron beam evaporation,^34,35 electrodeposition processing,³⁶ and hydrothermal^{37,38,39,40,41} and solvothermal techniques.⁴² Overall, relatively little research on sputtering-fabricated WO₃ films has been published. Furthermore, up to this point, we were unable to locate any other research in the literature demonstrating the influence of the deposition oxygen flow rate (OFR) on the electrochromic properties of WO₃ films produced by sputtering. Therefore, we present a thorough investigation of the effects of various oxygen flow deposition rates, post-annealed WO₃ films, and various electrolyte solutions on the optical, structural, electrochromic, and electrochemical attributes of WO₃ films deposited by the sputtering technique in the current work. Here, we discuss how various OFRs affect the structural, optical, and electrochromic characteristics of sputtering. WO₃ films were produced, and efforts were made to optimize the OFR, the electrolyte solution, and the nature of the film (amorphous or crystalline) for use in electrochromic applications.

Experimental Techniques

Method to Produce Thin Films of WO₃

A popular physical vapor deposition (PVD) technique known as direct current (DC) magnetron sputtering was used to deposit the WO₃ films on Corning glass and fluorine-doped tin oxide (FTO) substrates. Before being fed into the sputtering chamber, the glass samples were cleaned with a soap solution and deionized water (DI) using ultrasonication. The tungsten metal disc was used as the sputter target and it was sputtered in an atmosphere of argon (Ar) and oxygen (O₂). The base vacuum of the sputtering chamber was maintained at 2 × 10⁻⁶ mbar. In order to remove impurities that had been adsorbed onto the target surface pre-sputter, the tungsten target was subjected to an argon gas atmosphere for 10 min. The WO₃ films were produced under OFRs of 2, 4, and 6 standard cubic centimeters per minute (SCCM) at room temperature (RT). A constant 100 mA DC current was used during the whole deposition.

Characterization

X-ray diffraction (XRD; Rigaku MiniFlex, Rigaku Corporation) was used to analyze the crystallization of the films. Raman spectroscopy (Renishaw plc) was used to analyze the chemical bonding. The optical absorption behavior was characterized using an ultraviolet–visible spectrometer (SPECORD S600). An electrochemical workstation (SP-300 BioLogic) was used for all electrochemical analyses. The workstation has a three-electrode setup with a platinum wire as the counter electrode (CE), Hg₂/HgCl₂ as the reference electrode (RE), and WO₃ thin film as the working electrode (WE). An aqueous solution of 0.5 M H₂SO₄ and a non-aqueous solution of 0.5 M LiClO₄ were used as electrolytes, and the sweep voltage was maintained between −0.7 and +1 V at scan rates of 20 mV/s.

Results and Discussion

XRD Analysis

Figure 1a displays the XRD characteristics of the WO₃ films that were deposited using DC sputtering at various OFRs. The RT-deposited samples exhibit amorphous characteristics at various OFRs, according to the XRD evaluation. Amorphous films are produced on a glass substrate that has not been heated when different amounts of PaO₂ are deposited using DC and RF sputtering, according to studies by Madhavi et al.⁴³ and Mohamed et al.⁴⁴

Fig. 1

XRD spectra of WO₃ films deposited at various OFR: (a) RT and (b) annealed.

An eventual change in film crystallinity is the most fundamental effect of the annealing process. Consequently, XRD analysis was used to characterize the WO₃ films annealed at 400°C for 2 h in order to investigate the change in the crystal structure. The XRD patterns of heat-treated WO₃ films after 2 h at 400°C are clearly shown in Fig. 1b, which reveals the presence of distinctive diffraction peaks corresponding to the WO₃ planes. For different oxygen partial pressures, the XRD spectra that are produced show that the 400°C air-annealed sample is crystalline.²⁵ DC-sputtered WO₃ films have 2θ values of 14.04°, 23.38°, 24.47°, 28.16°, 33.89°, 36.93°, 50.09°, and 56.09°, and their hkl values are (100), (002), (020), (112), (220), (222), (002), and (102), respectively. There were differences in OFRs between 2 and 6 SCCM. For increased OFRs, WO₃ films have a crystalline character, due to the annealing at 400°C.

