Light Emitting DiodesDaniel Bowser

Fernando Robelo

Basics of LEDsCurrent LED technology

Band Gap SemiconductorOrganic

Frontier LED technologyQuantum Dot

Conclusions

Contents

Basics of LEDs

• Semiconductor based radiation sourceo photons emitted when electrons

recombine with holes

• Visible, Infrared, and Ultraviolet Emission

Light Emitting Diodes (LEDs)

Indicators and SignsLighting

street lights, aviation, display background, color changing, small durable devices, infrared emission, vehicle lighting

Fiber opticsSustainable lighting

energy, economical

Applications of LEDs

Current LED technology:Band Gap

SemiconductorUltraviolet and blueWhite

Organic (OLEDs)

Frontier Research:Quantum Dot

(QdotLEDs)

3 Types of LEDs

http://www.photonics.com/Article.aspx?AID=24915,http://www.arlty.com/faq/, http://en.wikipedia.org/wiki/Led

Current LED Technology Band Gap Semiconductor Organic

Frontier Research Quantum Dot

• First LED to be developed (1927)

• Most widely used • Most economical

to produce• p/n junction

semiconductor• typically made of

III/V compounds

Band Gap Semiconductor Based LEDs

http://en.wikipedia.org/wiki/Category:III-V_compounds

p-n junction

no voltage applied

Band Gap Semiconductor Based LEDs

Band Gap Semiconductor Based LEDs

p-n junctionforward voltage bias

Band Gap Semiconductor Based LEDs

Unfortunately its not that simple...

-modern LEDs do not use just one p-n junction composed of two materials

-hundreds of p-n junctions between highly complex crystal structures

-band gaps must be carefully engineered to produce light of the desired color

Current LED Technology:Band Gap SemiconductorOrganic

Frontier ResearchQuantum Dot

Organic LEDs

Thin, flexible, with some transparent components

Broad applications in displays and mobile devices

LED that uses an organic semiconductor as the luminescent layer

Magnified OLED phone display

Organic LEDs

http://www.circuitstoday.com/workng-of-organic-led-oled

Organic LEDs

Composition of small molecule OLED semiconductors

Emissive LayerOrganometallic chelates

Fluorescent and Phosphorescent Dyes

Conductive LayerDendrimers for charge transport

http://en.wikipedia.org/wiki/Organic_light-emitting_diode

Current LED TechnologyBand Gap SemiconductorOrganic

Frontier ResearchQuantum Dot

PhotovoltaicsPhotodetectorsLight Emitting Diodes

Uses of Colloidal Quantum Dots

Physical Structure of Qdot LED

Electronic Structure of Qdot LED

To determine the ability to dramatically enhance quantum efficiency of exciton recombination by tuning distance between PbS quantum dots in active layer LED

Purpose of Research

LED performance and interdot

distance

Current density-voltage characteristic

Emission Spectra and infrared image of LEDs

Low cost and may be directly integrated

Difficult to control exciton dissociation and recombination (unstable)

Greater color range, as well as color accuracy

100 times brighter than LCD displaysGreater flexibility and improve lifetime

Qdot LED Advantages and Disadvantages

Conclusions

Advantages:more energy efficientlonger lifetime

wider range of applicationsDisadvantages:

high initial pricequality of light produced

Current LEDs compared to current lighting alternatives

Competition between exciton dissociation an radiative recombination dominates the variation of EQE with different linker molecules

Performance improvement are possible based on the understanding of the limitations of charge transport and injection rates can optimize LED efficiency and radiance

Future Research

Primary paper being discussed:Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control. http://www.nature.com/nnano/journal/v7/n6/abs/nnano.2012.63.html

Other sources:http://onlinelibrary.wiley.com/doi/10.1889/1.2785443/abstracthttp://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6248162http://onlinelibrary.wiley.com/doi/10.1889/1.3500495/abstracthttp://onlinelibrary.wiley.com/doi/10.1889/1.1927730/abstracthttp://photonics.intec.ugent.be/education/ivpv/res_handbook/v1ch12.pdf

References

Questions?