Description:FIELD OF THE INVENTION
The present invention relates to a fully solid-state electrochromic device. More particularly, the present invention discloses an efficient and cost-effective lithium-poly vinyl alcohol (PVA) polymer film based solid state electrochromic device with switching time less than one second for various applications, including smart windows, displays, and other devices.

BACKGROUND OF THE INVENTION
Electrochromic materials are a class of functional materials with the remarkable ability to reversibly change their color or opacity upon application of a voltage. This unique property holds immense potential for a wide range of applications, including large-scale displays, glare-free mirrors, and even memory devices. However, the most significant impact of electrochromic materials lies in the development of smart windows and architectural glazing.

These devices can be divided into two categories:
? Liquid Electrolyte ECDs: These traditional ECDs contain a liquid electrolyte sandwiched between two electrochromic layers. Applying a voltage triggers ion movement within the electrolyte, causing a reversible color change in one or both electrochromic layers. However, liquid electrolyte ECDs face challenges that limit their widespread adoption. Leakage of the liquid electrolyte over time can lead to device failure and pose potential environmental concerns. Maintaining a proper seal between the electrolyte and electrodes can be difficult, especially for large-scale applications like smart windows. Additionally, the liquid electrolyte itself can degrade over time, impacting the overall lifespan of the device. These limitations have driven research towards the development of solid-state ECDs, which offer significant advantages in terms of durability, performance, and environmental safety.
? Solid-State ECDs: As the name suggests, these ECDs eliminate the liquid electrolyte, replacing it with a solid ionic conductor. This approach offers several advantages over liquid-based devices. Solid electrolytes eliminate leakage concerns and offer greater mechanical stability, extending device lifetime.

ECDs are made up of an electrochromic material and an electrolyte sandwiched between two transparent conducting electrodes, one of which contains the active EC material coated as a thin layer. When a voltage is applied, redox reactions take place between the two electrodes, as a result the system switches from a bleached (transparent) state to a colored state and vice versa.

The core functionality of an electrochromic device (ECD) hinges on its layered architecture. The said architecture includes at least:
• Substrate: The foundation of the device, typically composed of glass for its strength and transparency in applications like transportation and building sectors. However, for the substrate to function as an electron conductor, materials like Tin-doped Indium Oxide (ITO) or Fluorine-doped Tin Oxide (FTO) are commonly used due to their additional benefit of optical transparency. These Transparent Conducting Oxides (TCOs) need a specific balance: an electrical conductivity of ~10-30 O/sq and a visible light transmittance exceeding 90%. Fortunately, commercially available FTO or ITO coated glasses fulfill these criteria, making them ideal substrates for subsequent electrochromic layers.
• Electrochromic Active Layer: This layer forms the heart of the color-changing mechanism. It's often a metal oxide capable of absorbing a range of visible light, leading to subtractive coloration. The switching of colors occurs by inserting (doping) a guest cation (like a small ion such as Lithium+ or Hydrogen+) into the structure and then retrieving it by reversing the voltage. The speed of this insertion/retrieval process directly impacts the device's response time.
• Ion Storage Layer: As the name suggests, this layer acts as a reservoir for the guest cations that are shuttled in and out of the electrochromic layer. Similar to the active layer, it can also be a metal oxide, but with the crucial requirement of having a high solubility for the guest cations to facilitate efficient storage.
• Electrolyte Layer: The electrolyte layer serves as the critical interface between the ion storage and electrochromic layers. Faster movement of cations across this layer translates to a quicker device response. Traditionally, electrolytes are liquids with exceptionally high conductivity for the specific guest cation (e.g., LiCl4 for Lithium+ ions).
• Counter Electrode: To complete the electrical circuit as the cation is driven into the electrochromic layer by the applied voltage, a counter electrode is necessary. This layer conducts electrons and neutralizes the excess charge generated during cation insertion into the electrochromic material.

While the above description outlines a traditional ECD architecture, solid-state ECDs replace the liquid electrolyte with a solid ionic conductor. This change offers significant advantages like improved durability, faster switching times, and lower power consumption. The electrolyte plays an important role in ECDs, and solid-state electrolytes are handy substitutes for rendering scale-up without leakage. In addition, it should be cost effective and environmentally friendly. So the choice of SPEs should be appropriate for a feasible ECD.

Solid polymer electrolytes (SPEs) are a class of materials with the potential to revolutionize various electronic devices. The other commonly used SPEs are Poly (vinylfluoride) (PVDF), Poly (methylmethacrylate) (PMMA), Polyacrylonitrile (PAN), Poly (propylene oxide) (PPO), Poly (vinyl chloride) (PVC) composed of lithium salts such as LiClO4, LiPF4, LiBF4, lithium hexafluorophosphate (LiPF6 ). Usually, the reported ionic conductivity values for SPEs are in the 10-4-10-6 S/cm range. Engineering high-performance SPEs necessitates optimizing both solvation (ionic salt dissociation impacting polymer permittivity) and ion migration (facilitated by segmental motion).

Studies investigating SPEs containing lithium acetate, by Yahya et al. titled as “Effect of oleic acid plasticizer on chitosan-lithium acetate solid polymer electrolytes” (2003) and Ismail et al. titled as “Conductivity study in PEO-LiOAc based polymer electrolyte” (2009), have shown promising ionic conductivity in the range of 10?5 - 10?6 S/cm. This high conductivity is attributed to lithium acetate's high-water solubility and affordability, making it a viable choice for cost-effective flexible electronics that rely on ionic movement.

Reference is made to patent document CN112534345B, titled as “Preparation method of solid electrochromic device, solid electrochromic device and application of solid electrochromic device” published in 2022. This invention discloses the most popularly followed architecture in EC devices: Flexible substrate/transparent conducting layer/Ion storage layer/ Ion conducting layer or Solid polymer electrolyte/EC layer/transparent conducting layer/Flexible substrate. However, said device is bulky in nature, also the glass transition temperatures -20o C, resulting less ionic conductivity and higher response time.

Another reference is made to patent document CN213182263U titled as “A Functional and Transparent Gel Electrolyte System and Fast-Switching Electrochromic/Electrochemical Devices Thereof”, (2022). The invention discloses a gel based electrochromic device with switching speed is 1-3 s. The gel-based polymer offers flexibility, but has leakage concerns and less mechanical stability, also the switching time in said invention is higher.