Raman Analysis

The Raman spectra of the WO₃ films at various OFRs are shown in Fig. 2a and b. Because of the W–O bonds, the spectra of the as-deposited film exhibit a significant peak at 770 cm⁻¹, as shown in Fig. 3a.⁴⁵ The appearance of this peak corresponds with the two strongest peaks at 719 and 807 cm⁻¹ in the Raman spectrum of the crystalline WO₃ films. The W=O stretching mode of terminal oxygen atoms is attributed to a rather steep peak at 950 cm⁻¹, which may occur on the surfaces of the cluster and micro-void structures in the film.⁹ A weaker, broad peak is also present at 220 cm⁻¹, which is attributed to the presence of W⁴⁺ states. Figure 2b displays the Raman spectra that were recorded for the WO₃ annealed films. The spectra of WO₃ films annealed at 400°C for 2 h at various OFRs exhibit a broad peak in the range of 100–1200 cm⁻¹ due to the O–W–O stretching vibration mode, which denotes the growth of sub-stoichiometric WO₃ films of mixed metallic tungsten and tungsten oxide. Sharp Raman peaks, representing the bending and stretching vibration modes of the O–W–O bond, can be seen at 695 and 809 cm⁻¹⁴⁶ when the OFR increases from 2 to 6 SCCM.

Fig. 2

Raman spectra of WO₃ films at various OFRs: (a) RT and (b) annealed.

Fig. 3

Transmittance plots of WO₃ films deposited at various OFRs: (a) RT and (b) annealed.

Optical Properties

Figure 3 illustrates the transmittance plots of WO₃ films for both RT and post-annealing at 400°C for 2 h at different OFRs. The transmittance is increased with respect to the increase in OFR for RT-deposited samples, and transmittance is decreased for annealed samples.^47,48 The transmittance is more than 82% at a wavelength of 700 nm for RT samples at different OFRs, and transmittance is less for the sample annealed at 4 SCCM. The transmittance values for annealed and RT samples are shown in Table I.

Table I Comparison of transmittance of WO₃ films before annealing and after annealing at various OFRs

Electrochemical Properties

The electrochemical properties determine the structure of the electrochromic WO₃ thin film, which is employed as the working electrode. Hg/HgCl serves as the reference electrode, and a Pt needle serves as the counter electrode. For the half cell, 0.5 M H₂SO₄ and LiClO₄ solution were prepared as electrolytes. The cyclic voltammetry (CV) curves of WO₃ films were recorded in the potential range of −0.7 to 1 V at a scan rate of 20 mV s⁻¹. When a negative voltage is supplied to the working electrode, the color changes from transparent to blue. When a positive voltage is applied, the film becomes transparent.⁴⁹

