Conjugated polymers and oligomers have been widely studied based on their wide range of useful properties and applications. Given the importance of self-assembly and charge transfer in the development of conjugated materials for use in electronic applications, it is crucial to: (i) prepare functional materials by molecular design, (ii) evaluate the structure-property relationships of new materials, and (iii) develop fundamental understanding of electronic structure and charge transport behavior. The use of conjugated polymeric materials in electronic applications relies on control of the assembly and orientation of the polymer chains in the solid state. Conjugated polymers with liquid crystalline behavior could be used to implement an additional level of control over orientation and resultant properties. Substitution of the conjugated polythiophene backbone with semifluoroalkyl side chains (i.e., the diblock -(CH2)m(CF2)nF) has afforded materials with unusual properties. The mutual immiscibility of the aromatic backbone, the alkyl side-chain segments, and the fluoroalkyl side-chain termini provides control over supramolecular packing. A series of eight polymers has been synthesized, in which the lengths of the alkyl (m) and fluoroalkyl (n) segments are varied. One regiorandom analogue and two poly(3-alkylthiophene)s were also synthesized for comparative purposes. The structure, molecular weight, and regioregularity of the polymers were evaluated using a variety of techniques. The semifluoroalkyl-substituted polymers have been systematically studied to determine the effect of side chain length and m:n block ratios on their solution state, liquid crystalline, and solid state properties. The effect of side chains on conjugation was determined, where solubility allowed, by solution-state UV-visible and fluorescence spectroscopy. The thermal and liquid crystalline properties of the homopolymers were evaluated by DSC, variable-temperature X-ray diffraction, and polarized optical microscopy. Several semifluoroalkyl-substituted polythiophene homologues show liquid crystalline behavior. Molecular packing and charge transport are key factors governing the use of conjugated materials in electronic applications. A wide variety of oligomers have been studied as models for charge migration in conjugated polymers. One-dimensional models do not adequately represent two-dimensional charge transport; thus, a variety of two-dimensional, covalently-linked models have been developed. Previous work by our group, and others, led to the proposal of bis(oligothienyl) compounds as models to study the interaction of the ð-conjugated chains. Previous reports by other researchers described the synthesis and characterization of hydrogen-terminated analogues of 1,8-bis(oligothienyl)naphthalenes. However, these materials proved to be unsuitable for use as charge transport models, as they were subject to irreversible polymerization upon oxidation. Installation of methyl groups at the terminal a-positions of 1,8-bis(oligothienyl)naphthalenes allowed us to create a series of models in which conjugated chains are held in close proximity. This provides access to multiple redox states, and future systems based on these molecules may be used as models for charge transport or as functional materials for incorporation into devices.

Novel organic conjugated materials have led to new technologies in the field of flexible electronics, with applications in the area of sensors, field effect transistors, or photovoltaic devices. Several material parameters and properties come into play in these devices, including energy of the frontier molecular orbitals, thin film morphology, and charge transport. These properties can be controlled by the chemistry of organic materials, and through processing conditions. In particular, this dissertation focuses on the isoindigo unit as an electron deficient unit to tune polymer light absorption, charge separation, charge transport in the first part of this dissertation, and morphology control in organic photovoltaic (OPV) devices in a subsequent section. The first part of this dissertation introduces the synthesis and properties of isoindigo-containing polymers as n-type, p-type, or ambipolar semiconductors, and their application in all-polymer or polymer:fullerene blends OPV active layers. It is found that polymers with phenyl linkages along the backbone tend to have broader light absorption than polymers with alternating phenyl-thiophene rings; however, steric hindrance in the former leads to low charge mobilities, and poor device performance. In addition, this section highlights the importance of controlling phase separation in OPV devices by focusing on all-polymer blends, which show large phase separation, and polymer:fullerene blends, where the morphology can be controlled through processing additives generating a two-fold increase in device efficiency. Looking at poly(oligothiophene-isoindigo) polymers as model systems, emphasis is placed on photovoltage losses in these devices due to a decrease in effective energy gap between the polymers and fullerene as the oligothiophene donating strength is increased, as well as explanation of the device parameters through description of morphology as solubility is varied. The second portion of this dissertation focuses on solution properties of polymers and their correlation to thin film morphology. A first study investigates the influence of alkyl side chains on solubility, molecular packing, and phase separation in blends of poly(terthiophene-alt-isoindigo) with fullerenes. Specifically, as side chains are lengthened, solubility is increased, but with limited impact on the blends morphology. On the other hand increased backbone torsion leads to variations in energy levels, polymer packing and large phase separation in blends with fullerenes. These thermodynamic parameters are to put in perspective with the kinetic control of film formation during the coating process. This is discussed in a second study, which looks at the mechanism of thin film formation when processing additives are used. In particular, this study highlights the interactions that provide a driving force for polymer crystallite formation, depending on the mechanism followed when aliphatic and aromatic additives are used. These observations are then used to predict the morphology in spin-coated thin films.

published_or_final_version / Chemistry / Master / Master of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

