Colloidal Quantum Dot-enabling and Alternating Current Driven Gallium Nitride Based Light Emitting Diode Technology
Open Access
- Author:
- Liu, Jie
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 12, 2013
- Committee Members:
- Jian Xu, Dissertation Advisor/Co-Advisor
Suzanne E Mohney, Committee Member
S Ashok, Committee Member
Stephen Fonash, Committee Member
Osama O Awadelkarim, Committee Member - Keywords:
- light emitting diodes
LED
quantum dots
gallium nitride
alternating current LED
AC LED
optoelectronics - Abstract:
- The colloidal quantum dot-enabling light emitting diode (LED) technology has been developing rapidly in recent years due to the superior optical properties of colloidal quantum dots (QDs). Semiconductor QDs of various sizes and compositions can be used as the emissive media in the light emitting diodes, and can also be used as a novel type of phosphor in white light emitting diodes. Nitride based LED technology have also attracted much attention since the early 1990s. Due to their high power, high efficiency, and long lifetime properties, gallium nitride (GaN) LEDs are becoming promising candidates for next-generation solid state lighting. This dissertation attempts to advance the LED technology in three respects. Firstly, in order to improve the technology using colloidal QDs as the emissive media, the degradation mechanism of the colloidal QD LEDs was studied. The results reveal that the degradation of QD LEDs is associated with the reduction in the QD quantum efficiency. Two mechanisms were identified to account for the observed decrease of quantum yield: i.e., high temperature induced degradation during LED operation; and the diffusion of the carrier transport molecules into the QD emissive layer. To this end, the inorganic material of indium-gallium-zinc-oxide (IGZO) was introduced to replace tris(8-hydroxyquinoline) aluminum (Alq3) as the electron transport medium, making it possible to obtain air-stable QD LEDs. Secondly, to make the QD phosphors more efficient for color conversion in white LEDs, the nonradiative energy transfer between colloidal quantum dot-phosphors and nanopillar nitride LEDs was studied. In this approach, QD phosphors were brought to close proximity of blue-emitting quantum wells (QWs) by nanopillar LED design. This is accompanied with a novel design of low-resistance p-type contact selectively deposited on the top of the nanopillars. Strong non-radiative energy transfer was observed from the indium gallium nitride (InGaN) QWs to the colloidal QD phosphors, leading to a significant enhancement in effective internal quantum efficiency. Furthermore, the nonradiative energy transfer between colloidal quantum dot-phosphors and indirect bandgap silicon carbide was also studied. Nonradiative energy transfer has been demonstrated as an effective path to achieve color tunable emission from indirect semiconductors diodes. Finally, to drive the GaN LEDs under alternating current (AC) condition without external driver circuitry, a monolithic integration of LED arrays with on-chip Schottky diode Wheatstone bridge was demonstrated, for the first, to run the LEDs directly on the line voltage.