plenary 8_schubert_rpi_
TRANSCRIPT
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Light-Emitting Diodes for Solid-State Lighting and Beyond
E. Fred Schubert and Jong Kyu KimThe Future Chips Constellation
Department of Electrical, Computer, and Systems Engineering
Department of Physics, Applied Physics, and Astronomy
Rensselaer Polytechnic Institute
Troy NY 12180
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Lets keep the lights on and strongly reduce power
It is feasible to reduce energy consumption for lighting by 50%
and keep the lights on!
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Traditional applications for LEDs and OLEDs
Kodak
AT&TPulsarTexas Inst.
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Recent applications
ChinaUnited States Taiwan
Germany
Japan
Japan
Germany
China
Taiwan
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Solid-state lighting
Inorganic devices: Semiconductor plus phosphor illumination devices
All-semiconductor-based illumination devices
Organic devices:
Remarkable successes in low-power devices
(Active matrix OLED monitors, thin-film transistors, etc.)
Substantial effort is underway to demonstrate high-power devices
Anticipated manufacturing cost and luminance of organic devices areorders of magnitudedifferent from inorganic devices
Predicted growth of LED market
Comp. Semiconductors, 2006
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Lighting sources one has never seen before
Light-emitting large-area panels, wallpaper, and even curtains
OLEDs are light sources do not blind
OLED-panel lighting in movie theater
Siemens, 2005
Siemens, 2005
adopted from Edward Hopper
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OLED versus LED
LEDs are point sourcesThey are blindingly bright
Suitable for imaging-optics applications
Osram Corp.
OLEDs are area sourcesThey do do not blind
Suitable for large-area sources
Opto Tech Corp.
Luminance of OLEDs: 102 104 cd/m2
Luminance of LEDs: 106 107 cd/m2
Luminance of OLEDs is about 4 orders of magnitude lower
OLED manufacturing cost per unit area must be 104 lower
OLEDs
Low-cost reel-to-reel manufacturing
LEDs
Expensive epitaxial growth
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Energy conservation A singular opportunity
Solid-state lighting is singular opportunity for conservation of energy
Multiple light-emitting diodes LED with wavelength () converter
Nobel Laureate Richard Smalley: Energy is the single mostimportant problem facing humanity today and conservationefforts will help the worldwide energy situation.
Testimony to US Senate Committee on Energy and Natural Resources, April 27, 2004 1943 2005
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Quantification of solid-state lighting benefits
Energy benefits 22 % of electricity used for lighting
LED-based lighting can be 20 more efficient thanincandescent and 5 more efficient than fluorescent lighting
Financial benefits Electrical energy cost reduction, but also savings resulting from
less pollution, global warming
Environmental and economic benefits Reduction of CO2 emissions, a global warming gas Reduction of SO2 emissions, acid rain Reduction of Hg emissions by coal-burning power plants Reduction of hazardous Hg in homes
Antarctica United States
Hg
Cause: CO2
Switzerland
CO2 ,SO2, NOx, Hg, U
Czech Republic
Cause: SO2 Cause: Waste heatand acid rain
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Quantification of benefits
Global benefits enabled by solid-state lighting technology over period of10 years. First numeric value in each box represents annual US value.
US uses about of worlds energy.
70 4 = 28035 4 = 140Number of power plants notneeded
24.07 106 barrels 4 10 == 962.4 106 barrels
12.03 106 barrels 4 10 == 481.2 106 barrels
Reduction of crude-oilconsumption (1 barrel = 159 liters)
267.0 Mt 4 10 = 10.68 Gt133.5 Mt 4 10 = 5.340 GtReduction in CO2 emission
45.78 109 $ 4 10 == 1,831 109 $
22.89 109 $ 4 10 == 915.6 109 $
Financial savings
457.8 TWh 4 10 =
= 18,310 TWh = 65.92 1018 J
228.9 TWh 4 10 =
= 9,156 TWh = 32.96 1018 J
Reduction in electrical energy
consumption
43.01 1018 J 11% 4 10 == 189.2 1018 J
43.011018 J 5.5% 4 10 == 94.62 1018 J
Reduction in total energyconsumption
Savings under11% scenario
Savings under5.5% scenario
(*) 1.0 PWh = 1000 TWh = 11.05 PBtu = 11.05 quadrillion Btu = 0.1731 Pg of C = 173.1 Mtons of C1 kg of C = [(12 amu + 2 16 amu)/ 12 amu] kg of CO2 = 3.667 kg of CO2Quantitative data based on Schubert et al., Reports on Progress in Physics, invited review, to be published (2006)see also R. Haitz et al. Adv. in Solid State Physics, Physics Today2001; see also US DOE (2006)
Schubert et al., Reports on
Progress in Physics(2006)
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Fundamental innovations
Innovation in materials
Omnidirectional reflectors
New materials with unprecedented low refractive index
New materials with very high refractive index
Innovation in devices
White LEDs with remote phosphors
Solid-state lighting
Innovation in systems
Figures of merit
Future smart lighting systems
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Light-emitting diodes with reflectors
To avoid optical losses, ideal device structures possess either:
Perfect Transparencyor Perfect Reflectivity
Example of reflective structure:
Example of transparent structure (after Lumileds Corporation)
(after Osram Corporation)
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Omni-directional reflector (ODR)
Search for the perfect reflector: R= 100% for all , all , and TE and TM
Omni-directional reflection characteristics High reflectivity (> 99 %) Electrical conductivity Broad spectral width
High reflectivity is important!
