Multicolor Electrochromics: Rainbow-Like Devices - ACS Applied

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Multicolor Electrochromics: Rainbow-like Devices Yolanda Alesanco, Ana I. Vinuales, Jesús Palenzuela, Ibon Odriozola, German Cabañero, Javier Rodriguez, and Ramón Tena-Zaera ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01911 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on May 29, 2016

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ACS Applied Materials & Interfaces

Multicolor Electrochromics: Rainbow-like Devices Yolanda Alesanco, Ana Viñuales*, Jesús Palenzuela, Ibon Odriozola, Germán Cabañero, Javier Rodriguez and Ramón Tena-Zaera*. Y. Alesanco, Dr. A. Viñuales, Dr. J. Palenzuela, Dr. I. Odriozola, G. Cabañero, Dr. J. Rodriguez and Dr. R. TenaZaera, IK4-CIDETEC Research Center, Paseo Miramón 196, 20009 Donostia – San Sebastián (Spain) KEYWORDS. Multi-electrochromics, smart-windows, PVA gels, polyelectrolytes, viologens

ABSTRACT: Stimuli-responsive reversible coloration-change materials represent a highly demanded type of smart systems useful for a wide variety of applications, with a significant growing interest on multi-color abilities. In particular, electrochromic materials have received a great deal of attention due to their versatility and broad range of industrial uses. However, most of the existing electrochromic technologies provide a single coloration, while achieving multiple colors based on simple approaches remains a challenge. The present article reports on PVA gel-based electrochromic devices, containing a single viologen, providing a colorless and two different well-defined colored states. The successful fabrication of a device, based on two viologens (multi-EC gel) with a simple architecture (glass/TCO/multi-EC gel/TCO/glass), with five different multi-switchable colors based on four-zoned electrodes (rainbow-like ECD) is also demonstrated. This novel easy-to-make multi-chromic system represents a significant breakthrough towards the generation of full-color devices, expanding the potential of the electrochromic technology.

1.

INTRODUCTION 1

Chromogenic technologies provide a variation in the optical properties of a material in response to external stimuli such as light (photochromic),2 thermal (thermochromic),3 or electrical changes (electrochromic,4 electrophoretic suspended particles,5 and polymer dispersed liquid crystals6 among others). These technologies are attracting significant attention, being useful for smart glazing, creative industry, adaptive camouflage (variable structural coloration)7 and many other applications. Among the electrically-driven optical switchable technologies, electrochromism is the most popular owing to some major advantages such as requiring power only during switching, exhibiting adjustable memory and being the most efficient for thermal and lighting control. In this context, electrochromic (EC) materials have been subjected to intense investigation because of their potential applications such as displays,8 including electronic paper,9 smart windows for energy-efficient architectures 10 or the well-known viologen-based anti-glare rearview mirrors in vehicles developed by Gentex.11 Despite the significant improvements achieved in the last 30 years with regard to the electrochromic properties of materials, a further challenge was to find electrochromic devices (ECDs) that exhibit multi-electrochromic behavior. Multielectrochromism may significantly extend the market opportunities in different fields such as displays, smart windows and climate adaptive building shell (CABS),12

due to their aesthetic properties providing diverse colorations over time under different applied potentials. There are many reports dealing with the multielectrochromism principle and preliminary proof-ofconcept systems.13-15 However, most of the proposed multi-electrochromic devices do not show sufficiently diversified colors in their different colored states which may limit their practical use in the above described applications. The most performing strategies to obtain multielectrochromic devices are based on the integration of different electrochromic materials.16-18 Fabrication strategies include different device architectures such as dualtype (i.e. complementary anodic and cathodic EC materials19, 20) and patterned-based devices19, 21, 22 (e.g. pixel configuration: Nanochromics TM displays developed by NTERA, Ltd 22). However, additionally to the complexity in the device preparation (e.g. larger number of layers,17, 19, 20 multi-electrode configurations,23, 24 or the need of complex patterned electrodes), the color versatility of these ECDs may be significantly limited by the single coloration of the mono-electrochromic materials generally used.25 Many approaches have also been developed with the aim of diversifying the colors of a single EC material. In this context, ambipolar materials26, 27 and the conjugated polymers have been widely studied as promising multielectrochromic materials through their band gap control by structural modification.18, 19, 21, 28 However, EC polymers often exhibit a lack of a sufficiently colorless bleached

