We report a transmittance controllable electrochromic color filter (TCECF) by incorporating new electrochromic leuco dyes and their optimized composition. Each primary color red (R), green (G), and blue (B) electrochromic filter has an excellent transmittance of more than 84% at 650 nm, 540 nm, 450 nm, and the color coordinates are controllable from white (0.332, 0.347) to deep-red (0.621, 0.344), deep-green (0.327, 0.646), and deep-blue (0.179, 0.085), respectively. Also, each TCECF has good coloration efficiencies of 188.7 cm2 C−1 (R), 189.3 cm2 C−1 (G), and 147.8 cm2 C−1 (B) with high optical density change. A full color producible electrochromic color filter (ECF) is designed and fabricated by integrating primary RGB color filters with a refractive index matching adhesive layer. The fabricated three-stack full color producible ECF enables high transmittance of about 61% for clear white light extraction, and it can produce various colors including RGB. This TCECF technology will be very useful for high light out-coupling electro-optical applications, such as smart lighting, smart window, and display.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In the recent past, color production technology has been gathered enormous demand for the development of fascinating electro-optical applications including television, smart window and smart lighting [1,2]. The method of color production has been different for smart lighting and television display applications. In particular, smart lighting incorporates LEDs to adjust the color conditions, however television, and digital signage, contains white backlight with color filters to produce red (R), green (G), and blue (B). For smart lighting, LEDs should be replaced with multiple LED chips, but there is a limitation to the LED color change [3,4] and additionally, it is a very expensive solution. In the case of television display with white backlight and color filters, a spatial division has made to produce R, G, and B, which causes a significant reduction in light extraction efficiency. This result in high power consumption, and a much shorter lifetime with severely degraded color characteristics [5,6]. Conceptually, the above mentioned issues can be resolved by using a transmittance controllable color filter that can switch between very clear transparent mode and high-quality R, G, B colored mode. Regardless of the type of lighting states, the lighting can selectively produce the particular color of light with high extraction efficiency according to the atmospheric conditions, and concurrently the display can overcome spatial limitations and extract light of various colors with high efficiency.
For transmittance controllable color filter applications, electrowetting device, and the electrochromic device (ECD) can be highly useful. In the case of electrowetting device, it is possible to secure a fast response time, but simultaneously challenging to attain a clear transmission mode due to condensed ink when switching the transmission mode, and additionally, it also requires a high driving voltage [7–10]. On the other hand, ECD can change the color through electrochemical reactions of the electrochromic materials with low voltage and it also has clear light transmission characteristics. Among the above-stated devices, ECD has the most suitable and promising characteristics for such a color filter. For multicolor ECD, numerous research articles have been previously reported on different electrochromic dyes such as phthalate derivatives, viologen, and dioxythiophene based polymers [11–13]. Earlier, Kobayashi et al. demonstrated cyan, magenta, and yellow color ECDs using three phthalate electrochromic materials . They attained cyan (0.21, 0.24), magenta (0.41, 0.48), and yellow (0.38, 0.25) colored states at the driving voltage of 2.4 V. However, such color properties are still inadequate for use as a transmittance controllable color filter due to insignificant color properties [11–13]. Therefore, there is a requirement of deep-R, G, B color electrochromic materials and devices with excellent color tuning ability and good optical properties.
In this work, we demonstrate a transmittance controllable electrochromic color filter (TCECF) with primary color leuco dyes. Each R-, G-, and B-TCECFs show high coloration efficiency with deep color and high optical density(ΔOD) characteristics. Additionally, full-color producible ECF fabricated by stacking each primary color filter using a refractive index matching adhesive layer displays high light extraction efficiency, tunable any colors, and correlated color temperature (CCT) of white light.
