Organic light-emitting diodes (OLEDs) are grounded in a self-luminous display technology based on thin organic films as the light emitter. Like conventional inorganic light-emitting diodes (LEDs), OLEDs require a low drive voltage to produce bright visible light. But unlike discrete LEDs, which have crystalline origins, film-based OLEDs are an area emitter than can easily be patterned to produce flat-panel displays. Because OLEDs are self-luminous, they do not require a backlight as in LCDs. They have very low power requirements and are thin, bright, and efficient. As a result, many see them as the display technology of the future.

The benefits of OLED technology include the following:

  • High brightness and contrast, making displays very bright and easy to read;
  • Wide viewing angles, up to 160 degrees;
  • Full color, to communicate the most information in the most engaging way;
  • Fast response and wide temperature range of operation, for quick-response displays in a variety of environments;
  • Low power consumption and operating voltage, maximizing battery life and minimizing heat and electrical interference in electronic devices;
  • Light weight, compact, and thin, reducing the size and weight of devices that use displays;
  • Robust, tough enough to use in portable devices such as cellular phones and personal digital assistants (PDAs); and
  • Low cost.

OLED technology is already at work in car audio units produced by Pioneer Electronics. It will soon be found in cellular phones, PDAs, digital cameras, video cameras, head-wearable displays, and car instrumentation. As the technology is refined, it holds possibilities in computer monitors, video displays, ultrathin lighting panels, and even flexible displays that could be rolled and unrolled.

Figure 1. The basic OLED structure comprises several thin-film layers of organic materials (ETL, emitters, and HIL) covered over with an evaporated metal cathode. Total thickness of the deposited structure is less than the wavelength of green light.

Eastman Kodak Company scientists discovered OLED more than a decade ago. Since then, Kodak researchers have made a number of breakthroughs that led to patents on basic OLED materials, device structure, doping techniques to improve efficiency and control color, thin-film deposition methods, patterning methods, and design and fabrication methods for both passive- and active-matrix OLED panels.

Kodak's OLED technology grew initially from research on organic electronic devices used in solar cells and electrophotography. Now the company holds more than 40 U.S. patents, has many pending applications, and has patents and applications overseas on the basic structure of OLED devices, several unique classes of OLED materials, and fabrication methods and drive schemes.

The development of OLED technology supports one of Kodak's strategic initiatives in digital imaging technology: a revolutionary electronic image display. A new active-matrix full-color OLED display, demonstrated in October by Kodak and Sanyo Electric Co., will soon be used in digital cameras and many other portable electronics. Because of their superior viewing qualities, the full-color OLED displays will replace many conventional LCD screens.

The basic OLED cell structure (Figure 1) consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode. The organic layers comprise a hole-injection layer, an emissive layer, and an electron-transport layer. When an appropriate voltage (typically a few volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electroluminescence, or EL). The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the device. Figure 2 shows an active-matrix OLED as contrasted with a conventional LED display.

Figure 2. The active-matrix OLED display (left) as contrasted with a conventional LED display.

Remarkable enhancement of the electroluminescent efficiency and control of color output have been achieved by doping the emissive layer with a small amount of highly fluorescent molecules. This doping technique, patented by Kodak, is critical for producing color OLED displays.

There are two types of OLED displays, passive-matrix and active-matrix. The passive-matrix OLED display has a simple structure and is well suited for low-cost and low-information-content applications such as alphanumeric displays. The active-matrix OLED has an integrated electronic backplane as its substrate and lends itself to high-resolution, high-information-content applications, including video and graphics.

A passive display is formed by providing an array of OLED pixels connected by intersecting anode and cathode conductors. Kodak developed a relatively simple but unique method for its fabrication. Here a rib structure is preformed on patterned indium tin oxide (ITO) anode lines. As the organic materials and cathode metal are deposited, the rib structure automatically produces an OLED display panel with the desired electrical isolation for the cathode lines. To drive a passive-matrix OLED display, electrical current is passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers attached to each row and column.

In contrast to the passive-matrix OLED display, an active-matrix display includes an electronic backplane in the display panel.This type is made possible by the development of polysilicon technology (PolySi), which because of its high carrier mobility provides thin-film transistors (TFT) with high current-carrying capability and high switching speed. There are several key advantages in active-matrix displays: low voltage and power consumption, high resolution, large area, robust pixel design, and integrated drivers.

In an active-matrix OLED display, each individual pixel can be addressed independently via the associated TFTs and capacitors in the electronic backplane. In principle, each pixel element can be selected to stay "on" during the entire frame time. Since OLED is an emissive device, the display aperture factor is not critical, unlike LCD displays, where light must pass through an aperture. Therefore, there are no intrinsic limitations to the pixel count, resolution, or size of an active-matrix OLED display. Also, thanks to the TFTs, a defective pixel produces only a dark defect, considered much less objectionable than either a bright-point defect, such as in LCD panels, or a line defect. Furthermore, constant-current drivers for OLED and the necessary scanning circuitry based on PolySci can be built directly on the substrate, thus eliminating the need for high-density (and expensive) interconnects and peripheral drivers.

Kodak engineers expect that OLED technology will have an important impact on the display industry. Low-cost manufacturing methods are already in use for passive-matrix OLED displays, and the advance of the complementary low-temperature PolySci technology has enabled fabrication of high-resolution, full-color active-matrix OLED displays. Although OLED has just entered the display marketplace, Kodak believes it will continue to expand, and the race to manufacture active-matrix OLEDs will accelerate.

For more information on Kodak Professional Products, contact Dr. David J. Williams, Eastman Kodak Co., 1999 Lake Ave., Rochester, NY 14650; (716) 477-7575; or visit the web site at www.kodak.com/go/oel.


Photonics Tech Briefs Magazine

This article first appeared in the May, 2000 issue of Photonics Tech Briefs Magazine.

Read more articles from the archives here.