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state and slow response time in comparison to several single molecular materials, such as viologens.15 The latter exhibit three common redox forms: i) the dication, which is the most stable, ii) the radical-cation, formed by one electron reduction of the dication, and iii) the neutral state (di-reduced form), obtained by two electron reduction of the dication. The colorless and highly colored aspects of dication and radical-cation forms respectively, are commonly exploited in mono-electrochromic devices.29 Nevertheless, the consideration of di-reduced species, whose behavior and stability are relatively unknown, may open wide avenues for developing viologenbased multi-electrochromic materials. In this regard, the development of polyelectrolyte systems which could stabilize such species would offer new opportunities. Recently, polyvinyl alcohol (PVA)-borax gel has been demonstrated as an ideal polyelectrolyte to obtain highperformance and easy-to-make ECDs.30 The viscoelastic fluid character of these polyol-based gels first developed by Mattel Toy Corporation under the trade name “Slime”, results very convenient for the assembly of the devices while keeping fast response and good cyclability. The present communication reports on PVA gel-based dual-electrochromic devices, containing a single viologen, providing a colorless and two different well-defined colored states. Furthermore, the fabrication and characterization of a multielectrochromic device, based on a formulation containing two viologens (multi-EC gel) and a very simple configuration (glass/TCO/multi-EC gel/TCO/glass), which provides five well-defined colorations is described. The possibility of obtaining five different multi-switchable colors in different areas of the same device (rainbow-like ECD) is also demonstrated. Furthermore, the potential of the multi-EC gel in ECDs with a colored state with very low transmittance in whole visible is also discussed. 2.

EXPERIMENTAL SECTION

Materials: Polyvinyl alcohol (PVA, Mw 61000), sodium tetraborate (borax, 99.5%), potassium ferrocyanide (98.5%), potassium ferricyanide (99%) and 1,1’-diethyl-4,4’-bipyridinium dibromide (ethyl viologen dibromide, 99%), were purchased from Sigma-Aldrich and used without further purification. Fluorine-doped tin oxide (FTO) coated glass substrates (TEC, Rs 6-8 Ωsq-1) and Tin-doped indium oxide glass substrates (ITO Sol 30/1.1, Rs 25-35 Ωsq-1) were supplied by Solems and cleaned with warm acetone prior to use. Synthesis: 1,1’-bis-(p-cyanophenyl)-4,4’-bipyridilium dichloride (p-cyanophenylviologen dichloride) was synthesized according to previously reported procedure with minor modifications (Supporting Information). 29 1H NMR (500 MHz, DMSO-d6, δ): 9.77 and 9.18 (d of d, 4H and 4H, Ar H), 8.35 and 8.23 (d of d, 4H and 4H, bipyridine); IR (bulk ATR): ν = 3091 (C-H), 2227 (-CN), 1629, 1603 (C=C, C=N), 830 cm−1 (o-phenylene H). Methods:

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UV/Vis spectra are obtained in transmission/Absorption mode on a Jasco V-570 spectrophotometer using air as the background. The spectra were registered using a films holder accessory for solid samples, while the devices were connected to Biologic MPG potentiostat-galvanostat as a direct current source. Before recording the spectra measurement of the ECDs (%T or Abs as a function of the wavelength), the devices were exposed to the same voltage during 40 seconds with the aim of reaching the maximum colored state before starting the measurement. Color coordinates of the devices were determined using CIE 1976 L*a*b color space as quantitative scale to define the color, where L* represents the lightness of the color (L* = 0 yields black and L* = 100 indicates diffuse white), a* axis corresponds to the position between green and red (with green at negative a* values and red at positive a* values) and b* indicates the position between blue and yellow (with blue at negative b* values and yellow at positive b* values). Color coordinates were obtained using spectrophotometric method with the same spectrophotometer mentioned above. following the procedure developed by R. J. Mortimer and T. S. Varley.37, 38 The spectroelectrochemistry study was carried out with the same potentiostat-galvanostat mentioned above. Cyclic voltammograms (CV’s) were recorded for each gel at scan rates of 30 mV/s. Fourier transform infrared (FT-IR) spectra were recorded on a 4100LE FTIR from Jasco. The IR spectra were obtained using Attenuated Total Reflectance (ATR) technique on the pure solid. 1

