DOE’s Solid State Lighting Program: R&D for Efficient Lighting

A single, common incandescent light bulb. It signals the arrival of a bright idea. Its warm glow is a welcome sign when returning home after a long day. What we don’t usually consider when we click a switch and flood a room with brightness is that lighting consumes 12 percent of the energy we use at home and 25 percent in commercial buildings. We also seldom think about the greenhouse gases that are being emitted into the atmosphere to keep that bulb burning. But the National Energy Technology Laboratory (NETL) does, and they’re doing something about it.

So what does it take to change a light bulb? It’s a little more complicated than “one guy to hold the bulb and two to turn the ladder.” The U.S. Department of Energy (DOE) has initiated a Solid State Lighting (SSL) Program to develop more efficient light sources, especially for lighting large areas. NETL manages the program’s research and development (R&D) efforts, with cooperative agreements between the government and industry, academia, and various other research organizations.

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	Solid state lighting produces white light through two main techniques: color mixing and phosphor conversion. The color-mixing approach uses individual red, green, and blue devices mixed together in the appropriate ratios to produce white light. The semiconductor materials in the device determine what color light the LED will produce. In contrast, the phosphor-conversion approach uses blue, violet, or ultraviolet devices coated with a phosphor material. The phosphor coating converts the pump light into a broader spectrum, where the combined emissions produce white light of good color. Most high-brightness white LEDs on the market today use the phosphor-conversion approach. Some luminaires combine both approaches to obtain a full spectrum of colors.

The two most common light sources, incandescent and fluorescent, provide adequate light for daily life, but they are vastly inefficient. Incandescent light sources, such as the common household light bulb, create light by heating a metal filament. Only 10 percent of the energy used goes to producing light, while the remainder produces waste heat. Fluorescents have a slightly better efficiency, converting nearly 25 percent of the energy to light. These bulbs operate by electrically exciting mercury plasma which emits ultraviolet energy; this ultraviolet light is then converted to light we can see through the use of phosphors.

In contrast, solid state light sources produce tiny particles of light through the transport of electrons from one material to another within the device. SSL sources are semiconductors consisting of an emitting active region sandwiched between a “p-type” and an “n-type” layer. The p-type layer provides “holes” (electron-depleted regions) and the n-type layer provides electrons. Like pool balls falling into the pockets of the table, the electrons and holes combine in the active region to produce light.
 
DOE’s goal is to develop SSL products that are nearly 10 times more energy-efficient than incandescent light sources and twice as efficient as fluorescent sources. As if this isn’t enough, SSL lighting offers more than just efficiency. SSL light sources are estimated to have lifetimes that exceed 50,000 hours. This means they’ll last up to five times longer than fluorescent bulbs, and a whopping 50 times longer than incandescent bulbs. Not only will you save energy with an SSL bulb, you’ll spend a lot less time changing light bulbs—and there were will fewer bulbs thrown away.

The two main types of SSL devices are organic light emitting diodes (OLED) and inorganic light emitting diodes (LED). LEDs are most common today; people are used to seeing them in stadium signs and some traffic lights. The light from an LED originates from a tiny inorganic semiconductor, creating an intense point source. An OLED, on the other hand, is made up of organic polymers or small molecules, and it emits light over a relatively large area, up to several square inches. The result is a soft, diffuse light.

The R&D funded by NETL looks at improving both LED and OLED efficiency, first through individual parts or sections of SSL devices and then by developing the product as a whole. Current research is aimed at improving internal efficiency, phosphor conversion efficiency, light extraction, and fabrication process improvements. Once the product passes this part of the research, the goal is to combine all of the parts of a lighting system together to make a viable product, such as an under-cabinet light, luminaire, or outdoor lighting.

Since the ultimate success of its R&D program is a market-ready product, NETL plays a major role in the commercialization of SSLs. For example, NETL manages testing laboratories under the DOE Commercially Available LED Product Evaluation and Reporting (CALiPER) program. CALiPER supports performance testing of a wide array of commercially available SSL products for general illumination. DOE allows its test results to be distributed in the public interest for noncommercial, educational purposes.

To encourage development of cost-efficient and environmentally
savvy lighting products, NETL is also conducting the first government-sponsored SSL technology competition: the Bright Tomorrow Lighting Competition, called the L Prize™. Guidelines for the competition from the Energy Independence and Security Act of 2007 challenge industry to develop replacement technologies for two of today's most widely used and inefficient technologies: 60 watt incandescent lamps and PAR 38 halogen lamps. They also call for development of a 21st Century Lamp that delivers more than 150 lumens per watt (lm/W), among with other performance criteria. The competition will award significant cash prizes, plus opportunities for federal purchasing agreements, utility programs, and other incentives for winning products.

Led by a multi-year R&D plan that was established in conjunction with industry, NETL’s SSL R&D program has had many successes over the years. In just the past year and a half, the advancements in this field have been tremendous:

For more information about how the DOE SSL program is changing light bulbs, please visit our website: http://www.ssl.energy.gov/.