1. Brandon Hart, Department of Chemical Engineering
Mentor: Omar Manasreh, Ph.D., Department of Electrical Engineering
Graduate Student Mentor: Ramesh Vasan, Department of Electrical Engineering
7th Annual FEP Honors Research Symposium
April 18th, 2015
HART 7TH ANNUAL FEP HONORS RESEARCH SYMPOSIUM 1
Development of II-VI All-Inorganic Colloidal Quantum Dot
Light Emitting Devices
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[3.] [4.]
LED Displays
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Semiconductor- a material that has intermediate
conductivity between a conductor and an insulator
HART 7TH ANNUAL FEP HONORS RESEARCH SYMPOSIUM
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[5.]
Doping of a semiconductor
Doping- the process in
which impurities are
introduced to manipulate its
electric properties.
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P-N Junction
P-N junction- the
location where
electrons recombine
and release photons.
[6.]
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Emissive Layer
Emissive Layer- layer of nanocrystals that transports electrons
from the cathode to the anode.
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[8.]
Quantum Dot
Quantum Dot- A nanoscale
particle of semiconducting
material that can be
embedded in cells or
organisms for various
purposes.
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Quantum Dot Band Gap
[9.]
• Quantum Dot Band Gap-
Energy of the photon
emitted.
• The color of the quantum
dot depends on the size.
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[10.]
Advantages of Quantum Dot LEDs
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Advantages of Quantum Dot LEDs
(1) Narrower emission bandwidth = more saturated and
purer color than OLEDs.
(2) Adjustable emission colors through size and shape of
quantum dot.
(3) Cost of QLEDs are much lower than OLEDs.
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[10.]
Current progression of LEDs
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Current progression of LEDs
Quantum Dot LED Issues:
• High turn-on voltages
• Low device efficiency in practicable brightness
region
• Inefficient carrier injection
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Research Objectives
• Understand the working of QLED
• Understand the current carrier injection method
• Improve the carrier injection capabilities within the
semiconductor device
• Develop a new carrier injection technique
• Test new carrier injection technique to determine
improvements
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In conclusion, we were able to create quantum dots in a lab and
apply them to a light emitting application.
• The semiconductor produced a
bright green light with the band
gap energy of ~520 nm.
• The new carrier injection
method seemed to produce
better results than previous
methods.
Questions?
Editor's Notes
-Technology has advanced significantly in the last 20 years.
-It has become the era of the digital display.
-It’s impossible to not walk in the room and not see one.
-From the average television to a smart phone’s touch screen, the technology is all around us
-In 1968 the first light emitting diode (LED) was used to display information
-An LED is a semiconductor with the electric property of emitting light
Talk about picture
- Adding phosphorus adds electrons to the semiconductor and makes it an n-type.
Adding Boron removes an electron from the array of Silicon atoms and makes it a p-type semiconductor.
-When the correct voltage is applied to the LED, the electrons recombine with holes at the p-n junction.
-Electron holes are places in an atom or atomic lattice where an electron can reside and recombine.
-Recombination takes place in the emissive layer.
-The emissive layer transports electrons from the cathode to the anode.
-When the electrons have been transferred and recombine, photons are emitted in a process called electroluminescence.
Read definition
-Quantum Dots are known for their bright and saturated colors.
Talk about graphs.
-The amount of energy that the particle has determines its color that it releases.
-The size of the quantum dot determines its color
- blue particle is small, red particle is large
-The Quantum Dot LED utilizes an emissive layer made up of Quantum Dots.
-Based on the size of the quantum dot, the color produced during electroluminescence will vary.
-The QD emissive layer is only one particle thick hence monolayer.
Talk about graphs- spend time doing it
-The black dots on graph A are the potential QD colors.
-The actual shape is the scope of human vision in terms of color.
-The white triangle shows the colors of an HDTV
-Quantum Dots have the ability to be very close to the same color as sunlight, which makes them viable for lighting applications.
-Chart B- Luminous efficacy- measure of how well a light source produces visible light
-CRI- stands for color rendering index- it’s the measure of a light source’s ability to show object’s color
-Graph A: Peak EQE stands for peak external quantum efficiency
-External quantum efficiency describes how efficiently the device converts electrons to photons and allows them to escape.
-Graph B: describes the progression of the peak brightness of the different types of LEDs
Due to limited time, my research team decided to focus solely on improving the carrier injection method.
-The left picture shows the synthesis process of the quantum dots. The whole process took about 4 hours to complete.
We placed .1 mol of Cadmium Oxide and 4 mols of Zinc Acetate into a 5 mL solution of oleic acid in a 100 mL flask and heated the solution to 150 degrees Celsius for 30 min.
-We then injected 15 ml of 1-octadecene and heated the solution to 300 degrees Celsius.
-The reaction vessel was maintained under a Nitrogen atmosphere.
-Cadmium Oleic acid and Zinc Oleic acid were formed after being heated to 300 Celsius
- At 300 C, .2 moles of selenium and 3 moles of Sulfer were dissolved in 2 ml of trioctylphosphine and injected into the vessel with the Cadmium oleic acid and the Zinc oleic acid.
-The reaction went for 10 minutes in order to form Cadmium Selenide/Zinc Sulfide quantum dots.
-After 10 minutes of the reaction .5 ml of 1-octanethiol (a strong binding ligand) was added to the reactor to passivate the surfaces of the quantum dots.
-We then lowered the temperature of the reactor to room temperature.
-The picture on the left shows the quantum dots after the reaction.
The picture on the left shows the quantum dots under UV lighting and in a glass of water in order to cool it down.
-The picture on the right shows the quantum dots with UV light being applied.
-As you can see the quantum dots result in a very bright and saturated color
-The bright and saturated color is perfect for LED applications
-In order to apply the quantum dots to an LED it has to go through carrier injection.
-For our carrier injection technique we decided to try the spin coating technique.
-In the spin coating technique we applied a few drops of the quantum dots to a layer of Nickel Oxide that was coated on a layer of Indium Tin Oxide on glass.
-The Nickel Oxide was applied the Indium Tin Oxide in the same fashion.
-Once the drops of the quantum dots were on the Nickel Oxide layer, we put it in a spin coater machine that rotated the layers.
-The quantum dots coated evenly on the Nickel Oxide layer.
-Following the spin coating, the materials were placed in a furnace at 90 Celsius- 100 Celsius for 25 minutes.
-Then we repeated the step with a layer of Zinc Oxide
-Finally we applied a small layer of aluminum to the layers to act as a conductor.
-Once the layers were finished, we tested the semiconductor to see if it gave off light.
-In the pictures you can see that when voltage was applied that the LED did give off light.
The graph on the left shows the current density as the voltage increased.
-At around 5 volts, the LED began to work and sky rocketed.
-The graph on the right shows the wavelength given off by our LED on the x axis and the Normalized PL describes the intensity of the color.
-The green line is the measured wavelength of our LED.
-Because the green line reaches its peak at about 520 nm, it produces a bright green color.
-We measured the wavelength by using a spectrometer.
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Technology](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)