H NMR spectra was measured on a Bruker 500 MHz spectrometer in DMSO-d6, using tetramethylsilane as an internal reference. Preparation of EC gels (general procedure): all gels were prepared as follows: to a previously prepared 4% solution of PVA, aqueous solutions containing appropriate amounts of viologen and a 1:1 mixture of potassium ferrocyanide and potassium ferricyanide were added. The resulting mixtures were stirred until homogeneous solution was obtained. Then, each of these solutions were mixed with a 4% borax aqueous solution in a 4:1 volumetric ratio by vigorous stirring with a spatula, until the formation of a gel. The formulations were left to settle on its own until a completely bubble-free materials were obtained. Preparation of EC gels with varying concentration of p-CV viologen: A series of nine electrochromic gels were fabricated by varying concentrations of pcyanophenylviologen dichloride from 0.25 to 4.0 mmolL-1 (0.25, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mmolL-1) while keeping the concentration of ferro/ferricyanide potassium pair at a constant value of 0.4 mmolL-1. Preparation of EC gels with varying concentration of ferro/ferricyanide potassium salts: A series of ten electrochromic gels were fabricated by varying concentrations of ferro/ferricyanide potassium pair from 0.2 to 5.0 mmolL-1 (0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0 mmolL-1) while keeping the concentration of p-


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cyanophenylviologen dichloride at a constant value of 3.5 mmolL-1. Preparation of p-CV gel: This electrochromic gel was fabricated keeping the final concentration of pcyanophenylviologen dichloride at 3.5 mmolL-1 and the concentration of the ferro/ferricyanide potassium pair at 4 mmolL-1 value. Preparation of EtVio gel: This electrochromic gel was fabricated keeping the final concentration of ethyl viologen dibromide at 20 mmolL-1 and the concentration of the ferro/ferricyanide potassium pair at 6 mmolL-1 value. Preparation of Ferro/Ferri gel: This gel was fabricated keeping the final concentration of the ferro/ferricyanide potassium pair at 4 mmolL-1 value without adding any viologen. Preparation of Blend gel and Blend gel-2: These electrochromic gels were fabricated by mixing EtVio gel and p-CV gel in 2:1 and 1:1 weight ratios respectively, followed by vigorous stirring with spatula. The formulations were left to settle on its own until a completely bubble-free materials were obtained. Fabrication of Two-electrode Electrochromic Devices: The electrochromic devices were prepared according to previously reported procedure.30 The EC gel was spread on the FTO-coated side of one of the substrates provided with a 220 µm double-side adhesive tape frame used as spacer. Then it was covered with the other electrode substrate applying light pressure and both electrodes were clipped using paper clip clamps. The EC gel flows and adapts to the shape of the electrodes providing uniform films with excellent contact to the upper and lower electrodes.

device. Similarly laser scribed substrate was employed as counter electrode (CE). The following steps required for device assembling were similar to those indicated for twoelectrode electrochromic devices. 3.

