PEDOT:PSS-V2O5 Hybrid for Color Adjustement in Electrochromic Systems

Electrochromism is known as a modulation of the optical properties under an applied voltage. Used in various applications, aside to the commercialized smart windows based on transmissive electrochromic devices (ECDs), the opaque systems have received significant interest for displays purposes. Herein, in a novel approach to adjust color, electrochromic oxides with high EC performance were chosen as additives to commercialized conductive and electrochromic polymer ink. More precisely opaque films were deposited (using Bar Coater) from mixtures of V2O5, synthesized by polyol process, with poly(3,4-ethylenedioxythiophene, polystyrene sulfonate) PEDOT:PSS commercial ink in different weight percentages (wt%) ranging from 0 to 100 (V2O5 wt%). Films thicknesses varied from hundred of nms to few μms. As-deposited films colors range from blue for PEDOT:PSS to orange for V2O5. PEDOT:PSS-V2O5 films exhibit significant electrochromic properties associated with a good electrochemical stability. In the series, the hybrid film with 90 wt%–10 wt% PEDOT:PSS-V2O5 shows a reversible behavior with significant reflectance modulation (ΔR≈20.5% at 550 nm) in lithium based electrolyte and a good contrast (ΔE∗ > 30). The maximum contrast was reached for 3.7 μm thick layer. Our results indicate that the hybrid film, combining the benefit of mixing inorganic and organic materials, are promising materials for enhancing electrochromic devices color and hybrid electrochromic materials processability on large scale.


INTRODUCTION
Electrochromic materials that undergo reversible color change under an applied voltage (Lang et al., 2019) have received great interest due to their potential applications, such as smart windows (Runnerstrom et al., 2014;Nunes et al., 2019), information storage and displays (Mjejri et al., 2018b;Qingyi et al., 2019). Electrochromism occurs in a number of organic (Hu et al., 2018;Han et al., 2019), inorganic (Danine et al., 2014;Mjejri et al., 2016), and hybrid materials (Rodrigues et al., 2009) both in the visible and IR regions. Smart windows mature technology is typically based on combination of WO 3 and IrO x or NiO (Granqvist et al., 2019). Optimizing both materials and devices architectures is interesting to develop the technology and target more application (Jung et al., 2004;Varghese et al., 2018;Danine et al., 2019). In particular, focusing on display applications, our interest in reflective devices increases with the need of enhancing the range of colors developed in ECDs. For inorganic materials, multi-electrochromism (e.g., materials displaying different colors depending on the voltage applied) is observed when vanadium oxide (V 2 O 5 ) is deposited as a micrometric thick film. This material switch colors from blue, to green and orange depending on the voltage applied. Vanadium oxides offer several advantages: (i) they show both anodic and cathodic colorations (ii) their abundance and layered structure facilitate the exchange of a large amount of Li cations, and (iii) the existence of several oxidation states leading to the possibility of obtaining multicolor displays (Chu et al., 2016;Mjejri et al., 2019). The modulation of color may also be achieved by introducing new concepts of devices changing color independently on each side. In 2017, our group (Mjejri et al., 2017) reported for the first time a double-sided electrochromic device based on Metal-Organic Frameworks (MOFs) switching from light to dark blue on one side and from yellow to brownish on the other side. Integrating organic and inorganic materials into a hybrid material provides an opportunity to combine their advantages and explore new design. While transition metal oxides (TMOs) present generally a good redox activity they suffer, among other things from a too low electrical conductivity and a poor switching frequency. On the contrary, polymer-based electrochromic films are characterized by rapid kinetics, multicolor electrochromism, easy-processability using conventional printing technique (doctor blade, screen-printing, inkjet. . .), sufficient electrically conductivity while they typically suffer from shorter lifetime range and are highly sensitive to the working environment (Byker, 2001). To counter balance the pros and cons of both polymer and inorganic materials, hybrid systems are investigated. Recently, several groups have synthesized hybrid films by combining oxide and conductive polymer showing a better charge injection for a higher contrast for electrochromic displays (Lee et al., 2011) and better efficiency for solar cells (Lee et al., 2014). Following this path, we recently reported that the addition of low content of Fe 2 O 3 in a PEDOT:PSS ink allowed to achieve a color modulation from bluish to reddish, while in first approximation Fe 2 O 3 alone appears to be electrochromically inactive (Levasseur et al., 2019). Pursuing this hybrid approach herein, the electrochromic properties of bar coated opaque films, from mixture of electrochromically active V 2 O 5 to commercial PEDOT:PSS ink, are investigated. V 2 O 5 was preferentially used as it can be bring a larger color range to the PEDOT:PSS film. Indeed, other cathodic oxides such a Nb 2 O 5 (Mjejri et al., 2018b) or WO 3 (Danine et al., 2014) will only intensify the blue color of the PEDOT:PSS reduced state.
