Led

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LIGHT EMITTING DIODE (LED) Sept. 26, 2007 Sooseok Xia Lakshminarayanan Fang

EE 226

LED group

Outline 2

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Sooseok Introduction  Nitride Based LED Technology 

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Xia White LED Technology  Chromaticity and Color Rendering 

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Lakshminarayanan LED Basics & Ongoing Research & Applications  White LED Market Analysis and Patent related Issue 

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Fang Competitors for White Light Technology  OLED and Quantum dots based LED Technology 

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Introduction 3

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Background  LED

is a semiconductor diode that emits narrow spectrum light when electrically biased in forward direction. It is a form of Electroluminescence (EL)

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Advantage over conventional bulb  High

luminescence efficiency  Quick response time  Long lifetime EE 226 – LED Group

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Comparison between light bulb and LED Light bulb

LED

Power consumption RED YELLOW GREEN

70W 70W 70W

18W 20W 17W

Replacement Interval

6 ~ 12 month

Failure mode

Sudden total failure

5 ~ 10 years (estimated) Gradual intensity decrease

Visibility Cost Maintenance Manufacturing Luminous Efficacy (lm/w) EE 226 – LED Group

Use color filter, reflects sunlight

Direct

Expensive Cheap

Cheap Expensive

10 ~ 85lm/W

100 lm/w

Light Emission process 5

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Recombination of electron in conduction band with hole in valence band releases photon energy of well-defined frequency electron Ec

Eph = hv h: Planck’s constant (6.626x10-34 J-s) v: wave frequency e.g) GaN: 360nm

Forward biased hole

Ev k

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k (momentum) 0

Light Emission Process 6

Forward biased

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White LED Technology 7



Nitride-based Semiconductor material  Gallium

Nitride (GaN)  Indium Gallium Nitride (InGaN) 

Leading White LED  Cree:

131 lm/w @ 20mA  Nichia: 138 lm/w @ 20mA  Prof. Satoshi Kamiyama: 130 lm/w 

White LED generation  Mix

of RGB LEDs  Blue LED and Phosphors or YAG phosphor EE 226 – LED Group

White LED generation 8

LEDs

Phosphor Downconversion One BLUE or

Color Mixing Three RGB LEDs

Purple LED Cost

Cheap

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Expensive

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Property of Nitride-based semiconductor compound 

GaN compounds  Hexagonal 

Wurtzite crystal structure

 Direct 

crystal system

bandgap material

Semiconductor in which the bottom of the conduction band and the top of the valence band occur at the momentum k=0

 Advantage:

Excellent hardness  High thermal conductivity  High melting temperature 

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Issues of Nitride-based LED [1] 

Large lattice constant mismatch between GaN and Sapphire substrate Crystallinity is improved with buffer layer !

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Issues of Nitride-based LED [2] 

High quality low resistivity p-type GaN film for higher quantum efficiency Nitrogen and Hydrogen

TF-MOCVD (Two Flow-Metal Organic Chemical Vapor Deposition)

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Resistivity vs. Annealing temperature

Structure of Blue LED 12

Refractive index

C ⋅ ∆n ⋅ d Γ ≅1− e Γ : Confinement factor ∆n : difference in refractive index d : thickness of active layer

InGaAlP based LED

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GaN based LED

Research topic and Trend 13

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LED structure moves to Single Quantum Well (SQW) and Multiple Quantum Well (MQW) from Double Heterojunction (DH)

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Challenges on Nitride-based LED       

Lifetime Efficiency High output power Simulation Ultra Violet LED (UV-LED) White LED Other researches on LED industry

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Lifetime 15



Definition  Operation

time in hours for light output to reach 70% of its initial value  10,000 hours are required for commercial products 

Major factor for effecting lifetime  Heat 

at the p-n junction

Packaging for better heat dissipation is crucial

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Junction temperature 16

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Temperature measurement  T-point

method

 External

location on package  Easy access for measurement

Example of T-points for two different types of LEDs (left: Barracuda package; Right: Hemispherical encapsulant package

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Experiment results 17

Life output vs. Time & Different ambient temperature

Lifetime vs. T-point temperature

Thermal management becomes an important issue!