Existing inventions in this field lack in providing precise solutions due to limited design integration. Additionally, existing prior art utilizes plasticizers or ceramic fillers in SPE preparation, increasing production costs. These prior art devices are also bulky, involve complex SPE fabrication methods, and demonstrate slower switching times.

In order to obviate the drawbacks of the existing state of the art, there is a pressing need for a fully solid state based electrochromic device, that would integrate seamlessly into different designs, provide cost-effectiveness without using plasticizers, costly solvents, or ceramic fillers, integrates easily processable SPEs, and exhibit rapid switching speeds for optimal performance.

OBJECT OF THE INVENTION
In order to overcome the shortcomings in the existing state of the art the object of the present invention is to provide a fully solid-state electrochromic device (D).

Yet another objective of the invention is to provide an electrochromic device with a response time of less than one second.

Yet another objective of the invention is to provide a single, multifunctional layer for both ion storage and ion conduction within the device, leading to a less interfacial resistance, lower fabrication costs, thinner and faster electrochromic device compared to traditional designs.

Yet another objective of the invention is to fabricate a device containing a lithium acetate doped PVA based solid polymer electrolyte membrane.

Yet another objective of the invention is to provide a cost effective, portable device with good mechanical strength and electrochemical stability.

Yet another objective of the invention is to fabricate membrane without the use of conventional plasticizers, costly solvents, or ceramic fillers.

SUMMARY OF THE INVENTION:
The present invention discloses a fully solid-state electrochromic device (ECD) and the method thereof. The key step lies in the developing of the Solid Polymer Electrolyte (SPE) that eliminates the need for conventional components used in prior art devices. The invention utilizes cost-effective materials such as polyvinyl alcohol (PVA) and lithium acetate (CH3COOLi). PVA is doped with varying concentrations of lithium acetate to create the SPE. Unlike traditional SPEs, this formulation devoid the use of plasticizers, costly solvents, or ceramic fillers. The PVA and lithium acetate are mixed in specific ratios to create the SPE with desired properties. Lithium acetate (CH3COOLi) is incorporated into said PVA at varying weight percentages to create said SPE. Said SPE solution is then cast onto a suitable substrate and dried to form a thin film. The ionic conductivity and electrochromic performance of said SPEs are characterized using DC voltammetry and AC impedance spectroscopy. The ionic conductivity of said fabricated SPEs with different Li-acetate concentrations is measured using a combination of DC voltammetry and AC impedance spectroscopy.

The prepared SPE is sandwiched between two Fluorine-doped Tin Oxide (FTO) coated glass plates. A layer of tungsten oxide (WO3) is deposited on one of the FTO electrodes to act as the color-changing component (electrochromic layer). The FTO glass plates with the WO3 layer are assembled with said SPE film, creating a sealed ECD structure. The electrochromic properties of the assembled ECD are evaluated by applying a bias voltage and observing the color changes. Switching times between colored and bleached states are measured to assess the response speed of the device (D).

BRIEF DESCRIPTION OF DRAWINGS
Figure 1 (a) depicts XRD patterns of the pure PVA, 7 wt.% Li in PVA and 25 wt.% Li in PVA membranes.
Figure 1 (b) depicts DSC thermograms of the pure PVA, 10 wt.% Li and 20 wt.% Li SPE samples.
Figure 2 (a) depicts Optical transmission of SPE with different Li wt.%.
Figure 2 (b) depicts Transparent SPE with CH3COOLi (20 wt.%).
Figure 3 (a) depicts Representative I-V scans for multiple cycles in the PVA, PVA+7 and PVA+20 wt.% samples.
Figure 3 (b) I-V curves averaged from the cycles shown in (a) for all Li-salt containing samples.
Figure 4 depicts Impedance plots of SPE at (a) 15, (b) 20, (c) 25, and (d) 30 wt.% Li membranes.
Figure 5 (a) depicts variation of dc and ionic conductivity of the different SPE membranes.
Figure 5 (b) depicts comparison with previously reported ionic conductivity values of SPEs with present invention.
Figure 6 (a-g) depicts spectra of complex permittivity across the different SPEs.
Figure 6 (h) depicts relaxation peak positions from the tan ? loss spectra.
Figure 7 (a) depicts fabricated ECD in colored state.
Figure 7 (b) depicts fabricated ECD in bleached state.
Figure 8 depicts CV for different Li wt.% of the SPE,
Figure 8 (b) A typical optical absorption spectrum showing the extent of color contrast between bleached and colored state.
Figure 9 (a) depicts sample CA sequence of 3 color-bleach cycles.
Figure 9 (b) depicts zoomed portion of the coloring curve during CA; the inset clearly shows the complex initial transient as also the similarity of the 15,20 wt.% and 25, 30 wt. % samples.
Figure 9 (c) depicts a portion of the bleaching curve.
Figure 10 Schematic of the regions in the ECD used for the analysis of the
coloration behavior.
Figure 11 (a) depicts the CA data during bleaching of the 20 wt.% Li sample along with fits of different trial equations.
Figure 11 (b) CA data during coloring of the same sample with the corresponding fit.
Figure 11 (c) CA data during bleaching of an ECD constructed with 1M LiClO4 in PC electrolyte. A fit with eqn. (4) is shown, but eqn. (5) was a very poor fit.
Figure 12 Numerical solution of the CA results showing Li concentration in the WO3 sample at different depths after various durations of time in (a) 20 wt. % Li sample and (b) 30 wt. % Li sample.
Figure 13 depicts schematic of the fully solid-state Electrochromic device.

DETAILED DESCRIPTION OF THE INVENTION WITH ILLUSTRATIONS AND EXAMPLES
While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

Table 1: Legend of Reference numerals
Ser no. Item reference Reference Numerals
1. Device D
2. Solid polymer electrolyte SPE
3. Polyvinyl alcohol PVA
4. Wo3 coated Electrochromic layer ECL
5. FTO coated first glass substrate G1
6. FTO coated second glass substrate G2

The present invention discloses a fully solid-state electrochromic device (ECD) (D). Said ECD is constructed using solid polymer electrolyte (SPE) sandwiched between two FTO coated glass plates, where tungsten tri oxide (WO3) layer was coated above one of the FTO coated glass plate. Said solid polymer electrolyte (SPE) is PVA based and Lithium acetate (CH3COOLi) doped film. Both PVA and SPEs are prepared with different percentages of Lithium acetate without adding any plasticizers and ceramic fillers. The figure depicting the device (D) shown in figure (13). The fabrication of such device (D) includes following steps:
? Solid Polymer Electrolyte (SPE) film preparation: This involves fabricating Lithium acetate incorporated PVA membrane.
? Testing membrane and Device construction: This step involves testing membrane for its electrochromic characteristics using different techniques such as X-ray diffraction (XRD), Differential scanning calorimetry (DSC), UV-Visible spectroscopy, Impedance spectroscopy and I-V characteristics, Cyclic voltammetry (CV) and chronoamperometry (CA). Then using such membranes, for subsequent construction of a fully solid-state electrochromic device (D).