RESULTS AND DISCUSSION

PVA gel-based electrochromic devices with a single viologen, 1,1’-bis-(p-cyanophenyl)-4,4’-bipyridilium dichloride (p-CV, Figure 1a) were investigated using a conventional two-electrode configuration (Figure 1b). For this type of aryl substituted viologen, green color is expected for the radical-cation form. Potassium ferrocyanide and ferricyanide salts were employed as complementary redox species. As previously reported in other viologen-based devices (i.e. gel-based,30 and also aqueous liquid systems31), faster switching times were obtained as the amount of the complementary redox species increased, since potassium ferrocyanide exhibits redox potential close to that of the viologen compounds.31 After systematic optimization study of the gel formulations so as to obtain high-performance of the device in terms of optical contrast (Table S1) and switching time (Figure S1), the formulation named p-CV gel (3.5 mmolL-1 of viologen and 4.0 mmolL-1 of redox species) was selected for the entire study and characterization.

In the case of aqueous liquid systems employed for comparison, the electrochromic formulations were introduced in a previously assembled device by surface capillarity. Fabrication of Three-electrode Electrochromic Devices: The electrochromic devices were prepared as follows: 5 cm x 2.5 cm ITO coated glass substrate was electrically insulated by laser scribing into two sections, having an approximate active area of 5 cm x 2 cm and 5 cm x 0.5 cm. Ag ink, purchased from Acheson (Electrodag PF410), was employed to draw a layer on the 5 cm x 0.5 cm section which was employed as pseudo-reference electrode after being dried at 90 oC (RE). The wider section of the same substrate was employed as a working electrode (WE), while other ITO-coated glass substrate (without any laser treatment) was used as counter electrode (CE). The following steps required for device assembling were similar to those indicated for two-electrode electrochromic devices. Fabrication of Rainbow-like Electrochromic Devices: The rainbow-like electrochromic device was prepared as follows: 5 cm x 5 cm ITO coated glass substrate was electrically insulated by laser scribing into four different sections having all of them an approximate active area of 5 cm x 1 cm. Each section was employed as stand-alone working electrode (WE) for rainbow-like electrochromic

Figure 1. a) Chemical structure of p-CV viologen; b) Schematic illustration of the two-electrode ECD configuration including electrode substrates (1), spacers (2) and EC gel (3); c) Photographs of an electrochromic device containing p-CV viologen in its bleached state (I) and colored states at switching voltages of ─1.4V (II) and ─1.8V (III).

Interestingly, in addition to the green color shown by pCV and observed upon applying ─1.4V voltage (Figure 1cII), red coloration occurred as a result of applying more



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cathodic potential of ─1,8V (Figure 1c-III). Although the second reduction of p-CV was previously discussed in aqueous liquid systems,31, 32 no red coloration was reported. Furthermore, limited electrochemical reversibility and insolubility in water was suggested for the di-reduced (i.e neutral state) form of p-CV.31, 32 Electrochromic properties of p-CV gel such as optical contrast (Δ%T), switching time for the colored and bleached steps (tc and tb), coloration efficiencies (η), as well as color coordinates obtained by spectrophotometric method, are summarized in Table 1 for both applied potentials (─1.4V and ─1.8V). Additionally to the competitive optical contrast (i.e. ~ 60 %), it is worth noting that switching times are in the range of ECDs having gel or solid electrolytes.33, 34 The corresponding color interpretation of L*a*b coordinates is given in the Supporting information Figure S2. In order to characterize the electrochemical behavior of the p-CV gel and gain further insights into the origin of the red coloration, a new configuration of three-electrode ECD, enabling in situ spectroelectrochemical study, was designed to correlate the electrochemical behavior with color variation. It should be noticed that the main drawback of the vast majority of ECDs used so far is their lack of a reference electrode, thus not allowing consistent electrochemical characterization. In this study, Ag/Ag + redox couple based pseudo-reference electrode was incorporated and electrically insulated from the working electrode by laser scribing. A schematic illustration of the resulting substrate can be found in Figure 2a. It is worth noting that this three-electrode ECD architecture presents some advantages in comparison to previously reported ones,35, 36 which were based on stacked electrodes and separating insulating layers, which may cause unwanted loss of transparency and significant changes in the device thickness. Figure 2a shows the cyclic voltammetry of three-electrode ECDs containing p-CV gel, as well as a water solution containing the same amount of viologen (p-CV) and redox pair. In the potential range from 1.10 to ─1.45 V, three well-defined peaks could be distinguished on the cathodic sweep of the scan of both systems. The first reduction peak (~ -0.2 V) was assigned to the uncolored redox process of ferricyanide ion as demonstrated by CV and UV/vis transmittance responses of three-electrode ECD containing Ferro/Ferri gel (Supporting Information Figure S3). The second reduction peak (~ -0.5 V) was correlated with the green radical-cation (p-CV+.) formed by one-electron reduction of the dication (p-CV2+), while the third reduction peak (~ -0.9 V) was related to the direduced form (p-CV0) obtained by one-electron reduction of the radical-cation. Figures 2b and 2c display the visual appearance and UV/Vis transmittance spectra of devices under different potentials (i.e. 0, ─0.5 and ─0.9 V) respectively. The color coordinates obtained by spectrophotometric method, proposed by Mortimer’s group,37, 38 for devices based on both systems at two applied potentials are represented in the CIE color space plots (Figure 2d).