Different samples vs. Time

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Efficiency 18



To increase in the scattering and diffraction of the generated photons  Increasing

light collecting from the LED chip with less power loss  Increasing the extraction number of photons that trapped inside LED chip Surface roughening  GaN growth on a patterned sapphire substrate  Integration of 2-D photonic crystal patterns  Forming V-shape pits on surface that originate from low temperature growth conditions of topmost pGaN contact layer (EQE~30%) 

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Improvement inside LED chip 

GaN growth on a patterned sapphire 63% improvement substrate

14.1% improvement under 20mA current

PSS on C-plane

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Output power vs. Current

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Improvement inside LED chip 

GaN growth on a patterned sapphire substrate The SiO2 film with hole-patterns of 3µm diameter and 3µm spacing was deposited onto the sapphire substrate by PECVD method and defined by standard photolithography to serve as a wet-etching mask. The sapphire substrate was then etched using an H3PO4-based solution at an etching temperature at 300oC. The sapphire etching rate is about 1µm/min.

16.4% quantum efficiency improvement under 20mA current !!

PSS on {1102}R-plane

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Improvement inside LED chip 

Forming V-shape pits on surface V-shaped pits on p-type GaN contact

30% quantum efficiency improvement under 20mA current for 465nm output wavelength !!

Topview SEM image of LED

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High output power 22  

Multi-chip method is common for high bright LED Thermal management at high power level is crucial  New substrate material is needed  Si  Low cost  Large-sized wafer  Multifunction integration on same Si chip  GaN grow on Si is still challenge  Large lattice constant mismatch (16.9%)  Large thermal expansion coefficient mismatch (57%)  SiC  High cost

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Si-substrate based LED 23 Emitted light is absorbed by Si-substrate

Achieve high output power Under high injection current (>800mA)

Output power at high injection current is not saturated due to high thermal conductivity (1.5W/cm-K)

Output power vs. Injected current with Si/Sapphire substrate

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Improve electrode contact 24



The poor conductivity of p-GaN limits LED performance due to current crowding effect  Thin

Ni-Au layer  Highly transparent (> 80%) indium-tinoxide (ITO) layer

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Packaging for high output power

Package of typical high power LED

Package of low power LED

Osram Golden Dragon

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Luxeon Power LED

Ultra Violet-LED (UV-LED) 26

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AlGaN compound  

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Application    

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Ultra large bandgap property Emission wavelength can be down to 250nm Biological-agent sensing Air and water purification Biomedical diagnosis To excite UV phosphor to generate white light

Limitation 

Self heating   



High operation voltage Low radiative efficiency Poor electrical conductivity

Temperature dependency

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Simulation Software 27

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Simulator of Light Emitters based on Nitride Semiconductors (SiLENSe)  Covers

DH, SQW, and MQW structure  Standard LED structure, I-V curve, emission spectrum, internal light emission efficiency, temperature effect 

 Doesn’t 

cover yet

Many effects   

Many-body effects on InGaN AlInGaN QWs Boundary effects on the optical properties of InGaN MQWs Surface band-bending effects on the optical properties of InGaN MQWs

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Researches in LED industry 28

 

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Fabrication of high efficient LED Fabrication of high quality p-type GaN film Achieving same performance without Phosphor powder

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Thank You !

