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Towards the ideal white LED light source
Youri Meuret
Optical Design
New light sources
Appearance
Lighting
Measurement
Facilities
The Light & Lighting Laboratory
Towards the ideal white LED light source for general lighting applications
The ideal white light source for lighting?
Specification/properties
- Efficacy
- Spectrum
- Lifetime, stability, robustness
- Cost
- Radiation pattern
- Formfactor
- Luminance
- Total light output
Ideal
- ≈ 400 lumen/watt
- CRI = 100
- 100 khours at this moment
- low cost per lumen x hours
- Application depending
- Application depending
- Application depending
- Application depending
- E
- S
- L
- C
- R
- F
- L
- T
Overview
• LED fundamentals
• LED efficiency
• Three key issues of LED technology
• Efficient white LED light + state-of-the-art
• LED luminance
• LED alternatives
LED fundamentals
From a very simple
solid-state physics
point of view
Conduction in intrinsic semiconductors
Bandgap Eg
E Conduction band
Valence band
The recombination of a free electron and a hole can lead to the emission of a photon
Band gap Eg
E
For an intrinsic semiconductor at room temperature, the amount of
free electrons and holes is low → the chance that these meet is low
→ the amount of created photons is low.
. .photon
cE E h f h
p-type semiconductor
n-type semiconductor
+
+
-
-
Bandgap → the smallest frequency
F. Schubert “Light-Emitting Diodes” Cambridge University Press (2006)
Temperature → the (theoretical) spectral width
RED LED (25°)
→ Δλ = 28 nm
Shockley-Read-Hall
recombinations
(via defects in the
crystal lattice
Auger
recombinations
(cannot be avoided)
Spontaneous emission
(directe recombinatie
of electron-hole pair)
Non-radiative recombination
Radiative recombination
Direct band-gap materials
Indirect band-gap materials
Necessary requirements for potential LED materials
1. Material is a semiconductor.
2. Material has a bandgap in the visible region or in the UV
(for λ = 350 – 800 nm, Eg ≈ 3.5 eV – 1.55 eV).
3. Material is a direct band-gap material.
4. Robustness of the crystal lattice against defect formation.
5. Ease of fabrication/availability of substrate for crystal
growth.
6. Reliability for high temperature / high power operation.
7. Toxicity of the material.
Vinod Kumar Khanna “Fundamentals of solid-state lighting,” CRC Press (2014)
Three inorganic material combinations as a basis for light-emitting diodes
Aluminium Gallium Indium Phosphide (AlGaInP)
Indium Gallium Nitride (InGaN)
The Nobel Prize in Physics 2014 was awarded jointly to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura
"for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources"
Identified suitable substrate for crystal growth (Sapphire and SiC)
Developed suitable dopingmethods for p-type semiconductors out
of InGaN with sufficient conductivity.
LED efficiency
On the die level
In the 1960s, when the first III–V semiconductors had been
demonstrated, the internal quantum efficiencies at room temperature
were very low, typically a fraction of 1%.
At the present time, high-quality bulk semiconductors and quantum
well structures can have internal efficiencies exceeding 90%, and in
some cases even 99%. This remarkable progress is due to improved
crystal quality, and reduced defect and impurity concentrations.
(F. Schubert, “Light-Emitting Diodes,” (2nd ed.)
Cambridge University Press (2006))
F. Schubert “Light-Emitting Diodes” Cambridge University Press (2006)
The emitted light from the active region can be absorbed…
• …in the active region
• …in the confinement layers
• …in the substrate
• …at the electrical contacts
Flipped chip LED structure
Absorption of photons with energies that are smaller than the bandgap energy cannot be fully eliminated
A large part of the light is trapped inside the die due to the large refractive index (>2.5) of the
semiconductor material
Angles at which the light can escape
from a rectangular die
Solution 1 : Encapsulation of the die by a material with a refractive index that is higher then air
Solution 2 : Change the shape of the die
Optimal die shape
for perfect
outcoupling
Realistic die shape
for good
outcoupling
Solution 3 : Structuring or roughening of the die or substrate surfaces
Structuring of the sapphire substrate for GaN LEDs offers a double advantage
1. Higher extraction efficiency
2. Less surface defects
Donggeun Ko, et al., “Patterned substrates enhance LED light extraction,”
LEDs magazine (2014).
