Light Sources on Steroids

High Brightness LEDs

Course: Photonics and Optical Communications

Instructor: Prof. D. Knipp

Spring 2007, 20th April, 2007

Presenter: Borislav Hadzhiev

Overview

Principle of operation

Types of HB LEDs and materials

HB LEDs vs. Conventional Light Sources (CLS)

� Pros � Pros

� Cons

Applications

Trends in current market

References

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Principle of Operation (I)

Electron-Hole recombination

� Non-radiative – released energy dissipated in

the form of heat

� Radiative – photon emission (we are � Radiative – photon emission (we are

interested in the latter)

Ordinary LEDs – simple p-n junction

using materials undergoing radiative

electron-hole recombination

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Principle of Operation (II)

From ordinary LEDs to HB LEDs –additional active layers (hetero-junctions): quantum wells formation� Using materials with narrower band gap makes

it possible to form quantum wellsit possible to form quantum wells

� When electrons are trapped in quantum wells they have sharper density of states � photon generation is more probable

Performance of HB LEDs is strongly dependant on the materials used, their shape, etc.

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(AlxGa

x)0.5In0.5P active layer matched to GaAs –

cover amber to red color

� Developed in the mid-1990s

� In 2003 reached values

of 12 lm/W

Types of HB LEDs (I)

of 12 lm/W

� Disadvantage:

non-transparency of GaAs substrate for visible light

resulting in photon absorption

� By changing the substrate to a transparent one the

quantum efficiency is increased up to 30%

� HP developed TIP shape (truncated inverted pyramid) �

performance of TIP LEDs reached 100 lm/W in 20035

Types of HB LEDs (II)

GaN/InxGa1-xN and AlGaN active layers grown by

MO VPE on sapphire and SiC substrate – cover

blue to green color

� All structures include buffer

layers – dislocations at the

interface between the substrate

and nitrides

� Sapphire is inexpensive and highly transparent

� SiC gives many advantages in epitaxy

� QE of 15 lm/W in 2003 – goal 100 lm/W over the next

decade

� Alternatives – rare earth dopants (Eu (Europium), Er

(Erbium), Tm (Thulium)) 6

Organic HB LEDs (OLEDs)� Organic material is sandwiched between two injected

electrodes

� One of the electrodes is transparent – typical material for anodes is Indium Tin Oxide

� Active layer – PPV (poly-phenylenevinylene), PPP (poly-phenylene), PT (poly-thiophene), etc.

Types of HB LEDs (III)

� Active layer – PPV (poly-phenylenevinylene), PPP (poly-phenylene), PT (poly-thiophene), etc.

� Quenching effect due to disparity in the mobility of electrons and holes – solution: multi-layers

� Advantages: cheap to produce, can be produced on flexible substrates

� Disadvantages: react with moisture and oxygen – solution: encapsulated in inert gas atmosphere

� In 2003 60 lm/W, lifetime more than 50h, driving voltage less than 5V 7

White light emitting HB LEDs

� Combination of red, green, and blue HB LEDs –

complicated mechanism of combination

� InGaN HB LEDs (blue color) with fluorescent converter –

bluish white light (10 lm/W in 2003)

� UV HB LEDs with fluorescent converter – idea: the

Types of HB LEDs (IV)

� UV HB LEDs with fluorescent converter – idea: the

fluorescent coating absorbs 100% of the UV emission and

re-emits the light in a continuous spectrum over the

entire visible range; possible combinations of “RGB”

phosphorus

� OLEDs with special dopants also emit white light –

restrictions: OLEDs suffer from voltage dependant

variation in emission color and have small efficiency

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Significant reductions in power consumption (80% to

95% as compared to incandescents)

Longer life

Greater reliability

No filters needed

HB LEDs vs. CLS: Pros

Vibration proof

Directed light emission – no need of bending of light;

simplifications of circuits

Space saving – no need of fixtures

Dim of light without affecting color, efficiency, or

operating life

Side effects: reduction of pollution9

Bad color rendering for white light

� On a scale from 1 to 100 incandescents are rated 100

� Fluorescents – low to high 80s

� HB LEDs – from 60 to 70; some high performance LEDs

to low 70s

HB LEDs vs. CLS: Cons

to low 70s

Heat generation for high current HB LEDs

� Heat that LEDs generate is transferred to copper

leadframe

� Different coefficients of thermal expansion between

the chip and leadframe

� The mismatch is a potential thread for reliability10

Backlighting in LCD displays for laptops and desktop

computers

Traffic signaling – traffic lights, road signs

Automotive interior and exterior

Mobile world

Applications

Outdoor signage

Architectural , accent, and landscape lighting

Medical diagnostic instruments; medical and dental

applications

Marine navigational instruments; airfield and aircraft

interior lighting; safety beacons

Portable market11

Applications

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Improvement of quantum efficiency

Improvement of quality of light (especially

white light applications)

New applications

Trends in Current Market

New applications

Cost reduction

Introduction of new materials

“Fight” for market share

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for Your Attention!!!

Thank You

http://en.wikipedia.org/wiki/Quantum_well

http://nicosia.ccs.ucy.ac.cy/~ee06ip1/leds03260049.jpg

http://optics.org/cws/article/research/7312

http://outhouserag.typepad.com/gizmos/images/briteflash.jpg

http://techepics.com/files/samsung-led-lcd.jpg

http://www.audiworld.com/news/02/a8launch/a8020159.jpg

http://www.chinamanufacturerdirectory.org/showroom/feihua-led/images/traffic-light-panel.jpg

http://www.cir-inc.com/products/prod_detail.cfm?prod=7&id=171

http://www.elecdesign.com/Articles/ArticleID/5185/5185.html

http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=2254

http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02430Z90404338

References

http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02430Z90404338

http://www.partstrain.com/images/The_Auto_Blog/audi_nuvolari_led_headlight.jpg

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TW0-48D3BPT-

M&_user=1629077&_coverDate=06%2F02%2F2003&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000053

987&_version=1&_urlVersion=0&_userid=1629077&md5=90880f614217f406abae98ce3903662e#sec3

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TXF-42SXH2S-

X&_user=1629077&_coverDate=04%2F24%2F2001&_fmt=full&_orig=search&_cdi=5589&view=c&_acct=C000053987

&_version=1&_urlVersion=0&_userid=1629077&md5=c1593a1804dfb94adf7c286a4457f7a6&ref=full

http://www.sciencedirect.com/science?_ob=MiamiCaptionURL&_method=retrieve&_udi=B6TW0-48D3BPT-

M&_image=fig2&_ba=2&_user=1629077&_coverDate=06%2F02%2F2003&_rdoc=1&_fmt=full&_orig=search&view=c&

_acct=C000053987&_version=1&_urlVersion=0&_userid=1629077&md5=7790d8267fa0ce8b2ec28cec95fb51b7

http://www.sciencedirect.com/science?_ob=MiamiCaptionURL&_method=retrieve&_udi=B6TW0-48D3BPT-

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