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Continue on second session by

Xia

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White Light Technology 30

Generate White Light (GWL) Based on 

  

Color Rendering Index (CRI) Two Method to (GWL) Based On LED  

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Chromaticity diagram

Multi-Color (RGB) LED Based Phosphor Based

Current Researches in Phosphors

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Chromaticity Diagram 31

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White Region λ

= 700 nm (R), λ = 555 nm (G), and λ = 460 nm (B)

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Generate White Light (WL) theory  Combination

WL

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Quality WL Depend On  Intensity

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of R, G and B can generate

& how strong excited RGB

Planckian locus  Color

T  T ↑ Color glow from R → Y → W

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Combination of R, G and B can generate WL 32

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Chromaticity Diagram 33

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Color Rendering Index (CRI) 34

  

Important characterization of light Light source not render true color Measure a tested light source  Give

how different tested light source with reference light source



CRI > 85

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(a) High – CRI Source (b) Low – CRI Source

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Chromaticity difference result from reference and test light source 36

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CRI of Some Sample Light Source 37

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Two Method to Generate WL Based on LED 38



3 type white light LED  RGB

LEDS  B LED + Y Phosphor  UV LED + RGB Phosphor 

2 categories  Use

combination of 3 discrete RGB LEDS  Employees phosphor coating LED

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Method I: Multi-color (RGB) LED Based ⇒ WL 39

Generate WL Theory  R + G + B = WL  NOT Phosphors Coating 

 

RGB LED has most light output efficiency and offers specific white point control

Theory CRI > 95 Reality Trouble Problem 

Complicate control, high cost and power dissipation compare with phosphor based

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(a) Color mixing of three primary colors. (b) Additive color mixing using LEDs. 40

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Type 1: RGB LED ⇒ WL 41

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Method II: Phosphor Based ⇒ WL LED 42

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Principle: B

LED combined w/ coated Y phosphor technique ⇒ WL LED

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Process:  Blue

light emitted from LED chip is absorbed by the phosphor, then reemitted as yellow phosphorescence



Phosphor based white LED spectrum have 2 emission bands.

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Type 2: B LED + Y Phosphor ⇒ WL 43

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Introduce Bond Wire & LED Chip 44

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Phosphor Based White LED 45

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Phosphor-based White LED Spectrum 46

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Method II Continue 47



Phosphor Based Characterized  Only

one chip, simplest to implement.  Compact size, long lifetime, low power.  Efficient < RED LED  

Problem: CRI < 80 Reason:  Phosphor

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lacks enough red emission.

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Improve CRI Method 48



Solve to Improve CRI:  Enhanced

a red phosphor introduced in

package 

Employ the UV LED technique  UV

LED is very similar to B LED combined w/ Y phosphor technique

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UV LED ⇒ WL LED Principle:  UV

Light is absorbed by RGB phosphors and output WL

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Type 3: UV LED + RGB Phosphor ⇒ WL 49

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Summary method II to generate WL based on LED 50

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Current Researches in Phosphors 

 

B LED combines w/ phosphor technique is the most common method to generate WL Violate patent and get sued Research the phosphor material on inorganic nano-structure material

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LED Basics & Ongoing Research & Applications 52

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LED Basics: 4 efficiency type  based



on fundamental optical properties

Concept: 1) Internal Efficiency:  Quantum

efficiency  Count photons & electrons/S η

= #p emitted from active region/S #e injected into LED /S internal

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Four Efficiency Type Continue 53

2) Extraction Efficiency: 

ηextraction = #p emitted into free space/S #p emitted from active region/S

3) External Efficiency: 

ηexternal = #p emitted into free space/S #e injected into LED/S = ηinternal * ηextraction

4) Power Efficiency: 

Quantity efficiency

ηpower P EE  226 – LED= Group

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Reasons: Extraction Inefficiency 54

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Points:  Escape

problem is main reason for extraction inefficiency  Emission of photons has internal reflection occurs at air interface 

Trapped light:  Get

internal reflection at every incidence

angle range

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φi > φC

called trapped light

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Photons at trapped light range can not escape 55

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The Light Escape Cone 56

Definition of the escape cone by the critical angle φC or by area element dA or by whole area

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Light Escape in LEDs 57

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Pescape ≅

½ [1 - (1 - φC2/2)] = ¼ φC2

Psource

φC = critical angle of total internal reflection 

Problem: only small fraction of light can escape from semiconductor

Pescape Psource

2

1 nair = 2 4 ns

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Thank You !

