Dual-Type electrochromic device with single wall carbon nano tube employment in gel electrolyte.
Some materials exhibit color variation when they are subject to voltage application. This phenomenon is called as electrochromism (EC) (1). The change in the optical properties of such materials is attributed to some redox interactions in conducting polymer (CP), which are one of the most promising modern materials for the organic electronics' applications (2). Actually dual electrochromic device (ECD) concept is also developed amongst the mentioned applications such as (3). In fact inorganic-based ECD attempts were also presented with [WO.sub.3], Ir[O.sub.2], etc. but, they experience serious disadvantages such as high cost and low response time (4-6). There are several serious applications of ECD (7), (8), such as mirror construction (9), ECD-based smart windows, optical displays, camouflage materials, etc. (10-13). Actually when the CP are proposed with their EC properties they are found to be very well suiting due to high stability, efficient color change, optical memory, optical contrast, and color uniformity (14-17). In brief easy production, high degree color tailorability and fine tuning of CP are making them preferable over inorganics. On the other hand, CP-based ECD realization constitutes a basis for other CP based potential applications such as light-emitting diodes, solar cells, sensors, capacitors, etc. (18-20).
Polythiophene (PT) derivatives are among the promising candidates in CP family due to their chemical stability, easy synthesis, and versatile structure. There are some experimental works on their electrical properties and modification of these properties with single walled carbon nanotube, (SWCNT) doping indeed (21), (22).
In the scope of this work, a Dual-Type ECD is proposed with SWCNT-doped gel electrolyte and device characterization was performed with a couple of optical and electrical measurements. Whereby the effect of SWCNT in the gel electrolyte of the constructed device was also analyzed and observed.
Materials and Device Construction
Boron fluoride-ethyl ether (BFEE) and acetonitrile (AN) were purchased from Aldrich and used as received. Thiophene (Th) (Aldrich) was distilled prior to use. 3, 4-Ethylenedioxythiophene (EDOT), propylene carbonate (PC), tetrabulylammonium tetrafluoraborate (TBAFB), and poly(methyl methacrylate) (PMMA, Mw: 120,000) were purchased from Aldrich and used without further purification. SWCNT were provided from (Carbolex). SWCNT purity is 70-90 vol% according to Raman spectroscopy. Their typical rope diameter is ~20 nm and typical SWCNT diameter is ~1.4 nm. Indium tin oxide (ITO)-coated glass slide (Aldrich) has 8-12 ohm resistance.
Preparation of Electrochromic Layers (PTh and PEDOT)
A three-electrode cell containing ITO-coated glass slide as the working electrode, a platinum foil as the counter electrode, and a silver wire as the reference electrode were used for electrodeposition of PTh and PEDOT by potentiostatic methods. A 10 mM solution of EDOT in 0.1 M TBAFB/AN was used to deposit PEDOT film onto ITO coated glass electrode at 1.3 V versus Ag/[Ag.sup.+] for 8 sec. In the potentiostatic electrochemical polymerization of Th, BFEE was used as both solvent and supporting electrolyte. After introducing 10 mM Th to the reaction medium, electrolysis was performed at 1.9 V versus Ag/[Ag.sup.+] for 20 sec. The electrochemistry experiments were carried out at room temperature under inert argon atmosphere. The electrochemical polymerizations and characterizations were performed with CHI 760C potentiostat and the thicknesses of the coated polymer films onto ITO coated glasses were measured around 90 nm by Optical Elipsometry "Phe 101".
Preparation of Gel Electrolyte
For the construction of dual ECD, two different gel electrolytes were prepared in the following forms containing (i) AN + PC + PMMA + TBAFB and (ii) AN + PC + PMMA + TBAFB + SWCNT. The ratio of the composition of AN: PC: PMMA: TBAFB was 70:20:7:3 by weight. A high vapor pressure solvent, AN was used to dissolve the PMMA and to allow an easy mixing of the gel components. TBAFB and SWCNT as the key supporting elements of the proposed gel and PC as the plasticizer were added to the gel mixture. The procedure was as follows: After dissolving TBAFB in AC, PMMA was added into the solution. The complete dissolution of PMMA was achieved by vigorous stirring and heating. When all of the PMMA was completely dissolved. PC was introduced to the solution. The mixture was stirred and heated until the highly conducting transparent gel was formed. SWCNT was introduced to the gel mixture as a enhancement doping together with TBAFB in the ratio of 0.30% by weight after dissolution of PMMA.
Construction of Electrochromic Device
ECD were assembled with a configuration of ITO/PTh/ gel electrolyte/PEDOT/ITO as shown in Fig. 1. The effects of SWCNT doping in this gel electrolyte were investigated by comparing the electrical and optical properties with those of the pure gel electrolyte. Cathodically coloring layer PEDOT, and anodically coloring layer PTh. were synthesized on ITO according to the mentioned procedures. The devices were dried under atmospheric conditions.
[FIGURE 1 OMITTED]
RESULTS AND DISCUSSION
Spectroelectrochemistry of Devices
ECD were constructed with two configurations (i) ITO/ PTh/Gel electrolyte/PEDOT/ITO and (ii) ITO/PTh/Gel electrolyte with SWCNT/PEDOT/ITO.
