Solving the 'Green Gap' in LED Technology
One long-standing high-priority research area for DOE is to increase the efficiency of deep green LEDs. Although most products today use phosphor conversion (PC) to produce white light from a blue LED, having a good green source could lead to color-mixed white sources that would avoid the losses associated with the PC approach.
DOE-funded research at Rensselaer Polytechnic Institute (RPI) has shown the importance of both the epitaxial structure and the substrate material in improving green light efficiency. The RPI team is working on non-polar epitaxial multi-quantum well LEDs on Gallium Nitride substrates that produce deep green light directly. So far they have been able to produce excellent low-dislocation material and to replicate substrate quality throughout the active region, leading to a doubling of green light output for a given input power.
By growing on non-polar Gallium Nitride substrates, the RPI team is developing processes to double or triple the light output power from green and deep green AlGaInN LED dies in reference to the Lumileds Luxeon II. Lumileds Luxeon II dies and lamps have been identified by DOE as the uniform reference of current performance levels and therefore are being used by the research team.
Since LEDs in this spectral region show the highest potential for significant performance boosts, the project is paying particular attention to all aspects of the internal generation efficiency of light. Anticipated results are high output green and deep green (525 - 555 nm) LED chips as part of high efficacy red-green-blue LED modules. Such modules would perform at and beyond the efficacy target projections for white-light LED systems in DOE's accelerated roadmap.
Some key results to date include:
- Growing good quality green emitting epitaxial material on c-plane, a-plane and m-plane Gallium Nitride
- No wavelength shift in non-polar green emitting material with increased excitation density
- Replicating substrate quality throughout the active region
- Low-dislocation density bulk Gallium Nitride substrate