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Includes bibliographical references (p. 115-120).

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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Chan, Man Ki = UV改性的MEH-PPV在用雙層發光結構的多色聚合物發光器件的作用之研究 / 陳文基. / Thesis submitted in: October 2007. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Chan, Man Ki = UV gai xing de MEH-PPV zai yong shuang ceng fa guang jie gou de duo se ju he wu fa guang qi jian de zuo yong zhi yan jiu / Chen Wenji. / Abstract --- p.i / 論文摘要 --- p.iii / Acknowledgements --- p.iv / Table of Contents --- p.v / List of Figures --- p.viii / List of Tables --- p.x / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- History --- p.1 / Chapter 1.2 --- Conjugated polymers --- p.2 / Chapter 1.2.1 --- Structure and energy states --- p.2 / Chapter 1.2.2 --- Charge carriers --- p.4 / Chapter 1.2.3 --- Transport of charge carriers --- p.8 / Chapter 1.3 --- Polymer light emitting diodes --- p.8 / Chapter 1.3.1 --- Fabrication and operation --- p.8 / Chapter 1.3.2 --- Improvement of efficiency --- p.10 / Chapter 1.4 --- Full-colour display --- p.13 / Chapter 1.4.1 --- Three colour emissions --- p.13 / Chapter 1.4.2 --- Colour pixels --- p.15 / Chapter 1.5 --- Objectives of present studies --- p.17 / References --- p.20 / Chapter Chapter 2 --- Instrumentation and Experimental Procedures --- p.23 / Chapter 2.1 --- Nitrogen glove box --- p.23 / Chapter 2.2 --- High vacuum deposition system --- p.23 / Chapter 2.3 --- Fabrication of PLEDs --- p.24 / Chapter 2.3.1 --- Integrated fabrication system --- p.24 / Chapter 2.3.2 --- Fabrication procedure --- p.26 / Chapter 2.4 --- Keithley 236 source measure unit --- p.30 / Chapter 2.5 --- Photo Research SpectraColorimeter (PR-650 SpectraScan) --- p.30 / Chapter 2.6 --- Tensor Alpha Step 500 Profilometer --- p.31 / Chapter 2.7 --- Fourier Transform Infrared (FTIR) spectroscopy --- p.31 / Chapter 2.8 --- UV-visible spectroscopy --- p.33 / Chapter 2.9 --- Photoluminescence spectroscopy --- p.35 / References --- p.36 / Chapter Chapter 3 --- Intrinsic and UV-induced properties of MEH-PPV thin films --- p.37 / Chapter 3.1 --- Introduction --- p.37 / Chapter 3.2 --- FTIR studies of MEH-PPV thin films --- p.38 / Chapter 3.2.1 --- Sample preparation --- p.38 / Chapter 3.2.2 --- Results and discussion --- p.39 / Chapter 3.2.3 --- Summary --- p.45 / Chapter 3.3 --- Absorption and photoluminescence studies of MEH-PPV thin films --- p.45 / Chapter 3.3.1 --- Sample preparation --- p.45 / Chapter 3.3.2 --- Results and discussion --- p.46 / Chapter 3.3.3 --- Summary --- p.54 / Chapter 3.4 --- Current-voltage measurement of MEH-PPV thin films --- p.54 / Chapter 3.4.1 --- Sample preparation --- p.54 / Chapter 3.4.2 --- Results and discussion --- p.56 / Chapter 3.4.3 --- Remark --- p.62 / Chapter 3.4.4 --- Summary --- p.62 / Chapter 3.5 --- Conclusion --- p.63 / References --- p.64 / Chapter Chapter 4 --- Two-colour light emitting device using UV modified MEH-PPV layer --- p.67 / Chapter 4.1 --- Introduction --- p.67 / Chapter 4.2 --- Details of fabrication procedures --- p.68 / Chapter 4.3 --- Results and Discussion --- p.70 / Chapter 4.4 --- Conclusion --- p.80 / References --- p.82 / Chapter Chapter 5 --- Concluding remarks and future works --- p.83 / Chapter 5.1 --- Concluding remarks --- p.83 / Chapter 5.2 --- Future works --- p.84

In this thesis steady-state (i.e. steady-state with respect to the applied voltage waveform) transient current-transient voltage [i(t)-v(t)], transient brightness-transient current [b(t)-i(t)], transient brightness-transient voltage [b(t)-v(t)], transient current [i(t)], transient brightness [b(t)], and detrapped charge analysis are introduced as novel organic light emitting device (OLED) characterization methods. These analysis methods involve measurement of the instantaneous voltage [v(t)] across, the instantaneous current [i(t)] through, and the instantaneous brightness [b(t)] from an OLED when it is subjected to a bipolar, piecewise-linear applied voltage waveform. The utility of these characterization methods is demonstrated via comparison of different types of OLEDs and polymer light emitting devices (PLEDs) and from a preliminary study of OLED aging. Some of the device parameters obtained from these characterization methods include: OLED capacitance, accumulated charge, electron transport layer (ETL) thickness, hole transport layer (HTL) thickness, OLED thickness, parallel resistance, and series resistance. A current bump observed in i(t)-v(t) curves is attributed to the removal of accumulated hole charge from the ETL/HTL interface and is only observed in heterojunctions (i.e. OLEDs), not in single-layer devices (i.e. PLEDs). Using the characterization methods developed in this thesis, two important OLED device physics conclusions are obtained: (1) Hole accumulation at the ETL/HTL interface plays an important role in establishing balanced charge injection of electrons and holes into the OLED. (2) The ETL behaves as a leaky insulator while the HTL more efficiently conducts charge and acts as a voltage-dependent resistor. A preliminary investigation of the aging properties of OLEDs is presented as further evidence of the utility of the novel characterization methods developed in this thesis. In general, aging is characterized by a softer turn on of the forward bias portions of i(t)-v(t) and b(t)-v(t) curves. Also, some aging recovery is possible if the OLEDs are subjected to a zero or reverse bias. / Graduation date: 2000

