hirty years ago, as a colleague and I were traveling through Pennsylvania en route to a Bell lab in Reading, a severe snowstorm forced us to stop driving for several hours until visibility improved. In that enforced downtime we launched into a long conversation on our favorite technical subject, a new type of light source, the light-emitting semiconductor devices. Unbeknownst to us, we were also launching a series of discussions that would culminate a year later in a book presenting the first comprehensive treatment of light-emitting semiconductor devices (A.A. Bergh and P.J. Dean, Light-

Lighted by a layer thin-film OLEDs (organic light-emitting diodes), this glowing panel in a research lab in the state of Washington may be a precursor to windows that light up at night and walls that radiate light wean someone enters the room.Courtesy of Pacific Northwest National Laboratory

Emitting Diodes, Oxford Press, 1974).
        When we wrote the book, we were confident that we understood the physics, performance limits, and potential applications of light-emitting diodes (LEDs). In retrospect, however, it is clear that we grossly underestimated their performance limits, projecting efficiency limits far below those routinely achieved today. We erred in presuming only incremental advances and excluding the possibility of the kind of major breakthroughs in technology that have so often created new paradigms and lifted paths of technological development into new trajectories.
        Now, 30 years into the LED lighting era, LEDs have been joined by a sister technology, organic light-emitting diodes (OLEDs), and the two together make up the field of solid-state lighting (SSL). It is becoming apparent that all the work on SSL so far has laid solid foundations for a lighting revolution that has already begun. The coming decades will see not only the replacement of both incandescent (tungsten filament) and fluorescent bulbs but also radically different approaches to lighting not possible with current light sources.

What's solid-state lighting?

n the youngest lighting technology, solid-state lighting,

A Comparison of Incandescent Lamps and LEDs

ATTRIBUTES INCANDESCENT LAMPS LEDS--2003[2013]* Power 110V--AC ~5V--DC Size Large Small Available color White--close to sunlight Monochrome--all colors Life Thousand hours 20 [100] thousand hours Operating temperature Hot Cold Efficiency 16 lumens / watt 25 [160] lumens / watt Cost 0.4$ / kilolumen 200$ [2$] / kilolumen Integration with computer chips Cumbersome Easy

*LED Performance figures: Current [ten-year projected]

semiconductor materials produce light, but little heat, when they are stimulated by electricity. Unlike the silicon crystals at the heart of the computing industry, LED semiconductors are crystals comprising combinations of usually two or three elements, such as gallium phosphide (GaP) or gallium indium nitride (GaInN). These unique combinations of elements produce distinctive crystalline structures that can accommodate both electrons (negatively charged) and holes (positively charged electron vacancies), which exist at different energy levels separated by a "band-gap." Electrical stimulation injects both electrons and holes directly into the material. When the two combine under favorable conditions, the released band-gap energy is converted into a photon of emitted light whose frequency (color) corresponds to the band-gap energy.
        SSLs produce light at or near the visible portion of the spectrum, so the light can be used directly or with little modification. This property of producing directly usable light contrasts with fluorescent light sources, in which the original ultraviolet (UV) light produced by an ionized gas inside the tube must be converted to usable, visible light by phosphors coated on the inside of the tube. The phosphors absorb the UV light and then emit the visible light. Because they produce usable light so much more directly than fluorescent lights do, SSLs promise to outperform fluorescent lights by a factor of two.
        SSLs also are much more efficient than incandescent lights with their coiled tungsten filaments. Incandescent lights expend most of their consumed energy in producing heat, with only about 5 percent being converted to the light we see.
        As the older sister in the SSL family, LEDs provide the best window onto the promising features of solid-state lighting. LEDs offer a number of unique, new light-system options that challenge established views about the nature of light sources.

Rugged, shock-resistant devices achieve long life, even on mobile platforms.
        All these attributes point to a new lighting paradigm. Signaling will be more prevalent; more information will be dispatched in all segments of life. An example is road-crossing signs in Taipei. When the traffic light turns green, a white walking figure appears to signal pedestrian crossing. A digital clock simultaneously shows the seconds remaining for crossing. To emphasize the importance of the vanishing seconds, the small walking figure gradually accelerates its steps, turning into a runner 10--20 seconds before the time runs out.