UV-vis absorbance graphs acquired by "PerkinElmer Lamda 30" for these ECDs and results are plotted at various potentials from -2 V to +2 V as seen in Fig. 2. PTh is anodically coloring and PEDOT is cathodically coloring polymers. PTh has a red color in its neutral form while PEDOT is transparent. Natural red color of PTh have dominated to ECD at negative voltages, -2 V, because of nearly transparent peculiarity of PEDOT under that condition. As the potential rises to +2 V, the PEDOT is reduced and it is almost blue, while PTh is oxidized and it becomes blue as well. It has been seen that absorption peak is around 477 nm at -2 V in the absorbancc graph of ECD with pure gel (Fig. 2a). When the applied voltage is increased, the absorption peak shifts to higher wavelengths, this time it is around 590 nm at +2 V due to reducing of PEDOT. As for the SWCNT-doped gel containing ECD, although the wavelength of absorption peak is the same with that of the pure gel ECD at -2 V (Fig. 2b) the wavelength of the absorption peak at +2 V shifts to 622 nm by the effect of SWCNT indeed. If these devices are compared; the shift for example in the absorption peak at +2 V can be due to the enhanced electroactivity of SWCNT-doped gel electrolyte. Moreover, absorption intensity of ECD with SWCNT-doped gel is also higher than that of the pure gel, as depicted in Fig. 3.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Having understood the enhancing effect of SWCNT doping, Trisitumulus color coordinates of the SWCNTdoped device were acquired by "Photoresearch PR650" spectrophotometer and the related values are given Table 1, from where precise contrast analysis could be performed.
TABLE 1. Trisitumulus color co-ordinates of pure and SWCNT doped ECD. Color X Y L a B At -2 V (red) 0.520 0.359 99.585 0.327 -0.138 At +2 V (blue) 0.422 0.394 96.347 -14.757 -5.101 L, Luminance; a, Hue; b, saturation, CIE "The Commission Internationale de l'Eclairage".
Switching Properties of ECD's
Color switching times of ECD are of primary importance in possible display applications and nanotubes are suitable candidates for improving the time constant in such applications (23). The graphs demonstrating the switching times for pure gel and SWCNT-doped gel ECD's are given in Fig. 4a and b. Since the absorption peaks are different in pure and SWCNT-doped gels, the constructed devices were analyzed with corresponding wavelengths of 622 nm for SWCNT-doped gel device and 590 nm for pure gel device (with 2 V peak to peak amplitude and 3 s period, square wave potential). The response time for pure gel ECD is ~0.86 s as switching from +2 V to -2 V (red to blue) and it is 0.22 s in -2 V to +2 V (blue to red). A faster response time is seen in blue to red switching. As for the SWCNT-doped ECD, it was observed to have response time of 0.51 s from +2 V to -2 V (red to blue), and again with a shorter time constant of 0.19 s for -2 V to +2 V (blue to red). It was also seen from Fig. 4. that optical contrast of devices is nearly 30% for both of the pure gel and SWCNT-doped gel ECDs.
[FIGURE 4 OMITTED]
Stability of the Electrochromic Devices
The stability of the ECDs were studied by cyclic-volta-metry, from +2 V to -0.5 V with 400 mV/s of scan rate. ECDs were exposed to 3000 cycle of cyclic voltametry with SWCNT-doped gel device and 1600 cycle for pure gel device. When the electrolyte gel were doped with SWCNT, it was observed that the ECD is more stable than that of the pure gel device, as seen from the comparison of Fig. 5a and b. Electroactivity of pure gel device is nearly finished at 1600 cycle while it was lasting until 3000 cycle in SWCNT-doped gel device, although controversial explanations are present about the mechanisms of carbon nano tube (CNT)-doped polymer systems. In our point of view; one of the possible explanations on the enhancement by SWCNT doping comes from the efficient interaction between SWNTs and gel, which causes the effective degree of electron derealization to rise and protonation of gel elevates with the increase of electrical conductivity and ultimately electroactivity of the gel becomes better.
[FIGURE 5 OMITTED]
Throughout the work, an interesting and efficient ECD configuration was proposed with a SWCNT-doped sandwich structure and a couple of repeatable measurement results implied the fruitful combination of SWCNT doping in possible applications. It seems good to encounter the fact that there are some other encouraging experimental works and claims of CNT doping in similar systems (24). Positive effect of the SWCNT doping on the device performance is expected to come from the exclusive electrical properties of the so called CNT. In brief; electrical conductivity and electroactivity of the gel is increased with these nano tubes (NT) (25-29). Thiophene-based CPs, which are appropriate to electrochemical deposition applications, are found to be compatible in such a configuration with NT and various device applications can be inspired from the presented experiments and results.
The authors thank OPTOMED Ltd. for their contributions on the color measurements.
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Arif Kosemen, (1) S. Eren San, (1) Yusuf Yerli, (1) Mustafa Okutan, (1) Melek Uygun, (1) Faruk Yilmaz, (2) Asuman Celik (2)
(1) Department of Physics, Gebze Institute of Technology, Gebze, Kocaeli 41400, Turkey
(2) Department of Chemistry, Gebze Institute of Technology, Gebze, Kocaeli 41400, Turkey
Correspondence to: S. Eren San; e-mail: email@example.com
Contract grant sponsor: Tuhitak; contract grant number: 106T057.
Published (inline in Wiley InterScience (www.interscience.wiley.com).
[C] 2009 Soeiely of Plastics Engineers
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|Author:||Kosemen, Arif; San, S. Eren; Yerli, Yusuf; Okutan, Mustafa; Uygun, Melek; Yilmaz, Faruk; Celik, Asum|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Jul 1, 2009|
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