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Continue on third session by Lakshminarayanan

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Ways to improve efficiency: 59

   

Structures Encapsulation Packaging Thickness of active layer Performance of light-emitting diodes is defined to the great extent by two figures of merit, namely internal quantum efficiency of the active region and light extraction efficiency. While the former quantity reflects the quality of an epitaxially grown structure and normally lies in the range 20-90%, the latter strongly depends on particular design and can be as low as 2%.

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Different shapes of LED’s 60

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Effect of encapsulation: 61

Encapsulation increases the performance of an LED. A semiconductor coated with an encapsulant is typically epoxy or PMMA or silicone and the refractive index is reduced between the semiconductor and the free space . The epoxy acts as a buffer to change the refractive index from the high value in the semiconductor to a intermediate value in the epoxy and gradually to a low value in the air.

Packaging:   

Electrical path optical path thermal path.

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Thermal packages 63

The thermal path has a heat sink that could be silver , Al, Cu, which directs heat away to a large area PCB that will dissipate the heat and thus, LED will not reach a high temperature. Shown below are images of some general package and special packages.

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Thickness of active layer 64

Total efficiency is the product of internal efficiency and extraction efficiency. However, light extraction efficiency is self-dependent on internal quantum efficiency due to inevitable reabsorption of some of the light. In thin LEDs reabsorption effect is less severe. Fig. shows extraction efficiency vs. LED chip height. For high IQE, LED should be thin film, but for lower IQE a thick field is better because light escapes more readily from edges.

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Ongoing areas of research 65 

Current research activities include enhancements in structures,efficiency,pakaging, material to gain maximum benefits of LED technology.

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Applied field and research activity includes replacing traditional fluorescent lights in refrigerators with prototype led lighting system .Investigating the properties of phosphor to improve the white LED performance.

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Due to the mismatch between the refractive index of the epoxy and that of the die, less than 35% of light generated within most LED dies escapes the LED package as visible light. Recent studies on ultraviolet (UV) LED technology, as well as advances in quantum dot (QD) technology, have the potential to increase LED efficiency. QDs efficiently convert UV light into any wavelength of visible light based on the size of the dot. The research carried by LRC investigates the effects of temperature on the epoxy-QD and epoxy-phosphor encapsulants along with refractive index properties of LED epoxy mixed with varying concentrations of down-conversion materials (QDs EE 226 – LED Group and YAG:Ce phosphor).

Market analysis 66

case study – traffic signals:

Life time:

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Cost: Currently, the major barrier for LED system to replace traditional light is its initial cost. The cost of LED system is way much higher than traditional light

.

source

A 12” red LED signal can range from $60 to $125, and a 12” green LED module costs $160 to $250. Amber LED signals typically cost about $75 each. However, it only costs about $2 to $2.50 for a traditional light bulb. Considering a typical four-way intersection, it will cost around three thousand more on the setup fee to replace the light bulbs with LED light source. At this time, it is estimated that about 10% of the traffic signals in

.

the nation are LED traffic signals

Conclusion:

As a new generation light source, LED has a lot benefits that make it a much better choice for indoor and outdoor light source. The two most advantages EEimportant 226 – LED Group of LED are its energy consumption and its indurations. However, the cost is main factor to slow down the pace of LED to take over the whole light source market

HAITZ LAW 68

Like Moore’s law to semiconductor industry, Haitz Law typically states that the light output and efficacy of LEDs roughly doubles every 18 to 24 months, and that the future LED performance will likely follow a trend similar to that of the past 30 years. So far, though the growth of LED efficiency seems a little bit faster than Haitz Law’s expectation, the development of LED is still follow this statement. Most of the Million USD market reports right now did Unit: their prediction based on Haitz Law

Current LED market trend

2003

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2004

2005

2006

2007

Illumination /light source market 69

Actually, to replace the indoor light source is the final destination that all LED manufactures try to accomplish. The global market size of HB LED at 2005 is about 5 billion USD while the global illumination market is around 130 billion USD. The market of HB LED is very small right now but that also means how big the potential market is once the LED overcomes the barrier and occupies all the light source market.

