Towards the ideal white LED light source

Youri Meuret

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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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