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)

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To date, measurements of tunnel diode noise have dealt mainly with the negative conductance region, because the tunnel diode is an active circuit element only in this region. The noise has not been measured for reverse or near-forward biases due to the difficulties involving excessively low diode impedances in these regions. The purpose of this thesis is to show that, from the Esaki formulation for the direct-tunneling currents of a tunnel diode, in the bias regions where the electronic bands overlap, a simple theory can be developed relating the power spectrum associated with the direct-tunneling current noise to the direct current passing through the diode. This theory assumes that the two oppositely-flowing direct-tunneling currents in the Esaki junction are uncorrected and that both contribute full shot noise. The theory can be critically tested only in the bias regions where the noise is yet unstudied, and at sufficiently high frequencies that no contaminating 1/f noise exists. These conditions have been met experimentally and the noise measured quantitatively over the entire reverse and near-forward regions at a frequency of 4 Mc/s. Impedance-transforming networks and a very low-noise preamplifier suitable to the particular source strengths and impedances presented by the tunnel diode are developed for these measurements. A noise measurement technique is chosen from among several possible ones for the high degree of accuracy and smallest dependence on a good noise figure required for the tunnel diode source. The experimental results agree with the theory and vindicate the usual assumption that the two oppositely flowing direct-tunneling electron currents between two bands of a degenerately-doped semiconductor are uncorrelated. Noise measurements in the "valley" and far-forward region of the tunnel diode characteristic, where the diode current is not due to direct tunneling, do not agree with the simple two-current shot noise theory for direct-tunneling electron currents. Possible reasons for the enhanced noise measured in this region are advanced in the form of two models based on indirect-tunneling electrons via traps as the most important mechanism describing the excess or valley current. These models offer a possible explanation of the observed phenomena, but noise measurements alone appear insufficient to demonstrate unambiguously the detailed mechanisms producing either the excess current or the associated enhanced noise found throughout the valley and far-forward regions. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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Dramatic increase of energy consumption and environmental problems invigorate the development of organic semi-conductive materials to substitute for the conventional inorganic materials in the application of photovoltaic and light-emitting devices. In view of the merits of low driving voltage, high power conversion efficiency, large-area fabrication of thin and light organic films as well as saturated emission, organic light-emitting diodes (OLEDs) have received much more consideration by scientists in the past two decades. And even out of laboratory, the OLEDs are popular among the commercial electronic products for solid-state displays and illumination. Generally, three primary RGB emitters, involving red (R), green (G) and blue (B), are footstones to achieve solid-state displays and illumination because the spectra by compositing RGB emissions match very well with the solar spectrum. Also, the combination of two complementary luminophors, blue and orange or yellow is an alternative approach to simulate the solar spectrum for white light illumination. Except for the full-color light-emitting materials for solid-state displays and illumination, near infrared (NIR) organics are of great importance for applications in information-secured devices, communications, biosensors, and phototherapy. To date, uncountable research works focusing on the emitters for full-color emissions have demonstrated their synthesis, photophysical properties and OLED application, which shows enough efficiency and stability to commercial utility. However, there are still three challenging issues which are needed to be handled urgently. Firstly, the lack of efficient deep blue emitters makes the external quantum efficiency (EQE) of deep blue OLEDs around 10% when the Commission Internationale de l'Éclairage (CIE) coordinates of y is smaller than 0.1. On the one hand it is difficult to achieve the deep blue emitters with extremely broad energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). On the other hand the triplet and frontier energy levels of the host, electron transporting layer (ETL) and hole transporting layer (HTL) in the device are required to well match with that of the emitters. Secondly, high energy in the emissive layer (EML) of deep blue OLEDs may degrade the materials used inside the EML, resulting in their short lifetimes. Thirdly, by comparison with other colors, the investigations of NIR emitters, such as their preparation, property study and device fabrications are sparse.;By harvesting both 25% singlet and 75% triplet excited states, iridium(III) [Ir(III)] complexes have been proven to be one of the best candidates to achieve highly efficient phosphorescent OLEDs (PHOLEDs) for solid-state displays and illumination. Herein, based on Ir(III) complexes, 18 phosphors were synthesized to achieve a widely tunable phosphorescence from deep blue to NIR. In this thesis, their synthesis were fully characterized by NMR spectroscopy, mass spectrometry and X-ray crystallography. Further investigations on the photophysical, electrochemical and thermal properties reveal that these phosphors have the possibility of device fabrication. And rational design of device architectures afford the OLEDs with high efficiencies.;Firstly, N-heterocyclic carbene ligands (CˆC:) were used to elevate the LUMO of phosphors (Ir1-Ir7), resulting in true and deep blue emission spanning from 420 to 450 nm. Secondly, the widely tunable phosphorescence from 470 to 614 nm was accomplished by using polyfluorinated 2-phenylpyridine (CˆN) derivatives as cyclometallated ligands of Ir(III) complexes (Ir10-Ir17). Interestingly, electron-withdrawing trifluoromethyl (CF3) group on the phenyl ring of CˆN-type ligands results in significantly red-shifted emissions of Ir(III) complexes, which distinguishes with the blue-shift effect of fluoride approach. Lastly, by comparison with the reported literature on NIR Ir(III) phosphors with extensive conjugation, the addition of slight conjugation but electron-withdrawing moieties onto the pyridyl ligands is a powerful and convenient avenue to tune the phosphorescence of Ir(III) phosphor into the NIR region, emitting at 729 nm.;Meanwhile, the deepest blue OLED made from Ir1 showed a peak EQE of 7.1% with CIE of (0.16, 0.11). And the best deep blue OLEDs made from Ir7 by using single and double electroluminescent (EL) units gave the highest EQE of 19.0% and 31.5% with CIE coordinates of (0.15, 0.19) and (0.15, 0.22), respectively. Such high efficiencies are comparable to and even better than the currently reported deep blue PHOLEDs. Also, the sky blue, green, yellow, orange, red and NIR PHOLEDs fabricated from Ir10, Ir13, Ir15, Ir16, Ir17 and Ir18 afforded the maximum EQE of 11.2%, 20.1%, 15.4%, 9.9%, 6.8% and 4.0%, respectively. By stacking RGB EML, the white PHOLED (PHWOLED) made by Ir1, Ir13 and Ir17 gave a peak EQE of 16.0% and CIE of (0.36, 0.47).;All in all, this thesis has successfully combined the materials synthesis and devices design to achieve efficient full-color and NIR PHOLEDs which are of great interest for solid-state displays and illumination. These works have a great significance in terms of the improvement of efficiency and stability of deep blue OLEDs as well as simplifying the synthesis methods to prepare highly efficient NIR Ir(III) phosphors.