Efficiency Traditional light bulbs take around 27% of illumination market at 2005. That’s a big market with amount around forty billion USD. The efficiency of HB LED in the market right now is around twenty to thirty lumens per Watt. It’s already higher than the efficiency of traditional light bulbs, eight to ten lumens per Watts. Then why the LED doesn’t take over light bulb’s market? The reason is the efficiency to cost rate

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Cost In 2005, it’s around 0.2 USD/lm. The cost is expected to keep going down to reach 0.02 USD/lm at 2007. However, it has to be further reduced to around 0.01 USD/lm to really be applied in illumination market. This probably will be accomplished at around 2008 or 2009. To achieve this low cost, the main difficulty is the LED package. Though the package technology is already mature now, it still needs to be refined to find the better material and process to further reduce the cost.

Conclusion White LED has a huge potential market in indoor light source market. However, it still limited by the insufficient efficiency, higher setup cost and energy cost. The development of device and package technology will decide when LED could really dominate the light source market.

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LEDs and COLORs 71

Aluminium gallium arsenide (AlGaAs) — red and infrared  Aluminium gallium phosphide (AlGaP) — green  Aluminium gallium indium phosphide (AlGaInP) — high-brightness orange-red, orange, yellow, and green  Gallium arsenide phosphide (GaAsP) — red, orange-red, orange, and yellow  Gallium phosphide (GaP) — red, yellow and green  Gallium nitride (GaN) — green, pure green (or emerald green), and blue also white (if it has an AlGaN Quantum Barrier)  Indium gallium nitride (InGaN) — near ultraviolet, bluish-green and blue  Silicon carbide (SiC) as substrate — blue  Silicon (Si) as substrate — blue (under development)  Sapphire (Al2O3) as substrate — blue  Zinc selenide (ZnSe) — blue  Diamond (C) — ultraviolet  Aluminium nitride (AlN), aluminium gallium nitride (AlGaN), aluminium gallium indium nitride (AlGaInN) — near to far ultraviolet to 210 nm EE(down 226 – LED Group[8] 

Failure modes 72

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The mechanism of degradation of the active region, where the radiative recombination occurs, involves nucleation and growth of dislocations this requires a presence of an existing defect in the crystal and is accelerated by heat, high current density, and emitted light. Gallium arsenide and aluminum gallium arsenide are more susceptible to this mechanism than gallium arsenide phosphide and indium phosphide .Due to different properties of the active regions, gallium nitride and indium gallium nitride are virtually insensitive to this kind of defect; however, high current density can cause electro migration of atoms out of the active regions, leading to emergence of dislocations and point defects, acting as nonradiative recombination centers and producing heat instead of light.



White LEDs often use one or more phosphors. The phosphors tend to degrade with heat and age, losing efficiency and causing changes in the produced light color. Pink LEDs often use an organic phosphor formulation which may degrade after just a few hours of operation causing a major shift in output color.



Sudden failures are most often caused by thermal stresses. When the epoxy resin used in packaging reaches its glass transition temperature, it starts rapidly expanding, causing mechanical stresses on the semiconductor and the bonded contact, weakening it or even tearing it off. Conversely, very low temperatures can cause cracking of the packaging.

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Led applications 73        

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Motorcycle and Bicycle lights Toys and recreational sporting goods, such as the Flashlight Railroad crossing signals Continuity indicators Flashlights, including some mechanically powered models. Emergency vehicle lighting Elevator Push Button Lighting Thin, lightweight message displays at airports and railway stations and as destination displays for trains, buses, trams and ferries. Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.

In optical fiber and Free Space Optics communications.  In dot matrix arrangements for displaying messages.  Glow lights, as a more expensive but longer lasting and reusable alternative to Glow sticks.  Grow lights composed of LEDs are more efficient, both because LEDs EE 226 – LED produce moreGroup lumens per watt than other alternatives, and also because they can be tuned to the specific wavelengths plants can make the most 

Illumination applications 74 

Size of illuminated field is usually comparatively small and Vision systems or smart camera are quite expensive, so cost of LEDs is usually a minor concern, compared to signaling applications.



