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Towards the ideal white LED light source for general lighting applications

The ideal white light source for lighting?
Specification/properties 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

• LED fundamentals
• LED efficiency
• Three key issues of LED technology
• Efficient white LED light + state-of-the-art
• LED luminance
• LED alternatives

From a very simple solid-state physics point of view

Conduction in intrinsic semiconductors
E
Conduction band
Bandgap E g
Valence band

The recombination of a free electron and a hole can lead to the emission of a photon
E
Band gap E g E E photon

.
 h .
c

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.

+ n-type semiconductor p-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

Non-radiative recombination
Radiative recombination
Shockley-Read-Hall recombinations
(via defects in the
Auger recombinations crystal lattice (cannot be avoided)
Spontaneous emission
(directe recombinatie of electron-hole pair)

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.

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
Nanostructures fabricated via etching
Nano- structures fabricated via nano-imprinting http://www.luminus.com/

Issue 2: Impact of temperature on the LED light emission
8.0E-03
7.0E-03
6.0E-03
5.0E-03
4.0E-03
3.0E-03
2.0E-03
1.0E-03
0.0E+00
400 450 500 600 650 700
292.1 K
303.6 K
314.6 K
325.5 K
338.0 K
550 wavelength (nm)
1.
The radiant flux drops
2.
The spectrum shifts to longer wavelengths

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 !
Thermal conduction towards the PCB
Active cooling via liquid circulation

Issue 3: Droop
Reduction of the internal quantum efficiency at higher currents not as a consequence of temperature
   
I
2

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)

Combined with good color rendering

LEDs generate quite saturated/pure colors

First method to create white LED light

Luminous flux
( Φ v
)
takes eye sensitivity into account
 v

K m

V (

)
 e
(

) d
 with K m

683 lumen / watt
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
1: Ilumination
2: Object
Source: Philips Lighting Academy
Spectral
Stimulus
Human eye transforms spectral stimulus into color

The spectral stimulus or corresponding color depends as much on the illumination as on the object itself
Daylight
Reflection coefficient
1
Wavelength (nm)
0
Wavelength (nm)
1
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
Reflection coefficient
1
Wavelength (nm)
0
Wavelength (nm)
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
- 13 %
520 nm
450 nm
- 22 %
580 nm
- 30 %
640 nm
E E photon

.
 h .
c


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)

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)

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.
69 www.soraa.com

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

Reference Illuminant
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)

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
E
  n
2
S sin
2


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
 n
2  sin
2
   n
2
S sin
2


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

With free-form 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

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.

with high and low luminance

OLED advantage 1: Uniform emission.
Glare-free
(LG Chem) (Payne Alex
Lang)
(Tridonic)

OLED advantage 2 : Limited thickness – self cooling
(General Electric)
(JFB –
Designboom)
( Acuity Brands)

OLED advantage 3: Good color rendering

OLED advantage 4: Flexible
(LG Chem)
(Gergo Kassai)
(General Electric)

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
Optical configation based on blue laser diodes for the car-headlamps in a BWM i8 http://spectrum.ieee.org/transportation/advanced-cars/bmw-laser-headlights-slicethrough-the-dark

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