The following detailed description provides insights into each component of the present invention:

Fabrication of a fully solid-state electrochromic device (D) involves following steps:

WO3 thin film deposition: A stoichiometric WO3 (pure tungsten trioxide) target is used for pulsed laser ablation (PLD) with a KrF excimer laser to create high-quality, monoclinic phase WO3 thin films. The term “Pulsed Laser Deposition (PLD)”is a thin film deposition technique that utilizes a high-power pulsed laser beam to ablate a target material, creating a plasma plume that deposits the material onto a substrate. These films serve as a crucial component in the device (D). This is a technique where a powerful laser beam (KrF excimer laser in this case) is used in short bursts (pulses) to strike a target material (stoichiometric WO3 in this case). The laser pulses essentially "blast" tiny particles off the target, which then condense and form a thin film on a nearby surface.

Preparation of Lithium Acetate incorporated PVA based Solid Polymer Electrolyte (SPE): The preparation of solid polymer electrolyte (SPE) films utilizes materials like PVA (polyvinyl alcohol) weight of at least 115,000 g/mol, and Lithium acetate dihydrate (C2H3LiO2.2H2O). The conventional solution-casting technique is employed for this purpose, with distilled water serving as the solvent. PVA-CH3COOLi (lithium acetate) films are prepared at varying weight percentages of PVA (7%, 10%, 15%, 20%, 25%, and 30%). The process involves dissolving PVA in distilled water under magnetic stirring for 2-3 hours at 70-80°C. A predetermined amount of lithium acetate is then incorporated into the PVA solution and stirred for an additional 2 hours. The resulting solution should be clear, transparent, and free of air bubbles with proper stirring. This clear, viscous solution is then cast onto a petri dish and left undisturbed for drying at room temperature. After one week, the films are retrieved from the petri dish for characterization and subsequent use in the fabrication of electrochromic devices (ECD).

In the next step various tests performed on the said fabricated SPE film for further characterization and electrochemical measurement to ensure they possess the necessary properties such as crystallinity, glass transition temperature, transparency, and ion conductivity for optimal performance in the final electrochromic device (D). This may include following tests:

? X-ray diffraction (XRD): This technique helps identify the crystal structure and phases present in the PVA-lithium acetate membrane. In the context of the device (D), a well-defined crystalline structure can influence ion mobility and conductivity. X-ray diffraction (XRD) was carried out using a Bruker X-Ray diffractometer operating at 30mA, 45 kV which employed Cu-Ka radiation and scanned between a 2? range of 10-80°. Figure 1(a) shows the XRD pattern of pure PVA and PVA complexed with lithium acetate at concentrations of 7 and 25 wt.%. PVA typically exhibits three characteristic peaks at 19.5°, 23°, and 41°, corresponding to the (101), (200), and (111) planes, indicating its semicrystalline nature. While the addition of lithium acetate (7% and 25% by weight) doesn't introduce new peaks, a clear decrease in crystallinity is evident through the broadening of the existing peaks. This observation aligns with the established principle that increased short-range order (amorphous regions) within polymers benefits ion conduction. These disordered regions create "free volume" and allow for segmental relaxation in the polymer chains, ultimately facilitating ion movement.
? Differential scanning calorimetry (DSC): This measurement determines the glass transition temperature (Tg) of said film (SPE). The term “glass transition temperature (Tg)” used in this specification defines the point where the polymer changes from a rigid, glassy state to a rubbery state. A suitable Tg is crucial for the device's operation at desired temperatures. Glass transition temperatures (Tg) were measured using a Perkin Elmer differential scanning calorimeter (DSC) over a temperature range of 30o C to 300o C. The DSC thermograms are shown in Figure 1(b). Samples, including pure PVA and Li-doped PVA, show a gradual change at low temperatures corresponding to the glass transition temperature (Tg). Notably, the Tg of pure PVA (around 76°C) decreases to 62°C for the membrane containing 20 wt.% lithium acetate. This decrease in glass transition temperature (Tg) further signifies an increase in short-range order with lithium acetate addition, creating a favorable environment for ion conduction within the device (D).
? UV-Visible spectroscopy: UV-Visible spectroscopy for optical transparency was carried out using a spectrophotometer (FLAME-T-XR1-ES). This technique assesses the optical transparency of the membrane. In an electrochromic device, the membrane should ideally be transparent to allow light to pass through and interact with the electrochromic material (WO3 thin film) for proper color change. Good transparency of the electrolyte is a prerequisite for any ECD. Transparency of the prepared SPE membranes were inspected by their transmission spectra in the 450-1000 nm range as shown in Fig. 2 (a) and 2(b). The transmittance of pure PVA was very close to 100% but with the introduction of salt upto 25 wt.%, it decreases marginally to 90%, but for 30 wt.% Li:PVA membrane, the transmittance decreased to 70% owing to (probably) incomplete dissociation of the salt at the microscopic level and also because of ridges formed while casting the film.
? Impedance spectroscopy and I-V characteristics: These techniques measure the resistance (impedance) and current-voltage (I-V) behavior of the membrane when placed between electrodes. A good membrane for the device should have low resistance to allow efficient ion movement within the device (D). The I-V measurements define the current flow through the membrane under different voltage conditions. For said Impedance spectroscopy, NF ZM2376 LCR meter and I-V characteristic, Keithley meter are used. Studies were conducted by placing the membranes between cylindrical Au-coated brass electrodes. Said Keithley meter used for the I-V measurements scans a dc potential over several cycles with a current resolution of 10 nA. Said Impedance measurements were done with 100 mV ac potential and frequency scan from 100 mHz to >3 MHz.
? Cyclic voltammetry (CV) and chronoamperometry (CA): While not directly related to the membrane itself, these techniques are used to analyze the electrochromic material (WO3 thin film) deposited earlier. CV provides information about the redox reactions involved in the color change of the device, while CA helps understand the current flow during the color change process. Said measurements of the ECD were conducted using an electrochemical workstation CHI660C in the two-electrode configuration.

