Multiple color reflection in a single unit cellusing double-layer

electrochromic reactionJae Eun Jang,1,5 Seung Nam Cha,2 Ji Min Lee,2 Jun Jung Kim,3 Gehan A. J. Amaratunga,4 and Jae Eun Jung2,6

1Department of Information & Communication Engineering, Daegu Gyeongbuk Institute ofScience & Technology (DGIST), Daegu 711-873, Korea

2Material & Device Research Center, Samsung Advanced Institute of Technology, Yongin 449-712, South Korea3Department of Applied Chemical Engineering, Hanyang University, Seoul 133-791, South Korea

4Department of Engineering, University of Cambridge, Cambridge CB3 0FA, UK5e-mail: jang1@dgist.ac.kr

6e-mail: jaeeun.jung@samsung.com

Received August 12, 2011; revised November 22, 2011; accepted November 23, 2011;posted November 28, 2011 (Doc. ID 152763); published January 13, 2012

Multiple color states have been realized in single unit cell using double electrochromic (EC) reaction. The precisecontrol of bistability in EC compounds which can maintain several colors on the two separated electrodes allowsthis new type of pixel to be realized. The specific electrical driving gives a way to maintain both sides in the reducedEC states and this colors overlapping in the vertical view direction can achieve the black state. The four color states(G, B, W, BK) in one cell/pixel can make a valuable progress to achieve a high quality color devices such like elec-tronic paper, outdoor billboard, smart window and flexible display using external light source. © 2012 OpticalSociety of AmericaOCIS codes: 230.2090, 130.4815, 350.5130.

Electronic paper (E-paper) is one of the hottest researchissues in electronic devices. The goal is to enable a paper-quality electrical device with ultra-low power consump-tion and ubiquitous portability as well as the ultimate inrecyclability of the medium. The most important factorfor successful commercialization of a paper-like deviceis the achievement of a color image similar to a photo-graph using reflective electrical optical shutter. Althoughvarious reflective optical shutter have been showedfull color states or multicolor images, such as electro-phoresis [1], electrowetting [2], microelectromechanical(MEM) mirrors [3], and liquid crystals [4,5], most of tech-niques have displayed poor color images and with onlyblack (BK) and white (W) images approaching accepta-ble quality being possible currently. The main reason forthe poor quality of color images is assumed to be the pla-cement of parallel color bars in the unit structure (pixel)from which they are made up [6]. To improve the qualityof the color, the color forming layers, i.e., cyan (C),magenta (M), yellow (Y) or red (R), green (G), blue(B), can be stacked within the pixel structure verticallyto the direction of view. Unfortunately, this structure re-quires three separate drive circuits as well as compli-cated assembly, making it impractical [4,7]. Therefore,in order for E-paper to be widely adopted, it is essentialthat multicolor reflections can be incorporated in onepixel unit with simple structural design.Here, we report a new electrochromic (EC) cell design

concept together with its optical characteristics. It uses adouble state EC material structure, which achieves G, B,BK, and W color states within one unit cell with a simpleelectronic drive scheme. The precise control of bistabil-ity in EC compounds, which can maintain either one ortwo colors on the two separated electrodes.Figure 1(a) shows the schematic diagram of the pixel

design to get various color states in one unit cell. The

pixel has two types of EC material on TiO2 nanoparticlefilm deposited on indium tin oxide (ITO)-coated glass.Nano-TiO2 film was formed by a screen printing processusing commercially available TiO2 paste [8]. The film un-derwent 450 °C sintering in air for 1 h to remove organicresidues. The average particle size of TiO2 was 18 nm andthe film thickness was approximately 10 μm. The topside was coated with N , N 0-4; 40-bis(4-carboxyphenyl)-bipyridinium dichloride (EC I), which shows green color.N , N 0-4; 40-bis(2-phosphonoethyl)-bipyridinium dichlor-ide (EC II), which turns to blue upon electrical reduction,

Fig. 1. A schematic illustration of a color unit cell and W, G, Bstates made by EC reaction. (a) Magnified image of a color unitcell and SEM photo of TiO2 layer coated EC material. (b) Colorphoto images with applied bias states. (c) Spectrum of W, G, Bcolor state.