The goal of this thesis is the identification and synthesis of high-luminance, primary color alternating-current thin-film electroluminescent (ACTFEL) devices. Special attention is paid to the synthesis of primary color green ACTFEL devices because of the lack of an adequate primary green ACTFEL phosphor and the dominance of green wavelengths in the response of the human eye. The sulfide materials family is the focus of this investigation of ACTFEL phosphor hosts due to its importance in the field of electroluminescence, although oxide and selenide phosphor hosts are also considered. Particular attention is paid to the coactivation of the SrS:Cu ACTFEL phosphor because of the ability to control the emission spectrum with coactivators. Green ACTFEL devices exhibiting a high-luminance, high-efficiency, saturated green color are demonstrated via the coactivation of SrS:Cu with alkali metals. In addition, Zn���GeO���:Mn is demonstrated as a potentially bright and stable ACTFEL phosphor. Finally, the "phosphor sandwich" technique of ACTFEL fabrication is presented as a means of depositing highly-stable, high-luminance ACTFEL phosphors. The fabrication techniques presented in this thesis are subsequently employed to deposit custom ACTFEL devices for characterization studies aimed at determining fundamental physical properties of ACTFEL phosphors. A method for establishing the relative hot electron distributions inside operating ACTFEL phosphor hosts is presented. In addition, the fabrication of SrS:Cu ACTFEL devices for characterization has allowed new insight into the density and physical basis of phosphor space charge. This insight is gained by monitoring the electrical and optical characteristics of SrS:Cu ACTFEL devices coactivated with various non-isovalent impurities to assess the influence of native defects and extrinsic impurities on space charge related behavior. It is found that native sulfur vacancies are not the physical basis for dynamic space charge in SrS:Cu ACTFEL devices. / Graduation date: 2000

This thesis investigates two methods of optical excitation of alternating-current thin-film electroluminescent (ACTFEL) devices. The two experimental methods investigated in this thesis are the photo-induced charge (PIQ) and luminescence (PIL), and the subthreshold-voltage induced transferred charge (VIQ) techniques. PIQ/PIL experiments utilize an above-bandgap laser pulse to investigate the transport properties of photo-injected electrons and holes within the phosphor layer of the ACTFEL device. VIQ experiments use a broadband xenon lamp pulse to optically reset traps which are ionized by subthreshold bipolar voltage pulses. Both experiments characterize traps within the phosphor layer. PIQ/PIL experiments are performed on evaporated ZnS:Mn ACTFEL devices possessing phosphor layers with thicknesses of 950, 700, and 300 nm. From the PIQ/PIL experiment, an impact excitation threshold electric field for evaporated ZnS:Mn is found to be ~1 MV/cm. Evidence of hole-trapping is also obtained from the PIQ experiment. The holes in evaporated ZnS:Mn ACTFEL devices are found to possess a drift length of ~180 �� 70 nm, a hole lifetime of ~2 ps, and a capture cross-section of ~7 x 10������� cm��. It is speculated that the trap responsible for hole capture is a zinc vacancy or zinc vacancy complex. VIQ experiments are performed on evaporated, atomic layer epitaxy [ALE] (Cl), and ALE (DEZ) ZnS:Mn ACTFEL devices. Data obtained via VIQ experiments yield evidence for the generation of space charge below the EL conduction threshold, as well as providing a means of estimating the physical location, energy depth, density, and capture cross-section of traps responsible for VIQ. The depth of the traps responsible for VIQ in evaporated, ALE (C1), and ALE (DEZ) ZnS:Mn are estimated to be ~1.1, ~0.3, and ~0.8 eV, respectively. It is speculated that the traps responsible for VIQ are due to sulfur vacancies, chlorine, and oxygen, for evaporated, ALE (Cl), and ALE (DEZ), respectively. / Graduation date: 1999

Graduation date: 1997

In this report, the optical characteristics of ZnS:Tb AC driven thin-film electroluminescent devices are evaluated. Luminescence at low and room temperature under a constant phosphor field is recorded in order to probe the hot electron energy distribution. Samples fabricated by atomic layer epitaxy and by sputter deposition are investigated and their differences and similarities evaluated. The ALE sample exhibits a drastic increase in luminescence at low temperature, while the sputtered sample displays a decrease in luminescence at low temperature. The reduction of optical phonon scattering and nonradiative transition rates at low temperature are believed to be responsible for these changes. Differences between ALE and sputtered samples are due to the different fabrication methods. The approach suggested by Krupka in 1972 of determining the hot electron energy distribution by taking the ratio intensities of two luminescent transitions from different upper states in Tb�����, is shown to be inaccurate due to the possible nonradiative transitions taking place. The Tb����� impact excitation quantum yield at a wavelength of 489 nm is measured and is shown to agree with Monte Carlo simulations. Saturation occurs due to the increase in band-to-band impact ionization at high phosphor fields. / Graduation date: 1995