LED elements tend to be small and can be placed with high density over flat or even shaped substrates (PCBs etc) so that bright and homogeneous sources can be designed which direct light from tightly controlled directions on inspected parts.



LEDs can be easily strobed (in the microsecond range and below) and synchronized; their power also has reached high enough levels that sufficiently high intensity can be obtained, allowing well lit images even with very short light pulses: this is often used in order to obtain crisp and sharp "still" images of fast moving parts.



LEDs come in several different colors and wavelengths, easily allowing to use the best color for each application, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effect of ambient light.



LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation, therefore allowing plastic lenses, filters and EE 226 – LED diffusers to beGroup used. Waterproof units can also easily be designed, allowing for use in harsh or wet environments (food, beverage, oil industries).

Trends in LED design 75 Continually, while LED’s become brighter and cheaper; new colors and applications for LED’s are appearing in so many markets we truly believe LED’s will challenge every conventional light source during our lifetime . LED manufacturers are driven by volume demand.  Virtually any product that requires a status indicator can potentially use an LED. The success of LED’s lies in their longevity, energy efficiency, durability, low maintenance cost, and compact size.  LED’s last up to 100,000 hours, compared to 1,000 hours for incandescent bulbs,

.

which fail unexpectedly

Also, because there is no filament or gas heating prior to ignition, LED’s illuminate quicker than conventional lamps and use up to 90 per cent less energy.

 

With the introduction of bright AlInGaP chip technology in the1990s, red and amber LED’s began replacing incandescent bulbs in new automotive taillight assemblies. And InGaN blue and green LED production ramped-up through the end of the 1990s due in large part to the demand for traffic signal green LED’s.

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Moreover, in term of security, LEDs have fast reaction time: LEDs light up around 250 milliseconds quicker than bulbs. Therefore, at 100 km/h speed, that means a gain in braking distance of 7 meters. HBLEDs have large market opportunities for front lighting for cars as the HBLEDs performance will increase, fewer chips will be necessary (less than 15 should be necessary in 2009 for all functions). The cost objective for auto (10$/klm) will be reached in 2010 for a wide diffusion of HBLEDs in front lighting. However, HBLEDs should be implemented before this date (around 2007) on high-end cars.

Today a high-end car can have up to 200 LEDs and this figure is expected to grow in the future: it could be up to 800 LEDs in 2009. The market for HB-LEDs for front light is just starting. We forecast that the market for automotive external lighting will be 40% of the total HBLED automotive market in 2009. Regarding the white LEDs market, it will be shared by 5 major players: Lumileds, Osram, Nichia, Toyoda Gosei and Cree. Moreover, the recent interest for HUDs (Head-Up Displays) in cars should also benefit from the HBLEDs technology. HBLEDs are today bright enough to be used as a backlight on a FPD instead of EE 226 – LED Group bulbs.

Advanced led applications 77

Light-emitting diode (LED) technology has provided medicine with a new tool capable of delivering light deep into tissues of the body, at wavelengths which are biologically optimal for cancer treatment, wound healing and other clinical applications. The clinical use in therapeutic applications as well as in laboratory work of LED’s is given in view of low intensity laser irradiation effects in biomedicine.

High Flux surface mount (SMT) LED’s make three-dimensional RCL (Rear Combination Lamp) solutions possible. But what is the most suitable mounting substrate for the LED’s Considerations should allow stylists as much freedom as possible while meeting performance requirements. Multiple options are available including Flexible Printed Circuit bonded to Aluminum, Printed Circuit Board (FR4) and Insulated Metal Substrates.