Recent advances in growth techniques have lead to the production of high quality GaN and this has played a vital role in the improvement of GaN based devices. A number of device types can be produced from GaN. Spectrally selective devices can be produced by creating ternary or quaternary material systems by partially substituting either Al or In for Ga in GaN. This allows a wide spectral range that can be achieved ranging from the visible to the ultraviolet. The applications of detectors based on these material systems are vast and include areas such as biological, military, environmental, industrial and scientific spheres. In front illuminated Schottky barrier photodetectors, two major factors influencing the sensitivity of the device are the reverse leakage current and the transparency of the Schottky contact. In order to reduce the reverse current of semiconductor based devices, increase the barrier height, and enhance the adhesion of a metal on a semiconductor it is important to subject the contact to annealing. Annealing studies have been performed on AlGaN based photodiodes to investigate the evolution of the optical and electrical properties. In this study, the electrical and optical characteristics of AlGaN based Ni/Au and Ni/Ir/Au Schottky photodiodes were investigated. The electrical properties of the photodiodes were optimised by annealing in an Ar ambient. An increase in the Schottky barrier height and a decrease in the reverse leakage current were observed with increasing annealing temperature up to 500 oC. This effect was observed for both the Ni/Au and Ni/Ir/Au photodiodes. The optical characteristics of the photodiodes, which include the responsivity and the quantum efficiency, were also investigated. UV/visible rejection ratios of as high as 103 were obtained. The transmittance of Ni/Au and Ni/Ir/Au metal layers deposited on a quartz substrate were optimised by annealing. This was under the same ambient conditions as the Schottky photodiode. The transmittance increased with annealing temperature for the Ni/Au metal layer whereas it decreased at higher temperatures for the Ni/Ir/Au layer. The transmittance of the Ni/Au metal layer reached as high as 85 % after 500 oC annealing. The transmittance of the Ni/Ir/Au only reached a high of 41 % after 400 oC annealing. / Dissertation (MSc)--University of Pretoria, 2013. / Physics / unrestricted

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 167-177).