18 May 2022
Cubic-phase III-nitride LEDs overcome efficiency droop
Annually, lighting accounts for 15% of global electricity consumption and 5% of worldwide greenhouse-gas emissions. In the USA alone, the transition to solid-state lighting has already reduced lighting-related energy needs and greenhouse-gas emissions by about 25% with respect to non-solid-state lighting (SSL). However, the average household energy consumption for lighting is set to triple over the next 50 years due to population growth and an increase in housing size. Accelerating the transition to energy-efficient lighting is one of the most significant initiatives to improve the global economy and climate conditions.
If the US Department of Energy’s Solid-State Lighting Program goals are met by 2035, SSL will further reduce lighting-related energy needs and greenhouse-gas emissions by about 55%, reducing the US energy bill by ~$50bn (equal to about a 5% reduction in the total primary energy budget of the USA).
However, the performance of LED-based SSL is far below its theoretical limit because conventional, hexagonal (h)-phase indium gallium aluminium nitride (InGaAlN) multi-quantum well (MQW) LEDs suffer from efficiency rollover under high current density, i.e. efficiency droop. This issue imposes a trade-off between light output power, efficiency, and cost.
Numerous mechanisms, such as Auger recombination, carrier leakage, internal polarization, and, last but not least, phase-space filling have been proposed to explain the efficiency droop in conventional h-LEDs, yet none of them explains the efficiency droop alone.
Picture: Normalized internal quantum efficiency (IQE) (left y-axis) and efficiency droop (right y-axis) as a function of current density.
Now, Yi-Chia Tsai, Can Bayram, and Jean-Pierre Leburton at the University of Illinois at Urbana-Champaign (UIUC) have provided a detailed computational analysis of the various factors influencing the internal quantum efficiency (IQE) of InGaAlN MQW-based LEDs as a function of design and crystallographic structure (‘Interplay between Auger recombination, Carrier Leakage, and Polarization in InGaAlN Multiple-Quantum-Well Light-Emitting Diodes’, J. Appl. Phys. 131, 193102 (2022)). Their analysis shows that the primary factor for IQE degradation and the so-called ‘efficiency droop’ is the internal polarization arising in strained h-LEDs, which enhances either Auger recombination or carrier leakage from the QWs.
As both effects cannot be averted by modifying the device design alone, their work indicates that switching to novel, cubic (c)-phase LEDs would strongly enhance the IQE peak over 80%, while quenching the efficiency droop to just a few percent.