EXPERIMENT AND ANALYSIS
The present invention is described in detail with a non-limiting examples:

Analysis of the fabricated membranes using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques reveals promising results for ion conduction within the device (D).
Electrical Characterization: This section delves into the methods used to distinguish between ionic and electronic conduction mechanisms within the fabricated Solid Polymer Electrolytes (SPEs). DC scan and AC impedance spectroscopy techniques were employed for this purpose.

For DC Scan Measurements, small DC potential scans over several cycles were conducted using Au-coated brass electrodes to identify electronic conduction. Figure 3 (a) illustrates representative scan cycles employed in the experiment. Pure PVA exhibits complete linear electronic conduction throughout the entire voltage range scanned (-1V to 1V for most samples, exceeding 1.5V for pure PVA). Notably, the current increases with increasing Li-salt concentration, ranging from a fraction of a milliampere in pure PVA to over 150 mA for the 30 wt.% sample. Additionally, a hysteresis loop observed during cyclic scans suggests charge separation and capacitance build-up due to ion migration in membranes with higher Li-salt concentrations. This hysteresis becomes more prominent at higher Li concentrations.

To quantify the DC electron conductivity, the multiple I-V cycles were averaged and plotted as shown in Figure 3 (b). All curves exhibit a linear (Ohmic) behavior over a small voltage range near the origin. Linear fits within these regions were used to calculate the DC conductivity of the samples.

For a functional Electrochromic Device (ECD), a substantial amount of ionic conductivity (ideally exceeding 10?5 S/cm) is crucial. While DC voltammetry might not be sufficient to isolate the ionic response, AC impedance spectroscopy is used.

AC impedance spectroscopy, where the AC frequency can be swept over a wide range, effectively distinguishes ionic conductivity from electronic. At suitably high frequencies, ions can hop from one site to another (intra-chain or inter-chain hopping), leading to pure ionic conduction, while electronic movement becomes negligible. AC impedance measurements were conducted on all samples across a frequency range of 0.1 Hz to 5 MHz. A significant difference was observed between samples containing less than 15 wt.% Li-salt and those with higher concentrations. All samples exhibit a depressed semi-circle in the high-frequency region and a tilted spike in the lower frequency range, a characteristic behavior of most polymer electrolytes.

For compositions below 15 wt.%, the semi-circle is large with a small tail. As the Li concentration increases, the semi-circle shrinks, and the low-frequency tail becomes more prominent (Figure 4a-d). Multiple depressed semi-circles can be discerned in all plots, with one at low frequencies attributed to charge build-up at the electrode-sample interface leading to space-charge polarization of the dipoles. Interestingly, this low-frequency spike becomes more pronounced with increasing Li wt.%, suggesting the possible presence of a Warburg-type diffusive element in the circuit, in addition to electrode polarization.

To interpret the Nyquist plots, an equivalent circuit model was developed to represent the electrical inhomogeneity within the membrane. The model accounts for the bulk SPE resistance and capacitance dominated by electronic species, segmental relaxation of long-range ordered regions of the native PVA polymer segments due to the ionic response (-OH groups), and the relaxation of short-range segments associated with the Li-complexed regions (Li-ion concentrated regions). These segmental motions are captured by constant phase elements (CPEs) rather than pure capacitances due to the complex nature of both relaxations with closely distributed relaxation time constants. Moreover, there is no clear boundary between the short and long-range ordered regions; hence, the components attributed to these mechanisms are assumed to act in parallel. No separate element was included to account for air bubbles, as no visible traces were observed in any of the membranes.

As shown in Figure (4), the model circuit effectively fits the measured spectra across the entire frequency range of 100 mHz to 5 MHz. The inset of Figure 4 (a) depicts the equivalent circuit, where Rb-Cb represents the bulk resistance and capacitance of the SPE membrane. Q1-R1 and Q2-R2 represent the components corresponding to the relaxations of the native polymer chains and the short-range Li-hopping chains, respectively. Notably, Q1-R1 and Q2-R2 are essential components for reproducing the high-frequency semicircle, as shown in the inset of Figure 4b. This becomes particularly evident beyond 15 wt.% Li, where a high-frequency semicircle arises from the relaxation of the polymer segments after the hopping of the Li cation between complexation sites. Thus, a distinct observation of the high frequency semicircles as shown by the insets in Fig. 4, represents ionic conduction in the SPE.

It is also noted from the plots that these semicircles shift to higher frequencies with increase of the lithium salt concentration, clearly indicating a decrease in the resistivity with increasing concentration. The intercept of this high frequency semicircle on the real axis gives the resistance to ion movement, from which the ionic conductivity, ?ion is calculated. A comparison between the electronic conductivity (?el, from the dc scan studies) and the ionic conductivity, ?ion of the samples is presented in Fig. 5a along with a comparison with other reported ?ion of various SPEs (Fig. 5b). Interestingly from Fig. 5a, beyond 15 wt.%, the samples show an appreciable improvement in the ionic (?20wt.%/?15wt.% ?100) and a gradual increase in electronic conductivity with increasing Li salt. The trend suggests a sigmoidal behavior of Li ion conductivity with Li concentration as indicated in Fig. 5a, although compositions beyond 30 wt.% are not undertaken owing to deteriorating optical transmission, making them ineffectual for ECD. However, earlier reports suggest so: the previously reported value for the PVA:PVP-Li-acetate blend is in the 10-6 range for 25 wt.% and for 30 wt.% it reportedly decreases.

Maximum ionic conductivity is observed for the 30 wt.% sample in the range of 10-4 S/cm. The electronic conductivity is found lower than ionic conductivity which shows ions are the majority charge carriers in the SPE and contribute considerably to the total conductivity. At 15 wt.% ?ion surpassed ?el, and as reported in a later section, the ECDs fabricated beyond this composition showed coloration during voltage application and reversal.

