A polarization-matched LED co-developed by Rensselaer Polytechnic Institute and Samsung Electro-Mechanics has exhibited a 20% increase in light output and a 25% increase in wall-plug efficiency. And smart LEDs with the function of emitting light and handling date is in vision in the future, according to Professor Fred Schubert, the leader of the project.
The significant improvement is achieved by a notable reduction in “efficiency droop”, a problem of much concern in research of high efficiency LEDs. “This problem has been the focus of considerable research over the last two years; it’s the single most important hurdle facing solid-state lighting technology,” said project leader Fred Schubert, a professor at Rensselaer and head of the university’s National Science Foundation-funded Smart Lighting Engineering Research Center.
When receiving low-density electrical currents, LED is most efficient light source, while it loses efficiency when higher-density currents are needed. Electron leakage has been shown by study to be a major problem of the condition, though the cause of the droop is still not fully understood.
Electron escape from the active region is traced to account for this problem. To resolve this, electron blocking layers have been tried with unsatisfactory results. When they studied the active region of LEDs, which is the light-emitting region, the researchers discovered that it contained materials with mismatched polarization, the likely cause for electron leakage, then the idea of polarization matching occurred to the team. They studied into the quaternary materials for the active region and settled on a composition of quaternary and ternary materials, and the result was satisfactory.
A different quantum-barrier design was introduced to reduce the polarization considerably. The conventional GaInN/GaN layer of the LED active region was replaced with GaInN/GaInN which provided better matched polarization, reducing both electron leakage and efficiency droop.
Gallium nitride has high internal electric fields that are a problem, said Shubert. “A polarization match structure grown on c-plane sapphire substrate was implemented which allows a significant electric field reduction in the active region. As a result, the droop dropped, resulting in a 20% increase in light output power and a ~25% rise in wall-plug efficiency. These are very significant numbers for the LED industry. The industry will implement anything that consistently improves efficiency by 5%; 20% is an enormous benefit.”
According to Shubert, there are still many functionalities that can be implemented for LEDs such as changeable emission properties, color and temperature that can mimic sunlight, producing very natural, full-spectrum illumination.
Smart light sources which have a dual function of producing light and handling data is one of the goals, said Shubert. “Imagine lighting in an airport terminal, or a traffic light on a road, that in addition to its visual effect can deal with communications. Imagine a building in which light sources communicate a certain room number or authorization, and are capable of tracking objects within the building. Smart light sources could accomplish all of this. That is our long-term vision.”
