Of all the leading display technologies, none has generated more excitement as the display technology of the future than organic light-emitting diodes (OLEDs). OLEDs possess all of the positive attributes of any current display technology with little or no negative features—at least not yet.
For example, they don’t require any backlighting like other displays, such as liquid-crystal displays (LCDs). OLEDs present bright, clear video and images (brightness levels of more than 1000 candelas/m2 and contrast ratios greater than 10,000: 1) that are easy to see at almost any angle. They also dissipate low amounts of power and have fast switching rates. Their response times are in the range of a few microseconds, which together with their color-producing capability (over 16 million colors), makes them ideal candidates for TVs. NTSC-compatible TVs have already been demonstrated.
Furthermore, they’re lightweight and extraordinarily thin. At this year’s Display 2008 Conference, Sony showed off a 0.2-mm prototype—the thinnest OLED yet, according to Sony. Its manufacturing costs have the potential to be lower than other displays. Work on that front is ongoing and looks very promising. Several companies have tried roll-to-roll manufacturing with various levels of success.
But there are some drawbacks. A big one is a limited lifetime, particularly for blue and green colors. Part of the reason is the need to keep out water, which can damage an OLED’s organic materials, necessitating very tight sealing levels during their manufacturing.
Experimental green OLEDs with lifetimes of nearly 200,000 hours have been obtained. To date, the best lifetimes achieved for experimental blue OLEDs have been about 62,000 hours. A joint development by Toshiba, Matsushita, and Idemitsu Kosan yielded similar results using a thin-film transistor (TFT) substrate. Their work concentrated on a 2.2-in., 240- by 320-pixel quarter video graphics array (QVGA) for mobile phones, achieving 100 mW of power consumption.
OLEDs also typically emit less light per unit area than inorganic solid-state LEDs, which are usually designed for use as point-light sources. In fact, Epson Co. developed OLED materials that contributed to longer lifetimes by eliminating some early-stage deterioration of the organic materials.
Given these facts and the bright outlook researchers predict for OLED displays, design engineers should get to know more about OLEDs—how they work, how to apply them, what performance levels can be expected, and the status of this exciting technology, which is sure to satisfy a range of future displays.
THE BASIC STRUCTURE
An OLED consists of a metal cathode (typically aluminum or calcium) and an anode (typically indium tin oxide, or ITO) located on a glass substrate. Between these electrodes lie deposited emissive and conductive layers of organic molecules or polymers (Fig. 1). The deposition process occurs in rows and columns on a flat carrier by a “printing” process, forming a matrix of pixels that emit light of different colors, like red, green, blue, or white. Several layers can be stacked on top of one another.
OLEDs operate on the attraction between positively charged (holes) and negatively charged (electrons) particles. When voltage is applied, one layer becomes negatively charged relative to another transparent layer. As energy passes from the negatively charged (cathode) layer to the other (anode) layer, it stimulates organic material between the two, which emits light visible through the outermost layer of glass.
Electrostatic forces bring the electrons and holes toward each other and they recombine. The recombination occurs closer to the emissive layer, because in organic semiconductors, holes are more mobile than electrons. The recombination causes a drop in the energy levels of the electrons, accompanied by an emission of radiation whose frequency is in the visible region.
Should the anode have a negative potential with respect to the cathode, the OLED won’t work. In this condition, holes move to the anode and electrons to the cathode, so they move away from each other and thus don’t recombine.
Doping or enhancing organic material helps control the brightness and color of light. The organic materials can consist of small single structures or molecules, or complex chains of molecules (polymers), to best suit the manner in which they are produced.
The original OLEDs, developed by Eastman-Kodak in the late 1980s, made use of small organic molecules. Although small molecules emitted bright light, they had to be made in a costly vacuum deposition process. More recently, larger polymer molecules have been used, which can be made less expensively and in large sheets, suiting them for large-screen displays.
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