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AlGaInP and GaInN LEDs with ODR
AlGaInP LED = 650 nm, MQW active region
AlGaAs window layerGaAs substrate removed, Si submount
GaInN LED = 460 nm, MQW active region
Sapphire substrate
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Employment of specular and diffuse reflectors in GaInN LEDs
Waveguided modes are trapped modes in specular reflector
Light escape enabled by roughening reflector surface: diffuse reflector
Specular versus diffuse reflectors
AFM: Diffuse reflector roughened by dryetching with polystyrene nano mask
(a) Polystyrene 400 nm (b) Polystyrene 700 nm
Reflectivity measurement
Specular reflector Diffuse reflector
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Figure of merit for DBR: Index contrast n
Fresnel reflectance of interface1h1h
1h
nn
n
nn
nnr
+
=
+
=
( )
( )
2
2hl
2hl2
DBRDBR1
1
+
==
m
m
nn
nnrR
eff
Braggstop
2
n
n=
n
nnLL
r
LLL
++=
+ 212121pen
44
2100
1c
22
nn
n
nn
n
+
DBR reflectance
Spectral width of stop band
Penetration depth
Critical angle (max. angle forhigh reflectivity)
By increasing index contrastn, figures of merit improve
New materials are required
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From total internal reflection to waveguiding
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New class of materials: Low-nmaterials
Dense materials n 1.4: SiO2 (n= 1.45); MgF2 (n= 1.39)
Low-n: refractive index n< 1.25
Xerogels (porous SiO2)
Gill, Plawsky, et al. 2001, 2005
Oblique-angle evaporation
Xi et al., 2005, 2006
Technique was developed in the 1950s
Both techniques suitable for low-loss LEDs
Low-nxerogels, after Gill and Plawsky, 2005 Low-n SiO2
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New class of materials: Low-nmaterials
Pore sizes 100 lower mirror losses than DBRs
Suitable for low-loss LEDs
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Innovation in high-refractive index encapsulation materials
Fundamental problem of light extraction
Index mismatch between semiconductor and surrounding air
Fresnel reflection and total internal reflection
Encapsulation materials with highrefractive index would solve light-extraction problem
Titania nanoparticles in
Silicone
Epoxy
PMMA Titania, TiO2, n= 2.68
Polymer: n 1.6
Mixture n> 2.0
Lester et al. US Patent5,777,433 (1998)
Layered approach reduces scattering
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High-refractive index encapsulation materials
AFM of TiO2 nano-particles in epoxy
Without surfactant With surfactant
Optical scattering in film withpoissonian distribution of nano-particles?
Ordered distributions feasible?
Shiang and Duggal J. Appl.
Phys. 95, 2880 (2004)
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Solid-state lighting technologies and figures of merit
Lighting technologies
Figures of merit of new lighting technologies
Quantity Desirable value Luminous source efficiency 150 lm/W Luminous flux 1000 lm Color rendition capability (CRI) 75 or greater Color temperature 2500 6000 K Lifetime 100 000 hrs Cost < $ 10 per lamp
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White solid-state lighting sources
Different technical approaches Blue LED plus yellow phosphor
UV LED plus RGB phosphor
Phosphor has excellent color stability
Multiple LEDs
Which one is best?
Efficiencies Incandescent light bulb: 17 lm/W
Monochromatic green: 680 lm/W
Di-chromatic white source: 420 lm/W
Trichromatic white source: 300 lm/W withexcellent color rendering (CRI > 90)
Demonstrated with SSL sources: 100 lm/W
What is the optimum spatial distribution of phosphors?
