TECHNOLOGY LED DROOP
than 100 lm/W and 130 lm/W, respectively. The culprit for this massive difference between the lab record and the efficacy of commercial LEDs in bulbs is a mysterious malady known as droop, which causes a decline in the efficiency of an LED as the current density through it reaches very high levels. What this means is that the peak quantum efficiency of the LED occurs at a lower current than the value that it is driven at in a light bulb, compromising its efficacy.
If droop could be eliminated – or even trimmed substantially – this could be a game changer for solid-state lighting, moving this industry so that it is not just serving the early adopter in the home and the lighting engineer who thinks about all the costs associated with lighting large buildings, but selling to the general public. That’s because LEDs with far less droop wouldn’t only be more efficient and thus cut electricity bills: They could be also driven at far higher current densities, because their greater efficiency translates into less heating, and that ultimately means that far fewer chips would be needed in a bulb, cutting its cost substantially.
A little history
Droop is clearly distinct from the thermal roll- over seen in laser diode and VCSEL plots of output power as a function of current. Until the mid 2000s, droop was understood – without much controversy – as an inevitable phenomena associated with III-nitride materials. Nearly every commercial LED is grown on a lattice- and thermal-mismatched foreign substrate, such as either sapphire, SiC, or more recently silicon, using a technique commonly referred to as strained heteroepitaxy.
LED LIGHT BULBS have many attractive attributes: lifetimes of 50,000 hours, negligible warm-up times, the absence of mercury, and higher efficiencies than incumbent sources.
However, the retail prices of these lamps are too high to tempt the majority of the public to invest in solid-state lighting, partly because the gains in efficiency over compact fluorescents are not yet to be that alluring. What’s needed is for the LEDs that are used in the bulbs start to delivering the eye- watering efficiencies that they do in the lab. If that happens, bulbs based on these chips will have far lower running costs and sell for much less than they do today.
When it comes to efficiency, chips in the lab have produced 276 lumens-per-Watt (lm/W), which is three-to-four times that of compact fluorescent lamps (60-80 lm/W) and vastly higher than incandescent sources (11-17 lm/W). However, according to reports coming from the US Department of Energy, the efficacies of the ‘well-made’ warm-white and cool-white LED lamps are only slightly higher
This leads to a high defect density, with threading dislocations typically higher than 108
cm-2 . It is possible that these dislocations are not that
Figure 1. Quantum efficiency (QE) versus current density for blue LEDs without an electron-blocking layer (EBL), with an Al0.2
Ga0.8 Ga0.8 N EBL, and with an In0.18 Al0.82 N EBL, and with an In0.18 Al0.82
shows light output versus current (L-I) characteristics of LEDs without an EBL, with an Al0.2
N EBL. Inset N EBL. An alternative InAlN EBL significantly
mitigates the efficiency droop with the lowest efficiency droop ratio of ~18 percent. (Reprinted with permission from Appl. Phys. Lett. 96 221105 (2010). Copyright 2010 American Institute of Physics.)
October 2013
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