Available from October 2003, the PentaLED surgical lamp from the Italian manufacturer Rimsa will be much cooler, more energy efficient, and longer lived than current halogen surgical lamps.
Courtesy of LumiLEDs

        More information can be presented to commuters with inexpensive billboards; they can warn, for example, of emergency situations. "Lively" signage can show the location of a store or business and indicate daily specials or unusual events. The rugged, shock-resistant sources are ideal for mobile (transportation) platforms such as automobiles, trains, ships, and airplanes and can save both signaling and lighting functions.
        For general illumination, the small size and low operating temperature of individual light sources, plus the ease of assembling them into larger units of any desired size and widely diverse configurations, means that any part of a building, any furniture, or household item can become a fixture for lighting.
        Today's light fixtures, whether recessed in a ceiling or standing free as a floor lamp, all perform similar functions: They supply power to the lightbulb, dissipate its heat, and distribute light in a desired pattern. LEDs offer a different model because of their small size and relatively low-power operation. Typical recessed ceiling sockets will be replaced with simple flat panels attached directly to the ceiling without the need for any recessed mounting space. Simple plastic or glass waveguides (sheets or fibers) can distribute the highly concentrated LED light to several points, such as the 10 buttons of a telephone, or over a flat panel like the one used to backlight an LCD display. Finally, "smart" lights will dim automatically to adjust to usage and will change color to cater to the user's needs. This will cut energy consumption and provide a more productive and pleasant environment.
        Architects and interior designers will discover a new range of functional and creative lighting options. Exterior accent lighting will give a distinct--and variable--character to buildings. Interior designs will produce wall and ceiling lights with variable color and intensity that automatically adjusts to the time of day and the task or mood of the occupants. In short, the major strength of SSL is in its unique capabilities and its promise to create a new lighting paradigm. Those who approach it with a replacement mentality will greatly underestimate its future impact.

Growing toward the takeoff point

hrough steady advances in luminous efficiency, a measure of the amount of light [in lumens (lm)] produced by a given unit of energy [in watts (W)] over the last 20 years, LEDs have achieved significant penetration of such monochrome applications as traffic lights, exit signs, and automotive taillights. [For comparison in the timetable below, a typical 60-watt incandescent bulb is rated at luminous efficiency = 14 lm/W.] The major milestones of LED development were:

The mid '90s: a new class of organics, polymer OLEDs is developed. Power LEDs emerge for automobile taillights. The power increase comes from both improved efficiency and higher drive currents.

Lighting the world

rtificial lighting is viewed directly when it is used for signaling purposes and indirectly when it is used to brighten the darkness both indoors and outdoors.
        The real volume application for power LEDs is currently in the automobile, especially the tail end. In this application, the LED's light weight, low power draw, and long life are strong positives counteracting its higher initial cost. LED applications started in the mid-eighties and are growing, especially in luxury cars like the Cadillac Eldorado and the Mercedes S-Class. LEDs will soon dominate the tail end of cars, including backup lights and license plate illumination. In the interior, LEDs will appear in dome lights, reading lamps, and dashboards. The next step will be white headlamps using LEDs that change the beam direction with steering wheel position. Ford and Audi have already introduced concept cars with LED headlights, and other manufacturers are likely to follow suit within a year or two.

In its rooms for children undergoing stem cell transplants for cancer, Childrens’ Hospital Boston provides intelligent LED illumination in a number of ceiling tiles throughout the room. Patients can change the colors and effects to suit their mood, select from a variety of preprogrammed cycles of change, or even synchronize the color tiles to their music CDs.Courtesy of Color Kinetics