Figures 4a and b and 6a and b show typical broad featureless peaks in the CV characteristics that represent the insertion and extraction of H⁺ and Li⁺ ions into WO₃ films. The cathodic peak current of WO₃ films at RT in the H₂SO₄ electrolyte solution, as measured by the CV curves (measured at −0.7 V), initially decreased from −2.98 mA/cm² at 2 SCCM to −5.40 mA/cm² at 6 SCCM OFR. This increase in OFRs eventually led to an increase in cathodic peak current as shown in Fig. 4a. The cathodic peak current of WO₃ films at RT in LiClO₄ electrolyte solution is shown in Fig. 4b. The CV curves initially decreased from −0.83 mA/cm² at 2 SCCM to −1.42 mA/cm² at 4 SCCM. This increase in OFRs ultimately led to an increase in cathodic peak current back to −0.74 mA/cm² at 6 SCCM OFRs (Fig. 5). Figure 6 shows the first and 50th cyclic voltammetry curves of WO₃ film deposited at RT, which reveal that the performance of the thin film was severely degraded and the stability decreased after 50 cycles. The reversibility decreased slightly as the number of cycles increased. The film contained certain irreversible deep trap sites, which prevented the electrochromic reversibility from reaching 100%. The area of the voltammogram decreased with an increase in the electrochemical cycle. An increase in voltage was observed with a decrease in current. The charge insertion capability of the film also decreased with increased cycles.⁵⁰ The cathodic peak current of WO₃ films in the H₂SO₄ electrolyte solution that was annealed at 400°C for 2 h showed a CV curve that initially decreased from −0.74 mA/cm² at 2 SCCM to −0.92 mA/cm² at 4 SCCM, as shown in Fig. 7a. However, as OFRs increased, the cathodic peak current eventually increased back to −0.87 mA/cm² at 6 SCCM OFRs. The lowest cathodic peak current of all the CV curves is shown in Fig. 7b, which represents the cathodic peak current of annealed at 400°C in 2 h of WO₃ films in LiClO₄ electrolyte solution. The CV curves are recorded as −0.55 mA/cm² at 2 SCCM, −0.48 mA/cm² at 4 SCCM, and −0.39 mA/cm² at 6 SCCM OFRs. Figures 5 and 8 show the comparison plots of CV curves of WO₃ films RT and annealed for OFRs of 2 SCCM, 4 SCCM, and 6 SCCM. The Randles–Ševčík equation can be used to calculate the diffusion coefficient, D, for the H⁺ and Li⁺ ions.⁵¹

$$ D^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} = \frac{{i_{{\text{P}}} }}{{C_{o} \times A \times 10^{5} \times 2.69 \times n^{{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} \times {\upnu }^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} }} $$

(2)

where i_p, n, A, C_o, v, and D are peak current, number of electrons, area of the active WO₃ film, electrolyte concentration, scan rate, and diffusion coefficient, respectively.

Fig. 4

Cyclic voltammograms of WO₃ films deposited at RT for various OFRs: (a) 0.5 M H₂SO₄ (H⁺) and (b) 0.5 M LiClO₄ (Li⁺).

Fig. 5

Comparison plots of CV curves of WO₃ films deposited at RT at various OFRs: (a) 2 SCCM, (b) 4 SCCM, and (c) 6 SCCM.

Fig. 6

CV plots of WO₃ thin films: (a) 2 SCCM, (b) 4 SCCM, and (c) 6 SCCM after 50 cycles in H₂SO₄ electrolyte solution at RT.

Fig. 7

Cyclic voltammograms of WO₃ films annealed at 400°C for various OFRs: (a) 0.5 M H₂SO₄ (H⁺) and (b) 0.5 M LiClO₄ (Li⁺).

Fig. 8

Comparison plots of CV curves of WO₃ films annealed at 400°C for various OFRs: (a) 2 SCCM, (b) 4 SCCM, and (c) 6 SCCM.

Table II summarizes the determined diffusion coefficients of the WO₃ films and annealed 400°C in 2 h of the WO₃ films in both H₂SO₄ and LiClO₄ electrolyte solutions treated at increasing OFRs. It is obvious that the diffusion coefficient (DC) is influenced by the OFRs and different electrolytes. Amorphous WO₃ films in H₂SO₄ electrolyte treated at 2 SCCM OFR demonstrated a maximum diffusivity of 1.007 × 10⁻⁷ cm²/s, which is two orders of magnitude higher than the 1.59 × 10⁻⁹ cm²/s of the amorphous WO₃ films in LiClO₄electrolyte at 6 SCCM OFR. Additionally, crystalline WO₃ films in H₂SO₄ electrolyte treated at 4 SCCM OFR showed the highest diffusivity of 2.32 × 10⁻⁹ cm²/s, which is an order of magnitude higher than the 4.53 × 10⁻¹⁰ cm²/s of the crystalline WO₃ films in LiClO₄ electrolyte at 6 SCCM OFR. These films were post-annealed at 400°C for 2 h. Finally, amorphous WO₃ films in an electrolyte of H₂SO₄ at a 2 SCCM OFR revealed the highest DC of 1.007 × 10⁻⁷ cm²/s in comparison to crystalline WO₃ films in an electrolyte of LiClO₄ at 6 SCCM OFR, as shown in Fig. 9. Similar findings were reported by Madhavi et al., who found that the blockage of insertion sites was the reason for the decrease in DC at higher annealing temperatures.⁵² Hsu et al. explained that although there is a driving force from the applied voltage, the compact film structure prevented Li⁺ ions from intercalating in MoO₃ electrochromic films that were produced using a sol–gel technique, which reduced the DC.⁵³

Table II Diffusion coefficients of comparison for before annealing and after annealing WO₃ films in H₂SO₄ (H⁺) and LiClO₄ (Li⁺) electrolytes and various OFRs

Fig. 9

Diffusion coefficients comparison plots of (a) RT and (b) annealed WO₃ films at H₂SO₄ (H⁺) and LiClO₄ (Li⁺) electrolytes and various OFRs.

Electrochromic Studies

Figures 10 and 11 show the optical transmission spectra of the WO₃ films in their corresponding colored and bleaching states in the 300–900 nm range. The optical transmittance spectra of the DC-sputtered WO₃ films were investigated by UV-visible spectroscopy. Figure 10a, b, c, d, and e displays the transmittance for the bleached and colored states obtained for various OFRs (2 SCCM, 4 SCCM, and 6 SCCM) and two different electrolyte solutions (H₂SO₄ and LiClO₄). As shown in Fig. 11a, b, and c, the observed values of transmittance at a wavelength of 600 nm range from 0% for 2 SCCM, 1% for 4 SCCM, 5% for 6 SCCM for the colored state, and 58% for 2 SCCM, 85% for 2 SCCM, and 91% for 6 SCCM for the bleached state of WO₃ films in 0.5 M H₂SO₄ electrolyte solution. In the colored state of the WO₃ films in the 0.5 M LiClO₄ electrolyte solution, the observed values of transmittance at a wavelength of 600 nm range from 19% for 2 SCCM, 10% for 4 SCCM, and 22% for 6 SCCM; and in the bleached state, the values are 43% for 2 SCCM, 81% for 2 SCCM, and 90% for 6 SCCM, as shown in Fig. 10d and e. Also, as measured for different OFRs (2 SCCM, 4 SCCM, and 6 SCCM) and two different electrolyte solutions (H₂SO₄ and LiClO₄), the transmittance of annealed WO₃ films (crystalline) for bleached and colored states is shown in Fig. 11 a, b, c, d, and e. As shown in Fig. 11a, b, and c, the observed values of transmittance at a wavelength of 600 nm range from 9% for 2 SCCM, 8% for 4 SCCM, and 15% for 6 SCCM for the colored state, and 66% for 6 SCCM for the bleached state of crystalline WO₃ films in 0.5 M H₂SO₄ electrolyte solution. Figure 11d, e, and f shows the observed values of transmittance at a wavelength of 600 nm for crystalline WO₃ films in 0.5 M LiClO₄ electrolyte solution. The observed values range from 42% for 2 SCCM, 41% for 4 SCCM, and 64% for 6 SCCM for the colored state, and 68% for 2 SCCM, 60% for 2 SCCM, and 70% for 6 SCCM bleached state. We may deduce from this result that if OFRs in both electrolytes are increased, the films become less intensely colored because fewer ions are available for intercalation, making them less opaque and increasing the transmittance for the colored state.

Fig. 10

Transmittance plots of colored and bleached states of RT WO₃ films: (a) 2 SCCM, (c) 4 SCCM, and (e) 6 SCCM for H₂SO₄ (H⁺) solution and (b) 2 SCCM, (d) 4 SCCM, and (f) 6 SCCM for LiClO₄ (Li⁺) solution.