2. Result and discussion
2.1. Materials selection and color change properties of red, green and blue dyes
To develop deep R-, G-, and B-TCECFs, electrochromic dyes and composition must have strong absorption in the respective wavelength range in the colored state and high transparency in the bleached state. It has been previously investigated that the leuco dye with lacton moiety causes a color change when the ring is opened by the proton and concurrently also changes the conjugation length [14–17]. Particularly, in the colored state, the carbon bond at the center of the molecule changes from sp3 to sp2 leading to an increase in the conjugation length, and thereby variations in the absorbance spectrum from transparent to color state. Normally, the color of an electrochromic material can be estimated from the effective conjugation length. By considering such color change mechanism of leuco dye, we selected new electrochromic leuco dyes, 6'-(ethyl(p-tolyl)amino)-2'-methyl-3H-spiro[isobenzofuran-1,9'-xanthen]-3-one (red dye), 1-ethyl-8-(ethyl(p-tolyl)amino)-2,2,4-trimethyl-1,2-dihydro-3'H-spiro[chromeno[2,3-g]quinoline-11,1'-isobenzofuran]-3'-one (green dye), and 3,3-bis-4-dimethylaminophenyl-7-6-dimethylaminophthalide (blue dye)  for evaluating the performances of R-, G-, and B-TCECF. Generally, these leuco dyes (see experimental section for details) are widely used for paper coloring applications. Indeed, these electrochromic dyes are highly transparent in the visible range and have strong absorptions in the UV range due to the closed status of the lactone ring and their conjugation lengths. To confirm the color change features of these red, green, and blue electrochromic dyes, initially, we measured light absorption properties in the solution state. Typically, efficient color change properties in solution state can be attained by a Brønsted–Lowry acid-base (proton transfer) approach via opening and the closing reaction of the lactone ring . In particular, the redox reaction by proton generator may cause an efficient electrochromic reaction of the leuco dye than that of conventional electron transfer technique (Lewis acid-base). To confirm the color change properties of red, green and blue dyes, each dye of about 1×10−6 M was dissolved in tetrahydrofuran (THF) solvent followed by the addition of acetic acid in THF to promote proton transfer by a redox reaction. Figure 1 displays color alteration properties of red, green and blue dyes with different concentrations of acetic acid in the main composition. As can be seen in Fig. 1, each dye has an optical density of almost zero in the visible wavelength region before adding acetic acid in THF. However, an optical density is increased with the incorporation of acetic acid in THF composition. Similarly, when the concentration of acetic acid increases, further improvement in the respective optical density is realized with a darker color solution. Indeed, the number of protons in THF composition can be increased with an increasing concentration of acetic acid via proton generation. Such protons can easily oxidize electrochromic dye through proton transfer and as a result dramatic change in the color of the electrochromic solution. The absorbance of red dye, green dye, and blue dye solutions is dramatically increased at 500-550 nm, 400-500 nm (as well as 600-700 nm) and 600 nm wavelengths, respectively, after 40% concentration of acetic acid and also shows reddish, deep-green and bluish color for each dye solutions. These experimental results show that each leuco dye can form R, G, and B colors according to the acetic acid concentration in THF solvent . It is important to note that the color generation ability of red dye and blue dye is relatively weak than that of green dye in the THF solvent. Indeed, our electrochromic dyes can maintain both initial transparent state and R, G, B colored states if the composition of each color is well considered. Here, the equilibrium between colorless and color form of electrochromic dye is sensitive to the solvent,  and proton generator. Therefore, to achieve equilibrium between colorless and more stabilized color form with excellent performance in TCECF, suitable solvent selection with efficient proton generator is highly needed.
