high brightness leds...hb leds – from 60 to 70; some high performance leds to low 70s hb leds vs....
TRANSCRIPT
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Light Sources on Steroids
High Brightness LEDs
Course: Photonics and Optical Communications
Instructor: Prof. D. Knipp
Spring 2007, 20th April, 2007
Presenter: Borislav Hadzhiev
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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
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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
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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
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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
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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
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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
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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
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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-
M&_image=fig4&_ba=4&_user=1629077&_coverDate=06%2F02%2F2003&_rdoc=1&_fmt=full&_orig=search&view=c&
_acct=C000053987&_version=1&_urlVersion=0&_userid=1629077&md5=354c1ab181d9e99b4d82d8e488e8d97b
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