Figure 2. p-CV gel vs reference aqueous liquid system in three-electrode electrochromic device configuration. a) Cyclic voltammogram and schematic illustration (inset) of the working electrode (i) and pseudo-reference electrode (iii) of the device, along with the laser scribing which insulated both (ii); b) Photograph of the devices in their bleached state (left), colored state after applying switching voltages of ─0,5V (middle) and ─0,9V (right) of p-CV gel (I – III) and aqueous liquid system (IV – VI); c) UV/Vis transmittance response in bleached state and upon applying ─0.5V and ─0.9 V voltages; d) CIE color space plots representing the color coordinates of devices containing p-CV gel (left) and aqueous system (right) upon applying a ─0.5 V (1) and ─0.9 V (2).


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Table 1. p-CV gel: Electrochromic properties and color coordinates (D65) CIE L*a*b.

Potential (V)

%Tb

%Tc

Δ %T

(a)

tc (s)

(b)

tb (s)

(c)

2

-1 (d)

η (cm C )

L*

(e)

a*

(e)

b*

(e)

-1.4

68

7

61

16

8

78

62

-44

27

-1.8

61

2

59

14

4

83

29

38

-15

a)

Optical contrast: (%Tb - %Tc) being Tb = transmittance at bleached state and Tc = transmittance at colored state.

b), c)

Switching times: time required for 90% of the total transmittance change to occur at the maximum contrast wavelength for the colored (tc) and bleached steps (tb). d)

Color efficiency: [(ΔOD = log (Tc/Tb))/ (Q/A)] being A = device area and Q = injected/ejected charge.

e)

Color coordinates obtained by spectrophotometric method.

As shown in Figures 2b and 2d, the ECD containing p-CV gel exhibited green coloration when the external voltage was set at ~ -0.5 V, and the color turned into the red upon applying external voltage of ~-0.9 V. By comparing the UV/Vis spectra (Figure 2c), different absorption profiles were observed in the case of p-CV gel. When the applied potential was set at ~ -0.5 V, the ECD exhibited two strong absorption bands (i.e. transmittance valleys in Figure 2c) at around 420 and 600 nm, characteristic of the green materials that absorb both the blue and red regions of the electromagnetic spectrum. When a more cathodic potential of ~ -0.9 V was applied, the transmittance was quenched (indicating very significant absorption) around 500 nm and the ECD color turned into the red. After the potential was set again to 0 V, the ECD turned back to its original colorless form, showing reversible response. Nevertheless, in the case of reference aqueous liquid systems, the results revealed that upon applying a switching voltage of ~ -0.9V, the device showed mainly dark green coloration (Figure 2b and Figure 2d) and the corresponding transmittance profiles (Figure 2c). These results, also detected in the two-electrode configuration (Figures S4, S5 and Table S2 corresponding to the aqueous liquid system, and Figure S6 to the p-CV gel), showed that even though the applied potential was sufficiently negative for di-reduction to occur, the red coloration was not observed in the aqueous liquid system. This can be attributed to the comproportionation mechanism, which is more likely to happen in the aqueous liquid system. The electron transfer interaction between the di-reduced species (p-CV0) and the dication (p-CV2+) would result in the generation of the radical-cations (p-CV+.) and consequently the green color observed in the device. The comproportionation has been demonstrated to take place in other systems through ESR (Electron Spin Resonance) techniques,39 frequently followed by the formation of the electro-inactive radical-cation dimers ((p-CV+.)2).40 It has been scarcely reported that di-reduced bipirydilium species with aryl substituents can provide scarlet-red colors due to conjugation throughout the whole molecule, but