The two types of segmental relaxations that are anticipated (Li-depleted long and Li-rich short-range segments) are hard to distinguish from the Nyquist plots (the n values of the CPE of all samples fall into two clusters – one above 0.75, implying non-ideal capacitive behavior and the other around 0.35 indicating non-ideal resistive behavior). As reiterated earlier, the ion mobility of dipoles in polymers is correlated to the Tg. Therefore, the average relaxation time is calculated, from the ?max of the semicircle, giving relaxation times from ms to ?s, between the 15-30 wt.% samples. It is noted that dilute solutions relax relatively slow in PVA – an observation consistent with that obtained from ultrasonic relaxation spectroscopy. An analogous description based on complex permittivity (? *) or complex modulus (M*) can be used to clearly discern competing mechanisms but through different capacitive or resistive routes.

Accordingly, the approach using complex permittivity plots (?? ???+ j?'' = ?Re+ j?Im) are shown in Figure (6). The ?Re and ?Im spectra (in particular, the derivative spectrum, ?''der =-d?'/d(ln ?)) show the following regions: a low frequency spike arising from a combined electrode polarization and dc conduction loss, a mid-frequency peak between ?=101-103 evident even in the pure and dilute solutions (up to 15 wt.%), and a high-frequency peak (evident beyond 15 wt.% Li samples and boxed in the panels (e) and (f) of Fig. 6) at around ?=105-106 rad/s associated with ion migration and dipole relaxation. Interestingly, this high-frequency peak apparently shifts to higher frequencies with increasing Li concentration. For instance, with 25 wt.% Li, this lies in the ?=106-107 range. Based on this observation, we infer that there are two relaxation mechanisms in the SPE with different segmental relaxation times, differing by about 3 decades in time. Pure PVA has hydroxyl groups attached to carbon chains, [-CH2-CH(OH)-]n and hence structural relaxation in the order of a few ms or greater happens when dipole migration occurs in such PVA membranes. Upon addition of the Li salt additional dipoles (complexed to the backbone) are created in which Li+ migrate faster along a different route during which the relaxation occurs in timescales of a few ?s. In SPEs, microstructural heterogeneity (crystalline and amorphous phases, hard-soft block copolymers) and compositional inhomogeneity (dopant ion-rich and ion-depleted regions) can lead to different segmental relaxations and usually such competing mechanisms are clearly separated, often, by two or three decades in time.

The analyses based on the permittivity clearly show existence of at least two mechanisms of relaxation – one which is native to the pure PVA and another seen in Li-concentrated samples indicating relaxations related to long range (semi crystalline, Li-poor) and short range (amorphous, Li-rich) regions. More support for this is seen from the dielectric loss spectra, which is discussed below.

Dielectric loss peaks are indicative of resonance phenomena in samples with dipoles. All the SPEs give clear indication of dipole resonance peaks (the tan ? behaviors are also shown as insets in Fig. 6), which invariably lead to ion migration. The low frequency slope is indicative once again, of conductive losses. Beyond 15 wt.%, a clear high frequency resonance peak is observed while with the 30 wt.% sample, multiple resonances are observed. We note that the position of the resonance peak in the tan ? vs. ? curve equals the relaxation period in an ideal oscillator by, ?relax=1/RC=1/?. Based on the number of relaxation peaks, we can delineate three broad regions in the ?-domain shown in panel (h) of Fig. (6).

In the pure and dilute SPEs (=15 wt.% Li salt), the primary loss peaks are at ?15 wt.%, showed appreciable ion conductivity, an all-solid state ECD was constructed in the configuration Glass/FTO/SPE/WO3/FTO/Glass. Initially, the WO3 films were checked for coloration using 1 M LiClO4 liquid electrolyte in a conventional three electrode set-up with Ag/AgCl reference. After successful testing in the liquid electrolyte, trial runs over a potential window of -3 to +3 V were conducted to check for color change in the solid state ECD. All such ECDs with SPEs with >15 wt.% Li showed reversible coloration and bleaching during the potential scans as shown in figure Fig. 7, and hence were taken up for further systematic study. The higher concentration SPEs showed good contrast and a test of retention of coloration for a period of up to 1 month was also taken up with the 20 wt.% sample. Details of the studies on the ECD are given below.
? Cyclic voltammetry (CV):
Cyclic stability of the ECD was measured using the two-electrode configuration. The cyclic voltammograms were measured between -2.5V and +1.2V with a scan rate of 50 mV/s. During cathodic potential sweep coloration was observed and bleaching during the anodic sweep. Fig.8a shows the cyclic voltammograms measured for the ECD with different Li:PVA film. With increasing weight percentage, the peak current also increases. The 30 wt.% sample showed additional anodic oxidation and cathodic reduction peaks probably due to charges decomposed from the polymer chain. A comparison with the standard LiClO4 liquid electrolyte is shown in the supplementary file, along with the optical absorption of the ECD in the visible region while in the colored and bleached states.

A sample absorption curve of the 20 wt.% electrolyte device after coloration/bleaching is presented in Fig. 8 (b) showing excellent optical contrast. Interestingly, we noted the presence of moisture after cycling during humid conditions. The moisture retained in the SPE caused some deviations in the CV during prolonged cycling. However, they were quite stable for >150 cycles when tested for over a month as given in the supplementary write-up. We also note that the use of the traditional Randles-Sevick equation wherein the shift of the peak current, ip as a function of the voltage scan rate, v (using the relation, ???? = 2.69 × 105??0??v?? 3????????, where n is the number of electrons that participate in the redox process, and C0 is the concentration of ions in units of mol/cc), is appropriate to calculate the diffusion coefficient, DLi only in the case of the three electrode set-up. This is because in the 2-electrode set-up used here, the DLi evaluated is actually the total diffusivity of Li in the SPE electrolyte and EC layer and the measured current is also limited by the electronic conductivity of the electrolyte. This may lead to low estimates of diffusion coefficients (limited by the poor electronic conductivity of the polymer) that cannot be ascribed purely to the DLi of WO3 alone. Thus, they must be construed as effective diffusivities of Li (assuming no other charge carrying species across the interfaces) in the total device.

The effective DLi thus estimated for the different weight percentages are shown in Table 1. The maximum diffusivity is observed for 30 wt.%. The obtained value is comparable with reported value of 7.81 x 10-11 cm2/s for PMMA-LiClO4 (25 wt.%).

Table 2: Various parameters estimated for the ECD from electrochemical studies.