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was coated on the bottom side. Using a spacer, the gapbetween the electrodes was maintained at 80 μm and thecell was sealed. The electrolyte was injected into cell viaa hole on the electrode, which was subsequently sealed.For the white color state, transparency of EC materials atzero voltage allows an excellent white state from the re-flector layer [Fig. 1(b)]. The reflectance for the devicewith standard white paper as the reflector is about69% [Fig. 1(c)], which is much higher than the 40% orso reflectance in the white state of a commercial e-paperdevice relying on electrophoresis [1]. Compared withnewspapers and magazines, in which reflectance is∼60% and ∼80%, respectively, the reflectance of an elec-trophoretic device, applied to commercialized device, islow. Therefore, the good white state (∼69% reflectance)in this EC device implies that a true paper-like displaydevice can be achieved. With increasing drive voltageof negative polarity, green color is observed around−0.8 V from EC I, as shown in Fig. 1(b). The electricallyfloating state after optical change in EC I can maintainthe reduced state for a while, showing green color.The memory or bistability depends on the formulationof electrolyte and the cell structure as well as the struc-ture of the ECmaterial in the reduced state. The ions con-centration in electrolyte are quite important to make longmemory effect. The lower concentration gives longermemory time: however, it also makes the poor breachperformance. Therefore, the proper control of ionsconcentration depended on device characteristics is es-sential. In our experiment, the color maintained from sev-eral minutes to days. This memory effect can give a highadvantage to the power saving of device driving.To get the blue color, a positive bias,�1.5 V, is applied

to the cell structure. Because the blue generating EC II iscoated on the counter electrode, it can be used with thegreen generating EC I material coated on the workingelectrode to realize G, B states easily without any colorinterference through negative and positive bias. Thestructure exploits the fact that EC material changes theiroptical properties when reduced, responsible for the col-or generation, and separating the reduction statesthrough opposite biasing. Using these mechanisms, G,B, W color states are achieved in the cell structure. Thelow driving voltage can give some advantage, power sav-ing, to applied electronic device. The color spectrum dis-tributions were measured in reflective mode with whitereference light [Figure 1(c)]. Green and blue have propercolor states.Another crucial factor for high-quality images in color

is the generation of black color in the EC device. The neu-tral state of an EC material is generally transparent [7–9].From this optical characteristic, the white state can begenerated easily with a white reflector layer. How-ever, the black color state cannot be obtained if theEC material does not have black state by EC reaction.Unfortunately, the suggested unit cell already has twocolor states, green and blue. Therefore, a new solutionis essential to the black state. The suggested structureis consisted of two electrodes overlapped on top of an-other, with EC I and EC II adsorbed on each electrode.When EC I on the working electrode layer changes thecolor from colorless to green, the wavelength responsiblefor green in white light only passes through the working

electrode layer and the filtered light is absorbed by thereduced form of EC II (blue) on the other electrodedue to color mismatch. In theory, there should be no re-flective light coming from the unit cell since the visiblelight is completely absorbed by both reduced forms of ECmaterials on each electrode, making a black state. How-ever, both EC I and EC II generate colors in the reducedstate, meaning that only one side has a color state, whilethe other side maintains colorless state. If there is a wayto maintain both sides in the reduced states by applyingelectrical bias between them, the black state can beachieved easily.

To solve the aforementioned problems, the differentcombinations of the electrolyte and the driving methodshave been studied. Figure 2 shows the color state of EC II(blue color) with various types of electrolytes and theup–down driving condition. In this experiment, onlyEC II was coated on the TiO2 layer on the transparentworking electrodes (ITO-coated glass). The photonenergy was measured using a photon multiplier tube out-put over time. The electrolyte consisted of N -methyl-pyrrolidone (NMP), tetrabutylammonium perchlorate(TBAP), and ferrocene. NMP is used as a solvent. TBAPhas the role of an electrolyte. Ferrocene supplies Fc ionsworked as counter ion (electron donors) to the cell. Withthe electrolyte composed of 0.05 wt. % TBAP and0.05 wt. % ferrocene, EC II is changed to the blue stateat 1.5 V and the blue color disappears when −1.5V is ap-plied. When the potential goes to 0 V (ground state), thecolorless state is maintained, as shown in Fig. 2(a). Forthe 0.05 wt. % TBAP and 0.02 wt. % ferrocene condition[Fig. 2(b)], the blue color is also shown at 1.5 V applica-tion. The application of −1.5V bias enables the colorless