The preparation of oxide inks remains a big challenge for an easy film processability. A poor control of thickness and morphology is generally achieved with traditional coating techniques. Mixing a commercial PEDOT:PSS ink with oxides allows to promote higher quality film. Indeed, with the mixture of PEDOT:PSS-V 2 O 5 presented in this paper, industrial technique such as screen-printing can be used for the layer deposition at room temperature and baked at the temperature applied for PEDOT:PSS (e.g., 120 • C for 5 min). Another advantage consists in reducing the amount of V 2 O 5 used to obtain a film as the oxide will only be added in small quantity in the PEDOT:PSS ink which can be beneficial for cost reason as this oxide requires expensive vanadium precursors. Vanadium oxide is generally deposited using vacuum techniques for good film quality, which are not suitable for large-scale display production. Other deposition techniques have been developed lately such as inkjet printing (Costa et al., 2012) or atomic layer deposition (Blanquart et al., 2013) and can be interesting for further study on V 2 O 5 electrochromic properties. With our approach, a conductive layer is obtained thanks to the presence of PEDOT:PSS conductive polymer which can possibly reduce the number of layers needed for building the electrochromic display (Tashtoush and Kasasbeh, 2013). In summary, the goal of this study was to investigate a win-win strategy based on the polymer good processability and conductivity and the large color palette brought by the oxide.

Hybrid Inks Formulation
The hybrid electrochromic inks were formulated from the mixture of the commercial PEDOT:PSS ink Agfa Orgacon EL-P5015 and V 2 O 5 powders synthesized by polyol process (Mjejri et al., 2018a). The commercial PEDOT:PSS ink characterized by high-viscosity commercial paste (>100,000 mPa·s) developed for screen printing was first homogenized with a three-roll mill and then diluted with ethanol to lower the viscosity. PEDOT:PSS was chosen thanks to its printing properties, its conductivity and it shows a very different reversible change of color from oxidized to reduced states making it suitable for electrochromism display fabrication (Andersson, 2006). The V 2 O 5 weight percentages were calculated from the PEDOT:PSS paste mass. Seven premixes were prepared: weight ratio of (PEDOT:PSS paste)/(V 2 O 5 ) = (100-x)/x, with x = 0.01%, 0.1%, 1%, 5%, 8%, 10%, 15%. These preparations were then diluted with ethanol with a mixing ratio of 40 wt% of PEDOT:PSS-V 2 O 5 paste and 60 wt% of ethanol. The various hybrid inks with different weight ratio between PEDOT:PSS, V 2 O 5 and ethanol were stirred for 15 min at room temperature, then dispersed using an ultrasonic bath for 30 min at room temperature and stirred again for 24 h.
On the figures, 100% have been drawn in ochre, 15% in green, 10% in orange, 5% in purple, 1% in red, and 0% in blue. All percentages correspond to the wt%.

Hybrid Film Deposition and Characterization
The films were deposited with a bar coater (K control from RK Print Coat Instruments, Erichsen) with jointed whorl bars, noted from 1 to 8 with the following whorl diameters (mm): 0.08, 0.15, 0.31, 0.51, 0.64, 0.76, 1, 1.27. The coatings were done on ITO-coated (In 2 O 3 :Sn) glass substrates (commercialized by SOLEMS with a resistance of 30 /sq) and then dried at 120 • C/5 min on a hot plate. Depositions were made on 5 * 2.5 cm 2 ITO-coated glass substrates. The thickness of the PEDOT:PSS-V 2 O 5 films were measured using a Veeco Dektak 6M Stylus Profilometer (Veeco Instruments Inc., New York, NY, United States). For the identification of the crystal phases of the hybrid films, X-ray diffraction (XRD, Philips PW 1820, PANalyticalX'Pert instrument, 2θ range from 10 • to 50 • and Cu Kα1 λ = 0.154056 nm) was used. The layer morphology was investigated with a JEOL JSM-840 (operating at 15 kV) scanning electron microscope (JEOL JSM-840, JEOL SAV-Europe, Croissy sur Seine, France).

