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The Solid-State Lighting Science (SSLS) Energy Frontier Research Center (EFRC) works to advance the scientific foundation that underlies current and potential-future SSL technology, and to ultimately enable significant advances in the efficiency with which SSL is produced and used. We do this through the seven scientific research challenges:
Our last four scientific research challenges are focused on developing a foundational understanding or and exploring light-emission phenomena. To put these four in context, in the graphic above we present the equation that parses out light-emitting device’s power-conversion efficiency.
d. Finally, there is extraction efficiency- the fraction of photons that are created in the device that escape.
All of these component efficiencies are important. However, terrific technological progress has been made recently with extraction efficiency, so this isn’t as big an issue anymore. And injection currents aren’t high enough yet for Joule efficiency to make it to our radar screen. So, thus far, we have chosen to focus on: injection efficiency and internal quantum efficiency.
3. Competing radiative and nonradiative processes: Mary Crawfordhas focused our first light-emission-related scientific research challenge: the competing radiative and nonradiative processes that determine injection efficiency and internal quantum efficiency. We’ve paid particular attention thus far to carrier overshoot and escape, and to spontaneous emission. In fact, one of the things we’ve demonstrated is that this spontaneous-emission B coefficient is quite complex- it is not, as is normally assumed, carrier-density-independent, but instead decreases with carrier density, and therefore could play a role in efficiency droop.
4. Defect-carrier interactions: Among nonradiative recombination processes, defect-mediated recombination, and the physics of the point defects that cause it, is one that we think is particularly important, because it is present at all InGaN compositions and, hence, is a key issue for the green-yellow gap. So, Andy Armstrong has taken up the second light-emission-phenomenon-related scientific research challenge, which is focused on defect-carrier interactions. One of the principles we recently demonstrated is the ability to perform depth-profiling of point defect densities and properties in real InGaN devices, not just in special-purpose heterostructures. This opens up exciting new opportunities for correlating device performance with underlying defect properties.
5. Enhanced spontaneous emission: It is one thing to design devices to accommodate the spontaneous emission rates that are determined by the materials themselves in traditional 2D planar quantum-well architectures. It is another thing to try to enhance these spontaneous emission rates by modifying the environment around the light-emitting material. One way to achieve this enhancement might be through surface plasmonics, where electron-hole-pair excitations couple to surface plasmons, which then couple to free-space photons. Another way might through photonic crystals. So, Igal Crener is working the third light-emission-phenomenon-related scientific research challenge, which is focused on novel ways to enhance spontaneous emission. In fact, we have recently demonstrated a tour-de-force fully 3D photonic crystal fabricated in GaN, an important step towards achieving such enhancements.
6. Stimulated emission: Finally, it is also one thing to live with spontaneous emission at all. Why not go beyond spontaneous emission, through the addition of cavities and coherent processes, such as lasing, to the mix? So, Art Fischer working to surmount the fourth light-emission phenomenon-related scientific research challenge, which is focused on going beyond spontaneous emission. In particular, we are looking at lasers with the possibility of the ultra-low-thresholds that manufacturers like for ultra-high-efficiency, such as nanowire or polarition lasers.