High Brightness LEDs. Light Sources on Steroids

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Alexia Robinson
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1 High Brightness LEDs Light Sources on Steroids Course: Photonics and Optical Communications Instructor: Prof. D. Knipp Spring 2007, 20 th April, 2007 Presenter: Borislav Hadzhiev

2 Overview Principle of operation Types of HB LEDs and materials HB LEDs vs. Conventional Light Sources (CLS) Pros Cons Applications Trends in current market References 2

3 Principle of Operation (I) Electron-Hole recombination Non-radiative released energy dissipated in the form of heat Radiative photon emission (we are interested in the latter) Ordinary LEDs simple p-n junction using materials undergoing radiative electron-hole recombination 3

4 Principle of Operation (II) From ordinary LEDs to HB LEDs additional active layers (heterojunctions): quantum wells formation Using materials with narrower band gap makes it 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. 4

5 Types of HB LEDs (I) (Al x Ga x ) 0.5 In 0.5 P active layer matched to GaAs cover amber to red color Developed in the mid-1990s In 2003 reached values 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

substrate and nitrides Sapphire is inexpensive and highly transparent SiC gives many advantages in epitaxy QE of

6 Types of HB LEDs (II) GaN/In x Ga 1-x N 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

7 Types of HB LEDs (III) 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. 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 lm/w, lifetime more than 50h, driving voltage less than 5V 7

8 Types of HB LEDs (IV) 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 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 8

9 HB LEDs vs. CLS: Pros Significant reductions in power consumption (80% to 95% as compared to incandescents) Longer life Greater reliability No filters needed 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 pollution 9

10 HB LEDs vs. CLS: Cons 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 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 reliability 10

11 Applications Backlighting in LCD displays for laptops and desktop computers Traffic signaling traffic lights, road signs Automotive interior and exterior Mobile world 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 market 11

12 Applications 12

13 Trends in Current Market Improvement of quantum efficiency Improvement of quality of light (especially whitelight applications) New applications Cost reduction Introduction of new materials Fight for market share 13

14 Thank You for Your Attention!!!

15 References M&_user= &_coverDate=06%2F02%2F2003&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C &_version=1&_urlVersion=0&_userid= &md5=90880f614217f406abae98ce e#sec3 X&_user= &_coverDate=04%2F24%2F2001&_fmt=full&_orig=search&_cdi=5589&view=c&_acct=C &_version=1&_urlversion=0&_userid= &md5=c1593a1804dfb94adf7c286a4457f7a6&ref=full M&_image=fig2&_ba=2&_user= &_coverDate=06%2F02%2F2003&_rdoc=1&_fmt=full&_orig=search&view=c& _acct=c &_version=1&_urlversion=0&_userid= &md5=7790d8267fa0ce8b2ec28cec95fb51b7 M&_image=fig4&_ba=4&_user= &_coverDate=06%2F02%2F2003&_rdoc=1&_fmt=full&_orig=search&view=c& _acct=c &_version=1&_urlversion=0&_userid= &md5=354c1ab181d9e99b4d82d8e488e8d97b 15

Light-Emitting Diodes

445.664 Light-Emitting Diodes Chapter 1. History of Light-Emitting Diodes Euijoon Yoon Light Emitting Diodes (LEDs) There are two major technologies : - All-semiconductor-based illumination devices - Semiconductor/phosphor