Electrochromic Measurements
Electrochemical measurements of hybrid films PEDOT:PSS-V 2 O 5 deposited on ITO/glass were carried out in a three electrodes cell configuration using a BioLogic SP50 potentiostat/ galvanostat apparatus. The counter electrode and reference electrode consisted of a platinum foil and saturated calomel electrode, SCE (E SCE = 0.234 V/ENH), respectively. The operating voltage was controlled between −1.0 V and +1.5 V at a scan rate of 20 mV s −1 , in lithium-based electrolyte, namely, lithium bis-trifluoromethanesulfonimide (LiTFSI, Solvionic, purity N 99.99%) in 1-ethyl-3 methylimidazolium bis(trifluoromethanesulfonyl)-imide (EMIM-TFSI, purity N 99.99%) with a corresponding molar ratio of 1:9 (LiTFSI:EMIM-TFSI). All the electrochemical measurements were performed at room temperature. The ex-situ optical reflectance of PEDOT:PSS-V 2 O 5 hybrid films were measured using a Varian Cary 5000 UV-vis-NIR spectrophotometer between 300 and 800 nm. Colorimetry analysis was carried out using a Konica Minolta CM-700D spectrophotometer with SCE 10 • /D65 configuration (Konica Minolta Sensing Europe B.V), allowing the direct determination of colorimetric parameters of the CIE (L * a * b * ) color space.

Structural and Morphological Properties of PEDOT:PSS-V 2 O 5 Hybrid Films
To optimize the electrochromic performance of the hybrid PEDOT:PSS-V 2 O 5 films, the weight ratio between PEDOT:PSS and V 2 O 5 was investigated. Figure 1 shows the photographs of the various films deposited by bar coater using the same bar (n = 3) in association with the L * , a * , b * chromaticity parameters, defined by the CIE LAB. A significant color gradient in relationship with the decrease of the V 2 O 5 weight percentages is visible.
From 100 wt% V 2 O 5 yellow-orange film, the hybrid mixture remains close to the appearance of pure V 2 O 5 till 10 wt%. The color turns greenish for 5 wt% V 2 O 5 , and for the 1 wt% V 2 O 5 , the hybrid film adopts a light blue-gray aspect while the PEDOT:PSS films appear blue.
As displayed in Figure 2, using any bars (from n = 1 to 8), the V 2 O 5 wt% content has a strong influence on the film thickness. Overall, the thickness of hybrid films increases with increasing V 2 O 5 wt%.
If a low thickness increase is measured up to 5 wt% of V 2 O 5 , a first jump is observed for 10 wt% and 15 wt% while pure V 2 O 5 reaches thickness of 7 µm already for bar n = 3. Using the bar (n = 3) corresponding to Figure 1, the thicknesses of the films vary from 0.6 µm for pure PEDOT:PSS and 1 wt% V 2 O 5 , 0.8 µm for 5 wt% V 2 O 5 , 2.5 µm for 10 wt% and 15 wt% V 2 O 5 up to 7.2 µm for pure V 2 O 5 . The aspect and the chromaticity parameters of the films deposited with the bar n = 3 are relatively similar to the bar with a higher whorl diameter as the chromaticity parameters of the as-deposited state remain quite similar even when the film thicknesses increase.