2.2. Electrochemical system for color enhancement
Leuco dyes having lactone ring are known to be difficult to achieve a large color change from its own Lewis acid-base electrical redox reaction. Here, to obtain suitable color characteristics in ECD, we used electrochemically stable 2,3-dimethylhydroquinone (DMHQ), which can oxidize to generate protons as reported in our previous work . Typically, DMHQ induced a large color change through the Brønsted-Lowry acid-base redox reaction by providing proton to leuco dye. Additionally, polyvinyl butyral (PVB) polymer is incorporated to prevent the agglomeration of charged molecules and this composition is indeed made into a gel-type form, which provides better process ability than that of liquid-type. Likewise, tetra-n-butylammonium tetrafluoroborate (TBTF) is also employed as an electrolyte for the electrochemical cell . A solvent can have significant effects on the optical properties of the device and that not only dissolve all of the above components, but also stabilize an electrochromic dye in the neutral state and ionic state, resulting in transparent and colored modes. Therefore, to confirm the solubility test, each electrochromic dye is dissolved in 2-ethoxyethanol (2-EE) and tetrahydrofuran (THF) solvents. The red dye shows good solubility in both THF and 2-EE, however, green dye and blue dye are highly soluble in only THF. By considering solubility test results, TCECF devices are fabricated with different compositions as shown in Fig. 2. Among fabricated R-TCECF devices with different solvents, 2-EE shows the best color property compared with THF (Fig. 2(a)). On the other hand, green and blue TCECFs with electrochromic dye in mixed solvents (2-EE: THF) indicates good color characteristics than that of THF contained device (Figs. 2(b) and 2(c)). Such excellent color properties of R-TCECF with 2-EE as well as G- and B-TCECFs with mixed solvents are ascribed to the additional protons generated by hydroxyl group in 2-EE, which can also stabilize the redox state in the electrochromic dye.
To further alter the color properties of TCECF, ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMPBT), was added into the solution. The ionic liquid is a salt in the liquid state at the room-temperature condition. It has various properties such as good thermal and chemical stability, low melting point, high ionic conductivity, negligible volatility, moderate viscosity and high polarity [20–23]. In our electrochromic system, the ionic liquid can stabilize an ionic state of each electrochromic dye due to its ionic nature, and as a result, electrochromic dyes can be more easily oxidized into a ring-opened ionic state. Hence, the color property at 1.7 V has improved by 8.3% for R, 52.2% for G and 50.9% for B compared with the non-ionic liquid device (Fig. 2).
2.3. Optical properties of TCECF
The transmittance is the most important optical characteristic to be used as the TCECF because that can show the light transmission of the device at the respective wavelength range. To examine the optical properties of TCECFs, an optimized R, G, B devices were systematically fabricated and then their respective transmittance characteristics were also measured using a UV-vis spectrophotometer (Fig. 3). Figs. 3(a)–(c) show that each TCECF is highly transparent at 0 V condition (no applied voltage), even though two ITO substrates are used for the device. When the driving voltage is varied from 0.7 V to 1.7 V, the transmittance is greatly reduced at the respective wavelength because oxidation of DMHQ occurs from 0.7 V and the protons are generated to convert the dye into lactone ring-open form. As the voltage increases, oxidation of DMHQ also rises and more protons are produced, and thus it can be converted into a deep color. Besides, TCECFs show very low transmittance (approximately zero or under 0.001%) at a certain wavelength range owing to the strong absorption properties of our electrochromic materials.
Figure 3(d) presents CIE 1931 color coordinates and the real color images of R-, G-, and B- TCECFs (using a digital camera) according to the applied voltages. It can be seen that each TCECF gradually changes color according to the applied voltage, and also exhibits excellent color characteristics at 1.7 V. We calculated the color coordinates of TCECF to quantitatively express the precise color. The non-emissive display device, particularly ECD do not emit light directly; therefore, the measurement of its accurate color characteristic is slightly difficult. Indeed, the color of light coming out from TCECF depends on the spectrum of ambient light or backlight, and the condition of the input spectrum when it passes through TCECF [24,25]. Therefore, the CIE 1931 color coordinates are calculated from the D55 standard illuminant spectra based on the measured TCECF transmittance for objective evaluation.
The calculated color coordinates of R-, G-, B-TCECFs at different applied voltages are stated in the CIE 1931 color space (Fig. 3(d)) and their respective values are also summarized in Table 1. At the initial condition (0 V), the evaluated color coordinates of R, G, and B devices are (0.349, 0.371), (0.357, 0.388) and (0.350, 0.373), respectively. All TCECF devices show very clear white color except for G device, which is slightly yellowish due to the color of dye (Fig. 3(e)). When the medium voltage is applied to TCECF, a respective color coordinates are attained for R-, G-, and B-TCECF devices. Similarly, our TCECFs exhibit very good color characteristics of (0.621, 0.344), (0.327, 0.646) and (0.179, 0.085) for R, G and B (at 1.7 V) in comparison with the National Television System Committee (NTSC) colorimetry.