Amount of
Lithium
acetate
(wt.%) (Effective)
Diffusion
coefficient
(cm2/s) Response Time (ms) Charge density (/cm2) Reversibility
(%)
Coloration
Time (Tc) Bleaching Time (Tb) Qin Qdin
15 1.54 x10-13 370 300 4.14 2.22 54%
20 7.379 x 10-13 290 300 3.34 2.037 61%
25 4.058 x 10-12 100 100 39.42 10.34 26%
30 1.593 x 10-11 120 100 40.02 9.12 23%

Chronoamperometry (CA): The dynamics of reduction and oxidation, i.e., the switching speed of the ECD for increasing salt composition was determined using repeated chronoamperometry measurements by applying potentials -2.5V and 1.2V for 100s each and the results are shown in Fig. 9. The oxidation current and reduction current increased immediately after the application of step potentials and decreased with time. The switching time was determined by the time taken by the coloration and the bleaching current to decrease from 100% to 10%. Rapid switching speeds in the order of a fraction of a second are obtained and are shown in Table 1.

The colored and bleached state of the ECD during chronoamperometry measurement is shown in Figure 7. We also note from the insets in Figure 9 that the higher wt.% Li samples (>20 wt.%) show a complex transient response with an initial steep fall, plateau and further fall to steady state. The 15 and 20 wt.% show a comparatively smooth response. Electrochromic reversibility of said device (D) is determined from the intercalated and deintercalated charge during CA. Reversibility is the measure of charge intercalated (Qin) to the charge deintercalated (Qdin) during coloration-bleaching process and is an essential aspect of a said stable ECD. For a perfect ECD it should be 100% which means the intercalated charge during coloration is totally deintercalated during bleaching.

The maximum reversibility of 61 % is shown by the 20 wt.% sample. Reversibility for 25% and 30 wt.% was reduced, owing to various factors, implying the coloration over long term cycling would lead to permanent staining of said EC film. It was also quite surprising to note that the electrical response time, i.e., the time taken for the current to fall down to 10% of its maximum value in a CA run, was only in the order of a fraction of a second, quite unlike the response in liquid electrolytes such as LiClO4, which typically are one order of magnitude higher (a few seconds). Thus, in terms of cycle stability, both electrical and optical response times, potential window and mechanical robustness, the fabricated ECD device (D) is able to withstand the trial tests conducted on them.

Fundamentally it appears that the interplay of short diffusion lengths, reversible charge transfer across an apparently low-resistance interface and high fields together results in the favorable response of the said device (D). We briefly discuss the electrochemical behavior of the device to understand the factors at play and hence distinguish them from the response to standard liquid electrolytes.

Model of electrochromism in the ECD with the SPE: A simple but formal analysis of the electrochromic behavior in the FTO/PVA/WO3 device is presented below to seek an explanation for the coloration response of the device.

Said device (D) partitioned into three regions and proceed to analyze the response of the regions to the applied field:
? Region I is the Li:PVA SPE,
? Region II is the crucial SPE/WO3 interface and
? Region III is the WO3 EC layer (as shown in Fig. 10)

It is safe to assume that the largest voltage drop is across the PVA SPE owing to its poor electronic conductivity and that the WO3 film is relatively an extremely good electronic conductor.

In Region I, the response is dictated by the diffusivity of Li+ in the PVA (DLiPVA). To a rough approximation, if we assume that the ionic diffusivity and ionic conductivity (?Li) are related as in typical ceramics by the Nernst-Einstein relation,

?????? = ?????? 2 ?? 2???????????? ?????? /??????……………………………………………… (1)

where, zLi is the charge state (+1, in this case), CLi is the concentration (in units of ions/m3) in the polymer and the other terms have their usual meanings, then the concentration of the responding ions can be evaluated by knowing the other terms.

Typical values of equivalent SPEs from the literature suggest DLiPVA?2×10-8 cm2/s. Which is some two orders of magnitude lower than Li+ diffusivity in liquid electrolytes (DLiPC/LiClO4?2×10-6 cm2/s). With typical ranges of ionic conductivity evaluated from our EIS study (~?Li=5×10-6 S/cm), we get CLi?4×1025 ions/m3 – a value comparable to many fast ion conductors.

On application of the field, these ions respond by moving towards Region II, where they are blocked by the activation barrier that requires these ions to intercalate into the WO3 layer and the slow diffusive response of the intercalated ions in the WO3 layer. Thus, Li+ ions accumulate at the interface (Region II), due to the applied field and in turn building up a concentration gradient within the PVA membrane. Under equilibrium, the diffusive flux owing to the concentration gradient and the electromigration flux of the applied field balance each other:

(-) ?????????????????? ????/ ?????? * (????/????) = -???????????? * ???????? /???? ………………………… (2)

where dV/dx is the potential gradient across the Li-PVA SPE (in units of V/m) and dCLi/dx is the concentration gradient of Li+ ions. The (–) sign on the left indicates the cathodic scan leading to diffusion of Li+ into WO3 and hence coloration. If the applied potential drops primarily across the PVA membrane, then dV/dx (=E) can be approximated to the applied field (?3 V/1.5 mm). The above approximation is justified by the fact that PVA is a poor electronic conductor while the concentration distribution across the Li-PVA SPE can now be solved from (2) to give:
?????? = ???????? (?????? /?????? * ??)……………………… (3)
where A is the integration constant and our region of integration is from (-L,0). We note that the Li+ charge concentration builds up exponentially towards region II.

At region II, the CLi build-up and the applied field induces the dissolution of Li+ into the WO3 film leading to an emf that is a function of the intercalated Li in LixWO3. This emf vs. CLi in WO3 is reportedly a curve that is either smoothly varying (in the case of amorphous, a-WO3) or stepped if crystalline WO3 is phase transitioning from its Monoclinic to Tetragonal or Tetragonal to Cubic, as has been detailed in many reports. Irrespective of the nature of the curve, the emf deteriorates with increasing value of x in Lix(WO3), particularly beyond x=0.4. The exact form of this interfacial emf (in the same and similar alkali bronze systems) has been determined using a variety of techniques in a 3-electrode cell by many workers. In present invention, chronoamperometry results are used to arrive at a form of the time varying emf at the interface. Detailed report on the kinetics of coloration and bleaching of LixWO3 incorporating two rate constants – the coloration rate being determined by the rate of fall of the interfacial emf and the bleaching rate dependent on the diffusion impedance and space charge at the interface – have been reported by research paper published by Zhang et al, titled as Segmental dynamics and ionic conduction in poly(vinyl methyl either)-lithium perchlorate complexes. In their analysis, they obtain the rate of fall of current during the cathodic scan as:

