Some industry observers have predicted that solid-state lighting technology (SSL) will satisfy most lighting applications by the end of the decade. One particular SSL technology on the cusp of commercialization has the potential to be transformational: the organic light-emitting diode (OLED).
Imagine a light source that is manufactured on rolls, can be cut into flexible flat sheets in the factory or the field, and can be installed in almost any shape on almost any room surface. It is easy to control and install, is lightweight, and contains no hazardous substances. It is lighting that is completely integrated into furniture, window coverings and building materials—for instance, picture glass walls that become luminous at the flick of a switch; curved sheets that emit different colors from each side; wallpaper, room dividers, curtains and even clothing that doubles as illumination; ceilings that glow with color; or windows that are transparent during the day and luminous at night.
OLEDs are already used in cell phones and small video display color applications. For general illumination, the technology is still in its infancy. GE, Osram Sylvania, Philips and others—in addition to the U.S. Department of Energy (DOE)—are all investing in OLEDs. Several prototype and commercial products already exist, giving us a glimpse of their potential.
Eastman Kodak’s Dr. Ching Tang invented OLED technology in the late 1970s. He found that by depositing materials into thin films and passing electrical current through the resulting construct, carbon-containing (organic) compounds produce light.
The basic construction consists of a stack of organic thin films sandwiched between two current-delivering electrodes, which is typically enclosed between two layers of plastic or glass. Because OLEDs produce light by changing the electrical state of a chemical solid, they are solid-state sources, like LEDs. The stack has a diameter many times less than that of a human hair, yet its area can be very large, making OLEDs diffuse area light sources.
Typical OLED illumination devices may be configured as pixels, panels or complete fixtures. Panels are OLEDs with an area of at least 80 square centimeters, and they produce light output rated in lumens per square meter. These panels may be larger or joined into assemblies to create a larger luminous area. The assemblies, in turn, are connected to the OLED driver (which converts the line voltage to the voltage and current needed to start and operate the device) and to any electronic controls that enable dimming and other effects. Along with any housings and optics, the complete system, ready to be connected to the electrical supply and enter service, is called an OLED luminaire or lighting fixture.
One of the big differences between the OLED and its LED cousin is how light is emitted. LEDs are point sources, ideal for efficiently delivering a focused beam of light. The OLED, in contrast, is a perfectly diffuse area light source. This particular defining characteristic of the OLED is potentially highly transformational to lighting design. Typically, light sources are so bright that they cannot be viewed directly for long periods of time without producing a sensation of glare. Lighting fixtures are designed to house distinct light sources and auxiliary components, such as ballasts, and distribute the light in a controlled pattern without glare. With the OLED, the low-brightness light source can be viewed directly for a prolonged period without glare. Optics may be external or possibly even built directly into the light source to direct the light emission in a desired pattern.
According to Peter Ngai, vice president, research and development for Acuity Brands Lighting, today’s best OLED devices can produce about 6,000 lumens per square meter—about 120 lumens of output for a 200-square-centimeter OLED panel—with an efficacy of about 30 lumens per watt. Significant progress is already being made. Just two years ago, in 2009, the DOE assessed the earliest introductions and estimated performance at 23 lumens per watt and a 5,000-hour service life. The DOE further estimated the cost of an OLED panel at about $25,000 per kilolumen in 2009 compared to $4 for a typical fluorescent T8 system and $128 for a typical LED lamp, making it relatively expensive at the time.
The earliest prototypes have been mostly decorative, intended to demonstrate the technology’s unique characteristics. Ngai pointed out that a reasonable threshold that signals an effective degree of competitiveness with conventional sources is light output of 6,000 lumens per square meter, efficacy of 60 lumens per watt and rated life, based on useful light output, of 15,000 hours. All at a competitive cost, of course.
To increase performance and reduce cost, several technical hurdles must be crossed. The biggest opportunity is to increase the amount of light emitted by the device for the given surface area. One simple method is to increase the driver current, but there is a tradeoff in shorter device life; for example, increasing the light emission from 3,000 lumens per square meter to 10,000 lumens per square meter solely by raising the drive current would reduce life by 80 percent, according to the DOE. It is better to improve the device’s efficiency in extracting light. A significant amount of light produced by OLED panels actually remains trapped inside the substrate. If research and development can unlock this light output using a method that is reproducible at a reasonable cost to manufacture, not only would light output and associated efficacy increase, but the cost per kilolumen would significantly decrease. That decrease, coupled with economies achieved through high-volume manufacturing, could put OLED lighting within reach of mainstream use.
Other technical hurdles, the DOE points out, include finding good materials from which to realize white light and methods for rendering sensitive OLED materials that are resistant to oxygen, moisture and pollutants in the operating environment, which reduce service life. The long-term goal for service life, according to the DOE, is an L70 rating (point in time at which the light source is producing 70 percent of its initial lighting output, representing a lumen depreciation of 30 percent) of 50,000 hours for general lighting.