Solution 4: Use photonic crystal structures
Nano-
structures
fabricated via
etching
Nano-
structures
fabricated via
nano-imprinting
http://www.luminus.com/
Three key issues of current LED technology
Issue 1: The green gap
Issue 2: Impact of temperature on the LED light emission
1. The radiant flux drops
2. The spectrum shifts to longer wavelengths
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
400 450 500 550 600 650 700
wavelength (nm)
sp
ectr
al ra
dia
nt
flu
x (
W/n
m)
292.1 K
303.6 K
314.6 K
325.5 K
338.0 K
Reduction of the radiant flux
• The propability of Shockley-
Read Hall recombination is
higher at higher temperatures
• More charge carriers escape
from the quantum well in e.g.
double hetero-junctions.
The varying spectrum is due to the intrinsic variation of the semiconductor bandgap with varying temperature.
LEDs create a lot of heat so thermal managent is crucial !
Active cooling via
liquid circulation
Thermal conduction
towards the PCB
Issue 3: Droop Reduction of the internal quantum efficiency at
higher currents not as a consequence of temperature
2I I I
At constant temperature
J. Iveland, et. al., “Direct measurement of Auger electrons emitted from a LED :
identification of the dominant mechanism for efficiency droop,” Phys. Rev. Lett. (2013)
Efficient white LED light
Combined with good
color rendering
LEDs generate quite saturated/pure colors
Additive color mixing
First method to create white LED light
Luminous flux (Φv) takes eye sensitivity into account
wattlumenKwithdVK memv /683)()(
1 watt = 68 lumen 1 watt = 545 lumen
Theoretically possible efficacy (lumen/watt) of a dichromatic white LED
Theoretical
emission spectrum
Problem 1: The green gap is actually a yellow gap
Color of objects under illumination
Spectral
Stimulus
1: Ilumination
2: Object
Human eye transforms
spectral stimulus into color
Source: Philips Lighting Academy
Daylight
1
1
0
Wavelength (nm) Wavelength (nm)
Reflection coefficient
Reflected radiant flux
Wavelength (nm)
The spectral stimulus or corresponding color depends as much on the illumination as on the object itself
Low pressure sodium
1
0
The spectral stimulus or corresponding color depends as much on the illumination as on the object itself
Wavelength (nm) Wavelength (nm)
Reflection coefficient
Reflected radiant flux
Wavelength (nm)
Color rendering index (CRI)
• CRI is a quantitative measure (0-100) of the ability of
a light source to reveal the colors of various objects
faithfully in comparison with a reference light source.
Munsell color sampes used for
determining the CRI
Relative spectral power distribution
of illuminant D and a black body of
the same correlated color
temperature (in red)
Problem 2: Low color rendering index
Second method to create white LED light
Trichromatic (RGB) LED applications
Theoretically possible efficacy > 300 lm/W Theoretically possible CRI > 90
Trichromatic LED systems need complex electrical driving and feedback control circuitry
Both
• Luminous flux
• Peak wavelength
• Spectral width
depend on the junction-temperature.
This variation is strongly depending on
the used semiconductor material.
→ Important variation of the resulting
spectrum (color) as a function of
temperature.
Expensive electrical control systems are needed
And the winner of the white LED contest (category lighting)
is
The verdict of the jury: “The white phosphor converted LED offers the best trade-off between cost, efficacy and color rendering index at this moment”
Phosphor properties
The most common phosphor for white LEDs is Yttrium Aluminium Garnet (YAG) doped with cerium (CE)
• It is possible to tune the
emission spectrum by
adapting the YAG:Ce
composition.
• By varying the YAG:Ce
concentration or thickness
of the phosphor layer white
light of various colour
temperatures can be
achieved.
Also white phosphor converted LEDs allow a tunable color temperature
www.photonstartechnology.com/
Problem
Stokes Losses
450 nm
580 nm
640 nm
520 nm
- 13 %
. .photon
cE E h f h
- 22 %
- 30 %
Problem
Relative low CRI
Problem
Absorption of light that is sent back
towards the die
A quantitative analysis of a remote phosphor module showed an extraction efficiency of only 65%
P. Acuna, et. al., “Power and photon budget of a remote phosphor
LED module,” Optics Express (2014)
LEDs: State-of-the-art
What could be the next winner
of the white LED contest
At this moment, the efficacy of LEDs is larger than of any other white light sources
Based on an efficacy of 200 lm/W (with optimal color
rendering) and 60% market share
Solid-State Lighting Research and Development, “Multi-Year Program Plan”
US Department of Energy (2014)
Room for improvement ?