Fig. 2. Color state changes with different types of electrolyte.The more negative value signifies a higher intensity of the darkblue state. Photo images with the indicated steps were shown inthe graph. For the initial blue step, the images are almost same,so that just one image is shown. (a) 0.05 wt. % TBAP and0.05 wt. % ferrocene in NMP; (b) 0.05 wt. % TBAP and0.02 wt. % ferrocene in NMP; (c) change in color intensity invarious electrolyte components without ferrocene: C1,0.02 wt. % TBAP, C2, 0.05 wt. % TBAP, C3, 0.1 wt. % TBAPin NMP; (d) current change with up–down driving conditionand the different electrolyte compositions.

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state without any difference. However, the colorless stateturns to the light blue state when the electrical statechanges to 0 V. Without ferrocene, the blue color inten-sity at 0 V after the “up and down” electrical field cycle isalmost 80% of the initial blue state at 1.5 V [Fig. 2(c)]. Themain role of counter ions is “system neutralization” dur-ing the EC reduction state. Therefore, the insufficiencyor absence of counter ions can introduce an electricalunbalance to the device. Because of the unbalance, theexcess of electrons, which is built up on the upper ITOelectrode during the colorless state at −1.5V application,can move easily toward the lower EC material when theelectrical bias changes to 0 V (i.e., there is no strong re-pulsive force to prevent double oxidation states). As theresult of this charge redistribution, the high peak currentflow is detected when the short circuit state is made[Fig. 2(d)]. Compared with the cell structure, whichhas 0.05 wt. % ferrocene in electrolyte, a little currentflows at the application 0 V as the final bias step. We be-lieve that this phenomenon can be applied to the doubleEC reaction to make a black state.To verify the double color reaction states clearly, the

upper TiO2 layer with EC I is aligned with a half shift tothe lower blue EC TiO2 layer in the assembly process.The electrolyte used was 0.05 wt. % TBAP in NMP. With1.5 V applied to the device, the lower EC material (on thecounter electrode) changes to the blue state. On removalof the bias, the color is maintained in the electrical float-ing state. When −0.8V is applied to the cell, the colorstate changes from blue to green state. This green stateis also maintained under floating drive conditions. Theblack state is achieved by maintaining bistable stateson both EC materials at 0 V bias due to redistributioncharges, as mentioned earlier. Because the threshold re-action voltages are different between EC I and EC II—thegreen state bias is lower than the blue state—the bluecolor does not return to initial color intensity. The dura-tion of the −0.8V bias is increased twice as long as thetime of the blue color bias (1.5 V), and it makes the blackstate better, as shown in Fig. 3(a). However, the blackarea has still a greenish dark state [Fig. 3(a) top]. Thedarkness of the black state can be enhanced by a green-blue color combination. The driving cycle is almost sameas the blue-green combination except applying a bias se-quence suitable for generating the green-blue color viareduction electrically. After the reduction of green by−0.8V bias to the cell, 1.5 V is applied to the cell structureto get the blue color. When the applied bias returns to theground state, EC I is changed to a clear green color andEC II still maintains the initial blue state. Enough chargescan be stored to make both reduction states on ECI andEC II by applying the final 1.5 V bias with 2 s. Therefore,the black state is darker than the black state of the blue-green reaction [Fig. 3(b)]. Total reflection from thesedouble green-blue EC reactions is below 1%, whichmeans that a human would see the cell color as black.A double EC layer design is demonstrated. The newly

devised structure allows white, green, blue, and black

states within one simple cell, with the precise controlof bistability in EC materials. Black color, importantfor high-image quality, can be achieved by combiningtwo color reactions, providing a solution to a serious lim-itation to date for EC devices. We believe that this tech-nology will open up new possibilities for achievingelectronic devices with quality similar to photographs.

J. E. Jang acknowledges support from Ministry of Edu-cation, Science, and Technology (MEST) and DaeguGyeongbuk Institute of Science & Technology (11-BD-0404).

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Fig. 3. Double color reactions for the black state. The colorsof graph signify the main color state for cell structure with re-spect to time and applied bias (orange line). (a) Blue–greencombination with photo. (b) Green-blue combination withphoto.

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