they appear to be very unstable owing to their powerful reducing properties.29 Related to these matters, it has been proposed that one way of preventing this issue could be the inclusion of the viologen within the cavity of a cyclodextrin sugar or polymeric matrix.29, 32 In a similar manner, the viscosity of the present PVA gel-based polyelectrolyte seemed to avoid the di-reduced species and the dications to meet, preventing the comproportionation process and consequently making possible the observation of the red di-reduced species upon a applying suitable voltage of ─0.9 V. For further investigations, aqueous formulation containing the same amount of viologen (pCV), redox pair and PVA as p-CV gel without borax, and therefore with much lower viscosity, were studied (Figure S7) and the red coloration was not observed. Interestingly, when higher concentrations (i.e. 10 mmol) of viologen were used in the p-CV gel while maintaining the rest of the components at the same concentrations, red coloration was not detected, being this fact consistent with the comproportionation mechanism proposed, since the direduced species (p-CV0) and the dications (p-CV2+) are too close together. These results revealed that the presence of the PVA gel polyelectrolyte acts as a stabilizer enabling for the first time dual electrochromic behavior devices based on this viologen, while greatly simplifying the assembly process. Taking advantage of the dual electrochromic behavior observed in p-CV gel and with the purpose of finding ECDs that exhibit multi-electrochromic behavior, new PVA gel-based electrochromic formulations including two different kinds of viologens were investigated. These new formulations incorporated 1,1’-diethyl-4,4’-bipyridinium dibromide (ethyl viologen dibromide or EtVio), which provide bluish colorations at suitable external voltage,29, 30 in combination with p-CV as electrochromic materials, while potassium ferrocyanide and ferricyanide salts were employed as complementary redox species. Two multi-gel formulations (Blend gel and Blend gel-2) were prepared by mixing appropriate amounts of previously prepared pCV gel and EtVio gel.



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The electrochemical behavior of a three-electrode ECD based on the Blend gel was investigated by cyclic voltammetry (CV) (Figure 3a). In the potential range from 0 to ─1.7 V, five well-defined features were observed during the cathodic sweep. As shown in the inset and Figure S8, there was good agreement between these features and the sum of those detected in devices based on the single-gels p-CV and EtVio gels separately. The first reduction peak (~ -0.2 V) was assigned to the reduction of ferricyanide ion while the second peak (~ -0.6 V) and the third (~ -0.8 V) were correlated with first and second reduction of the p-CV respectively which is consistent with the observed in the Figure 2a for p-CV gel and explained above. The fourth and fifth peaks observed (~ -1.2 and ~ -1.6 V) were related to the radical-cation of the EtVio (EtVio+.) formed by one-electron reduction of the dication (EtVio2+) and the di-reduced form (EtVio0) obtained by one-electron reduction of the radical-cation, respectively, also in agreement with the observed for EtVio gel. As no significant overlap in the redox potentials of the p-CV and EtVio was detected in the CV of the Blend gelbased ECD, the latter was expected to behave as a multielectrochromic device or even provide new colorations when their absorption profiles are coupled due to a mixture of pre-reduced states. Thus, the multielectrochromic behavior encompassing five different colorations: colorless (0 V), green (~ -0.7 V), pink-violet (~ -0.9 V), orange (~ -1.1 V) and purple (~ -1.7 V) was visually confirmed (Figure 3b). The absorbance spectrum (Figure 3c) of the three-electrode ECD after applying a switching voltage of ~ -0.7 V was characteristic of green materials with two absorption bands at around 420 and 600 nm. As more cathodic potential of ~ -0.9 V was applied, the ECD exhibited broader band with its maximum located at 520 nm, which is in good agreement with the observed violet coloration. For a more negative potential of ─1.1 V the absorption band showed hypsochromic shift to give a maximum absorption band at 460 nm, characteristic of orange materials. When a more cathodic potential of ─1.7 V was set, ECD exhibited strong absorption band at around 520 and a new band grew up from 415 to 380 nm where the spectra ends. The color coordinates were determined by spectrophotometric methods (Supporting Information Figure S9) and are represented in the CIE color space plots in Figure 3d. The reproducibility and the electrochemical reversibility of Blend gel was assessed by measuring color coordinates of three ECDs at each potential (Table S3) and evolution of CV upon cycling (Figure S10). On the basis of the results, Blend gel system provides highly reproducible and electrochemically reversible multi-ECDs with welldefined colorations. Therefore, these multielectrochromic PVA gel systems contribute to address the above mentioned shortcomings frequently present in the multi-EC systems, such as the device complexity or not distinct enough coloration in their different redox states, among others.