The company SOORA co-founded by the Noble price winner Nakamura offers some clear advantages
• High internal quantum
efficiency by using GaN on a
GaN substrate.
• Good color rendering beyond
CRI based on violet emission.
• BUT large Stokes losses.
www.soraa.com 69
National Renewable Energy Laboratory proposes AlInP for efficiënt amber LED
• No Stokes losses
• High efficacy (lm/W)
• High CRI
• Color Tunable
http://www.nrel.gov/technologytransfer/technologies_led.html
Quantum dots are photo luminescent materials with a narrow emission spectrum of which the peak
wavelength can be easily varied
• Promising as lightsources
for LCD backlights.
• Quantum dots have a
potential role to play in the
development of new LEDs
with high efficiency and
good color rendering
www.qdvision.com
The research of
optimal color rendering
will play a vital role in the determination of
the ideal LED white light source !
Ra=50 Ra=60 Ra=70 Ra=85 Ra=100 Ra=83 Reference Illuminant
Large colour differences towards reference
low quality
Memory colour rendering index (MCRI)
The more similar a light source renders the familiar object colours
to their memory colours, the better the colour quality.
K. Smet, et. al., “A Memory Colour Quality Metric for White Light Sources,”
Energy and Buildings (2012)
LED Luminance
Do we want it high or low ?
Étendue determines the spatial and angular extent of a light bundle
For a light bundle with a uniform angular extent over the total light
bundle surface S the étendue can be calculated by
22 sinSnE
luminance = luminous flux/étendue
A green laser diode has a very
high luminance because the
étendue is extremely small
A HID lamp has a very high
luminance because the
luminous flux is very large and
the étendue is quite small
The étendue/luminance of a lightbundle cannot be reduced/increased with passive
optical components
2222 sinsin Snn
Two different requirements of a lighting luminaire
The radiation pattern created by the luminaire that results in a certain
illuminance distribution
The appearance of the luminaire
(Uniformity and brightness)
With freeform optics it is possible to generate an arbitrary radiation pattern for the light
emitted from a point source
“Energy-saving LED light sources,” 30 March 2011, SPIE Newsroom
With free-form optics it is possible to generate an arbitrary radiation pattern for the light
emitted from a point source
Accurate tailoring of the radiation pattern is only possible for low-étendue light sources
High source luminance however causes glare
“visual discomfort from LED luminaires by glare is one of the main
causes why these systems are sometimes perceived as less good than
their counterparts based on fluorescent lamps”
G. J. Scheire et. Al. “Calculation of the Unified Glare Rating based on luminance maps for
uniform and non-uniform light sources,” Building and Environment (2015).
Different optical systems can be used to reduce the observed luminance by the lighting luminaire
Conclusion
• Low étendue or high luminance light sources
are needed if accurate beam control is
important.
• Light sources with a low luminance help to
avoid glare-issues but advanced optics can do
the trick as well.
LED alternatives
with high and low luminance
OLED advantage 1: Uniform emission. Glare-free
(Payne Alex
Lang)
(LG Chem) (Tridonic)
OLED advantage 2: Limited thickness – self cooling
( Acuity Brands)
(JFB –
Designboom)
(General Electric)
OLED advantage 3: Good color rendering
OLED advantage 4: Flexible
(LG Chem) (General Electric)
(Gergo Kassai)
OLED advantage 5: Dynamic colors are feasible
(Verbatim)
(Verbatim)
OLED advantage 6: Transparent sources are possible
(Osram) (Fraunhofer)
(Philips Lumiblade)
Laser diodes have an intrinsic advantage over LEDs for the development of efficient white light sources
with high luminance
J. J. Wierer et. al., “Comparison between blue lasers and light-emitting diodes for
future solid-state lightings,” Laser and Photonics Reviews (2013).
This advantage is already being used for an application where accurate beam-control is essential :
Car-headlamps
http://spectrum.ieee.org/transportation/advanced-cars/bmw-laser-headlights-slice-
through-the-dark
Optical configation based on blue laser diodes for the car-headlamps in a BWM i8
LEDs, OLEDs, lasers ?
Efficient and qualitative lighting will remain
a very fruitfull research area
in the years to come
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