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Abstract

The movement of LED’s into the mainstream lighting industry
is now abundant with affordable LED lighting products readily available across
many industries there is now a strong movement for lighting engineers to create
the most efficient and therefore cost-effective light. At present, most LED products
use drive currents that are over and above optimum efficiency levels (Keeping, 2017). While LED research
continues to improve the lumens per watt ratio and try to resolve the cause of
the phenomenon known as efficiency droop, it is generally accepted within the
lighting industry that this issue is unlikely to be solved in absolute
certainty for some time.

Within the context of CREE’s recent announcement in 2013, reporting
a 300 lumen-per-watt (lm/w) breakthrough for a white LED (CREE, 2017),
this report discusses efficacy droop on the performance of LED lighting and
summaries common theories and concepts thought to cause the issue, and prevent
advancements in LED lighting efficacy.

Summary of this Report

 

·      
Context to CREE’s announcement

·      
Review of LED Architecture

·      
The issue with large drive currents which increase
brightness however at the expense of efficacy

·      
Current efficacy droop is a major limiting
factor in LED’s

·      
Contrary theories exist as to what causes this
efficacy droop

·      
Auger recombination (Justin Iveland, 2013) can be considered
the primary mechanism and most popular theory behind the efficacy droop

·      
Despite media hype efficacy droop is still a difficult
problem for better LED performance

·      
Understanding & directly measuring droop
allows engineers to optimize remedial solutions

 

Context   

Lighting manufacturer CREE state they have broken the 300 lumen
per Watt efficacy barrier for white LEDs. (CREE, 2017)
The announcement made in March 2014 stated that CREE had achieved this record by
obtaining a measurement of 303 lumens per watt from a white high-power LED at
room temperature which is a 10% increase from the manufacturers previous record
of 276 lumens per watt in 2012. (Semiconductor Today, 2014)

By achieving 303 lumens per watt CREE appears to make large
advancements in resolving the “LED efficacy droop” problem, which is the major
barrier to increasing the lumen per watt ratio.

Efficacy droop is now a well-known issue with LED lighting that
reduces efficiencies by as much as 20%  when
the LEDs are subjected to greater electrical currents (Clean Technica, 2017). This issue has impacted
the development of LED lighting and has been an area of much research.

Review of LED Architecture

It is stated (Compound
Semiconductor, 2014) that theoretically it would be possible for LED Lighting
to operate at a quantum efficiency of 100%, where each injected electron producing
a photon that is emitted from the chip. However, during the transfer of
electrical to optical energy, there are losses of electrons and photons. (Compound Semiconductor, 2014)

LEDs find
widespread applications, but they exhibit maximum efficiency only at very low
current. The electrical-to-optical power conversion efficiency drops
dramatically with higher input current. This so-called efficacy droop has been investigated
for many years, and it still represents a key challenge.

Figure 2 Showing the basic architecture of a LED.
Centred on top of the cathode lead is a semiconductor chip made from
nitride-based materials (Gallium Nitride symbol is GaN). When electricity is applied
across this structure it produces a blue light. Diffusing this light through a
yellow phosphor then produces a white light. In the current market place high-output
LEDs are variations on this type of LED architecture. (IEEE Spectrum, 2017)

Discussion

 

The principal of LEDs is that they function by injecting
electrons into the unit from one side and push their positive counterparts (holes)
from the other. Both types of carriers meet in a narrow trench, referred to as a
quantum well. In the quantum well, it is where they are trapped, join through
electrostatic attraction and recombine to produce light. (IEEE Spectrum, 2011)

Efficacy Droop Line = fall in efficacy

 

Figure 1 Showing the
droop line, when current is increased (to increase brightness or “lumens per
watt”) there is a point where efficiency starts reducing. (Digikey
Electronics, 2011)

Simply put, droop is a phenomenon in which LEDs lose
efficiency as current density is increased. The underlying physical cause is
most likely due to Auger recombination, which occurs when energized charge
carriers in the LED active region recombine but, instead of emitting light,
energize a neighbouring charge carrier, thus generating heat. The rate of Auger
recombination increases exponentially with the number of charge carriers, so
the phenomenon is much stronger at higher current operation. (Light Now,
2016)

The manufacturing process for LEDs has become more
sophisticated in recent years with a focus now towards improvements in cost and
performance. CREE’s announcement suggest a move by the company to dominate and capture
a large segment of the supply market and at the same time provide large energy
and environmental savings. The market potential in the USA alone is enormous, for
example a recent reporting published by the U.S. Energy Information
Administration state that in 2011, approximately 461 billion
kilowatt-hours (kWh) of electricity were allocated to lighting across
the residential and commercial markets. This equates to approximately 17% of
the total electricity used in these sectors and arround 12% of total U.S.
electricity consumption. (Forbes, 2017)

Literature Review of Efficiency
Droop in LEDs

As LED efficiency is understood in the field of quantum
physics, the theories can be difficult to understand, (Keeping, 2017) adding to this confusion is that contrary
theories exist as to what causes this efficiency droop. Different microscopic
mechanisms have been proposed, most prominently thermionic electron leakage
from the light-emitting active layers and Auger recombination inside these
layers, respectively. To this date Droop analysis is mainly based on modelling
and very few direct measurements of either mechanism are published thus far.

1.     Electron
leakage first noticed by engineers in 2008 on ultraviolet LEDs by measuring the
additional light produced by p-doped layers, which indicates electrons
traveling beyond the active region. A few similar reports followed, but could
not demonstrate a leakage magnitude that fully explains the measured efficiency
droop.

2.     The
first verification for auger recombination in quantum wells was provided only
in April 2013 by measuring high-energy (hot) electron emission from the LED
surface as report by Iveland et al in 2013. The authors reported that hot electrons
are generated by Auger recombination within the active region and subsequently
travel all the way to the LED surface.

3.     Another
experiment was conducted independently by German researchers based on the
assumption of a much shorter hot electron travel distance (Piprek.J., 2015). In that case, hot Auger
electrons release their energy quickly and are captured by a neighbouring
active layer. However, the Auger signal is relatively weak in both cases and
there was insufficient evidence that the Auger process is large enough to solely
cause the efficiency droop.

4.     Interestingly,
a group from Korea observed electron leakage when the LED is cooled down to
cryogenic temperatures (Dong-Soo Shin, 2012). The blue LEDs used in
this study did not exhibit any leakage at room temperature. This result was
quite unexpected since it is usually believed that higher temperatures make it
easier for electrons to escape from the active region. Advanced computer
simulation was recently able to explain this phenomenon. (Piprek, 2015)

5.     Due
to the high ionization energy of Mg acceptors used for p-doping, rising
temperatures free more holes and improve the hole conductivity significantly.
This was confirmed experimentally by a group from Finland (Lauri Riuttanen, 2015) As a result, the
hole injection into the active layers is enhanced, fewer electrons need to leak
out to find holes, and the efficiency rises with higher temperature.

6.     On
the other hand, if Auger recombination is known to initiating the droop, the experiments
show a reducing efficiency with increased temperature. Thus, the competing
efficacy droop mechanisms have the opposite effect on the efficiency when the
LED temperature rises. The temperature sensitivity of the LED efficiency
therefore offers a simple way to distinguish between both droop mechanisms. (Piprek, 2015)

As discussed above, the majority of published calculations
and results commonly indicate a declining efficiency with higher temperature. Thus,
Auger recombination can be considered the primary mechanism behind the
efficiency droop in LED devices. The technical calculation of droop occurs
because ?IQE (efficiency) decreases with increasing current
density ,  This effect can be seen from
the following expression:   

Figure 3
sourced from

·      
n is the carrier density, 

·      
Bn2 is the radiative recombination
rate,

·      
An and Cf(n) are
the linear and higher-order nonradiative recombination rates, respectively.

