The efficiency of deep ultraviolet light (< 280 nm) emitting diodes (UVC LED) for converting electrical power to UVC photon power (wall plug efficiency) is still low compared to diodes emitting light in the visible spectral region, for instance blue light (50%), see link.

With the use of state of the art UVC LED technology, only 1-2% of the consumed electrical power is converted to UVC light. An increase of emitted UVC power can be obtained by increasing the electrical current. This increases, however, also the amount of heat produced by the LEDs. In addition the lifetime of the UVC LED decreases as the level of electrical current that drives the LED is increased.   

If the efficiency of the UVC LEDs can be improved more applications for the diodes can be realized. More efficient and powerful LEDs reduce the treatment time, the need for cooling and consequently a more attractive and cost effective alternative to existing mercury based UVC solutions for disinfection purposes.

A research group at Cornell University, NY has released a promising route for dealing with the basic physics that limits the UVC LED efficiency. According to postdoctoral researcher Moudud Islam, there are three basic  problems that limit the efficiency of the UVC LEDs:  injection efficiency – the proportion of electrons passing through the device that are injected into the active region; internal quantum efficiency (IQE) – the proportion of all electrons in the active region that produce photons or UV light; and light extraction efficiency – the proportion of photons generated in the active region that can be extracted from the device and are actually useful. In the UVC spectral range, all three efficiency factors suffer, but this group found that by using atomically thin Gallium Nitride instead of conventional Aluminum Gallium Nitride for the active region, both IQE and light extraction efficiency are enhanced. Injection efficiency is improved through the use of a polarization-induced doping scheme for both the negative (electron) and positive (hole) carrier regions, a technique the group explored in previous work. Further study will include packaging both the new technology and existing technologies in otherwise similar devices, for the purpose of comparison.

Jimmy Bak, UVclinical:LEDs emitting in the visible has high wall plug efficiency. What is the expectation within the semiconductor society for the future efficiency of the deep UV LEDs?”

Moudud Islam, Cornell University:The wall plug efficiency of UVC-LEDs is limited mostly because of the low injection efficiency of holes into the active region, which is due to the high activation energy of p-dopants in AlGaN. Use of polarization doping combined with tunnel junctions seem to be a promising solution to overcome this. The other limiting factor is the light extraction due to TM polarized emission from AlGaN-based light emitters. Use of GaN-based active regions that emit TE polarized light combined with special light extraction mechanism through packaging and encapsulation will help to extract more light out. It is not possible to forecast how soon these challenges will be overcome without experimental data but with the current interest and efforts by researchers from around the world into DUV photonics, the WPE might see an increase to ~20-30% over the next 5 years.”

Link to the research group and the full press release from Cornell.

Link to published work in Applied Physics Letter (use of polarization-induced doping to realize UVC LEDs).

(Enhanced IQE with ultra-thin GaN compared to AlGaN)

http://aip.scitation.org/doi/full/10.1063/1.5000844

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