OLEDs Put On Quite A Display

Oct. 9, 2008
Though the technology is in its infancy, OLEDs are quickly making their way into displays of all sizes in portable products, TVs large and small, and energy-efficient white-light sources.

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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ACTIVE AND PASSIVE MATRIX Just like LCDs, OLEDs come in either the active- or passive-matrix variety. Each type lends itself to different applications. In an active-matrix OLED (AM OLED), cathode, organic, and anode layers are stacked above a low-temperature polysilicon substrate layer that contains TFT circuitry (Fig. 2). A corresponding circuit delivers voltage to the cathode and anode, stimulating the organic layer.

Pixels are defined by the deposition of the organic material in a continuous, discrete dot pattern. Each pixel is activated directly and independently via the associated TFTs and capacitors in the electronic backplane.

An AM OLED pixel turns on and off more than three times faster than the speed of conventional motion-picture film. This makes AM OLEDs ideal for fluid, fullmotion video and graphics. The substrate transmits electrical current very efficiently, and its integrated circuitry reduces an AM OLED’s weight and cost. There are no intrinsic limitations to the pixel count, opening commercial possibilities.

A passive-matrix OLED (PM OLED) is structurally simpler than an AM OLED and is therefore less expensive to produce. This suits it quite well for low-cost and low-information-content applications, such as alphanumeric displays. A PM OLED is formed by an array of OLED pixels connected by intersecting anode and cathode conductors (Fig. 3). Organic materials and cathode metal are deposited into a rib structure consisting of a base and pillar. Such a structure automatically produces an OLED display panel with the desired electrical isolation for the cathode lines.

A major advantage of PM OLEDs is that they can be patterned using conventional fabrication techniques. The entire panel fabrication process can be easily adapted to large-area and high-throughput manufacturing.

A PM OLED works by passing electrical current through selected pixels by applying a voltage to the corresponding rows and columns from drivers attached to each row and column. An external controller circuit provides the necessary input power, video data signal, and multiplexing switches. Data signals are generally supplied to the column lines and synchronized to the scanning of the row lines. When a specific row is selected, column and row data lines determine which pixels are lit. Subsequently, a video output is displayed on the panel by scanning through all of the rows successively in a single frame time, which is typically 1/60th of a second.

Cambridge Display Technology (CDT) has developed a technique called total matrix addressing. It blends the best characteristics of both passive- and active-matrix addressing at little or no penalty. CDT is working on bringing the technology to market.

PM OLED displays have some advocates. Dialog Semiconductor’s SmartXtend display driver technology will let the main displays of mobile devices, particularly those offering W-QVGA and QVGA resolution, use PM OLEDs at a much lower cost than AM OLEDs. Yet it will still provide the same advantages in video quality and performance. It reduces PM OLED peak currents and power consumption by up to 30% compared to conventional PM OLED driving schemes.

Intersil’s ISL97702 boost regulator IC is designed to power PM OLED displays used in portable and mobile devices. It minimizes OLED power consumption thanks to soft-start control and inputvoltage disconnect features.

Despite the relative simplicity of a PM OLED structure, AM OLEDs are coming on strong—nearly all major OLED display manufacturers, including Sony, Samsung SDI, Taiwan-based Chei Mei EL (CMEL), Pioneer, eMagin, and LG Displays, are adopting the technology. They’re being used in high-end 3G and 4G mobile phones from Nokia, Sanyo, and Toshiba. They’re also finding homes in digital cameras, digital photo frames, and portable media players, as well as handheld and free-standing TVs.

Market research company DisplaySearch expects OLED sales to surge 69% to more than $826.5 million this year, 83% next year, and 53% in 2010 as AM OLED displays find greater use in consumer electronic products. Other market forecasts are equally bullish on AM OLEDs. For example, iSuppli Corp. sees a worldwide market of $4.6 billion for AM OLEDs by 2014, up from last year’s $67 million, particularly for TV sets (Fig. 4).

Design tools for OLED displays used in mobile and portable products are surfacing, too. CDT in collaboration with Silvaco developed a new universal organic thin-film transistor (UOTFT) Spice software model. LG Philips is heading a project to design an accurate Spice model for both amorphous silicon hydrogenerated (a-Si:H) TFT and OLED devices.

Osram Opto Semiconductors offers an OLED reference design kit for both 2.7- and 1.6-in. 128- by 64-pixel OLED displays. It allows users to quickly upload patterns and evaluate the technology in their applications.

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OLED TV IN THE FUTURE? Given the major investments in LCD and plasma-display panels (PDPs), many experts wonder if OLED TVs will ever be able to match LCD and PDP TVs. Right now, the best answer is that the potential is out there. However, nearly every OLED expert agrees that OLED technology for TVs is still in the formative stage.

Last year’s introduction of the Sony 1.1-mm thin XL-1 11-in. diagonal OLED TV was a harbinger of this potential (Fig. 5). This year, Sony introduced an even slimmer version at 0.3 mm and showed off a prototype 27-in. diagonal OLED TV monitor capable of displaying video images in a 1920- by 1080-pixel format.

Sony is not alone. Samsung SDI plans to produce a 40-in. diagonal OLED TV by 2010. CMEL claims that it will begin production of 12.1-in. diagonal OLED displays for notebook computers in the first half of next year and volume production of 32-in. diagonal OLED TVs by the second half of 2009.

The one major challenge involves mastering the AM OLED manufacturing process for large-size displays needed in TVs. AM OLED manufacturing is still an inefficient process, as yields decrease with increasing panel sizes. So, at least for the next couple of years, we can expect OLEDs to make inroads as displays for portable and mobile electronic consumer products.

GETTING THE LIGHT OUT One of the most promising attributes of OLED technology is its potential as an efficient white-light source. In fact, display experts predict that OLEDs may prove to be serious (and possibly disruptive) competitors with inorganic LEDs, which themselves are making rapid advances as high-efficiency light sources. That can only happen, though, if most of the light presently trapped inside an OLED’s layers (about 60%) can be freed.

So far, efforts look very promising. One way to accomplish this is by using an embedded tandem system of low-index grid and micro lenses. That’s the approach being tried by researchers at the University of Michigan to deliver significantly more bright light than has been possible to date (Fig. 6). Developed jointly with Princeton University and funded by Universal Display Corp. (UDC) and the U.S. Department of Energy (DOE), their approach has yielded 70 lumens/W compared with 15 lumens/W for incandescent bulbs.

UDC recently announced a major breakthrough with a white OLED that has a power efficacy of 102 lumens/W at 1000 cd/ m2, using UDC’s phosphorescent OLED (PHOLED) technology. The device provides operating lifetimes of 8000 hours to 50% of initial luminance. Konica Minolta recently licensed UDC’s PHOLED technology to make and sell energy-efficient white OLED lighting products.

Osram has developed a transparent white OLED tile. A prototype has achieved luminous efficiency of 20 lumens/W at a brightness level of 1000 cd/m2 (Fig. 7). Osram is already using its OLEDs in home floor lamps designed by Germany’s Lösche Design. They’re also being used for table lamp lighting designed by Ingo Maurer.

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