Innovative polymer technologies for flexible liquid crystal displays

It is generally thought that the development of flexible display technologies will soon substantially broaden the possibilities for displays. The specific characteristics of the flexible technologies (in addition to their excellent flexibility, they can be easily handled and stored) mean that flexible displays could find several applications in everyday life (e.g., for posters and calendars, roll-screen TVs and signboards, large maps and design charts, wearable costumes and seals, as well as for packaging and painting materials). To realize these flexible displays, however, it is necessary for the display panel to be made sufficiently thin, light, and flexible for easy handling.

Organic LEDs are generally thought to be the most promising candidates for achieving super-flexible displays. Such devices, however, come with a number of drawbacks. For example, organic LEDs have limited lifetimes because of the poor gas barriers of their plastic substrates. In addition, the complex matrix driving systems of organic LEDs mean that they are associated with high production costs.

In our work, we have therefore previously shown that liquid crystal (LC) and plastic substrates^1–3 are good options for achieving the desired (i.e., improved) properties for flexible displays. For example, flexible LCDs have excellent storability, portability, and designability and can thus be used to create new viewing methods and human interfaces. Low-cost fabrication of large-area panels with flexible LCDs is also possible. The strongest benefit of flexible LCDs, however, is their excellent stability and reliability for the passage of moisture and oxygen through their substrates. Although LCDs exhibit these many attractive features (compared with ultraslim displays that include organic LEDs), the bending tolerance of flexible LCDs is limited. Indeed, we have previously fabricated a slightly curved LC device from polycarbonate substrates and conventional etched spacers (see Figure 1). As we increase the bending degree of this device, however, the substrate gap decreases at the curving center (see Figure 2), which causes the displayed image quality to be seriously degraded. Moreover, for devices with smaller radius of curvatures, the structural deformation can be even more severe.

Figure 1. A slightly curved liquid crystal (LC) device fabricated from polycarbonate substrates and conventional etched spacers. As the degree of bending increases, the substrate gap decreases at the curving center. This causes degradation of the displayed image (see Figure 2).

Figure 2. Schematic illustration of a deformation model for bent flexible LC devices that include plastic substrates.

In our more recent work, we have thus developed polymer wall spacers^4,5 to rigidly bond two substrates and to increase the bending tolerance of LCDs. We construct these spacers from a molecular-aligned solution of a liquid crystalline photoreactive monomer and a nematic LC, and we control the molecular alignment of the solution by coating the two substrates with polyimide alignment layers. Specifically, we conduct the fabrication of our bonding wall spacers under UV radiation that is patterned with an orthogonal-latticed photomask. During this process (see Figure 3), molecules of the drifted monomers move toward the UV-irradiated regions of the solution. This photopolymerization-induced phase separation phenomenon causes the networking polymers to be segregated into the UV-exposed areas. The aligned polymer aggregation then forms a condensed wall spacer that creates the bond between the two substrates. Furthermore, the orthogonal-lattice-shaped walls have large bonding areas and therefore a good LC-confining effect. As a result, the molecular-aligned polymer wall surfaces prevent LC material flow (even during large amounts of bending) and do not suffer from disordered LC alignment in high-contrast displays. We have also investigated the use of several different monomers and found that the polymer aggregation efficiency depends on the molecular structural features of the monomers (e.g., their skeleton flexibility and motility).

Figure 3. Schematic diagram illustrating the fabrication process (left) for the polymer wall spacers. A scanning electron image of a lattice-shaped polymer wall formed from this process is shown on the right.

With our bonding polymer wall spacers we can therefore maintain a constant substrate gap and enable the use of extremely thin plastic substrates in LCDs (i.e., when the use of the spacers becomes even more beneficial). We have thus also designed an LCD-fabrication process in which we use ultrathin plastic substrates (formed via coat-debonding). We have demonstrated that our 10μm-thick coat-debond polyimide substrates exhibit excellent heat resistance, high light transmittance, and low optical anisotropy.^{6, 7} In addition, the final super-flexible LC device we fabricated (see Figure 4) has an excellent bending tolerance (curvature radius of several millimeters). With the use of our polymer wall spacers it is therefore possible to make any LC-mode display highly flexible. To date, we have realized nematic systems of twisted or guest-host LCs, cholesteric systems of cholesteric-to-nematic phase change or blue-phase (BP) LCs, and a smectic system of ferroelectric LCs.

Figure 4. Left: Photograph of an ultrathin (10μm) polyimide substrate formed by a coat-debond process. Right: A super-flexible LCD, without polarizers, formed from the polyimide substrate. The device is shown rolled up during a bending tolerance test.

We have also recently fabricated flexible BP LC devices that have optical isotropy and exhibit a wide range of viewing angles.^{8, 9} To obtain a high contrast ratio and a wide viewing angle with our devices, however, it is necessary to perform optical compensation. We have therefore developed a number of total compensation techniques^{10, 11} for the LC substrates in commonly used vertically aligned (VA) and in-plane switching LCDs. Our optical techniques are based on precise ellipsometry measurements for various plastic substrates. For example, the positive-c-plate optical anisotropy of a VA-LC and the negative-c-plate property of a polycarbonate substrate compensate each other. We obliquely observed the black states for a VA-LC device with and without optical compensation (see Figure 5) and found that even with the use of a plastic substrate, a wide viewing angle (160°) and a high contrast ratio (650:1) can be obtained. With our results we have thus, for the first time, confirmed that flexible LCDs can be used to display high-quality images (at a similar level to conventional glass-based LCDs).

Figure 5. Measurement of the black states of a flexible polycarbonate-substrate vertically aligned LCD with (bottom) and without (top) optical compensation. Even with the use of a plastic substrate, a wide viewing angle and high contrast ratio is achieved in both cases.

To enhance the displayed image quality of LCDs even further, a flexible backlight (including LED chips) is an important component. We have therefore developed a direct-illumination backlight sheet and a transparent rubber light-guide plate for this purpose.^{12, 13} Although recently available light-guide plastic plates are flexible (because of thickness reduction), local dimming control of the backlight is essential for achieving higher contrast ratios and lower power consumption (i.e., so that they are suitable for mobile terminals and large TVs). In our more recent work, we have thus proposed a flexible light-guide backlight system that includes an alignment-controlled polymer-dispersed LC (PDLC) cell with high transparency.^{14, 15} In this PDLC light-guide plate the light-illumination area (generated by light scattering) is selected with the use of segmented transparent electrodes that are driven according to the displayed images.

In summary, we have investigated and developed a number of polymer technologies to improve LC devices for use in flexible displays. For instance,^{15, 16} our polymer wall spacers enable the use of extremely thin plastic substrates and make any LC-mode display highly flexible. In addition, we have implemented optical compensation techniques to increase the contrast ratio and viewing angle of LCDs, as well as flexible light-guide backlight systems to increase image quality even further. In the next stage of our research we will form small image pixels with thin-film transistors on a flexible plastic substrate. With our various techniques for flexible LCDs it is therefore possible to achieve low-cost and high-resolution large-screen displays (i.e., that are difficult to realize with more-conventional organic LED displays). We believe that flexible LCDs will therefore have a substantial impact in image-information service technologies in the coming years.