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Connecting Theory and Practice in Optoelectronics
The energy efficiency is the fraction of the electrical input energy that is emitted as laser light. It is usually given as power conversion efficiency (PCE) and it is surprisingly low for GaN-based lasers. OSRAM just announced a record number of PCE=43% at SPIE Photonics West. This is certainly a remarkable achievement, considering the struggle to break the mysterious 40% limit. However, 43% is far below the record PCE of 84% reported for GaN LEDs. The inherently low hole conductivity and large series resistance on the p-doped side of GaN lasers are usually blamed for the efficiency deficit. However, the series resistance is known to shrink with rising temperature, which can be attributed to the increasing density of free holes in p-doped layers. Thus, one would expect that the PCE improves at elevated temperatures. But the OSRAM paper reported that the measured PCE drops with higher ambient temperature despite the shrinking series resistance. Ergo, there seems to be an even stronger loss mechanism involved.
Ever since Shuji Nakamura mentioned in his Nobel lecture in Stockholm that lasers are the future of lighting, I am puzzled by this claim, especially by the now widely circulated statement that laser diodes are free from efficiency droop. It suggests an advantageous energy efficiency of laser diodes, as shown in the last figure 17 of his lecture (which is actually invalidated by his own reference). If you are familiar with the much debated efficiency droop burdening GaN-LED lighting, you will agree that the underlying carrier loss mechanisms are also present in laser diodes. Even worse, laser diodes require a higher carrier density in the active layers and therefore exhibit stronger Auger recombination and possibly also electron leakage already at lasing threshold. Read more of this post
We still don’t fully understand the efficiency droop observed in GaN-based light-emitting diodes (LEDs). That is why I keep looking for new ideas and I just found one at nature.com . The authors of this paper performed Raman measurements and observed that the GaN strain turns from compressive to tensile with increasing current density while the efficiency droops by 80% (see figure). Based on first-principle calculations, they claim that the strain reversal is caused by carrier accumulation within the InGaN quantum wells. But how can the rising density of electrons and holes expand the InGaN lattice constant in such dramatic way? Read more of this post
GaN-based light-emitting diodes (LEDs) are at the heart of modern lighting applications, but their energy efficiency deteriorates with higher current and temperature (see picture). This efficiency droop problem gave rise to intense research worldwide, including various LED modeling and simulation efforts. Different models feature different droop explanations, including Auger recombination, electron leakage, or defect-related mechanisms. But none of the LED models covers all possible mechanisms simultaneously in sufficient detail. The uncertainty of key material parameters also gives room for contradicting results. Authors often apply their model to unique LED designs which may result in unique conclusions. All this undermines the general reputation of numerical LED simulation, I think, and we should put some joint effort into finding more consensus on the GaN-LED efficiency droop. Read more of this post
I have been struggling with this questions ever since I wrote a first review paper on efficiency droop in 2010. Five years and hundreds of research papers later, experts are still divided over the primary cause of the droop. The two competing explanations are electron leakage and Auger recombination, respectively, but only very few direct measurements of either mechanism are published, none of which establishes a dominating magnitude. Modeling and simulation of measured characteristics has not led to a decision thus far, because parameter uncertainties often leave enough room to substantiate either mechanism. Thus, I recently looked deeper into the widely reported GaN-LED efficiency reduction with higher ambient temperature.  Read more of this post