Light Emitting Diodes
Daniel Bowser
Fernando Robelo
Contents
Basics of LEDs
Current LED technology
Band Gap Semiconductor
Organic
Frontier LED technology
Quantum Dot
Conclusions
Basics of LEDs
Light Emitting Diodes (LEDs)
• Semiconductor based radiation source
o photons emitted when electrons recombine
with holes
• Visible, Infrared, and Ultraviolet
Emission
Applications of LEDs
Indicators and Signs
Lighting
street lights, aviation, display background, color
changing, small durable devices, infrared emission,
vehicle lighting
Fiber optics
Sustainable lighting
energy, economical
3 Types of LEDs
Current LED technology:
Band Gap Semiconductor
Ultraviolet and blue
White
Organic (OLEDs)
Frontier Research:
Quantum Dot (QdotLEDs)
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
Band Gap Semiconductor Based
LEDs
• First LED to be
developed (1927)
• Most widely used
• Most economical to
produce
• p/n junction
semiconductor
• typically made of
III/V compounds
http://en.wikipedia.org/wiki/Category:III-V_compounds
Band Gap Semiconductor Based
LEDs
p-n junction
no voltage
applied
Band Gap Semiconductor Based
LEDs
p-n junction
forward
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 Semiconductor
Organic
Frontier Research
Quantum Dot
Organic LEDs
LED that uses
an organic
semiconductor
as the
luminescent
layer
Thin, flexible, with some
transparent components
Broad applications in displays
and mobile devices
Magnified OLED phone display
Organic LEDs
http://www.circuit
stoday.com/work
ng-of-organicled-oled
Organic LEDs
Composition of small molecule OLED semiconductors
Emissive Layer
Organometallic chelates
Fluorescent and
Phosphorescent Dyes
Conductive Layer
Dendrimers for charge transport
http://en.wikipedia.org/wiki/Organic_light-emitting_diode
Current LED Technology
Band Gap Semiconductor
Organic
Frontier Research
Quantum Dot
Uses of Colloidal Quantum Dots
Photovoltaics
Photodetectors
Light Emitting Diodes
Physical Structure of Qdot LED
Electronic Structure of Qdot LED
Purpose of Research
To determine the ability to dramatically
enhance quantum efficiency of exciton
recombination by tuning distance
between PbS quantum dots in active
layer LED
LED performance
and interdot
distance
Current density-voltage
characteristic
Emission Spectra and infrared
image of LEDs
Qdot LED Advantages and
Disadvantages
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 displays
Greater flexibility and improve lifetime
Conclusions
Current LEDs compared to current
lighting alternatives
Advantages:
more energy efficient
longer lifetime
wider range of applications
Disadvantages:
high initial price
quality of light produced
Future Research
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
References
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.htm
l
Other sources:
http://onlinelibrary.wiley.com/doi/10.1889/1.2785443/abstract
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6248162
http://onlinelibrary.wiley.com/doi/10.1889/1.3500495/abstract
http://onlinelibrary.wiley.com/doi/10.1889/1.1927730/abstract
http://photonics.intec.ugent.be/education/ivpv/res_handbook/v1ch1
2.pdf
Questions?