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

Brandon Hart, Department of Chemical EngineeringMentor: 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

BackgroundIn 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.

ObjectivesOur 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 DotsFor 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 DotsFollowing 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

ResultsWe 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.

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CdSe/ZnS QD LEDEmission at ~520 nm

Anode/HTL/QD/ETL/CathodeITO/NiO/CdSe@ZnS/ZnO/Al

ConclusionWe 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

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