1. Proceedings of the 7th AnnualFEP Honors Research Symposium
Copyright, 2015, Hart, B. Pleasedo not usethematerials withoutexpressed permission of theauthors.
Hart, B. 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
Abstract
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.
1. Background
Technology has advanced significantly within the last twenty years. One of the greatest
technological advancements during this time period is the digital display. Screens that display
information have become a necessity. It is nearly impossible to enter a room and not see one. From
the average television to a smart phone’s touch screen, the technology is all around us.
One of the key advancements that has been made is the type of digital display. In 1968, the first
light emitting diode was used to display information. An LED is a semiconductor withthe electric
property of emitting light. A semiconductor is a material that has intermediate conductivity
between a conductorand an insulator [6.]. The process that gives semiconductors the light
emitting electric property is called “doping.” Doping is the process in whichimpurities are
introduced to a pure semiconductor to manipulate its electric properties from a good insulator to a
viable conductor.LEDs consist of a semiconductor that has a p-n junction, the location where the
electrons recombine and release photons. When the correctvoltage is applied to an LED, electrons
“recombine with holes”. Electronholes 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 fromthe cathode whichresults in the emission of photons. This photon (light
particle) emitting process is called “electroluminescence,” and the colorof the light depends on the
“band gap” energy, whichis the energy of the photon emitted [2.].
There are twotypes of thin film LEDssuitable for thin film displays and lighting: organic, and
quantum dot. An organic LED(OLED)consists of an emissive layer made of an organic material like
polymer. The structure of a quantum dot LED(QLED) is very similar to the OLEDtechnology.But
the difference is that the light emitting centers are cadmium selenide (CdSe) nanocrystals, or

2. Hart, B. 2
quantum dots [2.]. The electroluminescence performance of a Quantum Dot LEDis better than an
OLEDbecause it requires a lower voltage to operate and produces higher intensity light.
2. Motivation
The Quantum Light platform exploits the unique light-emitting properties of semiconductor
nanocrystals to deliver a new value proposition for LED-based products, including extraordinary
colorquality, high-power efficiency,manufacturing versatility and design flexibility [2.]. The
potential advantages of QLEDsare: (1) much narrower emission bandwidth (full width at half
maximum ~30 nm compared with 60-80 nm of OLEDs),whichmeans that QLEDs have more
saturated and purer colorthan OLEDs;(2) easier tunability of emission colors in the entire visible
range by simply controlling the particle size and shape with the same chemical composition for the
quantum dot; (3) and therefore the cost of emitters are much lower forQLEDswhile organic
phosphorescent emitters used for best OLEDsare very expensive [4.].
While quantum dot LEDs have extensive potential, multiple issues still remain in the development
of them: high turn-on voltages, low deviceefficiency in the practicable brightness region and non-
negligible parasitic electroluminescence emission fromthe adjacent conjugated organic layers or
surface-trap states of quantum dots, mainly due to inefficient carrier injection into the quantum
dots and resultant poor electron-hole balance [4.]. Due to limited time, we willfocus solely on
improving the carrier injection method.
3. ResearchObjectives
Our research objectivesare:
 Understand the workingof 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
4. Research Activities and Results
Synthesis of Quantum Dots with Chemical Composition Gradient:
For the synthesis of QDs withemission wavelength (PLlmax) 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 for30 min. 15 mL of 1-octadecenewas injected into the reaction flaskand 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 for10 min in order to form the CdSe@ZnS QDs witha 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. Purificationprocedures 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. [7.]

3. Hart, B. 3
Application to Semiconductor
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 coatthe 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 furnaceto allow it to cool.Next, the quantum dots were applied to the layers.
The quantum dots were coated on the layers with the spin coatermachine. Once the quantum dots
were coated evenly,the materials were placed in the furnace again at 90C-100C for25 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.
5. 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 whichcreated an emissive layer. We were able to
create a semiconductor withlight 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 methods.
6. Future Research
We are interested in applying quantum dots to several different areas. Quantum dots have the
capability to be used in solid-state quantum computation to make computers faster than ever
before. They can be used in biology as an organic dye and in vitroimaging of pre-labeled cells. We
are interested in using them for single-cell migration of cells in areas such as embryogenesis, cancer
metastasis, stem cell therapeutics, and lymphocyteimmunology. Quantum dots can be used in
photovoltaic cells to create a cheaper and more efficient way of producing energy. They also have
the ability to be used in lighting to produce light similar to sunlight. Quantum dots are a new
technology that have several applications. With the right kind of research, quantum dots have the
capacity to change the world.
7. References
[1.] Freudenrich, Ph. D., Craig. HowOLEDs Work. 24 March 2005.
<http://electronics.howstuffworks.com/oled.htm>.
[2.] Introduction.10 October2014. <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." Barriereffect on holetransportandcarrier distributionin InGaN∕GaNmultiple
quantumwellvisiblelight-emittingdiodes (2008):93.
[4.] Kwak,J., et al. "Nano Letters." Bright andEfficient Full-ColorColloidalQuantum Light-Emitting
DiodesUsingan InvertedDeviceStructure (2012):2362-2366.
[5.] Shirasaki, Yasuhiro, Geoffrey J.Supran, Moungi G. Bawendi, and Vladimir Bulovic."Nature
Photonics7." Emergenceof colloidalquantum-dotlight-emittingtechnologies (2012):11.

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[6.] Skromme, Brian J. Basicsof Semiconductors.22 June 2004.
<http://enpub.fulton.asu.edu/widebandgap/NewPages/SCbasics.html>.
[7.] Wan Ki Bae, Jeonghun Kwak, Ji Won Park,Kookheon Char, Changhee Lee, and Seonghoon Lee.
"Advanced Materials." HighlyEfficient Green-Light-EmittingDiodes BasedonCdSe@ZnS
QuantumDotswith a Chemical-CompositionGradient(2009):1690-1694.