Design considerations for full-color e-paper
A new color microcapsule stepwise transfer process for fabricating full-color electrophoretic displays produces the widest color gamut for these devices to date.
Electrophoretic display (EPD) technology—in which black and white particles suspended in a liquid medium are migrated to the front (viewing) or rear side of the display using an electric charge—is widely used in e-paper applications because of its flexibility, low power consumption, and good visibility in sunlight. The commercial success of monochromic (single-color) EPDs has motivated the development of full-color EPDs to satisfy the demand for this technology in a range of devices. However, several challenges still need to be overcome before this technology can be commercialized.
Many research groups and companies have intensively explored full-color EPD technology, resulting in the development of a number of different strategies (see Figure 1 and Table 1).1–8 The first of these strategies, the addition of color filters, is a mature technology used in LCD devices. However, when color filters are added to monochromic EPDs the reflected light is reduced by 70%, resulting in very low contrast ratios; hence, this strategy is not appropriate for commercialization. Alternatively, the white particles in each pixel can be suspended in different colored dyes, but this approach currently has technical problems relating to the spatial distribution of the different colored pixels.
| Strategy | Year | %NTSC | Contrast Ratio | Notes | Ref. |
| Color filter | 2006 | ∼4% | n/a | None | 1 |
| 2011 | 13% | 10:1 | None | 2 | |
| 2011 | 14.1% | n/a | Printed plastic color filter | 3 | |
| 2012 | 3.14% | 8:1 | Direct lithographic color filter | 4 | |
| Color dye | 2006 | n/a | n/a | None | 5 |
| Color particles | 2010 | 6.63% (RGB) | n/a | Calculated from single color particles | 6 |
| 11.83% (CMY) | |||||
| 2014 | 1.56% | 23:1 | Red, black, and white particles in one pixel | 7 | |
| 2016 | n/a | n/a | CMY and white particles in one pixel | 8 | |
| 2016 | 19.1% | n/a | Our full-color EPD prototype | 9 | |
Another approach to full-color EPDs is the multi-particle system, in which particles of different colors in a single pixel are migrated by adjusting the mass and zeta potential (the degree of electrostatic repulsion between particles) of each color particle. This strategy produces excellent color performance, but it is difficult to precisely manipulate the particles in such a complicated system.8 To avoid this issue, a micro-encapsulated dual-particle system has been developed. In this strategy, microcapsules containing white and single-colored particles in a colorless medium are used as subpixels, producing a spatially mixed full-color display. Here, we describe our recent work using color microcapsules to fabricate a full-color EPD prototype with a color gamut (the range of colors a device can reproduce) measured in %NTSC—the percentage of total color space that can theoretically be reproduced by devices—of 19.1% (see Figure 2), the highest full-color EPD %NTSC to date.9
The major problem to overcome in fabricating color microcapsule EPDs is patterning the microcapsules. We developed a microcapsule patterning process using gravure printing, in which trenches are etched or engraved into a mold and filled with a liquid. In this case, the trenches in the polydimethylsiloxane (PDMS) mold are filled with the microcapsule slurry (made with a hydroxypropyl methylcellulose binding agent) by blade coating (see Figure 3). This patterning process gives single-color microcapsule squares with a side length of 300μm—shown in Figure 4(a)—and the size and spacing of the squares can be varied easily: see Figure 4(b,c). However, the microcapsules of other colors cannot be added using simple gravure printing, so we extended the patterning method to include a stepwise transfer process. The microcapsule squares are coated with an adhesive in the mold, transferred to a temporary PDMS substrate, and then to the final substrate (e.g., polyethylene terephthalate). Repeating this process for all the required microcapsule colors gives the full-color EPD prototype: see Figure 3 and Figure 4(d).
The choice of temporary substrate and adhesive are key considerations in the stepwise transfer process as it is imperative that the microcapsules do not permanently bond to the temporary substrate. PDMS was chosen as the temporary substrate owing to its chemical inertness and tunable surface energy, which enables us to control the adhesion between the acrylic adhesive and the PDMS surface. By altering the PDMS surface energy with UV irradiation or ozone treatment,10 we can easily detach the microcapsules from the temporary substrate when required.
In summary, we have developed a stepwise transfer process for fabricating full-color EPDs with spatially mixed microcapsule subpixels. This approach avoids the low contrast ratios and complicated particle manipulation associated with other full-color EPD strategies, and we have used the process to fabricate a prototype with a %NTSC of 19.1%, the highest full-color EPD %NTSC to date. Our future work in this area will involve designing a color-mixing system based on the microcapsules, and the fabrication of further full-color EPD prototypes.
Bo-Ru Yang completed his PhD at National Chiao Tung University, Taiwan, which included visits to University of Oxford, UK and Tohoku University, Japan, as a doctoral researcher. He subsequently joined SiPix and became a research and development manager. He is currently an associate professor at SYSU, an associate editor of the Journal of the Society for Information Display, and vice chair of the flexible display and e-paper committee at the Society for Information Display.