2.4. Response time
Herein, response time is the time interval required to change the transmittance by 80% from an initial state of TCECF. Normally, the response time is categorized into coloring and bleaching response times, where, coloring and bleaching response times are nothing but the transition time from initial bleached state to colored state and from the colored state to bleached state, respectively. The response time of TCECF can be influenced by numerous device parameters, for instance, size of the active area, the distance between the working electrode and counter electrode (i.e. cell gap), diffusion rate, viscosity of electrolyte, and applied voltage [26,27]. Here, the response time of R-, G-, B-TCECF were measured at a specific wavelength with the device active area of 9 cm2 and a cell gap of 70 µm. The coloring response times of 1.9s (R), 1.3s (G) and 0.9s (B) and bleaching response times of 16.0s (R), 11.5s (G) and 11.9s (B) are attained for TCECFs at 1.7 V and 0 V, respectively (Fig. 4). However, R-TCECF exhibits slower response time than those of G- and B-TCECF. Indeed, R-TCECF has contained 2-ethoxyethanol (1.85 mPa.s), which has a higher viscosity than that of THF (0.46 mPa.s) used in G- and B-TCECF [28,29]. Due to low viscosity of THF, diffusion of protons in the device cell occurs relatively faster and resulting in an improved response time compared with R-TCECF. In the case of B-TCECF, the coloring response time is slightly faster than the G-TCECF because the ionic property of ionic liquid allows the dye to rapidly convert to ring-open form. The response times of our TCECFs are relatively lower due to their large active area, but it can be further improved to the level needed for display applications by decreasing the active area and incorporating previously reported mesoporous metal oxide electrode with vertical porosity, that can offer effective diffusion of dyes to the electrode [30–32]. If the current cell-gap and active size are reduced, the response time can be improved further because the required current amount is reduced. Additionally, it is important to note that the bleaching times of TCECFs are relatively longer than the coloring times because of no external electric field (zero bias condition or 0 V). In particular, during the coloring process, protons can move quickly from the working electrode to the counter electrode under the influence of high electric potential (forward bias condition (+1.7 V)) and thus an improvement in coloring response time. Moreover, in the reverse bias condition (upon applying the voltage of −1.7 V), the bleaching response time can be intensely decreased compared to that of zero bias condition . This difference in bleaching times is attributed to an enhanced diffusion speed of protons under reverse bias conditions. Additionally, this tunable bleaching speed depending on the applied potential is very useful in TCECF application. The TCECF not only can save power in the steady colored state using memory effect under no bias condition but also quickly switch to the bleached state under the reverse bias condition.
Driving stability is one of the crucial parameters of TCECF in terms of its practical application. Herein, we examined the driving stability of TCECFs at respective wavelength under light illumination and dark conditions. To verify the driving stability, the transmittance of TCECF was measured at bleached (0 V) and colored (1.7 V) states by applying repetitive voltage pulses (+1.7 V for 8s followed by −1.7 V for 2s and 0 V for 7s. Inset of Fig. 5). The stability of red TCECF at both bleached and colored states is relatively better than the G- and B-TCECF devices (Fig. 5). In particular, the transmittance of R, and G devices at the bleached state (under white LED backlight (Viltrox L116 T LED) illumination condition: 405 lux) is considerably decreased to 40% after 10000 and 5000 switching cycles, respectively. Likewise, the colored state stability of the R device is almost similar with and without illumination condition. However, the transmittance of G- and B-TCECF devices at colored state (with and without illumination) is drastically increased with increasing switching cycles. The relatively low driving stability of B- and G-TCECFs can be attributed to the almost no contribution of THF solvent for the stabilization of dye in the ionic state. In addition, considering our previous report, TCECFs (especially, red device) are expected to have good stability under non-driving and ambient conditions due to the stabilization of EC dye in the ionic state via protic solvent and lower oxidation potential of the proton donor [17,31].
