Researchers at the Rensselaer Polytechnic Institute (RPI, Troy, NY) have announced their research advances in light-emitting diodes ( LEDs ) that can greatly improve LED lighting performance and energy efficiency. They collaborated with Samsung Electro-Mechanics (Suwon City, South Korea) to develop polarization-matched LEDs with a 20% increase in illumination output, while characterizing LED conversion efficiency, and electro-optical conversion efficiency increased by 25%.
The new device significantly reduces the "efficiency settling" phenomenon, which is the main limiting factor for the efficiency of LED devices. It is well known that low density currents are most efficient when flowing through LEDs. But some applications, such as bulbs with higher brightness, require high-power devices so that LEDs lose efficiency. Although the principle of efficiency settling is not fully clear, studies have shown that a large part of the reason comes from electron leakage.
“In the past two years, this issue has received wide attention from the research community; it is also the main obstacle to the application of solid-state lighting technology,†said Fred Schubert, director of the research project, who is also a professor at Rensselaer, and The National Science Foundation (NSF) of the school sponsors the director of the Intelligent Luminous Engineering Research Center.
"We track the process by which electrons escape from the active region of the luminescence. In the active region, we want the electrons to remain in the cavity so that they can recombine and illuminate; however, if the carriers leave the active region, it will obviously not There is another reorganization. We think this is the source of efficiency sinking: one of the carriers leaves the active zone; this creates leakage in the active zone."
This phenomenon is very common because current high-brightness LEDs operate at high current densities that exceed peak efficiency. The natural idea is to reduce the operating current density to the peak area of ​​efficiency, but its brightness is unacceptable, so this problem has become a huge obstacle to LED applications.
Other researchers have tried electronic barriers, but the results are not satisfactory. PRI's research team—consisting of academics, industry partners, and students—uses the concept of polarization matching. They use a four-polar material in the active region, using a combination of four- and three-polar materials, and using different methods to reduce polarity, each method yields positive results.
The new LEDs feature a newly designed polarity-matched active region that allows the device to achieve peak efficiency in high current density regions. They studied the active areas of light generated in LEDs, and the researchers found that materials with polar mismatches are likely to be electron leakage, which is why efficiency is reduced. More research has shown that by using different quantum barrier designs, polarity mismatch can be greatly reduced. The GaInN/GaN layer of the LED active region is replaced with a conventional GaInN/GaInN. A more matched polarity can be obtained, reducing electron leakage and efficiency settling.
An energy band diagram of a conventional GaInN/GaN active region and a novel polarization-matched GaInN/GaInN active region in an LED. (Source: Rensselaer Polytechnic Institute)
"Nitride is a special class of semiconductors, unlike silicon and gallium arsenide," Schubert said. “Gallium nitride has a high internal electric field, which is a problem. Our polarity matching structure just grows on the c-plane sapphire substrate, which can greatly reduce the electric field in the active region. As a result, the efficiency of sedimentation is suppressed. The luminous output power is increased by about 20%, and the electro-optical conversion efficiency is improved by about 25%. This is a very pleasant number for the LED industry. In order to increase the efficiency by 5%, the industry is willing to make any effort; 20% is Very huge improvement."
Schubert pointed out that the academic community has not yet reached agreement on the single cause of efficiency subsidence. “The next step is to find out the physical reasons behind it and comprehensively remove the cause of the efficiency setback and learn more about all aspects.†The research team will continue to explore other possible LED structures that can further improve the device. According to Schubert, LEDs can also perform other additional functions, such as illuminating properties that change, colors and temperatures similar to sunlight, so that full-spectrum illumination closer to nature can be achieved.
“We can modulate the spectrum,†he said. “This gives us a natural, adjustable light source and can be used to process the data. This is one of our goals: the dual function of generating light and processing data. Imagine the lights in the airport terminal and the traffic lights on the road. In addition to the lighting function, it can also be used for communication; the light source in the building can carry the room number and information, and can track the objects in the building. The intelligent light source can achieve this. This is our long-term goal." Schubert expects As with healthcare, transportation systems, digital displays, and computer networks, LED- and solid-state lighting-based lighting devices can create a new wave of environmental, energy-saving, and low-cost.
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