promoptica “nouvelles techniques d’eclairage” inorganic leds: working principles and prospects...

Promoptica“Nouvelles Techniques d’Eclairage”

Inorganic LEDs: working principles and prospects for general lighting applications

Laboratory for Light and Lighting

KaHo St.-LievenUniversity College

Gent (B)

P. Hanselaer

Liège

Novembre 8, 2007

Laboratorium voor Lichttechnologie

1. Main categories of light sources


Light sources

DischargeIncandescent

Mercury Sodium

Low pressure

High pressure

FL, CFL MetalhalideOutdoor

illumination

Outdoor,

Shops

Solid State

LED

Low pressure

High pressure


2. Elementary Solid State Physics


Intrinsic semiconductor

“Free” electrons and holes

Bandgap Eg

EConduction band

Valence band


Recombination between an electron and a hole: energy can be released by the creation of a photon

Band gap Eg

E

At room temperature: only a small amount of free electrons and holes: limited number of photons!

Photon

Light!


Extrinsic semiconductor (n)

Donor-atoms (P); n-type

E


Acceptor-atoms (B); p-type

Extrinsic semiconductor (p)

E


+ -

Injection of free electrons in p-type and free holes in n-type

p-n junction!

p n


3. Electrical characteristics


• Diode characteristic

• Low, dc voltage

• Forward voltage dependent on bandgap of the semiconductor:

AlGaInP: 2.95 V typ.

InGaN: 3.42 V typ.

Electrical characteristics


Electrical characteristics


4. Optical characteristics


1240( ) . .

( )g

cE E eV h f h

nm

Photon energy (and colour) is determined by bandgap Eg

Rather monochromatic radiation

Optical characteristics: spectrum

Band gap Eg

E

1240( )

( )g

nmE eV

Band gap Eg

E

1240( )

( )g

nmE eV

Band gap Eg

E

Band gap Eg

E

1240( )

( )g

nmE eV


0

300

600

900

1200

1500

350 400 450 500 550 600 650 700 750

Golflengte (nm)

Sp

ectr

ale

stra

ling

sin

ten

site

it (

mW

/sr.

nm

)

LED1

Peak wavelength λp : from UV to IR

Full Width at Half Maximum: from 20 to 50 nm

Optical characteristics: spectrum


Optical characteristics: chromaticity

CIE chromaticity

• Purity

• Dominant wavelength


Optical characteristics: chromaticity

CIE chromaticity

Additive mixing with wide colour gamut


Bandgap engineering to obtain an extensive range of

wavelengths and colours:

use of compound semiconductors

Optical characteristics: colour


Compound Semiconductors


AlGaInP

InGaN


Optical characteristics: white LEDs


Three or more LEDs of different Colors

+ The more colours one has to mix, the more control one has in producing white light with a high color rendering index.

+ Photons from each LED contribute directly to the light intensity, i.e. no conversion efficiencies have to be considered.

+ Extensive range of hue’s can be obtained

- Optical control, coloured shadows


Osram 6 lead multiLED


Optical characteristics:Radiant/luminous Flux Φ(e)

forward electrical current

# recombinations

# photons

luminous flux


rode LED : I = f(Y)

0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100 120 140 160 180 200

Y [cd/m2]

I [m

A]

LED’s: current driven


Efficacy red: 55 lm/W (room temperature)

Energy-efficiency: 24 %

LossesNon-radiative recombination (heat)

Internal reflections

Optical characteristics: efficacy


Non-radiative recombination

Temperature of the semiconductor junction increases!


Total Internal Reflection

Substrate

Active material

absorption

Partially reflectedTotally reflected


Internal reflections

1. semiconductor-encapsulant

2. encapsulant-air


Optical characteristics: spatial

Dependent on

• position of die and reflector

• shape of the external dome

www.nichia.com


Secundary optics

0

20

40

60

80

100

120

140

-90 -70 -50 -30 -10 10 30 50 70 90

zonder lens

medium beam

Optical characteristics: spatial


5. Effect of Temperature


Effect of Temperature: luminous flux

Increase of non-radiative recombination!


Effect of Temperature: peak wavelength and light flux

Decrease of the bandgap, increase of wavelength!

Band gap Eg

E

1240( )

( )g

nmE eV

Band gap Eg

E

1240( )

( )g

nmE eV

Band gap Eg

E

Band gap Eg

E

1240( )

( )g

nmE eV


EN 12368

15s 1min

90min3min

30min10min

0.38

0.39

0.4

0.41

0.42

0.43

0.44

0.45

0.46

0.53 0.55 0.57 0.59 0.61

x

y

Effect of temperature: chromaticity

Chromaticity versus warm-up time


Effect of temperature:

lumen maintenance

http://www.lrc.rpi.edu/programs/solidstate/

ongoingProjects.asp?ID=57


Thermal management


Thermal management

( )T

P WR

10 °C/W


Determination of junction temperature

LuxeonIII U(junction T) for series 1 (used)

y = -0.0026x + 2.4443

R2 = 0.999

1.90

1.95

2.00

2.05

2.10

2.15

2.20

2.25

2.30

2.35

2.40

2.45

20 30 40 50 60 70 80 90 100 110

temperature (°C)

U (

V)

1: 10 µA

1: 100 µA

2: 10 µA

2: 100 µA


6. Photometry of LEDs

Photometer/colorimeter or

spectroradiometer


780

,

380

780'

,

380

Y . ( ). ( )

Respons . ( ). ( ). ( )

e

e

Tristimulus value k y d

k S d

Photometer


Important errors in tails of eye sensitivity curve

0.0000

0.0001

0.0010

0.0100

0.1000

1.0000

400 500 600 700

Wavelength (nm)

Re

lati

ve

Re

sp

on

se

CIE Photopic LuminosityFunction


Spectroradiometer

780

,

380

. ( ). ( )eY k y d

Bandwidth: 5 nm


Photometry of LEDs: intensity

Some LEDs have a very narrow radiation pattern (FWHM 2°)

• Large distance to detector and small detector aperture required.

