1. Development of II-VI All-Inorganic Colloidal
Quantum Dot Light Emitting Devices
Brandon Hart, Department of Chemical Engineering
Mentor: Omar Manasreh, Ph.D., Department of Electrical Engineering
Graduate Student Mentor: Haydar Salman, Department of Electrical Engineering
April 18, 2015 7th Annual FEP Honors Research Symposium
Background
In a world consumed by digital technology, further
advancements for digital displays are required. We report
the development of II-VI All-Inorganic Colloidal Quantum
Dot Light Emitting Devices for digital display application.
Quantum Dot Light Emitting Devices have several potential
advantages such as extraordinary color quality, high-power
efficiency, manufacturing versatility and design flexibility.
QLEDs still face multiple issues before it can be implicated.
A big issue that still remains is an inefficient carrier injection
into the quantum dots and resultant poor electron-hole
balance. We have decided to focus on this issue and
attempt to improve the current carrier injection method.
Objectives
Our research objectives are:
• Understand the working of a 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
How QLEDs work:
Quantum Dot LEDs take advantage of an emissive layer
made up of quantum dots, a nanoscale particle of
semiconducting material. The quantum dot has a certain
band gap energy based on the size of the particles. When
current is passed through the semiconductor, photons are
given off at the quantum dot emissive layer with the same
band gap energy as the quantum dots, resulting in bright
and pure colors.
Synthesizing CdSe/ZnS Quantum Dots
For the synthesis of QDs with emission wavelength (PL
lmax) at 524 nm, 0.1 mmol of CdO and 4 mmol of
Zn(acetate)2 were placed with 5 mL of oleic acid (OA)
in a 100 mL flask, heated to 150 °C, and degassed for
30 min. 15 mL of 1-octadecene was injected into the
reaction flask and heated to 300 °C as the reaction
vessel was maintained under N2, yielding a clear
solution of Cd(OA)2 and Zn(OA)2. At the elevated
temperature of 300 °C, 0.2 mmol of Se and 3 mmol of S
dissolved in 2 mL of trioctylphosphine was swiftly
injected into the vessel containing Cd(OA)2 and
Zn(OA)2. The reaction proceeded at 300 °C for 10 min
in order to form the CdSe@ZnS QDs with a chemical-
composition gradient. After 10 min of reaction, 0.5 mL
of 1-octanethiol was introduced in the reactor to
passivate the surfaces of the QDs with strongly binding
ligands (1-octanethiol), and the temperature of the
reactor was lowered to room temperature. Purification
procedures followed (dispersing in chloroform,
precipitating with excess acetone, repeating ten times).
The resulting QDs were then dispersed in chloroform,
toluene, or hexane for further experiments.
Synthesis of QDs QDs without UV light
Cooling QDs after synthesis QDs with UV light
Carrier Injection of Quantum Dots
Following the synthesis of the quantum dots, we
placed a few drops of Nickel Oxide on an ITO
(Indium Tin Oxide) substrate layer on glass. In
order to coat the substrate layer with the Nickel
Oxide evenly, the materials were placed in a spin
coater machine.
Once the NiO was on the substrate layer, we put it
in the furnace at 500 C for 15 minutes. Once the
layer was done, we removed it from the furnace to
allow it to cool. Next, the quantum dots were
applied to the layers. The quantum dots were
coated on the layers with the spin coater machine.
Once the quantum dots were coated evenly, the
materials were placed in the furnace again at 90C-
100C for 25 minutes. We then repeated the same
steps as the quantum dots with a layer of ZnO. The
ZnO layer acted as the electron transport layer.
Finally, we applied a small layer of aluminum to
the layers using an electron beam evaporator. We
then tested the semiconductor to see if it gave off
light.
Spin Coater machine
Without Current With Current
Results
We found that as more voltage was applied to the
semiconductor, the Current Density increased in a
sharp slope. This sharp spike indicates the turn on
voltage in order to produce light.
Our quantum dots were found to have a band gap
energy of ~520 nm and produced a bright green color.
The peak of the green line indicates the moment
where the light is produced and at what wavelength.
0 1 2 3 4 5 6 7 8 9
0
25
50
75
100
125
150
175
CurrentDensity(mA/cm
2
)
Voltage (V)
CdSe/ZnS QD LED
Emission at ~520 nm
Anode/HTL/QD/ETL/Cathode
ITO/NiO/CdSe@ZnS/ZnO/Al
Conclusion
We were able to synthesize quantum dots in the lab
and apply them to a light emitting application through
coating a material in the quantum dots which created
an emissive layer. We were able to create a
semiconductor with light emitting properties. The
semiconductor gave off photons with the band gap
energy of the quantum dots found in the emissive
layer. The band gap energy of the emissive layer was
~520 nm and produced a bright green color. The new
carrier injection method seemed to produce better
results than previous carrier injection methods.
Future Research
• Solid-state quantum computing.
• Cell staining for single-cell migration in areas such
as cancer metastasis.
• Photovoltaic Cells
• Lighting application
References
[1.] Freudenrich, Ph. D., Craig. How OLEDs Work. 24
March 2005.
<http://electronics.howstuffworks.com/oled.htm>.
[2.] Introduction. 10 October 2014. <www.qled-
info.com>.
[3.] J. P. and Ryou, J. H. and Dupuis, R. D. and Han, J.
and Shen, G. D. and Wang, H. B. "Applied Physics
Letters." Barrier effect on hole transport and carrier
distribution in InGaN∕GaN multiple quantum well
visible light-emitting diodes (2008): 93.
[4.] Kwak, J., et al. "Nano Letters." Bright and Efficient
Full-Color Colloidal Quantum Light-Emitting Diodes
Using an Inverted Device Structure (2012): 2362-
2366.
[5.] Shirasaki, Yasuhiro, Geoffrey J. Supran, Moungi G.
Bawendi, and Vladimir Bulovic. "Nature Photonics 7."
Emergence of colloidal quantum-dot light-emitting
technologies (2012): 11.
[6.] Skromme, Brian J. Basics of Semiconductors. 22
June 2004.
<http://enpub.fulton.asu.edu/widebandgap/NewPag
es/SCbasics.html>.
[7.] Wan Ki Bae, Jeonghun Kwak, Ji Won Park,
Kookheon Char, Changhee Lee, and Seonghoon Lee.
"Advanced Materials." Highly Efficient Green-Light-
Emitting Diodes Based on CdSe@ZnS Quantum Dots
with a Chemical-Composition Gradient (2009): 1690-
1694.