Lead Performer: University of California, Santa Barbara – Santa Barbara, CA
DOE Total Funding: $999,996
Project Term: September 1, 2017 – August 31, 2019
Funding Type: SSL R&D Funding Opportunity Announcement (FOA) (DE-FOA-0001613)
This project will address the fundamental challenges for green LEDs by a combination of innovations in epitaxial growth and layer design, advanced processes including tunnel junctions, and advanced materials characterization. The work will be guided by 3D simulation methods to efficiently calculate wave functions, charge density, carrier transport, and recombination throughout LED structures. The team will research, develop, and demonstrate green LEDs capable of ≥ 54% internal quantum efficiency (IQE), ≥ 46% external quantum efficiency (EQE), and power conversion efficiency (PCE) of ≥ 35% at 35 A/cm2. With such performance, high-efficiency white LED lamps based on red-blue-green LEDs will develop with much better color rendering and adaptive color mixing than today’s best lamps. The project’s main tasks will be to reduce Shockley-Read-Hall (SRH) in green emitting materials via growth optimization of the active region; to develop new laboratory LED designs using the efficient device simulation methods offered by the novel landscape theory of disordered materials, an essential ingredient to quantitatively understand electrical and optical properties of the alloy-based active quantum wells (QWs); and to address decreased radiative recombination rate (relative to SRH and Auger) and excess forward operating voltage.
The poor efficiency of green LEDs – also known as the “green gap” – is the primary efficiency limitation for color-mixed white LED lamps. The origin of the green gap remains controversial, but some recent studies point to increased Auger recombination with increasing emission wavelength as the main cause. This project focuses on solving the key issues of green LEDs that likely have a mix of intrinsic and extrinsic origin: the poorer materials quality due to the low temperature MOCVD growth to realize the high In concentration for green-emitting InGaN quantum wells (QWs); the decreased radiative recombination (RR) rate due to the larger internal electric fields in green LEDs, which decreases electron-hole overlap; the increased nonradiative Auger recombination (AR) rate with increasing In content in the QWs; the increased carrier localization due to natural In composition fluctuations in the QWs, which leads to high local carrier densities and thus a relative increase of the Auger rate compared to the RR rate; and the excess operating voltage ΔVF above the photon energy, which corresponds to useless energy injection in the LED.
DOE Technology Manager: James Brodrick, James.Brodrick@ee.doe.gov
Lead Performer: Jim Speck, University of California, Santa Barbara