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Projection of LED for the future 78 According to projections from Sandia National Laboratories, the energy-saving benefits of LED lighting would be impressive: If the technology can be improved so that half of all lighting is solid-state by 2025, it will cut worldwide power use by 120 gigawatts, saving $100 billion a year and reducing carbon dioxide emissions from power plants by 350 megatons a year. Moreover, lighting experts say, semiconductor LED’s and organic light-emitting diodes (OLED’s) would change the way people think about lighting their homes. Rather than static fixtures holding single-color bulbs, solid-state lighting will be more flexible . Researchers are pursuing two tracks for increasing the light output of LEDs. One is to improve the internal quantum efficiency -- the percentage of electricity that gets turned into photons; the other is to boost the external quantum efficiency -- the percentage of photons that get out of the LED and into the world. "Silicon for electronics is like carbon for organic chemistry," says Moungi Bawendi, professor of chemistry at MIT and an expert on semiconductor nanomaterials. "It's sand--you can't get better than that, so you certainly have a cost advantage if you can base [an LED semiconductor] on silicon."

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Thank You !

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Continue on third session by Fang

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Competitors for white light technology 

Organic Light Emitting Diode (OLED )

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Quantum Dots based Light Emitting Diode (QDLED)

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Organic Light Emitting Diode (OLED )

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Organic Light Emitting Diode (OLED) 

An OLED is a special type of light emitting diode wherein the emissive layer consists of a thin film of organic compounds.

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Types of OLEDs 83

     

Passive-matrix OLED Active-matrix OLED Transparent OLED Top-emitting OLED Foldable OLED White OLED

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Passive-matrix OLED (PMOLED) 

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

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The anode strips are arranged perpendicular to the cathode strips External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. consume more power than other types of OLED, mainly due to the power needed for the external circuitry PMOLEDs are most efficient for text and icons and are best suited for small screens : cell phones, PDAs and MP3 players

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Active-matrix OLED (AMOLED) 

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EE 226 – LED Group

full layers of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image. efficient for large displays best uses are computer monitors, large screen TVs and electronic signs or billboards.

Transparent OLED 86

EE 226 – LED Group

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have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85 percent as transparent as their substrate.

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When turned on, it allows light to pass in both directions.

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can be either active- or passive-matrix.

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This technology can be used for heads-up displays.

Top-emitting OLED 87

EE 226 – LED Group

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have a substrate that is either opaque or reflective.

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best suited to activematrix design.

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Manufacturers may use top-emitting OLED displays in smart cards.

Foldable OLED 88

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Foldable OLEDs have substrates made of very flexible metallic foils or plastics.

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Foldable OLEDs are very lightweight and durable. Their use in devices such as cell phones and PDAs can reduce breakage, a major cause for return or repair.

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Potentially, foldable OLED displays can be sewn into fabrics for "smart" clothing, such as outdoor survival clothing with an integrated computer chip, cell phone, GPS receiver and OLED display sewn into it.

EE 226 – LED Group

White OLED 89

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White OLEDs emit white light that is brighter, more uniform and more energy efficient than that emitted by fluorescent lights.

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White OLEDs also have the true-color qualities of incandescent lighting.

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Because OLEDs can be made in large sheets, they can replace fluorescent lights that are currently used in homes and buildings.

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Their use could potentially reduce energy costs for lighting.

EE 226 – LED Group

Advantages of OLED 90 

(1) A large advantage of OLED is they can be made at low temperature. This allows for plastics, which in turn allow for flexible and thinner displays. It is also a reduction in weight.

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(2) As OLED pixels in a display can be made very small, they allow for high resolution displays.

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(3) Their response time is much faster then LCD pixel.

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(4) As they generate light themselves, it eliminates the need for a backlight. This means that they draw far less power and when powered from a battery can operate longer on the same charge.

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(5) They are brighter and more efficient than LEDs.

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(6) They offer great potential for lighting applications ranging from general purpose illumination to small flat panel displays found in mobile phones and digital music players.

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(7) They have the ability to tune the light emission to any desired color.

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(8) They are current-driven devices, where brightness can be varied over a very wide dynamic range and they operate uniformly without flicker.

