A high-speed passive-matrix electrochromic display using a mesoporous TiO2 electrode with vertical porosity.

Recently, the application of electronic paper (E-paper) has attracted considerable attention. Many types of reflective displays, such as reflective liquid crystal displays and electrophoretic displays, have been introduced and applied to E-paper. Among them, electrochromic materials, which change in color intensity when an appropriate potential is applied, are the subject of an increasing number of reports. Recently, polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) or catenanes were reported to show electrochromic behavior. The slow response time for coloring has been a serious problem with these kinds of polymers. As probable electrochromic materials, viologens have commonly been utilized for electrochromic displays (ECDs). Many studies have focused on viologen-modified microspheres or nanostructures to increase the switching speed. Viologens are basically blue in color, and it is thus difficult to realize a full-color display. Furthermore, these kinds of displays have a common drawback: poor background whiteness. To date, electronic displays have not been able to meet the requirements necessary for extensive practical applications. Currently, full-color reflective displays that demonstrate a fast response time are in much demand. Usually, display devices are driven by either active-matrix drive mode or passivematrix drive mode. Active-matrix drive mode is very fast, but it needs expensive thin-film transistors (TFT) for all the pixels of the display, which leads to a high price. Passive-matrix drive mode does not need such expensive electric elements, and it has a simple, low-cost structure. However, when ECD devices are driven by passive-matrix mode at high scanning speed, the drift of electrochromic materials around the electrode leads to poor resolution. That is, the display images are blurry. Herein, we aim to realize high scanning speed and high display quality. We focused on leuco dyes, which are well known as recording materials in thermal imaging systems, because the leuco dyes show a wide variety of colors and are commercially available. We demonstrated a high-speed and high-resolution electrochromic passive-matrix display using a leuco dye with a mesoporous TiO2 electrode with vertical pores (Figure 1). The vertical pores of the electrode can support effective diffusion of leuco dyes perpendicular to the electrode and can prevent the diffusion of the dye around the electrode. Since the colorless state of this kind of display is transparent, it exhibits better background whiteness, which improves readability and reduces eyestrain. Furthermore, the application of leuco dyes to ECD devices has high potential to realize a full-color reflective display with low production costs. These features are very desirable for future E-paper applications. Our device, which consists of two electrodes (working electrode and counter electrode) and electrolyte (Figure 1), was driven by the passive-matrix driving method (an addressing scheme used in earlier liquid crystal displays). Each electrode has striped indium–tin-oxide (ITO) layers 420 mm wide on a glass substrate (Figure 1b and Figure S1 in the Supporting Information). The mesoporous TiO2 film was grown only on the observation side of the working electrode. By improving the previous method, continuous TiO2 films with highly ordered mesostructure and vertical pores were uniformly prepared on the working electrodes by spin coating with a precursor solution. The film thicknesses were changed by using different spinning speeds. Thicknesses of approximately 300, 200, and 100 nm were realized by speeds of 2000, 4000, and 6000 rpm, respectively. Cross-sectional and topsurface SEM images showed that mesopores were oriented vertically with respect to the substrate (Figure S1c in the Supporting Information). The mesochannel walls are composed of periodically arranged cages with connecting necks between the neighboring cages (see the Supporting Information, in particular Figure S2, for details). The two electrodes sandwiched the electrolyte so that the striped ITO layers were orthogonally crossed (Figure 1b and Figure S3 in the Supporting Information). The electrolytic solution consisted of black leuco dye (2-(3’-trifluoromethylphenylamino)-6’[*] W. Weng, T. Higuchi, M. Suzuki, T. Fukuoka, Dr. T. Shimomura, Dr. M. Ono Funai Electric Advanced Applied Technology Research Institute Inc. 2-1-6 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) E-mail: weng.w@funai-atri.co.jp