III-V nitride based light-emitting diodes (LEDs) have experienced rapid developments during past decade, proving their potential to substitute conventional incandescent bulbs and fluorescent lamps to fulfil energy-efficient and sustainable lighting needs. Tremendous endeavours have been made to improve the performance of LEDs, most of which focused on enhancing the internal and external quantum efficiencies. However, other optical properties of LEDs remain to be explored for a more flexible way of using LEDs in various applications. Therefore, this thesis proposes two nanostructuring strategies through the use of nanospheres to tailor the optical properties of LEDs. The nanostructured LEDs are demonstrated enable light emission with reduced divergence, or becomes polarized. The monolithic modifications are free of external optics and thus eliminate light loss, meanwhile providing manipulability of optical emission from LEDs. Firstly, close-packed indium-tin-oxide (ITO) micron-lenses with dimension of the order of wavelength have been integrated onto InGaN LEDs aiming at reducing the emission divergence. The sub-micron lens arrays are patterned by nanosphere lithography with silica nanosphere serving as an etch mask on ITO layer, leaving the semiconductor layer damage-free. An enhancement of up to 63.5% on optical output power from the lensed LED has been observed. The LED with 500 nm lenses exhibits a 26.8° reduction in emission divergence (full width at half maximum) compared with the bare LED. Three-dimensional finite-difference time-domain simulations performed for light extraction and emission characteristics is found to be consistent with the observed results. Secondly, polarization behavior of light emitted from InGaN LEDs propagating through a self-assembled polystyrene nanosphere opal film has been studied. Angular-resolved optical transmission of transverse electric (TE) and transverse magnetic (TM) polarized light has been measured. An integrated p/s ratio of 2.16 is observed at a detection angle of 70°, attributed to the suppression of TE mode at particular frequencies by the three-dimensional photonic crystal. Polarization is found to depend strongly on both the photonic bandgap of the opal and the angle of incidence. Theoretical calculations by transfer matrix method yield results consistent with the experimental data. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

It is observed that the power consumption of road lighting is increased with the length of trafficable road in Hong Kong. The energy used in road lighting is increasing, which means that the greenhouse gases (GHGs) emitted from power plant for generating electricity for road lighting is at the same time increasing. To compare the performance of light emitted diode (LED) road lighting with road lighting of other lamp sources, literature review, life cycle assessment (LCA) and technical assessment are adopted to give an overall comparison. This research focuses more on the environmental impacts of road lighting. LCA is adopted in order to give a comprehensive view on the environmental impact of road lighting. A total of 3 different lamp sources are compared: high pressure sodium (HPS) lamp, induction lamp and light emitted diode (LED) lamp. From the model result, it is found that due to the low power consumption and long life time, LED and induction lamp road lighting gives generally less environmental impact than HPS road lighting. As induction lamp has a longer life span than LED, the environmental impact of induction lamp road lighting is found slightly less than that of LED road lighting. Taking account the future development in LED technology, leading to longer life time, higher efficacy and lower production cost, LED road lighting is expected to be a replacement for road lighting in Hong Kong for the future. / published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

Abstract—A dependency of quantum efficiency of nn+pp+ silicon complementary metal–oxide–semiconductor integrated lightemitting devices on the current density through the active device areas is demonstrated. It was observed that an increase in current density from 1 6 10+2 to 2 2 10+4 A cm 2 through the active regions of silicon n+pp+ light-emitting diodes results in an increase in the external quantum efficiency from 1 6 10 7 to 5 8 10 6 (approximately two orders of magnitude). The light intensity correspondingly increase from 10 6 to 10 1 W cm 2 mA (approximately five orders of magnitude). In our study, the highest efficiency device operate in the p-n junction reverse bias avalanche mode and utilize current density increase by means of vertical and lateral electrical field confinement at a wedge-shaped n+ tip placed in a region of lower doping density and opposite highly conductive p+ regions.

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Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references (leaves 76-80). Also available in electronic version.

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 59-64). Also available in print.

Thesis (M.S.)--California Polytechnic State University, 2010. / Title from PDF title page; viewed on May 10, 2010. Major professor: Jianbiao Pan, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Industrial Engineering." "March 2010." Includes bibliographical references (p. 88-93).

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references. Also available in electronic version.

Organic light-emitting diode (OLED) devices have been applied in the fields of display and solid-state lighting. In addition to phosphorescent OLEDs using heavy transition metals, a new approach of harvesting both singlet and triplet excitons generated in the OLED device by using pure organic materials has drawn a lot of attentions in recent years. It is thermally activated delayed fluorescence (TADF) process, which makes it possible to obtain potential 100% internal quantum efficiency (IQE);TADF is a process existing in certain organic materials with small singlet-triplet exchange energy (EST), which is generally observed in the molecules with weak-coupled electron-donating (D) group and electron-accepting (A) group. Individual molecule containing D/A, which is named intramolecular exciplex, or intermolecular exciplex with D/A on separated molecules, can fulfill this requirement. Although at present the intramolecular exciplex attracts considerable research interests, it takes a lot of efforts to design an individual molecule with high fluorescent quantum yield as well as small EST. Intermolecular exciplex, which is achieved by physically blending individual D and A molecules with appropriate selection from present materials, has excellent performance comparable to the phosphorescent emitter.;In this work, we studied the TADF process in an intermolecular exciplex and its application in highly efficient OLED devices. By doping electron-donating material tris(4-carbazoyl-9-ylphenyl)amine (TCTA) with electron-accepting material 2,4,6-tris(3'-(pyridin-3-yl)biphenyl-3-yl)-1,3,5-triazine (Tm3PyBPZ), an exciplex with a green emission around 514 nm was demonstrated. The time-resolved photoluminescence of the exciplex under different temperatures from 12 K to 300 K demonstrated the existence of temperature-dependent delayed fluorescence. By applying this exciplex as the emissive layer, a highly efficient all-fluorescent organic lighting emitting diode with maximum efficiencies of 13.1% and 53.4 lm/W was realized with an extremely low turn-on voltage of only 2.4 V. The efficiencies of the device have outperformed conventional fluorescent OLED devices due to the contribution of triplet excitons. By doping this exciplex with other conventional green or yellow fluorescent dopants, we observed that the performances of these dopants also surpass the limitation of conventional fluorescent OLED (5̃ % external quantum efficiency)