Understanding LED efficiency “droop”
When LEDs are operated at high power their efficiency drops–it’s called efficiency droop. The reason has been a matter of debate for some time now as the theoretical predictions haven’t matched the experimental measurements. However, a new study shows that a typically ignored cause of efficiency loss is responsible for the droop.
The finding applied to nitride LEDs, the kind that seem to have most potential for general lighting applications. If droop can be minimized then the whole efficiency goes up which means either brighter LEDs or less power needed to run them at required illumination levels.
The key to understanding efficiency droop is in a process called Auger recombination. Typically, an LED creates light when an electron meets a hole inside the LED. The released energy from the recombination of electron and hole is given off as light. However, some fraction of the time, the released energy isn’t given off as light but is transferred to a third electron. That means one extra photon that isn’t produced and a consequently dimmer LED.
Previous calculation showed that Auger recombination shouldn’t be sufficient to account for the efficiency droop. New experiments show that the basic process of Auger recombination just described indeed doesn’t account for the droop but a different version of it does. Hence the confusion.
The variant is called indirect Auger recombination and it is usually insignificant compared with direct Auger recombination and so is often neglected. In nitride LEDs, however indirect Auger recombination is the dominant form of inefficiency. It works like the direct version except that the electron and hole recombination is assisted by either the electron scattering from ions in the metal or by giving off vibrations into the metal. The process is stronger for green LEDs which explains their greater efficiency droop.
The indirect Auger recombination can’t be prevented entirely but it can potentially be mitigated and physicists have a few ideas about how to do that. The significance of the work is that the problem is now understood and efforts can be put into reducing the droop and making more efficient LEDs for lighting.