We have been researching on flexible displays with the goal of creating high-quality, large-screen displays for watching television anywhere, indoors or out. We are conducting R&D on organic light-emitting diodes (OLED) and liquid crystal (LC) film display devices, organic thin-film transistors (TFT), and active matrix panels comprising arrays of TFTs.

Organic light-emitting diodes (OLED)

We are researching phosphorescent OLEDs to increase the emission efficiency of OLED displays. We have already developed phosphorescent polymers that can be used in thin film wet-fabrication processes. These polymers confer an external quantum efficiency of about 10% for red, green and blue. We have also fabricated a prototype of a full-color, flexible OLED display using an ink-jet printing method. In FY2009, we evaluated time-resolved electroluminescence spectra of the phosphorescent devices driven with an extremely short pulse in order to study the energy transfer processes when driving these devices. The results suggested that there are differences between photo excitation and current excitation in the energy-transfer and emission processes in the emission layer. Furthermore, we evaluated the mobility of electrons and holes in the materials by using a time-of-flight method and showed that the mobility can be controlled by the polymerization ratio of the hole- and electron-transport monomer materials. By considering the balance between mobility and electron injection, we were able to obtain external quantum efficiency for green of 14.5%.

Liquid crystal (LC) film display devices

In order to create flexible LC televisions that can be easily scaled to larger sizes, we are researching a self-supporting composite film of fast-response LC material (ferroelectric LC) and polymers bonded with two plastic substrates. This device is formed using a wide-area printing process and an ultraviolet-light polymer-curing process. So far we have fabricated flexible LC devices up to A4 size. We have also developed flexible backlights using light-guide plates or direct illumination plates with small light-emitting diodes, and have fabricated a full color video display by driving the LC composite film with polycrystalline silicon TFTs and organic TFTs. In FY2009, we proposed using strong, interdigitated spacers composed of concavo-convex structures formed on the surfaces of the two substrates. We used a roller nanoimprint technique to form striped concavo-convex structures, to maintain the process precision needed for interdigitation. We also fabricated flexible display devices using twisted nematic liquid crystal, which is resistant to bending, and determined the conditions for forming spacer walls for a high-contrast display.

Organic TFT devices

We have been improving the performance of organic thin-film transistors (TFT) for actively driving flexible displays. Moreover, we have evaluated and analyzed the mechanisms affecting the operating voltage of organic TFTs with highly capacitive gate insulators (Tantalum pentoxide, Ta₂O₅). We have also employed surface treatments using self-assembling monolayers to improve the crystallinity of semiconducting polymer film and have studied degradation mechanisms of organic TFTs in the atmosphere. In FY2009, we reduced the energy barrier between organic semiconductor and electrodes by using a solution process selectively forming a hole-injection layer (MoOx, F₄TCNQ) over the source and drain electrodes. We increased the output current in small-molecular and polymer semiconductor TFT devices by a factor of ten. We also manufactured a bottom-contact organic TFT array on plastic by using amorphous fluoro polymer in the gate insulator. The array's performance was superior to that of amorphous silicon TFTs.

Active matrix driven panels

An active matrix technology using flexible organic TFTs must be developed in order to make a bright, high-contrast display. We have already created 42-ppi flexible display panels (OLED and LC) with low operating voltages by using organic TFT arrays with high-dielectric-constant gate insulators. In FY2009, we introduced an isolating wall structure using an organic material that allows the size of an organic semiconductor to be reduced and manufactured a TFT array on a plastic substrate with improved on-current and surface uniformity (two-TFT per pixel structure, Figure 1). We used pentacene as the organic semiconductor and achieved a carrier mobility of 0.15 cm²/Vs and an ON/OFF ratio of over 10⁶. We used vacuum deposition to form phosphorescent organic LED devices on this TFT layer and made a 5.8-inch diagonal 213 (RGB) x 120 pixel panel capable of displaying color video at a frame rate of 60 Hz and operating while curved (Figure 2).

Figure 1. Prototype TFT array magnified pixel image

Figure 2. Curved flexible color OLED panel displaying an image