Light emitting diodes (LEDs) have seen increased commercialization and investment into R&D as energy efficiency begins to play a larger role in cutting emissions. The phase-out of incandescent light bulbs have spurred adoption of compact florescent and LED lights. While the growth of the LED market has spurred many companies into different parts of the value chain, there are still technical hurdles that need to be addressed. One of the most significant challenges is obtaining a high efficiency white LED.
One approach to fabricating high efficiency white LED for solid state light applications is to combine individual LEDs that emit in the red, green and blue portions of the spectrum, with the caveat that they each be highly efficient as well. While the red and blue components have already been realized, efficient green emission is still problematic, partially because of the direct to indirect band gap cross-over.
NREL scientists have found a way to addresses the efficiency losses associated with inter-valley transfer incurred in most III-V material systems where green emission occurs at an energy in the vicinity of the direct to indirect band gap crossover point.
This invention presents a novel approach for obtaining LEDs that emit in the green, yellow and red regions of the visible spectrum. It allows for highly efficient injection luminescence from LEDs operating in these spectral regions without the traditional penalty of photocarrier losses due to inter-valley carrier transfer. Moreover the approach benefits from requirements of minimal mismatch strain, thus significantly simplifying device growth and fabrication. The approach should enable highly efficient LED devices operating near the peak of the “human eye spectral response", and provide efficient light emission in the region of the green gap that it has been very difficult to achieve.
Such a device has the best color rendering index (CRI) of any LED architecture, but it requires that each of the individual LEDs also have high quantum efficiencies, defined as the ratio of emitted photons to electrons injected into the device. The ideal green emission wavelength for a three-color mixing scheme is approximately 560 nm, which maximizes the CRI and relaxes the requirements for the red and blue emission as well. Al1-xInxP is a promising material for green LEDs due to its more favorable peak in the direct bandgap. It undergoes a direct to indirect transition at 2.4 eV (x = 0.46, assuming no bandgap bowing), the largest energy of any of the nonnitrides. Accounting for bandgap reduction necessary to prevent inter-valley carrier transfer, photon emission in the 2.1-2.3 eV range (540-590 nm) is possible.
For more information, contact Bill Hadley at Bill.Hadley@nrel.gov
U.S. Patent # 8,866,146
Applications and Industries
LED lighting, solid state lighting, vehicle lighting, commercial and residential lighting, outdoor lighting
Increased efficiency, increased luminescence, lower photocarrier losses