Date of Graduation

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor

Omar Manasreh

Committee Member

Simon Ang

Second Committee Member

Surendra Singh

Third Committee Member

Jingxian Wu

Keywords

Light Emitting Diodes, Microelectronics, Optoelectronics, Quantum Dots, Solid State Lighting, Thin Film Electronics

Abstract

Quantum dot light emitting diodes are investigated as a replacement to the existing organic light emitting diodes that are commonly used for thin film lighting and display applications. In this, all-inorganic quantum dot light emitting diodes with inorganic quantum dot emissive layer and inorganic charge transport layers are designed, fabricated, and characterized. Inorganic materials are more environmentally stable and can handle higher current densities than organic materials. The device consists of CdSe/ZnS alloyed core/shell quantum dots as the emissive layer and metal oxide charge transport layer. The charge transport in these devices is found to occur through resonant energy transfer and direct charge injection.

Nickel oxide thin film is engineered with defect states within the bandgap by changing the stoichiometry of film. These defect states take part in the charge transport via resonant energy transfer mechanism. The energy transfer mechanism is modeled by measuring the lifetime of quantum dots in the presence of nickel oxide thin film. Energy transfer between nickel oxide defect states and quantum dots occurs at time scales as low as 0.26 ns. This mechanism is exploited to fabricate high efficiency light emitting diodes. Efficient green, yellow, and red emitting devices are fabricated and characterized. The peak external quantum efficiencies of 11.4%, 1.6% and 6.04% are obtained for green, yellow, and red emitting devices, respectively. The performance of green LED is much better than that of the other two colors as the photoluminescence quantum yield of green quantum dots are much higher than the other two quantum dot samples.

Nickel oxide is also synthesized as nanoparticles for potential application in hole transport. Charge transport in devices with nickel oxide nanoparticles occurs via direct charge injection mechanism. The nanoparticles are near-stoichiometric with very low defect densities.

The quantum dots exhibit shorter average lifetimes when mixed with nickel oxide nanoparticle powder indicating a favorable band alignment for direct charge injection. Quantum dot LED is fabricated with nickel oxide nanoparticle hole transport layer and red emitting quantum dots. The light output characteristics are evaluated and a peak external quantum efficiency of 2.36% is obtained.

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