2.5. Coloration efficiency
Coloration efficiency is one of the essential parameters to evaluate the characteristics of TCECF. It is defined as the ratio of change in ΔOD at a particular wavelength to the injected/ejected charge density (Q) as shown in Eq. 1 [33,34]. Therefore, if a small amount of charge is used to generate a large ΔOD, then it is possible to efficiently produce R, G, B colors.
Here, the coloration efficiency is evaluated from the slope of optical density change as shown in Fig. 6. A high coloration efficiency of about 188.7cm2 C−1, 189.3 cm2 C−1 and 147.8 cm2 C−1, is obtained for R-, G-, and B- TCECF, respectively. The R- and G-TCECFs show almost similar coloration efficiency, however, it is relatively low in B-TCECF. Indeed, such a tendency is attributed to the electrochemical reaction by the ionic liquid, where, the change in ΔOD is greater but simultaneously, an amount of charge consumption is also high. Previously, several studies have been reported on ECDs to achieve a high level of coloration efficiency by using a smaller amount of charge rather than increasing the ΔOD. For example, FMFPhV2+, a viologen-based electrochromic material, reported a ΔOD of 0.45 and a coloration efficiency of 79.39 cm2 C−1 for ECD device. In another study, the Prussian Blue-based ECD showed a ΔOD of 0.4 and a coloration efficiency of 131.5 cm2 C−1. Similarly, polymer-based ECD stated a ΔOD of 0.5 and the calculated coloration efficiency of 434 cm2 C−1 [33–35]. However, our developed TCECF can achieve a high level of coloration efficiency by changing the optical characteristics that are more than twice as large as (ΔOD) 1.2. These results indicate that our TCECF has an effective optical property with controlling capability.
2.6. Three-stack full color producible ECF
Here, we demonstrate a three-stack full-color producible ECF using R-, G-, B-TCECFs to overcome the spatial limitation. This full-color producible ECF was fabricated by attaching each single TCECF device using a refractive index matching adhesive layer proving not only good adhesion property between different color TCECFs but also suppresses the optical loss generated from the interface of each device. As a result, under white backlight conditions, our three-stack ECF yields R, G, B and white colors using the same active area as shown in Fig. 7(a). The transmittance of three-stack ECF was measured using a UV-Vis spectrophotometer (Fig. 7(b)). The transmittance in visible wavelength region (400∼700 nm) is average for each color. Considering each optical loss factor, the conventional color filter is ideally capable of transmitting light of about 11.1% for R, G, and B, and 33.3% for white. However, our three-stack ECF exhibits a very high initial transmittance of 61.5% due to the proper refractive index matching adhesive layer even though three single devices are overlapped . Indeed, such an arrangement allows extraction of about 1.8 times brighter light from a white light source than that of the conventional color filter system. Besides, it is possible to extract more light such as 33.9% for R, 13.2% for G and 21.3% for B with good color characteristics. This allows high light extraction efficiency for primary colors using white light sources.
A three-stack full-color producible ECF can adjust CCT by controlling the transmitted light. The adjustable CCT can have many benefits for optical applications. The white light source, including lighting and display, needs CCT control capability to selectively implement warm white and cool white depending on the user needs and the surrounding environment. In three-stack ECF, CCT adjustment is possible in the same white source by selectively applying voltage to each ECF. Here, CCT was calculated using CIE 1931 color coordinates obtained from the D55 standard illuminant spectra and the transmittance of three-stack ECF. The calculation formula is as follows [37,38].
Figure 8 illustrates the calculated results with the Planckian Locus line. The driving voltage of TCECF used in each condition is summarized in Table 2. The CCT of 2872 K is calculated for the driving voltage conditions of R (0.8 V), G (0.0 V) and B (0.0 V). Similarly, the CCTs of 3873 K, 5212 K, and 7485 K are also obtained by controlling the driving voltages. Three-stack full-color producible ECF demonstrated a good color controlling ability and highly improved color light extraction capability including white light (Transmittance 61.5%) as well as adjustable CCT.