        Other examples of signaling applications being overtaken by LEDs are traffic signals at street crossings, signs announcing a business establishment, moving text on a billboard warning of a road condition, or large-area displays in a stadium depicting a ball game. For these applications, LEDs are a near-perfect match. Small, multicolored, and bright, they consume little energy and have a long life.
        LED-based solutions can beat filtered incandescent lamps by a substantial luminous efficacy margin in practically all monochrome signaling applications. For instance, in a 12-inch red traffic light, the red filter transmits 10 percent of the light from a 140-watt incandescent bulb. The corresponding LED solution uses 18 or fewer LED lamps and a glass or plastic waveguide to distribute the light, giving the appearance that it is generated by a much larger number of sources. The LED light saves $72 a year in energy cost.
        Additional, unexpected applications for LEDs are already emerging. One is in flat-panel, liquid crystal display monitors, whose acceptance has been limited because their picture cannot match the color gamut of the cathode ray tube (CRT). By backlighting the liquid crystal display screen with an LED, engineers have transformed the dull, liquid crystal display monitor into a screen whose breathtakingly vivid colors outperform the CRT at a fraction of the power consumption.Within a few years, commercial LED-enhanced, liquid crystal display monitors are likely to emerge as strong competitors to CRTs.
        In the outdoor lighting sector, automobile headlights and mobile platforms, as previously mentioned, are the most promising application for LEDS. Other outdoor lighting applications for illuminating streets, parking lots, and sports facilities are largely powered from the electric grid; they emphasize high efficiency and high brightness, with moderate demand on color rendering. These applications may be among the last in which LEDs will displace current light sources.
        Examples of indoor illumination are fluorescent lights in an office or factory and incandescent lights in a home or a retail shop. Here, the function of the light source is to replace or enhance daylight. The light source is viewed indirectly and its quality is judged by comparing the reflected light from multicolor objects to the reflected light of the sun. Ideal sources imitate daylight as measured by the Color Rendering Index (CRI = 100 is best). Further, they are energy efficient, inexpensive, and long lived.
        In the United States, indoor illumination accounts for roughly 92 percent of all electricity drawn from the grid to support lighting. In this arena, LEDs promise to be future contenders, especially to replace incandescent lights. Achieving that, however, will require major technological breakthroughs that might be 5 to 15 years away. Major issues include cost reduction (by two orders of magnitude), efficiency improvement at all visible colors, color performance (high CRI), powering (low-voltage direct current), and competition from improved conventional technology such as compact fluorescent lamps.

The organic sister

tarting in the mid to late 80s, a new type of solid-state light source was developed based on organic semiconductors, which are built around chains of carbon and

Phasing Out the Lightbulb

Electrical energy is expensive, and modest savings in many single devices add up to substantial sums. Consider the energy and cost-saving potential in simply replacing the incandescent-lighted exit sign with an LED-lighted version. Exit signs are required by law in all commercial and institutional buildings and must operate continuously.         Today, over 100 million exit signs are used throughout the United States, consuming 22--35 billion kilowatt-hours of electricity annually. The majority of these signs use incandescent lamps. Replacing them all with LED signs could yield a savings of approximately three-quarters of this electricity--15 billion kilowatt-hours. According to the Pacific Northwest National Laboratory, incandescent exit signs cost $42 per year to operate, versus $5 per year for LED exit signs. Thus, the yearly savings from replacing all incandescent exit signs with LED signs would be roughly $3.7 billion.         These figures are consistent with the real marketplace phenomenon in the United States: LED lighting is rapidly becoming the standard in exit signs, due to its energy efficiency and long life. --A.B.

hydrogen instead of the exotic (and more expensive) materials used in LEDs. The OLEDs provide an efficient source of light at low voltage. OLEDs quickly became a new field of study, with intense worldwide participation in developing full-color flat-panel displays. The result was an enormous improvement in OLED efficiency over the last decade. By the late '90s efficiency demonstrations made OLEDs potential contenders for general illumination, and they are already in use in some cell phones.
        OLEDs are in some ways similar to LEDs. Both are efficient, low-voltage, solid-state light sources that generate light through the recombination of holes and electrons. However, OLEDs are made of amorphous materials [see "Glasses for the Masses," The World & I, June 2003, p. 130], while LEDs are made of single-crystal material.
        An OLED consists of one or more organic layers sandwiched between two electrodes--one of which must be transparent. When a voltage is applied across the device, electrons and holes are injected into the organic layer(s) from opposing electrodes. These carriers hop between molecules or polymer segments in the organic layer under the influence of the electric field until they recombine at a luminescent center, resulting in the emission of a photon. The color of the light emitted as well as the electrical characteristics of the device can be tuned by modifying the chemical structure of the organic materials and the details of the device design. Already, devices that span the whole visible color range, from violet to red, and including white, have been demonstrated.

Remaining barriers facing OLEDs

ver the past 10 years, the rate of progress of OLED technology has been enormous, with luminous efficacy increasing by almost two orders of magnitude. Similarly, the operating lifetime at display brightness levels has progressed from less than 1 hour to over 10,000 hours in the same period. The current OLED performance is adequate for many display applications, and a few monochrome OLED displays are already available commercially. Today's OLED performance is not yet adequate for general lighting, however. The key challenges that OLED technology must meet to enable general illumination applications are: