plenary 8_schubert_rpi_

8/7/2019 Plenary 8_Schubert_RPI_

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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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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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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

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