???????????? = (????????-??(??)) ?? exp ( ?? ??0 )???????? (v ?? ??0 )………………… (4)

where Vapp is the applied voltage where coloration/bleaching is observed, e(t) is the time/concentration – dependent emf (determined through in situ observation of the optical transmission and fitting a high order polynomial) and t0 is a characteristic time incorporating the diffusive mechanism and the resistance of the sample. A research paper published by Nagai and Kamimori titled as “Kinetic study of LixWO3 electrochromism”, obtained a simple bleaching current decay curve during CA as:

????????????h/?????????? = ????????-??(??) /?? …………………………………… (5)

We used Nagai’s simple equation to fit our data with reasonably good agreement, as shown in Fig. 11 and fitting parameters shown in table (2). We point out that in both Nagai, Kamimori’s and Zhang’s paper, the kinetics of coloring and bleaching were analyzed to be different: a diffusion impedance limited process during intercalation, and extraction through a space charge region during bleaching. As shown in the inset in Fig. 11 (a) and (b), we also note that the fit converges to the measured data only after 1 mA/cm2 or after the current has decayed to about one third of its peak value. The t -0.6 behavior observed for t>0.03 s must be understood as a change in the rate controlling mechanism. For practical convenience, we assume that the emf varies according to the form given in equation (5). As a comparison, we also show the bleaching current obtained using a three-electrode set-up in a standard liquid (1 M LiClO4 in PC) electrolyte, which clearly indicates that neither equations (4) or (5) used for the Li-PVA SPE are suitable fits to describe their bleaching behavior (as shown in Figure 11c). It is also worth noting the peak current during CA in the presence of the liquid electrolyte is almost one order larger than that using the SPE for approximately the same applied potential, indicating the relatively lower diffusivity and ionic current with the SPE.

Table 2: Fit parameters corresponding to equation. (5) used for the fits shown in Figure 11 (a) and (b)
J=A+Btn
Fit Parameters A B n R2 of the
fits
Coloring curve
-0.0612 ± 0.0022 -0.0932 ± 0.0018 -0.5363 ± 0.0039 0.9763
Bleaching curve 0.0320 ± 0.0001 0.1146 ± 0.0002 -0.6030 ± 0.0005 0.9966

Finally, region III is the WO3 film which completes the coloration by Li+ intercalating into the thickness of the film. The coloration spreads into the thickness of the film by the associated e – from the electrode resulting in a [Li+ • e –] complex. The concentration of Li+, denoted CLi(x, t) that diffuses into the WO3 film can be determined by solving the diffusion equation (in 1D along the thickness, x of the film):
????????/???? = ?? ??2?????? /????2 …………………….(6)

subject to the partially insulating boundary conditions:

???????? ???? = ????????(??)-??(??) /???????? = ??(??)/ ?????? ? ??????(0,??)……………………. (7)
and
???????? /???? = 0 ? ??????(??,??)………………………………………………... (8)

where F is the Faraday constant, D is the diffusion coefficient and n=1. We also impose the initial condition that the film is completely devoid of Li+ initially:

?????? = 0 ? ??????(??, 0)……………………………………………….. (9)

As the applied voltage, Vapp is scanned from a coloring potential (~-2 V) to the bleaching potential (~+2 V), at a fixed rate (~0.3 V/min), the Li+ concentration also changes at the PVA/WO3 interface (region II), resulting in a time-varying emf(t) and consequently a time-varying current density, j(t) injected into the film at x=0. Using the fits from the CA results, it is seen that the current density, j (and the junction emf e(t)) scales as t -0.6. With this approximation it is instructive to see how the concentration of Li+ varies within the film during the coloration process. The set of equations (6)-(9) were solved using an explicit numerical scheme and the results are shown in Figure 12.

It is evident that the concentration of Li+ very near the surface (=10 nm into the WO3 film) is where the flux of ions is concentrated during t=0.3 s. It is well known that beyond a permissible limit (x?0.4 in LixWO3), the coloration is permanent, shown by the hatched area in Fig. 12. For the 20 wt.% film, the concentration of Li drops drastically within the permissible limit (shown by the colored region in Fig. 12) within 10 nm in under 0.3 s, implying that Li injection, >0.4 is only limited to the surface layers during the initial time, which diffuses rapidly leading to uniform coloration. In comparison, in the 30 wt.% sample even after 3 s, the concentration within the first 25 nm of the film is far above the threshold limit, implying high likelihood of permanent coloration. The reversibility thus decreases with increasing wt.% Li, the optimal being around 20%, as shown earlier in table 1. In fact, with the 30wt% electrolyte, after a few cycles, patches of permanent color was observed on the film after the cathodic scan. The summary of the analysis is that the coloration/bleaching characteristics are governed by the incoming flux at region III, which in turn is governed by the emf(t) at region II. In typical liquid electrolyte ECDs, the emf(t) at the interface drops down slowly implying a proportionally slow concentration increase (the j(t) curve is broad as seen in Figure 11c), whereas in ECDs with SPE, the emf is a much more rapidly varying function of time. The transient current is hence very sharp and swiftly decays to equilibrium. The relatively fast coloration observed in the Li:PVA SPE ECD must thus owe its origin to the following factors:
? High electric field across the SPE coupled to very short electrolyte diffusion lengths, in agreement with other earlier reports.
? Rapid drop of emf(t) at the interface,
? Rapid injection of Li+ ions into the surface which then diffuses deeper into the film and
? High surface concentration of Li within the first tens of nm in the film which is responsible for the coloration observed.
Before concluding, we also must reiterate the approximations made herein:
? The fact that ionic conductivity (as measured by impedance spectroscopy) is ascribed solely to Li+ ions is a major approximation that might infect the details of the phenomenon modelled. One should realize that the overall ionic conduction must involve the transport of all the ionic species – including various cations and anions in the polymeric chain, which can be appropriately evaluated only by using blocking electrodes;
? The analogy between ion migration in ceramics and polymers invoked in eqn.(1) to evaluate the concentration of migrating ions is also inaccurate, but nevertheless helps in arriving at reasonable initial guesses (at least of a rough order-of-magnitude value); (iii) we also must acknowledge that the initial transients are also influenced by the nature of physical contact (intimate or not) between the EC layer and the SPE membrane. This may lead to a prominent difference if the SPE were, for instance, spin-coated on the EC layer rather than being cast-dried separately and sandwiched, as reported herein. We believe that the previous scenario might be more advantageous.

