We developed a single-panel LCD microdisplay system using a field-sequential color (FSC) driving method and an organic light-emitting diode (OLED) as a backlight unit (BLU). The 0.76′′ OLED BLU with red, green, and blue (RGB) colors was fabricated by a conventional UV photolithography patterning process and by vacuum deposition of small molecule organic layers. The field-sequential driving frequency was set to 255 Hz to allow each of the RGB colors to be generated without color mixing at the given display frame rate. A prototype FSC LCD microdisplay system consisting of a 0.7′′ LCD microdisplay panel and the 0.76′′ OLED BLU successfully exhibited color display and moving picture images using the FSC driving method.
Organic light-emitting diodes (OLEDs) have attracted extensive attention during the past two decades for potential applications in flat panel displays owing to their intrinsic properties such as self emission, low weight, high luminance, high contrast, wide viewing angle, and fast response speed [
In order to expand OLED market, it is worth noting the case of light emitting diode (LED) industry. The demand for high-performance LEDs has increased greatly with the emergence of new technologies and applications such as signage, automotive lighting, mobile display devices, and LED backlight units (BLUs) for LED TVs. From this point of view, our group has been attempting to develop OLEDs as BLUs for LCD microdisplays [
Microdisplays with a diagonal size of less than 1′′ (25.4 mm) are widely used not only in small screen devices such as camcorder viewfinders, digital cameras, and other portable devices but also in large screen devices such as projectors, TVs, head-mounted displays (HMD), and 3D displays [
However, full-color microdisplays may have some limitations when used in LCD or OLED devices, because of reduction in the pixel size, such as a decrease in the luminance efficiency and production yield with a decrease in the aperture ratio. In order to resolve these problems, we have developed an LCD microdisplay based on the OLED BLU and that uses a field-sequential color (FSC) driving method.
A full-color display can be achieved using the FSC driving method wherein, three monochromatic images corresponding to the three primary colors, red (R), green (G) and blue (B) are combined in a repetitive sequence and at a frame rate that is more than three times that of the conventional frame rate (60 Hz) [
Most researches on FSC LCDs have been performed using an LED BLU that uses many expensive optical films to convert a point source into a flat source. We have developed an FSC LCD microdisplay using an OLED BLU, because it is cheaper than the conventional LED BLU and it eliminates the need for expensive optical films and color filter layers. Figure
Comparison between (a) LCD display with LED BLU and (b) FSC LCD display with OLED BLU.
In this paper, we describe in detail the development of a single-panel LCD microdisplay system using an OLED BLU and the FSC driving method in terms of the optimization of the aperture ratio as well as the electro-optical and image display performances of the microdisplay.
The 0.76′′ OLED BLU with high aperture ratio (73%), which has stripe-patterned RGB arrays, was fabricated using conventional materials and methods as shown in Figure
Structure of stripe-patterned RGB OLED device; (a) Top (b) Cross-sectional view.
Materials: 4,4′,4′′-Tris (2-naphthylphenylamino) triphenylamine [2-TNATA] was purchased from E.L.M. Co., Ltd., Korea. N,N′-Di-(naphthalene-1-yl)-N,N′-diphenylbenzidine [
All the organic materials were used in as received state without further purification. The organic layers and metal electrode layers were formed using a vacuum thermal evaporator (5.0 × 10−6 Torr) equipped with a CCD alignment system and a metal shadow mask. After the cathode electrode deposition, we encapsulated the OLED device using cover glass and getter material to protect the device from deterioration by oxygen and moisture.
As shown in Figure
(a) Configuration of FSC LCD microdisplay system based on OLED BLU and (b) FSC driving scheme.
During the first subframe, previously addressed red LC pixel is on, and the red OLED BLU is simultaneously illuminated, while the green video data for second subframe is applied and stored through the
We measured the current density-voltage-luminance (J-V-L) and emitting spectrum properties using a CS1000 Spectroradiometer and a Keithley 236 Source Meter. The electro-optical response property of the FSC driving OLED BLU was investigated by a digital oscilloscope (Tektronix, DPO-4104) and a photodiode with a power meter (Newport).
Prior to making a full-color OLED device, we tested four kinds of devices with an aperture ratio of 56%, 67%, 73%, and 81%, which is the ratio of emitting area to nonemitting area in one pixel, and found the optimum aperture ratio for high brightness without traces of stripe patterns when the OLED is covered with a thin diffuser film. The pitches in Figure
Emitting images as function of emitting width (or aperture ratio) with and without diffuser film (the aperture ratio and emitting width are 56% and 90
The emitting images of R, G, B, and W with and without diffuser film for device with an aperture ratio of 73%, and their emitting spectra were shown in Figure
(a) Emitting image of R, G, B, and W with and without diffuser film and (b) emitting spectrum of fabricated RGB stripe-patterned OLED device.
The emitting images with diffuser film showed clear R, G, B, and W colors without any stripe patterns though having some defects. Their emitting spectrum had three peaks at 623 nm, 519 nm, and 460 nm. Our red, green, and blue emitting layers were host-dopant systems of fluorescent material designed to improve the luminous efficiency [
Figure
Luminance as function of applied voltages for R, G, B, and W state OLED.
Figure
Electro-optical response of FSC driving OLED BLU.
Figure
Captured images of color display operation of 0.7′′ microdisplay system.
Our OLED BLU and LCD panel have potential for faster driving performance, so we are now trying to develop a 3D display employing an additional cylindrical microlens array (MLA) attached to our newly developed microdisplay system.
We have developed a single-panel LCD microdisplay system using a newly designed RGB color OLED device as a BLU. A low-cost microdisplay system with high optical efficiency and high pixel density was realized using the OLED BLU and the FSC driving method, as they eliminate the need for color filter layers, subpixels, and various optical films. Our microdisplay system shows a fast response speed, distinct color display, and clear moving picture images. The proposed microdisplay can open up a new application area of OLED devices as BLUs. It might be useful in a wide range of applications from small size mobile displays (microprojector, head-up display, mobile phone, etc.) to flexible large size displays by introducing a flexible LCD panel and a flexible OLED BLU. We also expect the development of 3D display systems using our microdisplay with an additional MLA in the near future.
This work was supported by the DGIST Basic R&D Program of the Ministry of Education, Science, and Technology (MEST) of Korea. The authors would like to express thanks to Mr. Kil Whan Oh (ILJIN DISPLAY Co.) for his fruitful discussions on the 0.7′′ SVGA HTPS LCD panel, Ms. Gwi Jeong Cho (Kyungpook National University) for assisting OLED fabrication processes, and Ms. Yun Seon Do (DGIST) for helping the measurement of electro-optical response property. The authors also would like to give a special thanks to Professor Youngkyoo Kim of Kyungpook National University for his great advice on OLED devices.