·      
At high injection, Cf(n)
dominates and diminishes ?IQE.

·      
The efficiency ?EQE can also be
impaired by ? diminishing with increasing current due to
carrier leakage. (Justin Iveland J. S., 2014)

Addressing the Efficacy Droop Problem

Even as recent as April 23, 2015 (IEEE Spectrum, 2017) The industry is
still uncertain on the cause or how to overcome the droop issue, As mentioned
above the current most plausible theory is the Auger recombination and hot
electron overflow. However, according to the technical professional
organisation IEEE Spectrum the cause of the biggest defect in the LED is said
to be resolved by research undertaken at the University of California, Santa
Barbara in France. This research team state that they have positively identified
the cause of droop. (IEEE
Spectrum, 2017)

Based on the research team’s theory there are two ways to overcome
droop.

1.    
With
the commonly used C-plane substrates, it is possible to use multiple quantum
wells (QWs) to increase carrier spreading and enable higher current density. Or

2.    
Transition
to semi- or non-polar GaN substrates, which enable a single QW to realize high
radiative recombination of holes and electrons at higher current densities and
reduce Auger losses. (LEDs Magazine, 2017)

 

Industry Outlook

The outlook for LED lighting is strong as it can provide
benefits in other areas than just improved efficiency. At present it provides
large opportunities to improve and reduce a buildings energy demand through sophisticated
controls and function of lighting. For example LED’s are dimmable and therefore
easily controlled, in addition to this can be readily combined with sensor
systems, therefore providing simple energy savings. The techniques to achieve
this already exist  through the use of
occupancy sensing, daylight harvesting, and local control of light levels. LED
is at the forefront of advancements in the lighting industry around the world with
respect to high performance building. (Department of Renewable Energy USA, 2015)

As there is no conclusive solution to the cause of
efficiency droop, the industry needs to carefully scrutinise LEDs when
specifying them as a way to promote advancements in performance. In theory this
would require understanding both the LED’s maximum efficiency and the
efficiency droop curve However, this is very difficult in practice as there is
usually a lack of details from manufacturers because they tend to only specify a
minimal information such as a single lumen per watt figure. What is important
for the industry is to be aware of efficacy across an entire current spectrum. (Digikey Electronics, 2011)

The claims made by CREE need to be investigated further at
some other manufactures could produce 300 lm/W efficacy albeit green in colour and
a very poor CRI value. CREE has not specified the chromaticity in its statement,
although it is understood the company does usually deliver certain milestones
with commercial products in a period of two years from the announcement (LED Magazine, 2015)

Conclusion and Recommendation

In the short term it is expected that LED products will continue
to be more expensive than traditional lighting however the higher operating
efficiency and longer operating lifetime (reduced maintenance and replacement
costs) confirm that LED lighting is already highly competitive on a total cost
of ownership (TCO) basis. In some application there is a payback period of less
than one year in some high use settings. (Department of Renewable Energy USA, 2015)

This report provides an overview of the issues related to
the LED efficacy droop and summaries the current theories and technical issues
on the phenomenon. CREE’s announcement is still relevant as it is stated that by
2020 it is now forecast that LEDs will have captured around 90 percent of the
lighting market (Fraunhofer, 2014). The increasing
demand for LED has been driven by a number of applications including display
backlighting, communications, medical services, signage, and general
illumination (Centre for Advanced Life Cycle Engineering, 2012).

Whilst LED”s lighting is already very efficiency, the gains
still matter for the industry, there may be fewer opportunities these days
however each additional percent becomes vastly beneficial (LED Inside,
2015)
and there is a strong demand across many industries. It is therefore important
for companies involved in the research and development of LED’s to understand
and be aware of this announcement by CREE to ensure current research remains
relevant to manufacturing high lumen-to-watt ratio lighting and ultimately
bringing cost effective lighting and designs to market. 

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