Proximate and remote distributions
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Innovation in white LEDs Phosphor distribution
Proximate distribution(after Goetz et al., 2003)
Proximate distribution(after Goetz et al., 2003)
Remote distribution(after Kim et al., 2005)
Remote phosphor distributions reduce absorptionof phosphorescence by semiconductor chip
Luo et al., Appl. Phys. Lett. 86, 243505 (2005)
Narendran et al., Phys. Stat. Sol. (a) 202, R60 (2005)
Kim et al., Jpn. J. Appl. Phys. Exp. Lett. 44, L649 (2005)
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Ray tracing simulations
Ray tracing simulations proveimprovement of phosphorescenceefficiency for
Remote phosphor
Diffusive reflector cup
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Experimental results
Improvement of phosphorescence efficiency:
15.4 % for blue-pumped yellow phosphor 27.0 % for UV pumped blue phosphor
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Novel loss mechanisms in white lamps with remote phosphor
Whispering Gallery Mode
Trapped optical mode
Solution: Non-deterministic element that breaks symmetry
Suppression of trapped whispering-gallery modes
Lord Rayleigh
(18421919)
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Remote phosphors with diffuse and specular reflector cups
Specular reflector cup
Reflectance versus angle
Surface texture by beadblasting
Diffuse reflectance increasedby two orders of magnitude
Diffuse reflector cup
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Color rendition
A light source has color rendering capability
This is the capability to render the true colors of an object
Example: False color rendering
What is the color of a yellow banana when illuminated with a redLED?
What is the color of a green banana when illuminated with a yellow
LED?
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Examples of different color renditions
High CRIillumination source
Low CRIillumination source
Franz Marc Blue Horse (1911)
CRI = 90 CRI = 62Siemens, EU Olla project (2005)
Solid-state lighting iscrosscutting technologythat will enable brilliantdisplays with the mostvivid colors ever seen
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Color Temperature
Hot physical objects exhibit heat glow (incandescence) and a color
Planckian radiator = Black, physical object with temperature T
Color temperature = Temperature of planckian radiator with samelocation in chromaticity diagram
red, 1000 K 730 C
orange, 1300 K
yellow, 2100 Kwhite, 6000 Kbluish white, 10 000 K
As temperatureincreases, hot objectssequentially glow in thered, orange, yellow,white, and bluish white
Example: Red-hothorseshoe
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Demonstration of trichromatic source
Color rendering index (CRI) depends strongly on alloy broadening
64 lm/W demonstrated at this time (CRI = 84) for trichromatic sources
For some applications CRI is irrelevant
For such applications, 680 lm/W would be possible with perfect SSL device
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Luminous efficacy and CRI for tri-chromatic source
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The Future: Smart Sources
Smart light sources can be controlled and tuned to adapt to differentrequirements and environments
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The Future: Smart Sources
Smart light sources will enable a wealth benefits and new functionalities
Example: Circadian lights
Example: Communicating automotive lights and room lights
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The Future: Smart Sources
Bio-imaging
NASA
Agriculture
Displays
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Future transportation systems
seat belts air bags anti-lock brake electronic stability control
and
communicative traffic lights
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Conclusions
Solid-state lighting is revolutionary technology that enables
Huge energy savings
Less global-warming gas and acid-rain-causing gas emissions
Reduced dependency on foreign oil
Fundamental innovation required to satisfy needs. Examples:
New low-nmaterials n= 1 .08
Omnidirectional reflectors with 100 lower mirror losses than metal reflectors
High-refractive index encapsulants would be very beneficial
Remote phosphor distributions with higher luminous performance
Novel applications enabled by smart light sources Circadian lighting systems
Communications systems
Intelligent transportation systems
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Acknowledgements
Dr. Jong Kyu Kim, Profs. Shawn Lin, Christian Wetzel, Joel Plawsky, William Gill,Partha Dutta, Richard Siegel, and Thomas Gessmann, Dr. Alex Tran (RPI), Drs.Jaehee Cho, Cheolsoo Sone, Yongjo Park, (Samsung SAIT) Drs. Art Fischer,Andy Allerman, Dan Koleske, and Mary Crawford (Sandia) Students: SameerChhajed, Charles Li, Pak Leung, Hong Luo, Frank Mont, Alyssa Pasquale,Chinten Shah, Jay Shah, JQ Xi, Yangang Andrew Xi (RPI)
Non-commercial agencies:
National Science Foundation Army Research Office
Department of Energy
Sandia National Laboratories
New York State, NYSTAR
Companies:
Crystal IS
Samsung Advanced Institute of Technology
Applied Materials
Troy Research Corporation