VCU news

By Sathya Achia Abraham

Virginia Commonwealth University electrical engineers are paving the way for the development of more energy-efficient lighting with increased brightness in the form of light emitting diodes.

The team, led by Hadis Morkoç, Ph.D., the Founders professor in the Department of Electrical and Computer Engineering in the VCU School of Engineering, has been studying the efficiency of gallium nitride-based light emitting diodes, or LEDs, which are used for indoor and outdoor lighting and in computer and television displays. For indoor lighting, LEDs have been used as replacement for incandescent and compact fluorescent bulbs. Examples of outdoor lighting include street lights, parking lights and accent lighting found on bridges, buildings and other landmarks.

Engineers have known that the light intensity of gallium nitride (InGaN)-based LEDs does not increase as fast as the electrical drive power and results in less efficient use of electric power.

The use of LEDs has been hampered in indoor lighting due to their inefficiency at high power levels. According to researchers, it is imperative that electric power to light conversion efficiency be maintained at high intensities for longevity and power savings.

In a study published in the Sept. 23 issue of the journal Applied Physics Letters, Morkoç and his team identified the origin of the efficiency droop problem that has contributed to decreased brightness in LEDs. They also devised structures in which the particular drop in conversion efficiency has been reduced to the point where the limitation is now posed by the ability to remove heat from the device.

“Although the problem has been known for quite a while, the source of the problem has been largely misdiagnosed prior to our work,” said Ümit Özgür, Ph.D., a professor in the Department of Electrical and Computer Engineering who collaborated on the study. “Now that the source of the bottleneck has been determined and a means to eliminate it has been identified, industry can begin implementing the devised structures in highly perfected LEDs along with appropriate packaging,”

“The findings could one day lead to increased efficiency and also the brightness of LEDs, as well as longevity for a given light output making their entry into ordinary lighting application closer to reality,” said Özgür. “Also, fewer LEDs or lower electrical power would be needed for displays using LEDs for background lighting.”

In related work, now that Morkoç and colleagues have identified the source of the efficiency droop problem, they have completed a follow-up study showing how a high efficiency can be maintained at high electrical power levels. Those findings will also appear in a forthcoming issue of Applied Physics Letters.

This work was supported by grants from the Air Force Office of Scientific Research.

Morkoç collaborated with researchers from the Department of Electrical and Computer Engineering including Jinqiao Xie, Ph.D., Xianfeng Ni, Qian Fan, Ryoko Shimada, Ph.D., and Özgür.