Fig. 11

Transmittance plots of colored and bleached states of annealed WO₃ films: (a) 2 SCCM, (c) 4 SCCM, and (e) 6 SCCM for H₂SO₄ (H⁺) solution and (b) 2 SCCM, (d) 4 SCCM, and (f) 6 SCCM for LiClO₄ (Li⁺) solution.

The transmittance of these transition metal oxides in both transparent and opaque phases, which is necessary for electrochromic applications, is one of their most important properties. As a result, there is a significant optical modulation, or ΔOT, which is measured as the difference between the transmittance of the colored and bleached states.⁵⁴ The host matrix’s trapped ions play a part in the material’s degraded electrochromic performance. Because the intercalated H⁺ ions are not entirely removed from the formed film, degradation in optical modulation of the host matrix can be detected during specific voltage cycles. CV measurements were performed, and the bleached and colored switching characteristics of WO₃ and post-annealed WO₃ films in H₂SO₄ and LiClO₄ in two different electrolyte solutions of 0.5 M concentration were recorded at a scan rate of 10 mV/s and applied potential of −0.7 to 1 V. The results are shown in Table III for the various OFRs, as well as the WO₃ and post-annealed WO₃ films. The optical modulation of amorphous WO₃ films in H₂SO₄ electrolyte solution is greatest at 6 SCCM OFR. However, WO₃ films that were annealed at 400°C displayed very low bleaching and coloring transmittances, which is indicative of weak electrochromic characteristics. The crystalline WO₃ films in LiClO₄ electrolyte were unable to recover to their initial transparent form despite having poor transmittance in the colored state. Figure 12a and b shows the bleached and colored states obtained during oxidation and reduction, respectively. The coloring and bleaching times of the electrochromic films are plotted using chronoamperometry in Fig. 10. We fixed the pulse interval to 10 s, the start voltage to −0.7 V, and the upper limit voltage to 1 V for the chronoamperometry (CA) curve experiment using H₂SO₄ as the electrolyte. The response period for oxidation and reduction is the amount of time required for the anodic and cathodic currents to equalize when voltages are applied. The coloration time for 2 SCCM, 4 SCCM, and 6 SCCM of WO₃ samples is 3.2, 3.7, and 3.3 s, respectively, and the bleach time for 2 SCCM, 4 SCCM, and 6 SCCM of WO₃ samples is 1.7, 2.8, and 1.3 s, respectively. The outcome demonstrates that all deposited films took longer to color than to bleach (Fig. 13).

Table III Comparison of optical modulation, coloration efficiency, bleaching, and coloration transmittance before annealing, after annealing WO₃ films in H₂SO₄ (H⁺) and LiClO₄ (Li⁺) electrolytes at various OFRs

Fig. 12

(a) Bleached and (b) colored states of WO₃ thin film.

Fig. 13

CA plots of WO₃ thin films: (a) 2 SCCM, (b) 4 SCCM, and (c) 6 SCCM after 50 cycles in H₂SO₄ electrolyte solution at RT.

The ratio of the change in optical density to the incorporated charge per unit area is known as the coloring efficiency (CE), and it is one of the most important parameters for assessing the effectiveness of electrochromic products. It can be calculated using the formula shown below⁵⁵:

$$ {\text{CE}} = {\raise0.7ex\hbox{${\Delta {\text{OD}}}$} \!\mathord{\left/ {\vphantom {{\Delta {\text{OD}}} \frac{Q}{A}}}\right.\kern-0pt} \!\lower0.7ex\hbox{$\frac{Q}{A}$}} $$

(3)

$$ \Delta {\text{OD}} = {\text{Log}}\frac{{T_{{\text{b}}} }}{{T_{{\text{c}}} }} $$

(4)

where T_b is the bleached transmittance, T_c is the color transmittance, Q_in is the incorporated charge, and ΔOD is the optical density. Using T_b and T_c from Table III, the CE of the WO₃ films at 600 nm was then calculated as follows. For different OFRs, the calculated CE of the WO₃ films (amorphous) was found to be 22 cm²/C at 2 SCCM, 21.16 cm²/C at 4 SCCM, 20.54 cm²/C at 6 SCCM in H₂SO₄ electrolyte solution, and 19.10 cm²/C at 2 SCCM, 27.44 cm²/C at 4 SCCM, and 18.46 cm²/C at 6 SCCM in LiClO₄ electrolyte solution, respectively. The calculated CE of the heated WO₃ films (crystalline) at 400°C for 2 h was found to be 50.18 cm²/C at 2 SCCM, 16.64 cm²/C at 4 SCCM, and 33.75 cm²/C at 6 SCCM in H₂SO₄ electrolyte solution, and 20.06 cm²/C at 2 SCCM, 18.25 cm²/C at 4SCCM, and 4.92 cm²/C at 6 SCCM in LiClO₄ electrolyte solution, respectively. According to CE results, crystalline WO₃ films in an electrolyte concentration of H₂SO₄ performed better in an electrochemical cell than amorphous WO₃ films in an electrolyte concentration of LiClO₄, as shown in Fig. 14.

Fig. 14

Comparison of CE for H₂SO₄ (H⁺) and LiClO₄ (Li⁺) electrolytes: (a) and (b) RT WO₃ films and (c) and (d) annealed WO₃ films.

In this study, we propose the coexistence of amorphous and crystalline phases in WO₃ films annealed at 400°C at various OFRs (2, 4, and 6 SCCM) in two different electrolyte (H₂SO₄ and LiClO₄) solutions. Since they demonstrated a good balance between ion storage capacity, optical modulation, and CE, they are recognized as the optical electrochromic features for electrochromic device applications. A higher CE was seen in the WO₃ that was annealed at 400°C with a lower OFR (2 SCCM) and a higher concentration of the H₂SO₄ electrolyte solution but at the expense of switching properties, as shown by the prior data. The quick switching properties of crystalline WO₃ films, however, occur at the expense of low optical modulation.

Conclusion

This study revealed the effects of various OFRs (2, 4, and 6 SCCM) and annealing temperature on the structural, electrochromic, and optical characteristics of DC magnetron-sputtered WO₃ films. The WO₃ films are amorphous, according to XRD measurements; however, they exhibit crystalline characteristics at 400°C after annealing. SEM scans of WO₃ films revealed a uniform and smooth surface, which is a sign of its amorphous nature. The elimination of additional organic residues caused the WO₃ films to become denser with more evident cracks. Energy-dispersive x-ray spectroscopy (EDS) investigation demonstrated the absence of contaminants in the grown film and provided confirmation that W and O were present in the deposited film. We observed and carefully examined the effects of changing OFR and two different electrolyte solutions (H₂SO₄ and LiClO₄) on the electrochromic characteristics of the WO₃ and post-annealed WO₃ films. UV-visible spectrometry showed that the optical transmittance of higher OFRs and amorphous WO₃ samples was greater than that of lower OFRs and air-annealed samples. The amorphous WO₃ films in an electrolyte of H₂SO₄ at a 2 SCCM OFR is the highest DC of 1.007 × 10⁻⁷ cm²/s in comparison to crystalline WO₃ films in an electrolyte of LiClO₄ at a 6 SCCM OFR. The optical modulation of amorphous WO₃ films in H₂SO₄ electrolyte solution is 86% at 6 SCCM OFR. In the CE results, crystalline WO₃ films in an electrolyte solution of H₂SO₄ performed better in an electrochemical cell than amorphous WO₃ films in an electrolyte solution of LiClO₄. Finally, we reach the conclusion that the electrolyte solution of H₂SO₄ contributes to improving the electrochromic properties of WO₃.