The crystallinity of the hybrid films was studied by X-ray diffraction analysis (Figure 3). The PEDOT:PSS layer (Figure 3a) shows an amorphous behavior, whereas for the hybrid film with 1 wt% of V 2 O 5 a first peak of V 2 O 5 can be identified with a very weak signal associated with a low content (Figure 3b). For the 5 wt% V 2 O 5 hybrid film, the peaks intensities increase with the appearance of a second peak (Figure 3c). With the film containing 10 wt% V 2 O 5 , all the peaks can be indexed as V 2 O 5 (Space Group:Pmmm) (Figure 3d) revealing an agreement between color and cristallinity. Indeed, the d-spacing values of all diffraction peaks match the ones of the orthorhombic crystalline phase (JCPDS# 85-0601) and of the ITO substrate (JCPDS # 44-1087).
The SEM analysis of the hybrid films (V 2 O 5 , PEDOT:PSS and hybrid films PEDOT:PSS-V 2 O 5 ) (Figure 4) demonstrates that  the films morphology changes with the amount of V 2 O 5 . The PEDOT:PSS film appears smooth and homogenous (Figure 4a). The SEM micrograph of the hybrid composite film 99 wt%-1 wt% PEDOT:PSS-V 2 O 5 (Figure 4b), 5 wt% V 2 O 5 (Figure 4c), 10 wt% V 2 O 5 (Figure 4d) shows a homogenous dispersion of the V 2 O 5 particles in the PEDOT:PSS ink. Not surprisingly, the higher is the amount of V 2 O 5 , the rougher is the surface and the more particles are visible. The V 2 O 5 film appears homogenous and resembles to a collection of the sub-micronic particles (100 nm) of V 2 O 5 (Figure 4e). The V 2 O 5 particles seem to form bigger aggregate especially when the vanadium oxide amount increases. The 10 wt% V 2 O 5 and the 100 wt% V 2 O 5 films surfaces appear very similar. For the 10% hybrid film, the oxide particles seem to be embedded in the PEDOT:PSS which has an impact on the chromatic parameters as it will be mentioned later in the paper.

Electrochromic Properties of PEDOT:PSS-V 2 O 5 Hybrid Films
The electrochromic activity of the hybrid PEDOT:PSS-V 2 O 5 films was recorded in a lithium-based ionic liquid electrolyte. Figure 5 shows the cyclic voltammograms of single V 2 O 5 , single PEDOT:PSS and hybrid PEDOT:PSS-V 2 O 5 films deposited onto ITO and cycled, at a scan rate of 20 mV s −1 between −1 V and +1.5 V, in a three electrodes cell using ionic liquid based LiTFSI:EMIM-TFSI (1:9) as a supporting electrolyte and SCE as reference electrode, for thickness of 1.5 µm. The shape of the cyclic voltammograms adopts significant changes as soon as the addition of 0.1 wt% V 2 O 5 (Figure 5A). A small amount of V 2 O 5 induces a high reactivity associated with strong increase in current and featured CV illustrated by the presence of redox peaks both in anodic and cathodic sweeps ( Figure 5B). Higher peak resolution is achieved by cycling thinner films as reported in Figure 5B inset. The CV shape of a 1 wt% submicron film of 500 nm displays a specific signature with both anodic and cathodic distinguishable peaks. Such signature recalls the one observed for crystallized sputtered V 2 O 5 thin films (Benmoussa et al., 2002). These changes in the electrochemical behavior do not have a strong impact for film with a V 2 O 5 amount lower than 1 wt%. The comparison of the CV intensities indicates lower performances for hybrid films, PEDOT:PSS-V 2 O 5 as compared to pure V 2 O 5 film alone. Figure 6 shows the chronoamperograms recorded for 30 s at +1.5 V ( Figure 6A) and for 30 s at −1 V (Figure 6B). The CA was applied for five cycles, the second cycle is drawn for a film thickness of 1.5 µm. Starting from 1 wt% V 2 O 5 , the switching time increases with a higher current density. This trend well agrees with the faster kinetic reported for electrochromic conductive polymers as compared to oxides (Thakur et al., 2012).