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(9) They can be deposited on any substrate such as glass, ceramics, metal, thin plastic sheets, fabrics and therefore, can be fabricated in any shape and design.

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(10) Since OLED can be printed onto any suitable substrate, they can have a significantly lower cost than LCD or plasma displays.

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(11) The viewing angle possible with OLED is greater because OLED pixels directly emit light.

EE 226 – LED Group

Disadvantages of OLED 91 

(1) The biggest problem is the limited lifetime of the organic materials. Particularly, blue OLEDs typically have lifetimes of around 5000 hours.

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(2) The intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the duration of more flexible displays.

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(3) Commercial development of the technology is also restrained by patents held by firms.

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(4) As they are made with organic material, they are susceptible to heat. All the energy which is not emitted in the form of light is converted to heat, degrading the organic layer.

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(5) Because efficiency of OLED is not yet very high, higher current is needed to make the OLED emit the desired amount of light. This also results in more heat, which slowly destroys the LED.

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(6) When large displays are made, the lifetime also drops.

EE 226 – LED Group

92

Quantum Dots based Light Emitting Diode (QDLED)

EE 226 – LED Group

93

Quantum Dots based LED 

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions.

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EE 226 – LED Group

2 to 10nm (10 to 50 atoms) in diameter

Quantum Confinement 94 

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EE 226 – LED Group

If the size of a semiconductor crystal becomes small enough that it approaches the size of the material's Exciton Bohr Radius, then the electron energy levels can no longer be treated as continuous - they must be treated as discrete. This situation of discrete energy levels is called quantum confinement. meaning that there is a small and finite separation between energy levels

95

Size Dependent Control of Bandgap in Quantum Dots 

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EE 226 – LED Group

As with bulk semiconductor material, electrons tend to make transitions near the edges of the bandgap. However, with quantum dots, the size of the bandgap is controlled simply by adjusting the size of the dot. Because the emission frequency of a dot is dependent on the bandgap, it is therefore possible to control the output wavelength of a dot with extreme precision. In effect, it is possible to tune the bandgap of a dot, and therefore specify its "color" output depending on the needs of the customer.

96

Quantum Dot Material Systems and Emission Ranges Quantum Dot Material System CdSe

Emission Range

Quantum Dot Diameter Range

Quantum Dot Type

465nm - 640nm

1.9nm - 6.7nm

Core

Standard Solvents

Quantum Dot Example Applications

Toluene

Research, Solar Cells, LEDs

CdSe/Zns

490nm - 620nm

2.9nm - 6.1 nm

CoreShell

Toluene

Visible Fluorescence Applications, Electroluminescence, LEDs

CdTe/CdS

620nm - 680nm

3.7nm - 4.8nm

CoreShell

Toluene

Deep Red Fluorescence Apps.

PbS

850nm - 2100nm

2.3nm - 9.8nm

Core

Toluene

Near Infrared Applications, Security Inks, Solar Cells, IR LEDs

Toluene

Opto-electronics, Optical Switching, Non-linear Applications, Photonics, Telecommunications

PbSe

1200nm 2340nm

EE 226 – LED Group

4.5nm - 9nm

Core

97

Advantages of Quantum Dots 

(1) The very first advantage is their small size. Due to this, they can be tuned to emit at any visible or infrared wavelength.

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(2) The small size also allows for incredible flexibility.

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(3) Extremely small size allows them to be inserted into any medium necessary to accommodate any underlying light emitting source.

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(4) Bandgap can be altered with the addition or subtraction of just one atom.

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(5) Predetermining the size of QLEDs dots would fix the emitted photon wavelength at the appropriate customerspecified color even if it is not naturally occurring.

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(6) They provide with high stability.

EE 226 – LED Group

98

Disadvantages of Quantum Dots 

Quantum dots are generally made up of Cd, Se or Pb. These materials are toxic in nature. They cause environmental harm. They are more expensive.

EE 226 – LED Group

99

Thank You  EE 226 – LED Group