• CIE 127 standardisation: “averaged LED intensity” at 316 mm (A) or 100 mm (B) distance and 1 cm2 detector area.


Photometry of LEDs: luminous fluxReference light source required


Cosine-corrected photometer head

baffle

Substitution

Standard LED

Test LED

Auxiliary LED

d

50 mm

Precision aperture

Fast measurements: partial flux


7. LED penetration into general lighting


LED penetration into General Lighting: main obstacles

• Luminous flux

• Efficacy

• Colour and flux maintenance

• Thermal management

• Reproducibility

• Price


LED penetration into General Lighting: obstacles : luminous flux (white)

P(W) I(mA) Φ(lm)

0.070 20 1.5


LED penetration into General Lighting: obstacles : luminous flux

P(W) I(mA) Φ(lm)

1.2 350 60

3.6 1000 100

5.0 700 120Luxeon


LED penetration into General Lighting: obstacles : luminous flux

P(W) I(mA) Φ(lm)

4.7 420 108

26 2300 567

860 13300“Chip on board”

technology

Lamina Ceramics, Osram

Multiple LED package


LED penetration into General Lighting: obstacles : efficacy/ CRI


CRI (Ra)Test sourceColour samples(8 of 14)

Standard illuminant<colour differences>

Colour coordinates

Colour coordinates


Low CRI and yet high colour preference?

CRI and LED’s: subject of international research

CIE TC 1-69

Colour Rendition by White Light Sources


Obstacles: efficacyBUT Lighting Systems

Higher Light Output Ratio possible due to a higher directionality of the “naked” light source


Obstacles: efficacyBUT coloured applications

0

5

10

15

20

25

30

35

40

350 400 450 500 550 600 650 700 750

Golflengte (nm)

Sp

ec

tra

le s

tra

ling

sin

ten

site

it (

mW

/sr.

nm

)Halo1*0.01

Halo2

LED /Halogen green traffic signal: efficiency (cd/W): 8 / 1


LED penetration into General Lighting: obstacles : lumen maintenance

• LED lifetime is sometimes specified in MTBF (mean time between failure).

• Various LED manufacturers predict LED source life up to 100K hours

• “Lumen Maintenance” is even more important.• End-of-Life specification: light output has

dropped to 70% compared to the original light output:

50.000 hrs !


LED penetration into General Lighting: obstacles : thermal management - chip


LED penetration into General Lighting: obstacles : thermal management - luminaire

Project 2.2 Californian Energy Commission


LED penetration into General Lighting: obstacles : reproducibility

“binning”


LED penetration into General Lighting: obstacles : price


Pro and contra: pro• Saturated colours, dynamic colour effects with a

large colour gamut• High efficiency for applications with coloured light

(e.g. traffic lights)• Liftetime up to 50 000 hours (70% definition) • Vibration-proof• Low voltage• No mercury• No UV and IR radiation• Instantaneous switch-on • Easy dimmable


Pro and contra: contra• Reproducibility is difficult (semiconductor

processing); binning (sorting by intensity, colour, forward voltage) is required

• Colour and intensity shift with temperature, driving current and life time

• Low output/device

• Low efficacy for white (but is improving)

• Price


www.lichttechnologie.be

Instituut voor de Aanmoediging van Innovatiedoor Wetenschap en Technologie in Vlaanderen

KaHo St.-LievenGebr. Desmetstraat 1

B-9000 GENTTel: + 32 9 265 86 10

[email protected]


Laboratory for Light&Lighting

Founded in 1997 with the support of IWT Vlaanderen (Flemish institute for the promotion of innovation in science and technology).

Main activities:– Education– Scientific research – Supporting industrial developments


Topics

Photovoltaics

Lighting Optical design

Appearance

Measurement Facilities


Lighting

Research: – Criteria efficient lighting– LED’s (PhD)

Supporting industry– Groen Licht Vlaanderen: promotion of

energy efficient lighting (Greenlight)– Shoplighting


Optical design

Research: – Luminaire design with ray-tracing (PhD)

Supporting industry– Secundary optics for LED clusters– Surface with uniform luminance


Appearance

Research: – Gloss (PhD)– Colour rendering with LED’s

Supporting industry– Automotive– Wood– Retro-reflection


Photovoltaics

Research: – Spectral response– Light trapping in cells and modules

Supporting industry– Stand-alone systems– Signalization


Measurement facilities

• 8/d spectral reflectance and transmittance

• Goniometer

• Spectrometers: VIS, UV, near IR

• Electrical characterization

• Bidirectional Scattering distribution

• Photometric/colorimetric camera

• LED integrating sphere

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