Proceedings of the 7th Annual FEP Honors Research SymposiumCopyright, 2015, Hart, B. Please do not use the materials without expressed permission of the authors.

Development of II-VI All-Inorganic Colloidal Quantum Dot Light Emitting Devices

Brandon HartDepartment of Chemical Engineering

Mentor: Omar Manasreh, Ph.D.Department of Electrical Engineering

Graduate Student Mentor: Haydar SalmanDepartment 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 with the electric property of emitting light. A semiconductor is a material that has intermediate conductivity between a conductor and an insulator [6.]. The process that gives semiconductors the light emitting electric property is called “doping.” Doping is the process in which impurities 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 correct voltage is applied to an LED, electrons “recombine with holes”. 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 which results in the emission of photons. This photon (light particle) emitting process is called “electroluminescence,” and the color of the light depends on the “band gap” energy, which is the energy of the photon emitted [2.].

There are two types of thin film LEDs suitable 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 OLED technology. But the difference is that the light emitting centers are cadmium selenide (CdSe) nanocrystals, or

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quantum dots [2.]. The electroluminescence performance of a Quantum Dot LED is better than an OLED because 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 color quality, high-power efficiency, manufacturing versatility and design flexibility [2.]. The potential advantages of QLEDs are: (1) much narrower emission bandwidth (full width at half maximum ~30 nm compared with 60-80 nm of OLEDs), which means that QLEDs have more saturated and purer color than 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 for QLEDs while organic phosphorescent emitters used for best OLEDs are very expensive [4.].

While quantum dot LEDs have extensive potential, multiple issues still remain in the development of them: high turn-on voltages, low device efficiency in the practicable brightness region and non-negligible parasitic electroluminescence emission from the 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 will focus solely on improving the carrier injection method.

3. Research 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

4. Research Activities and Results

Synthesis of Quantum Dots with Chemical Composition Gradient:

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. [7.]

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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 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.

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 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 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 vitro imaging 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 lymphocyte immunology. 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. 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.

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[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/NewPages/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.

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