The DOE and the manufacturing community are now investing in research and development to solve these problems. In terms of development, the OLED is about where the LED was several years ago, Ngai believes, but is progressing more rapidly and will benefit from standards and research developed to accommodate LEDs.
The DOE predicts that, by 2015, the OLED will achieve light emission of 10,000 lumens per square meter, or 200 lumens for a 200-square-centimeter panel. The DOE further predicts the OLED will achieve an efficacy greater than 100 lumens per watt and a cost of about $8–9 per kilolumen by that time. The quality of light and service life will likely continue to improve as well. Therefore, within five years, we may see a number of OLED products that are commercially competitive for mainstream applications.
OLED products are unlikely to prove competitive with basic forms such as troffers and downlights in the near future. Naomi Miller, senior lighting engineer for the Pacific Northwest National Laboratory, a DOE contractor who is engaged in the DOE’s SSL program, said current applications are focused more on demonstration than practical use. The first white light applications will likely be specialty and high-end applications where the OLED’s unique aesthetics and capabilities are most desirable. Early applications include spaces that require small amounts of light from a thin luminous surface, such as step lights and marker lighting, undercabinet lighting and decorative panels and components of conventional fixtures. Note that these OLED devices may require external optics and integration within a fixture housing, which may present light emission losses and also the same current droop and thermal sensitivity problems that affect today’s LED fixtures.
Regarding control, OLED devices require drivers just like LED devices. The driver functions similarly to a conventional ballast, converting line voltage to the proper voltage to start the device (usually low voltage) and then regulating current flowing through the device during operation. As with LEDs, OLEDs are instant-on, and service life is not negatively affected by frequent switching, making them well suited to automatic shutoff devices, such as occupancy sensors. The driver may be dimmable, enabling the light emission to be controlled automatically or manually by users. And as a solid-state light source, OLED drivers can be digitally controlled, enabling precise control and generation of information through integral sensors that can be fed back to a central operating station.
Ngai pointed out that it is unlikely that OLED products will become competitive with other sources in applications requiring intense, focused illumination, such as accent lighting, high-bay lighting and roadway and outdoor area lighting. As a result of the intense point source characteristics of the LED and the diffuse area source characteristics of the OLED, it is likely that these sources will be specified to work side-by-side in many applications, Miller said. In a high-end retail stores, for example, OLED lighting may provide whimsical decorative elements and luminous display shelving and surfaces, while aimable LED fixtures may be used to punch key merchandise with strong focused illumination, and either or both may provide general lighting.
The form factor of the OLED supports physical installation similar to typical commercial recessed, surface-mounted and suspended fixtures. The technology does allow for more lightweight designs, however, which may result in changes in these fixtures’ support and mounting structures. As most OLED panels are low voltage, there may be opportunities to reduce line-voltage wiring and conduit at the room level with plug-and-play wiring structures that deliver power and communications. Overall, installation is likely to be simplified with OLED lighting compared to today’s conventional equipment.
Similarly, as with LED devices, maintenance requirements are likely to change as the source does not “fail to off” on a predictable mortality curve like fluorescent, but rather will simply decline in lighting output until it is no longer useful to continue operating. While this will simplify lamp replacement—by largely eliminating the need for ongoing spot replacement with a long mean time between failures—the owner will need to know when the device’s light output falls below the L70 rating. This may require building maintenance personnel to periodically test light levels, or install some form of automatic feedback mechanism that signals maintenance personnel to replace the fixtures.
By 2020, Ngai said, OLEDs are expected to achieve efficacies approaching 200 lumens per watt at a cost per lumen of several cents. This is an ambitious goal for the next decade but not overly so when one considers that LED light output has increased by 20 times each decade for the past 40 years while the cost per lumen has decreased by 10 times during the same period. By that point, Ngai said, advanced OLED products will have become widely available, including transparent OLEDs that can be applied to windows—enabling the window to convert from a daylight aperture to an electric light source at night—and flat and flexible luminous sheets that can be custom-cut into a wide variety of configurations and then simply attached to a surface.
“The unique characteristics of OLEDs are unlike any other current light source,” Ngai said. “OLEDs offer a new platform upon which new approaches to lighting design will be conceived—from integration of lighting and architecture to new lighting application design philosophies to visionary luminaire designs that are functional, aesthetically pleasing and emotionally compelling.”
He added that the future of emerging technologies often arrives sooner than we anticipate. Stay tuned as OLEDs continue to commercialize into products and solutions that may have a transformational impact on how lighting is designed, installed and used.
DILOUIE, L.C., a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.