Electrochromic device of PEDOT–PANI hybrid system for fast response and high optical contrast
Introduction
The electrochromic devices (ECDs), which exhibit reversible color change through electrochemical redox reaction, have attracted considerable research interests because of their potential applications in various optical devices such as information displays and storage, sensors, consumer sunglasses and helmets, smart windows, variable reflectance mirrors or sun roofs for cars, and so on [1], [2], [3], [4].
Among various electrochromic materials ever reported, the tungsten oxide films have been by far the most extensively studied [5], [6]. However, their slow response time made it less attractive in the viewpoint of applications. In this sense, the conducting polymers could be considered as an alternative material for ECDs with fast response time [7], [8], [9], [10]. In particular, poly(3,4-ethylenedioxythiophene) (PEDOT) has been one of the promising candidates for electrochromic applications due to the fast response time, the small electronic band gap (Eg=1.6 eV, 775 nm), low redox potentials, and facile fabrication in a doped form [11], [12], [13]. In spite of such advantages, its poor coloration efficiency (Δ%T) of PEDOT has still remained as a disadvantage, because the color of PEDOT is pale blue even at its bleached state. To circumvent this problem, it is necessary to fabricate a full cell system using a complementary counter electrode, which showed both colored state and bleached state together with PEDOT layer [11], [14], [15], [16], [17], [18].
In this study, we have attempted to enhance an optical contrast while maintaining a fast response time of conducting polymer ECDs in which PEDOT and polyaniline (PANI) were employed as complementary coloring electrodes. Electrochemical deposition of monomers was used for fabricating the electrodes, because it is thought to be advantageous in terms of fabrication time and eventually the cost, compared with conventional vacuum deposition methods such as sputtering and evaporation. Moreover, to obtain optimal optical contrast on this system, we have controlled the thickness of PANI electrodes by varying the deposition time. Finally, the electrochemical and optical behaviors of the ECDs were quantitatively investigated with respect to the thickness of PANI electrodes.
Section snippets
Experimental
We have prepared the 2.5×2.5 cm2-sized ITO glass (Samsung Corning, ∼19 Ω sq−1) as an electrode substrate for electrodeposition by cleaning it with a 0.5 M of potassium hydroxide (KOH) solution and deionized water, and by subsequent drying with nitrogen (N2) gas. Copper conducting tape (3M Company) was then attached on one side of ITO glass to serve as a bus bar.
Three electrodes were used for the electro-polymerization of conducting monomers: ITO glass as a working, fluorine-doped tin oxide (FTO,
Results and discussion
In order to maximize complementation properties of electrodes where ECD quality such as charge capacity, optical contrast, coloration efficiency, and response time could be varied, we controlled the deposited thickness of PANI on ITO substrate by changing the deposition time of electrochemical polymerization under three different conditions (50, 60, and 90 s, hereafter abbreviated as PANI50, PANI60, and PANI90 electrode, respectively). The optimal thickness of PEDOT was fixed to about 200 nm,
Conclusion
In conclusion, an organic–organic hybrid ECD with enhanced optical contrast and fast response time was developed successfully by employing a PANI anode with controlled thickness and a PEDOT cathode in combination with hydrophobic lithium electrolyte. In the optimal case where the 60 nm thickness PANI electrode was used, the PANI60–PEDOT ECD exhibited an ideally matched integrated charge of Q=0.91 between −1.3 and 0.9 V, a noticeable increase in optical contrast of 66Δ%T, an enhanced coloration
Acknowledgments
This work was supported by the SRC program of MOST/KOSEF through the Center for Intelligent Nano-Bio Materials at Ewha Womans University (Grant no. R11-2005-008-00000-0). Joo-Hee Kang is grateful to the Ministry of Education for the Brain Korea 21 (BK21) fellowship.
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