Date

2015

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Electrical Engineering

First Adviser

Tansu, Nelson

Other advisers/committee members

Decker, Richard; Ou-Yang, Daniel; Zhou, Chao

Abstract

The importance of having low-cost and practical technology for improving the efficiency of solid-state lighting is key for the practical implementation of this technology for general illumination market. The thin-film flip-chip (TFFC) LED has been pursued as the state-of-the-art LED technology, which has been shown to have improved extraction by 1.6 times over the conventional planar LED technology. The combination of the thin-film concept with flip-chip technology provided surface brightness and flux output advantages over conventional LED, and currently the TFFC LEDs are widely used in industry for improved performance. To improve the light extraction further in TFFC LEDs, both surface roughness and photonic crystal methods had been implemented. In this thesis, the use of self-assembled colloidal microlens arrays with rapid convective deposition (RCD) method will be demonstrated in both GaN and organic LEDs. The use of rapid convective deposition method enables roll-to-roll printing process of microsphere and nanosphere arrays on large wafer area applicable for manufacturing of large area LED technology. Comprehensive studies were carried out to analyze the light extraction efficiency of conventional top-emitting III-Nitride LEDs with microsphere arrays and TFFC LEDs with microsphere arrays deposited via rapid convective deposition process. The device structure was engineered to achieve optimum light extraction by varying refractive indices of spheres, the diameters of spheres, packing density and packing geometry of microsphere arrays. The optimized device structure is TFFC LED with hexagonal close-packed TiO2 sphere arrays. The use of hexagonal close-packed monolayer of TiO2 microsphere arrays on TFFC LED results in light extraction of 75%, which is 3.6 times higher than that of TFFC LEDs with planar surface. Further optimization by using microlens arrays on TFFC LED results in light extraction efficiency of 86%, which is 1.3 times higher than that of state-of-the-art TFFC LED with surface roughness approach. The key advantage of the self-assembled colloidal process is the ability for implementation of roll-to-roll printing method for large wafer scale manufacturing process.

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