In order to evaluate the electrochromic properties, the chromaticity parameters L * , a * , b * were measured during chronoamperometry cycles. These three parameters defined by    the CIE colorimetric space, are used to define and compare quantitatively the colors. The optical contrast ( E * ) can be deduced from these three chromaticity parameters thanks to equation (1) Where L * red , a * red , b * red represent the chromaticity parameters in reduced state and L * ox , a * ox , b * ox the ones in oxidized state, at a voltage of −1 V and +1.5 V for 30 s for each cycle, respectively. The optical contrast is determined for each film after two cycles of CA. Figure 7 displays the evolution of the optical contrast vs. the layer thickness for each wt%. A certain thickness is required to obtain a good change of color from the oxidized to the reduced state.
However, over a thickness of 4 µm, contrast decrease for all the films containing at least 10 wt% of vanadium oxide. The decrease in contrast for a thick layer may illustrate a limitation in the redox process that does not occur only on the surface of the film but remain limited to a certain thickness. For a high oxide amount (above 10 wt%) layer for the mixture between PEDOT:PSS and V 2 O 5 the film resistance increases also, which is in agreement with a longer color switching time.
In order to minimize the amount of V 2 O 5 for cost reason and to obtain a similar contrast value as for pure vanadium oxide, the ink formulation with 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 was chosen as the best candidate for a satisfying color change and a good thickness control.
Electrochromic Properties of 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 A more detailed study focusing on the 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 was then carried out to evaluate the impact of the thickness on the electrochromic performance. Figure 8 shows the CVs for various thicknesses, the electrochemical behavior of a submicron film is very different from the behavior of a film with a thickness of 2.5 µm and more ( Figure 8A). The current density value does not evolve much for films over this thickness. The same observation can be deduced from the CA curves ( Figure 8B) with furthermore, a shift toward higher current density value for layers above 2.5 µm for both oxidation and reductions parts after 20 s. This change in slope may indicate a different redox reaction on the surface of the film that may change with the amount of vanadium oxide. Figure 9 displays the evolution of the chromaticity parameters vs. thickness for the initial (i.e., as-deposited), oxidized (+1.5 V) and reduced (−1 V) states of 100 wt% PEDOT:PSS (in blue), 100 wt%V 2 O 5 (in ochre) and a 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 (in orange). The hybrid PEDOT:PSS-V 2 O 5 and pure V 2 O 5 exhibit similar trend with only a small shift toward lower values of the L * parameter for the three states (Figure 9A), more than five points lost for the a * parameter (Figure 9B) and again a shift toward lower values for the b * parameter ( Figure 9C).
The presence of PEDOT:PSS, responsible of a slightly darker color, has a definitive impact on the b * parameter. The shift  FIGURE 10 | Visual comparison between the colors in the oxidized and reduced states for 0 wt% V 2 O 5 , 100 wt% V 2 O 5 , and 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 vs. thickness (µm) during CA after three cycles and contrast value E*. Initially, the corresponding curve follows the same trend and gives to the mixture a bluer color for low thickness especially in the reduced state. However, and as already observed on the electrochemical behavior, for film above 2.5 µm the V 2 O 5 behavior takes the lead and has the strongest impact on the chromaticity parameters.
On the Figure 10 are gathered, the colors in oxidized and reduced states of the 0 wt% V 2 O 5 , 100 wt% V 2 O 5 and 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 films for various thicknesses. For a given thickness, the left picture illustrated the oxidized state (E = +1.5 V) and the right one the reduced state (E = −1 V). The pictures together with the chromaticity parameters were recorded during CA measurements. In agreement with the Figure 8B, the thicker is the layer, the harder the current density is driven to a value close to 0. Therefore after few cycles, the greenish layer of the surface is correlated with an irreversible reaction from V 5+ to V 4+ with the Li + insertion.
When comparing the CA curves (not shown here) between 0 wt% V 2 O 5 , 100 wt% V 2 O 5 , and 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 , it could be noticed that the 0 wt% V 2 O 5 corresponds to a very fast kinetic with less than 1 s for color change. 10 wt% V 2 O 5 adopts a visual change in 5 s while it takes at least 15 s to observe a change of color from the oxidized to the reduced state for 100 wt% V 2 O 5 . As expected the changing color mechanism for PEDOT:PSS is much faster than the one of crystallized V 2 O 5 . Besides the presence of the polymer helps to reduce the time necessary to change the color from oxidized to reduced states (and vice-versa) without affecting the optical contrast when only 10%wt V 2 O 5 is added to the polymer ink.
