RECOMMENDATION 3-1: The Department of Energy should continue to make investments in LED core technology, aimed at increasing yields, and in fundamental emitter research to increase efficacy, including improvements in the controlled growth and performance of the emitter material. DOE should carefully consider the range and depth of funding in its portfolio of investments in these areas, given the existing technological challenges, in order to determine how the targeted goals of device performance can indeed be met.
FINDING: Efficient operation of LEDs depends on a number of critical factors related to materials defects, structure, and strain. Such factors not only limit device efficiencies, but also lead to thermal and current droop; all have a major impact on the cost and performance of LED lighting.
FINDING: The color output of LEDs is extremely sensitive to the control of materials composition and thicknesses of the LED structure, which in turn are influenced by the control of the MOCVD growth process.
FINDING: A number of approaches have successfully been used to achieve and modulate color rendition for LED lighting. Phosphor-converted and color-mixed LEDs show promise but face different challenges. The ultimate choice of approach will depend on a multiplicity of issues regarding sensitivity of color control, efficiency, reliability, manufacturability, and cost.
RECOMMENDATION 3-2: The Department of Energy has provided excellent guidance in its roadmap targets for both phosphor-converted and color-mixed light-emitting diodes. Core investment in these technologies should be continued, with consideration for promising new technologies (e.g., quantum dot layers replacing phosphors).
FINDING: Production-scale MOCVD growth of LEDs is a complex process. The uniformity and yield of the structures grown (and hence of the optical performance of the LEDs) is strongly and negatively affected by small variations in the MOCVD growth process. The thermal and lattice mismatch between substrate and overlayer exacerbates the sensitivity of the growth process. Further difficulties of growth control are anticipated with use of substrates with increased diameter.
RECOMMENDATION 3-3: The Department of Energy should fund research to develop instrumentation for in situ monitoring and dynamic control of the metal organic chemical vapor deposition growth process.
FINDING: Significant improvements in LED efficiency, yield, and reliability are possible by using GaN substrates and latticed-matched epitaxial growth processes. Currently, there are no viable techniques for producing high-quality, low-cost GaN substrates. While realization of low-cost GaN substrates is not assured, the potential payoff of this research is immense.
RECOMMENDATION 3-4: The Department of Energy should make a long-term investment in the development and deployment of gallium nitride substrates.
FINDING: LED efficiency and performance is still limited by materials issues. Improvements in efficiency at the device level, as targeted by the DOE SSL roadmap, will have a “lever effect,” influencing design, performance, and cost of the luminaires. Improvements in efficiency and performance are linked to further fundamental investigations in core technology on emitter materials.
RECOMMENDATION 3-5: The Department of Energy should continue to make investments in light-emitting diode core technology and fundamental emitter research. Its portfolio of investments in these areas should be extensive enough to ensure that the targeted goals of device performance can indeed be met.
FINDING: A number of promising approaches have been developed to increase outcoupling efficiency.
RECOMMENDATION 3-6: The Department of Energy should focus on efforts that result in significant light outcoupling enhancements for OLED that are low-cost to implement and are independent of both wavelength and viewing angle.
FINDING: OLEDs show a decrease in efficiency as the current is increased. This results in a reduction in efficiency at high brightness.
RECOMMENDATION 3-7: The Department of Energy should support research to understand the fundamental nature of efficiency droop at high currents in organic lightemitting diodes and to seek means to mitigate this effect through materials and device architectural designs.
FINDING: The lifetime of OLEDs is very sensitive to extrinsic factors such as exposure to air and moisture. The low-cost fabrication of large area OLED lighting sources requires a high degree of fabrication competency that can ensure package hermiticity along the entire large package periphery and scavenge excess water and oxygen that might have been enclosed during the package manufacture.
RECOMMENDATION 3-8: To create a highly environmentally robust organic light-emitting diode (OLED) lighting technology, the Department of Energy should invest in materials and packaging technologies that make OLEDs resistant to degradation over their long operational lifetimes.