Figure 3. Multi-color three-electrode electrochromic device containing Blend gel: a) Cyclic voltammetry (CV) of Blend gel and comparison with CVs of p-CV gel and EtVio gel separately (inset). b) Observed coloration from top to bottom and c) absorption profiles after applying a switching voltage of 0 V (colorless) , ─0.7 V (green), ─0.9 V (pink-violet), ─1.1 V (orange) and ─1.7 V (purple) respectively; d) CIE color space plots representing color coordinates obtained at off state (1) and upon applying a switching voltages of , ─0.7 V (2), ─0.9 V (3), ─1.1 V (4) and ─1.7 V (5).

Going one step further, an easy-to-make multielectrochromic device based on zoned electrodes (hereafter named rainbow-like ECD) was also developed. It should be noticed that the fabrication process of rainbowlike ECDs was relatively straightforward, it only being necessary to sandwich a single layer of Blend gel between two ITO-coated glass substrates previously zoned in rows by laser scribing. As shown in Figures 4 and S11, each zone exhibits multi-electrochromic behavior, enabling the adjustment of their color independently and changing over time under suitable applied potentials according to the needs. This achievement could extend the market opportunities for electrochromics in different fields such as full-color displays, smart windows, and emerging climate adaptive building shell (CABS),12 since the building sector is showing an increasing demand to develop highly energy-efficient and pleasant spaces, including adaptive system designs.


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Figure 4. Photographs of rainbow-like electrochromic device containing Blend gel. Multi-electrochromic device provided with zoning design and colors observed in each row of the device while the suitable voltage is being applied.

The rainbow-like ECDs developed here exhibit the ability to change not only the tint level but also the color over time under different applied potentials filtering different wavelengths of the visible region. This represents significant novelty and performance enhancement versus the current state of the art smart windows which provide different intermediate tin levels, but always of the same bluish color.41, 42 Windows and zoning strategies made of this technology could enhance the aesthetics and enable users and operators to optimize light transmission, thermal and visual comfort, adjusting to location and needs of the occupants. In addition, as shown in the video provided as Supporting Information (Video S0), the rainbowlike ECD can be programmed to function in customized segments. Apart from the application in multi-color devices, the developed two-viologen PVA-borax-based formulations may have significant impact in other electrochromic applications such as visible light filters. As an example, ECD with very low transmittance in most of the visible range were also fabricated by using Blend gel-2 (Figure S12). The reached transmittance was as low as to that previously reported by using more complex approaches such as those based on 3 viologens,43 or following different strategies such as double layered working electrode and counter electrodes,20 or two working electrodes and two counter electrodes.23 4.