3.1. Electrochromic solution preparation
The R, G, and B electrochromic dyes were received from Yamada Chemical company and Sigma-Aldrich, respectively. Similarly, all solvents, proton donors and supporting polymers were purchased from Sigma-Aldrich. Electrochromic solutions were prepared for each color at room temperature with the following compositions. After the preparation of solutions with suitable weight ratio, stirring has performed using a magnetic bar for 6 hours.
R-TCECF: Red dye (3.2 wt%), PVB (7.7 wt%), DMHQ (5.2 wt%), TBTF (1.3 wt%), BMPBT (12.9 wt%), 2-EE (69.7 wt%)
G-TCECF: Green dye (3.2 wt%), PVB (7.7 wt%), DMHQ (5.2 wt%), TBTF (1.3 wt%), BMPBT (12.9 wt%), 2-EE:THF (69.7 wt%)
B-TCECF: Blue dye (3.2 wt%), PVB (7.7 wt%), DMHQ (5.2 wt%), TBTF (1.3 wt%), BMPBT (12.9 wt%), 2-EE:THF (69.7 wt%)
3.2. Device fabrications and characterization
To fabricate TCECF, initially, 150 nm thick ITO deposited glass substrates (thickness: 0.7 mm) were successively cleaned with acetone and isopropyl alcohol for 10 minutes each in sonicator and then dried using nitrogen gas. Afterward, a plastic spacer film (thickness: 70 um) was used to form an active area of 3 cm x 3 cm on the ITO glass. Then, the electrochromic solution was filled in the active area using a pipette and covered with another cleaned ITO glass and subsequently, the complete device was fixed using a clamp. To fabricate a three-stack full-color producible ECF, all single devices were attached using a refractive index matching adhesive to prevent the light loss between each single device. The total thickness of the three-stack ECF device is under 4.5 mm (including the thickness of glass substrates). The transmittance and absorption measurements were performed using a UV-Vis spectrophotometer (Thermo scientific, Evolution 220). The current density of the devices was obtained by using amperometry (Bio-logic SAS, SP-50) and ΔOD was measured from the UV-Vis spectrophotometer. Finally, the coloration efficiency was evaluated using Eq. 1 and the slope of optical density change. The CIE 1931 color coordinates were calculated using the D55 standard illuminant spectra based on the measured TCECF transmittance [27,28]. To obtain the color coordinates, the CIE tristimulus values X, Y, and, Z must be obtained. CIE tristimulus values X, Y, and Z can be calculated using the color matching functions, and for CIE 1931 2° standard as shown in Equations 5∼7. These functions are intended to correspond to the sensitivity of the human eye.
I(λ) can be calculated from Eq. 8.8), k is the normalizing constant, S(λ) is the spectral power distribution of D55 standard illuminant, and T(λ) is the transmittance spectra of TCECF at a specific voltage.
In summary, we present a high-performance R-, G-, and B-TCECFs using new leuco dyes with new electrochromic compositions. Our TCECFs with R, G, and B electrochromic leuco dyes display maximum transmittance of about 84% at respective wavelengths and excellent coloration efficiency of about 188.7 cm2 C−1, 189.3 cm2 C−1 and 147.8 cm2 C−1 for R, G, and B, respectively. Each TCECF also exhibits deep R, G, and B color with tunable color coordinates. Also, we demonstrate an advanced and facile approach for the production of full color using R-, G-, and B-TCECFs. A three-stack full-color producible ECF developed by stacking each color TCECF with a refractive index matching adhesive layer shows maximum transmittance of about 61% for white light and produces numerous colors including RGB. Furthermore, when the corresponding voltage is applied to each TCECF in a three-stack structure, the CCT can be adjusted between cool and warm white in the same backlight. It is expected that this color filter technology will be very useful for future electro-optical applications such as display, smart lighting, and smart window.
Korea Evaluation Institute of Industrial Technology (10052147); National Research Foundation of Korea (2016R1A2B4016567).
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