published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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by Sin Lai Yi, Mandy. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (leaves 120-121). / Text in English; abstracts in English and Chinese. / by Sin Lai Yi, Mandy. / Abstract --- p.ii / 論文摘要 --- p.iii / Acknowledgements --- p.iv / Table of Contents --- p.v / List of Figures --- p.ix / List of Tables --- p.xiii / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Adhesion --- p.1 / Chapter 1.2 --- Adhesion issues in PLEDs --- p.1 / Chapter 1.3 --- Literature review of the adhesion measurement techniques --- p.4 / Chapter 1.3.1 --- Methods involving detachment normal to the interface --- p.4 / Chapter 1.3.1.1 --- Direct pull-off method --- p.5 / Chapter 1.3.2 --- Methods based upon the application of lateral stresses for detachment --- p.7 / Chapter 1.3.2.1 --- Scotch tape method --- p.7 / Chapter 1.3.2.2 --- Peel test --- p.8 / Chapter 1.4 --- Motivations and aims of studies --- p.11 / Chapter 1.5 --- Outline of the thesis --- p.12 / Chapter Chapter 2 --- Experimental and Instrumentation / Chapter 2.1 --- Sample preparation --- p.13 / Chapter 2.2 --- Instrumentation --- p.13 / Chapter 2.1.1 --- Modified Scotch tape method --- p.13 / Chapter 2.2.2 --- Atomic force microscope --- p.23 / Chapter 2.2.3 --- X-ray photoelectron spectroscopy --- p.26 / Chapter 2.2.4 --- Radio-frequency (RF) plasma etching system --- p.27 / Chapter Chapter 3 --- Verification of the modified Scotch tape method / Chapter 3.1 --- Introduction --- p.31 / Chapter 3.2 --- Verification Test One ´ؤ Measurement against different adhesive tapes --- p.31 / Chapter 3.3 --- Verification Test Two ´ؤ Measurement with different crosshead speeds --- p.40 / Chapter 3.4 --- Verification Test Three ´ؤ Measurement of adhesive tapes with different sizes --- p.46 / Chapter 3.5 --- Summary --- p.51 / Chapter Chapter 4 --- Adhesion of gold films coated on the plasma treated ITO / Chapter 4.1 --- Introduction --- p.53 / Chapter 4.2 --- Characteristics of RF plasma etching --- p.53 / Chapter 4.2.1 --- Test One ´ؤ Study the relationship between sputtering time and sputtering depth --- p.54 / Chapter 4.2.2 --- Test Two - Study the relationship beteen self-bias voltage and sputtering depth --- p.54 / Chapter 4.3 --- Modification of the surface morphology of the Ar plasma treated ITO --- p.57 / Chapter 4.4 --- Quantitative adhesion measurement of gold films on plasma treated ITO --- p.62 / Chapter 4.5 --- Summary --- p.68 / Chapter Chapter 5 --- Adhesion of PFO on ITO with self-assembled monolayers as interfacial layers / Chapter 5.1 --- Introduction --- p.69 / Chapter 5.2 --- Reactions between SAMs and ITO --- p.71 / Chapter 5.3 --- Sample preparation --- p.73 / Chapter 5.4 --- Tests of alkylsilane --- p.74 / Chapter 5.4.1 --- Determination of the point of film detachment --- p.74 / Chapter 5.4.2 --- Adhesion test of PFO/alkylsilane/ITO --- p.79 / Chapter 5.4.3 --- Adhesion test of PF0/alkylsilane/Si02 --- p.91 / Chapter 5.5 --- Tests of phenylsilane --- p.104 / Chapter 5.6 --- Summary --- p.114 / Chapter Chapter 6 --- Conclusions and further studies / Chapter 6.1 --- Conclusions --- p.116 / Chapter 6.2 --- Further studies --- p.118 / References --- p.120

Magnetoresistance (MR) and doping effects have been investigated in a poly(3-hexylthiophene-2,5-diyl) (P3HT) based organic light emitting diodes. In single device of fixed composition (Au/P3HT/Al as spun and processed in air), the measured MR strongly depends on the drive conditions. The magnetoconductance (MC) varies from negative to positive (-0.4% ≤ MC ≤ 0.4%) with increasing current density, depending on which microscopic mechanism dominates. The negative MC is due to bipolaron based interactions and the positive MC to triplet-polaron based interactions (as confirmed by light emission). Oxygen doping is prevalent in P3HT devices processed in air and the effect of de-doping (by annealing above the glass transition temperature) is investigated on the MC of an Au/P3HT/Al diode. De-doping reduces the current through the device under forward bias by ~3 orders of magnitude, but increases the negative (low current) MC from a maximum of -0.5% pre-annealing to -3% post-annealing. This increased negative MC is consistent with bipolaron theory predictions based on Fermi level shifts and density of states (DoS) changes due to de-doping. The decrease in current density is explained by increased injection barriers at both electrodes also resulting from de-doping. Deliberate chemical doping of P3HT is carried out using pentacene as a hole trap centre. The trapping effect of pentacene is confirmed by reproducible and significant hole mobility-pentacene concentration behaviour, as measured by dark injection (DI) transient measurements. The enhanced carrier injection resulting from the pentacene doping also leads to increased electroluminescence (EL). The resultant MC in pentacene doped devices is strongly dependent on carrier injection and can be significantly enhanced by doping, for example from -0.2% to -0.6% depending on device and drive conditions. Throughout this thesis Lorentzian and non-Lorentzian function fitting is carried out on the measured MC, although the underlying microscopic mechanisms cannot always be discerned.

Halide perovskite solar cells, with rapid efficiency improvements from ~10% to ~23% in 6 years, have attracted significant attention due to their remarkable performance, low processing cost and their potential to become a strong alternative candidate to silicon solar cells. Significant development has also been achieved in halide perovskite-based LEDs with EQE improved from below 1% to ~20% in less than 4 years. This remarkable progress can mainly be attributed to the optimisation of halide perovskite properties. This dissertation focuses on the correlation between optical and electrical properties of halide perovskites and their remarkable performance. Bandgap tunabilities of halide perovskites in blue to green regions through mixing Br-and Cl-and in near infra-red region by substituting Pb2+ with Sn2+ are demonstrated. The absorption and PL spectra are consistent with each other supporting the bandgap tunability. Corresponding EL spectra, which are consistent with their PL spectra, are also demonstrated for blue to green regions. Terahertz measurements coupled with PLQE and transient PL results reveal that the high carrier mobilities are the main reason behind the high efficiency of tin-rich samples. A novel perovskite-polymer-bulk heterostructure is introduced and studied comprehensively. Correlations between their optoelectronic properties and remarkable performance on timescales ranging from femtosecond to microsecond are presented. Transient optical spectroscopy reveals the energy transfer from 2D regions to 3D regions happens in 1 ps. The 20% EQE of the LEDs based in this structure is consistent with conventional thin-film optical models giving internal quantum efficiency of ~100%. This in agreement with near-unity PLQE value of the pristine emissive layer material and the dominant bimolecular recombination process observed in nanosecond-scale transient PL measurements. Two typical interfacial engineering methods to improve the quality of halide perovskite and device performance are then presented. Optimised NiOx is adopted to improve the anode interface. From transient photovoltaic measurements, we find the charge collection ability of NiOx is superior to that of PEDOT:PSS. This is also the main reason behind their better photovoltaic device performance. A unique anti-solvent treatment with additive modifies both the bulk and surfaces of halide perovskites and improves the device performance significantly. Transient PL and PLQE measurements demonstrate that non-radiative recombination pathways are significantly reduced.

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