Aiming at a better investigation of the optical properties, the reflectance spectra was recorded on the film with the highest contrast between the oxidized and reduced states for the mixture 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 (e.g., a film thickness of 3.7 µm) (Figure 11). The reversible color changes of hybrid composite films between the two states are associated with reflectance values of about 8.3% for the reduced state and 28.9% for the oxidized state at 550 nm, respectively, leading to an optical reflectance modulation, R, of 20.6%.
The electrochromic performance are characterized thanks to the optical density (OD) which can be calculated directly from the reflectance spectra and represents the logarithm of the ratio FIGURE 12 | Illustration of the memory-effect by comparison between the standard PEDOT:PSS and 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 with chromaticity parameters taken at the initial stage, at E = -1 V and after 3 months after applying first E = -1 V, leaving the samples rest at room temperature and ambient atmosphere without any cycle in between.
of the reflectance at the oxidized state (Rox) to the reflectance at the reduced state (Rred) as shown on the Equation 2.
A OD of 0.54 is estimated at 550 nm. The color efficiency (CE) can be calculated with the charge transferred per unit area (Q) deduced from the CA curves.
In this case, Q = 1.85 C.cm −2 and CE obtained equals to 0.29 cm 2 .C −1 . This rather low value may be explained by a measurement of the capacity in chronoamperometry mode rather than in dynamic mode using cyclicvoltammetry. Besides, there is a lack of comparison with other hybrid systems mixing two electrochromic materials as a single layer.
Memory Effect of 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 Film Figure 12 gathers the chromaticity parameters of the 0 wt% V 2 O 5 and the 90 wt%-10 wt% PEDOT:PSS-V 2 O 5 films for different thicknesses recorded as initial state, in reduced state (−1 V), and after resting the film for 3 months. No voltage was applied meanwhile and films were stored at ambient atmosphere and room temperature. The PEDOT:PSS film goes back to its most stable state exhibiting no memory effect with a very low value difference between initial and 3 months values. On the contrary, as reported by our group in a prior work, V 2 O 5 shows a strong memory effect (Mjejri et al., 2018a). After 3 months, if the chromaticity parameters exhibit different values as compared to the first applied voltage at E = −1 V they also display a big difference with the initial data. From a blue-green film at E = −1 V, a green film is visible after 3 months. The mixture of 10 wt% V 2 O 5 in the PEDOT:PSS induced a memory-effect.

CONCLUSION
In this paper, hybrid composite films with different compositions between a polymer ink of PEDOT:PSS and V 2 O 5 synthesized by the polyol process were successfully deposited with a bar coater technique. The influence of the film thickness according to the amount of vanadium oxide in PEDOT:PSS-V 2 O 5 hybrids was studied via electrochemical techniques and surface morphology.
The introduction of 10 wt% V 2 O 5 with a film thickness of 3.7 µm is sufficient to mimic the behavior of pure V 2 O 5 change of color with a faster reversible color change from oxidized to reduced states. Indeed 10 wt% V 2 O 5 are sufficient to gain color tuning, significant optical contrast and memory-effect suggesting that there is no need to mix further inorganic material in the polymer. Our approach is a promising route to improve the electrochromic properties including the tuning of color of displays based on PEDOT:PSS-based ink in a very simple way, combining advantages of V 2 O 5 inorganic and PEDOT:PSS polymer materials. The memory-effect was also improved for hybrid films as compared to single PEDOT-PSS. Furthermore, dispersing the vanadium oxide in a polymer ink reduces the overall toxicity, the cost, improves the process control and allows the use of the hybrid in industrial technique such as screen-printing which is promising for developing large-scale electrochromic displays.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/supplementary material.

AUTHOR CONTRIBUTIONS
RF, IM, and HR conducted the investigation and methodology. AR supervised the project. RF wrote the original draft.

FUNDING
This activity included in the SUPERSMART project has received funding from the European Institute of Innovation and Technology (EIT) (Grant Agreement: 17161). This body of the European Union receives support from the European Union's Horizon 2020 Research and Innovation Program. RF wishes to thank the financial support of Luquet-Duranton.