CONCLUSIONS

In conclusion, PVA-borax-based systems enabling the development of easy-to-make multichromic devices are described. Single viologen-based two-colorelectrochromic devices, providing two different and welldefined colored states (i.e. green and red in addition to the colorless state), have been reported. Furthermore, the successful fabrication of a multi-electrochromic device, based on a formulation containing two viologens (multiEC gel), providing five well-defined colorations is demonstrated. The resulting devices showed multielectrochromic behavior including colorless, green, pinkviolet, orange and purple, involving just a single EC layer sandwiched between two electrodes following the next configuration: glass/TCO/Multi-EC gel/TCO/glass. In

addition, the proof-of-concept of a rainbow-like ECD, based on a simple zoning design, has been demonstrated. Finally, the potential of these multi-EC devices to reach very low transmission in most of the visible wavelength range enabling an effective blocking of the whole visible radiation in smart windows is also discussed. These results may have broad impact in the field of displays, smart windows and climate adaptive building shell (CABS) and clearly indicates that viologen-based EC gel offers many advantages over other multi-electrochromic devices previously reported and could be a potential candidate for easy-to-make multi-electrochromic or full-color electrochromic devices, which bring us closer to rainbow-like windows.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.” Optimization study of the p-CN based gel formulations, aqueous liquid system in two-electrode ECD configuration. UV/Vis response, cyclic voltammetry and color coordinates of different systems.

AUTHOR INFORMATION Corresponding Author * R. Tena-Zaera: [email protected] * Ana Viñuales: [email protected]

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Table of Contents (TOC) Multi-color ECDs exhibiting five different colorations is reported. The easy design involves just two-viologens in a single EC layer. This allows the fabrication of rainbow-like ECD wherein the color of each area can be adjusted independently between different colors under suitable applied potentials, bringing us closer to rainbow-like smart windows.


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Figure 1. a) Chemical structure of p-CV viologen; b) Schematic illustration of the two-electrode ECD configuration including electrode substrates (1), spacers (2) and EC gel (3); c) Photographs of an electrochromic device containing p-CV viologen in its bleached state (I) and colored states at switching voltages of ─1.4V (II) and ─1.8V (III). 98x115mm (300 x 300 DPI)


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Figure 2. p-CV gel vs reference aqueous liquid system in three-electrode electrochromic device configuration. a) Cyclic voltammogram and schematic illustration (inset) of the working electrode (i) and pseudo-reference electrode (iii) of the device, along with the laser scribing which insulated both (ii); b) Photograph of the devices in their bleached state (left), colored state after applying switching voltages of ─0,5V (middle) and ─0,9V (right) of p-CV gel (I – III) and aqueous liquid system (IV – VI); c) UV/Vis transmittance response in bleached state and upon applying ─0.5V and ─0.9 V voltages; d) CIE color space plots representing the color coordinates of devices containing p-CV gel (left) and aqueous system (right) upon applying a ─0.5 V (1) and ─0.9 V (2). 171x346mm (300 x 300 DPI)



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Figure 3. Multi-color three-electrode electrochromic device containing Blend gel: a) Cyclic voltammetry (CV) of Blend gel and comparison with CVs of p-CV gel and EtVio gel sepa-rately (inset). b) Observed coloration from top to bottom and c) absorption profiles after applying a switching voltage of 0 V (colorless) , ─0.7 V (green), ─0.9 V (pink-violet), ─1.1 V (orange) and ─1.7 V (purple) respectively; d) CIE color space plots representing color coordinates obtained at off state (1) and upon applying a switching voltages of , ─0.7 V (2), ─0.9 V (3), ─1.1 V (4) and ─1.7 V (5). 117x161mm (300 x 300 DPI)


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Figure 4. Photographs of rainbow-like electrochromic device containing Slime 5a. Multi-electrochromic device provided with zoning design and colors observed in each row of the device while the suitable voltage is being applied. 45x15mm (300 x 300 DPI)


Multicolor Electrochromics: Rainbow-Like Devices - ACS Applied

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