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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
Laboratorium voor Lichttechnologie
Light sources
DischargeIncandescent
Mercury Sodium
Low pressure
High pressure
FL, CFL MetalhalideOutdoor
illumination
Outdoor,
Shops
Solid State
LED
Low pressure
High pressure
Laboratorium voor Lichttechnologie
2. Elementary Solid State Physics
Laboratorium voor Lichttechnologie
Intrinsic semiconductor
“Free” electrons and holes
Bandgap Eg
EConduction band
Valence band
Laboratorium voor Lichttechnologie
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!
Laboratorium voor Lichttechnologie
Extrinsic semiconductor (n)
Donor-atoms (P); n-type
E
Laboratorium voor Lichttechnologie
Acceptor-atoms (B); p-type
Extrinsic semiconductor (p)
E
Laboratorium voor Lichttechnologie
+ -
Injection of free electrons in p-type and free holes in n-type
p-n junction!
p n
Laboratorium voor Lichttechnologie
3. Electrical characteristics
Laboratorium voor Lichttechnologie
• 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
Laboratorium voor Lichttechnologie
Electrical characteristics
Laboratorium voor Lichttechnologie
4. Optical characteristics
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Optical characteristics: chromaticity
CIE chromaticity
• Purity
• Dominant wavelength
Laboratorium voor Lichttechnologie
Optical characteristics: chromaticity
CIE chromaticity
Additive mixing with wide colour gamut
Laboratorium voor Lichttechnologie
Bandgap engineering to obtain an extensive range of
wavelengths and colours:
use of compound semiconductors
Optical characteristics: colour
Laboratorium voor Lichttechnologie
Compound Semiconductors
Laboratorium voor Lichttechnologie
AlGaInP
InGaN
Laboratorium voor Lichttechnologie
Optical characteristics: white LEDs
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Osram 6 lead multiLED
Laboratorium voor Lichttechnologie
Optical characteristics:Radiant/luminous Flux Φ(e)
forward electrical current
# recombinations
# photons
luminous flux
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Efficacy red: 55 lm/W (room temperature)
Energy-efficiency: 24 %
LossesNon-radiative recombination (heat)
Internal reflections
Optical characteristics: efficacy
Laboratorium voor Lichttechnologie
Non-radiative recombination
Temperature of the semiconductor junction increases!
Laboratorium voor Lichttechnologie
Total Internal Reflection
Substrate
Active material
absorption
Partially reflectedTotally reflected
Laboratorium voor Lichttechnologie
Internal reflections
1. semiconductor-encapsulant
2. encapsulant-air
Laboratorium voor Lichttechnologie
Optical characteristics: spatial
Dependent on
• position of die and reflector
• shape of the external dome
www.nichia.com
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
5. Effect of Temperature
Laboratorium voor Lichttechnologie
Effect of Temperature: luminous flux
Increase of non-radiative recombination!
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Effect of temperature:
lumen maintenance
http://www.lrc.rpi.edu/programs/solidstate/
ongoingProjects.asp?ID=57
Laboratorium voor Lichttechnologie
Thermal management
Laboratorium voor Lichttechnologie
Thermal management
( )T
P WR
10 °C/W
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
6. Photometry of LEDs
Photometer/colorimeter or
spectroradiometer
Laboratorium voor Lichttechnologie
780
,
380
780'
,
380
Y . ( ). ( )
Respons . ( ). ( ). ( )
e
e
Tristimulus value k y d
k S d
Photometer
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Spectroradiometer
780
,
380
. ( ). ( )eY k y d
Bandwidth: 5 nm
Laboratorium voor Lichttechnologie
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.
Laboratorium voor Lichttechnologie
Photometry of LEDs: luminous fluxReference light source required
Laboratorium voor Lichttechnologie
Cosine-corrected photometer head
baffle
Substitution
Standard LED
Test LED
Auxiliary LED
d
50 mm
Precision aperture
Fast measurements: partial flux
Laboratorium voor Lichttechnologie
7. LED penetration into general lighting
Laboratorium voor Lichttechnologie
LED penetration into General Lighting: main obstacles
• Luminous flux
• Efficacy
• Colour and flux maintenance
• Thermal management
• Reproducibility
• Price
Laboratorium voor Lichttechnologie
LED penetration into General Lighting: obstacles : luminous flux (white)
P(W) I(mA) Φ(lm)
0.070 20 1.5
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
LED penetration into General Lighting: obstacles : efficacy/ CRI
Laboratorium voor Lichttechnologie
CRI (Ra)Test sourceColour samples(8 of 14)
Standard illuminant<colour differences>
Colour coordinates
Colour coordinates
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Obstacles: efficacyBUT Lighting Systems
Higher Light Output Ratio possible due to a higher directionality of the “naked” light source
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Laboratorium voor Lichttechnologie
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 !
Laboratorium voor Lichttechnologie
LED penetration into General Lighting: obstacles : thermal management - chip
Laboratorium voor Lichttechnologie
LED penetration into General Lighting: obstacles : thermal management - luminaire
Project 2.2 Californian Energy Commission
Laboratorium voor Lichttechnologie
LED penetration into General Lighting: obstacles : reproducibility
“binning”
Laboratorium voor Lichttechnologie
LED penetration into General Lighting: obstacles : price
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
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
Laboratorium voor Lichttechnologie
Topics
Photovoltaics
Lighting Optical design
Appearance
Measurement Facilities
Laboratorium voor Lichttechnologie
Lighting
Research: – Criteria efficient lighting– LED’s (PhD)
Supporting industry– Groen Licht Vlaanderen: promotion of
energy efficient lighting (Greenlight)– Shoplighting
Laboratorium voor Lichttechnologie
Optical design
Research: – Luminaire design with ray-tracing (PhD)
Supporting industry– Secundary optics for LED clusters– Surface with uniform luminance
Laboratorium voor Lichttechnologie
Appearance
Research: – Gloss (PhD)– Colour rendering with LED’s
Supporting industry– Automotive– Wood– Retro-reflection
Laboratorium voor Lichttechnologie
Photovoltaics
Research: – Spectral response– Light trapping in cells and modules
Supporting industry– Stand-alone systems– Signalization
Laboratorium voor Lichttechnologie
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