In summary, SPE films based on the PVA-lithium acetate system were prepared and their ionic conductivity and dielectric properties were measured using a combination of dc voltammetry and ac impedance spectroscopy.

Considerable ionic conductivity in the range of 10-5 S/cm was obtained and attributed to Li-ion hopping through short-range ordered regions with segmental relaxation times of a few ms. An improved ionic conductivity value is obtained compared to previous studies on SPEs without using any type of plasticizers or costly solvents or ceramic fillers. An ECD was successfully fabricated using the prepared SPE. The coloration was observed from 15 wt.%. The SPE was stable during the electrochemical studies. The CV measurements were done for 170 cycles and after 100 successive cycles the coloration became weaker due to the removal of tungsten oxide film from FTO coated glass substrate. Fast switching times between coloration and bleaching observed for all weight percentages indicated a rapid electrochromic response attributed to a combination of a complex interface potential and short diffusion lengths in the electrolyte that results in a rapid transient response. An immediate coloration was seen with biasing. The reduced reversibility observed can be improved by applying equal anodic and cathodic potential. It is evident from the results that with PVA-CH3COOLi SPE, a good ECD can be constructed. A more robust ECD can be fabricated by ensuring intimate contact between SPE and EC layer by using a different route to prepare the SPE.

ADVANTAGES OF THE PRESENT INVENTION
The present invention offers following advantages:
? Improved Performance: The PVA-lithium acetate SPE exhibits significant ionic conductivity, leading to faster switching times (less than one second) between colored and bleached states within the ECD. This surpasses the performance of existing ECDs that often utilize plasticizers.
? Simplified Fabrication: By eliminating the need for additional components like plasticizers or ceramic fillers, the invention potentially streamlines the ECD manufacturing process.
? Solid-state design: Solid-state construction offers advantages like improved durability, leak-proof operation, and easier integration into various applications compared to liquid-based electrochromic devices.
? Cost-effective: The use of readily available materials such as polyvinyl alcohol (PVA) makes the ECD fabrication process more affordable compared to devices relying on complex SPE formulations.

, Claims:WE CLAIM:
1. A fully solid-state electrochromic device (D) comprising:
? first glass substrate (G1);
? second glass substrate (G2);
? solid polymer electrolyte (SPE) film;
? tungsten trioxide (WO3) doped electrochromic layer (ECL);
wherein
? said solid polymer electrolyte (SPE) film is
- lithium acetate (CH3COOLi) doped polyvinyl alcohol (PVA) prepared using conventional solution-casting technique and distilled water as solvent,
- interposed between said first (G1) and second glass substrate (G2) is lithium acetate (CH3COOLi) doped polyvinyl alcohol (PVA) based solid polymer electrolyte (SPE) film interposed between said first and second glass substrate,
? said tungsten trioxide (WO3) doped electrochromic layer is deposited in form of a thin layer onto at least one Fluorine-doped Tin Oxide (FTO) coated glass substrate .

said device (D) being capable of integrating seamlessly into different designs, easily processable SPEs, exhibit rapid switching speeds for optimal performance and providing cost-effective solution with good mechanical strength and electrochemical stability without using plasticizers, costly solvents, or ceramic fillers,

2. The fully solid-state electrochromic device (D) as claimed in claim 1, wherein said first glass substrate (G1) is selected from but mot not limited to Flourine-doped SnO2 (FTO) or Sn doped In2O3 (ITO) coated glass substrate.

3. The fully solid-state electrochromic device (D) as claimed in claim 1, wherein said second glass substrate is selected from but mot not limited to (G2) is Fluorine-doped SnO2 (FTO) or Sn doped In2O3 (ITO) coated glass substrate.
4. The fully solid-state electrochromic device (D) as claimed in claim 1, wherein said WO3 films is fabricated by pulsed laser ablation (PLD) of a stoichiometric WO3 target.
5. The fully solid-state electrochromic device (D) claimed in claim 1, wherein said device (D) is switchable between a colored state and a bleached state upon application of a bias voltage.
6. The fully solid-state electrochromic device (D) claimed in claim 1, wherein said SPE film functions as both ion-storage layer and as ion-diffusing electrolyte layer, making the device (D) thinner.

7. The fully solid-state electrochromic device (D) claimed in claim 1, wherein said device’s (D) switching response time is less than one second.

8. A method for preparing the solid polymer electrolyte (SPE) film as claimed in claim 1, comprising the steps of:
? dissolving a measured amount of PVA in distilled water by magnetic stirring for 2-3 h at 70-80 o C.
? adding of Lithium acetate (CH3COOLi) to said PVA polymer solution and stirring for 2 hours.
? preparing PVA-CH3COOLi films with salt concentration selected from 7, 10, 15, 20, 25 and 30% by weight of PVA.
? obtaining a clear, watery, transparent solution devoid of air bubbles.
? casting the prepared solution onto a petri dish and maintaining said cast solution on the petri dish undisturbed for drying at room temperature for a period of one week; and
? removing the dried film from the petri dish for further characterization.
? characterizing said SPE film using DC voltammetry and AC impedance spectroscopy to determine ionic conductivity properties relevant to Electrochromic device (ECD) performance.
? obtaining the characterization results, selecting the SPE film with the most favorable properties for ECD performance and fabricating an ECD utilizing the selected SPE film,
wherein said solid polymer electrolyte (SPE) film is devoid of plasticizers, costly solvents, and ceramic fillers.

9. A method of fabricating fully solid-state electrochromic device (D) as claimed in claim 1, comprising the steps of:
? preparing first Fluorine-doped Tin Oxide (FTO) coated glass plate (G1).
? preparing second Fluorine-doped Tin Oxide (FTO) coated glass plate (G2).
? selecting best ionic conductivity showing SPE film .
? depositing a thin film of tungsten oxide (WO3) onto at least one Fluorine-doped Tin Oxide (FTO) coated glass substrate;
? disposing said SPE film between said first and second FTO glass, with said WO3 based electrochromic thin layer coated on one of said FTO glass; and
? sealing said device using glue to prevent moisture ingress.