In this thesis, atomic force microscopy (AFM), transmission electron microscopy (TEM) and optical microscopy techniques were used to investigate systematically the self-assembled nanostructure behaviour of two different types of spin-cast polymer thin films: poly(isoprene-b-ethylene oxide), PI-b-PEO diblock copolymers and [poly(9,9-dioctylfluorene-co-benzothiadiazole)]:poly[9,9- dioctyfluorene-co-N-(4-butylphenyl)-diphenylamine], F8BT:TFB conjugated polymer blends. In the particular case of the polymer blend thin films, the morphology of their composites with cadmium selenide (CdSe) quantum dot (QD) nanoparticles was also investigated. For the diblock copolymer thin films, the behaviour of the nanostructures formed and the wetting behaviour on mica, varying the volume fraction of the PEO block (fPEO) and the average film thickness was explored. For the polymer blend films, the effect of the F8BT/TFB blend ratio (per weight), spin-coating parameters and solution concentration on the phase-separated nanodomains was investigated. The influence of the quantum dots on the phase separation when these were embedded in the F8BT:TFB thin films was also examined. It was found that in the case of PI-b-PEO copolymer thin films, robust nanostructures, which remained unchanged after heating/annealing and/or ageing, were obtained immediately after spin coating on hydrophilic mica substrates from aqueous solutions. The competition and coupling of the PEO crystallisation and the phase separation between the PEO and PI blocks determined the ultimate morphology of the thin films. Due to the great biocompatible properties of the PEO block (protein resistance), robust PEO-based nanostructures find important applications in the development of micro/nano patterns for biological and biomedical applications. It was also found that sub-micrometre length-scale phase-separated domains were formed in F8BT:TFB spin cast thin films. The nanophase-separated domains of F8BT-rich and TFB-rich areas were close to one order of magnitude smaller (in the lateral direction) than those reported in the literature. When the quantum dot nanoparticles were added to the blend thin films, it was found that the QDs prefer to lie in the F8BT areas alone. Furthermore, adding quantum dots to the system, purer F8BT and TFB nano-phase separated domains were obtained. Conjugated polymer blend thin films are excellent candidates for alternatives to the inorganic semiconductor materials for use in applications such as light emitting diodes and photovoltaic cells, mainly due to the ease of processing, low-cost fabrication and mechanical flexibility. The rather limited optoelectronic efficiency of the organic thin films can be significantly improved by adding inorganic semiconducting nanoparticles.

The discovery of semi-conducting and conducting organic materials has opened new possibilities for electronic devices and systems. Applications, previously unattainable for conventional electronics, have become possible thanks to the development of conjugated polymers. Conjugated polymers that are both ion- and electron conducting, allow for electrochemical doping and de-doping via reversible processes as long as both forms of conduction remain available. Doping causes rearrangement of the -system along the polymer backbone, and creates new states in the optical band gap, resulting in an increased electronic conductivity and also control of the color (electrochromism). Doping can also occur by charge injection at a metal – semiconducting polymer interface. Electrochemical electronic devices and solid state devices based on these two types of doping are now beginning to enter the market. This thesis deals with organic based-devices whose working mechanism involves electrolytes. After describing the properties of conjugated polymers, fundamentals on electrolytes (ionic conductivity, types, electric double layer and the electric field distribution) are briefly presented. Thereafter, a short review of the field of organic field effect transistors as well as a description of transistors that are gated via an electrolyte will be reviewed. Paper I present a novel technique to visualize the electric field within a two-dimensional electrolyte by applying the electrolyte over an array of electronically isolated islands of electrochromic polymer material on a plastic foil. By observing the color change within each polymer island the direction and the magnitude of the electric field can be measured. This technology has applications in electrolyte evaluation and is also applicable in bio-analytical measurements, including electrophoresis. The focus of paper II lies on gating an organic field effect transistor (OFET) by a polyanionic proton conductor. The large capacitance of the electric double layer (EDL) that is formed at organic semiconductor/polyelectrolyte upon applying a potential to the gate, results in low operation voltages and fast response. This type of transistor that is gated via electric double layer capacitor is called EDLC-OFET. Because an electrolyte is used as a gate insulator, the role of the ionic conductivity of the electrolyte is considered in paper III. The effect on the electronic performance of the transistor is studied as well by varying the humidity level.

No description available.

Philosophiae Doctor - PhD / The continuous increase in energy consumption and decrease in fossil fuels reserves are a primary concern worldwide; especially for South Africa. Therefore, there is an urgent need for alternative energy resources that will be sustainable, and environmentally friendly in order to tackle the ecological degradation generated by the use of fossil fuels. Among many energy ‘niches’, solar energy appears to be one of the most promising and reliable for the African continent because of the constant availability of sun light. Organic conjugated polymers have been identified as suitable materials to ensure proper design and fabrication of flexible, easy to process and cost-effective solar cells. Their tendency to exhibit good semiconducting properties and their capability to absorb photons from the sunlight and convert it into electrical energy are important features that justify their use in organic photovoltaic cells. Many different polymers have been investigated as either electron donating or electron accepting materials. Among them, poly(3-hexylthiophene) is one of the best electron donor materials that have been used in organic photovoltaic cells. It is a good light absorber and its Highest Occupied Molecular Orbital (HOMO) energy level is suitable to allow electron transfer into an appropriate electron acceptor. On the other hand, the molecular ordering found in dendrimers attracted some interest in the field of photovoltaics as this feature can ensure a constant flow of charges. In this work, I hereby report for the first time, the chemical synthesis of a highly crystalline dendritic star copolymer generation 1 poly(propylene thiophenoimine)-co-poly(3-hexylthiophene) (G1PPT-co-P3HT) with high molecular weight and investigate its application as donating material in bulk heterojunction organic photovoltaics.

Ho Ming Kei = 新的有機發光器件的界面研究 / 何銘基. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Ho Ming Kei = Xin de you ji fa guang qi jian de jie mian yan jiu / He Mingji. / Abstract --- p.i / 论文摘要 --- p.iii / Acknowledgements --- p.iv / Table of Contents --- p.v / List of Figures --- p.viii / List of Tables --- p.xiii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Overview --- p.1 / Chapter 1.2 --- Conjugated Polymers --- p.2 / Chapter 1.2.1 --- Electronic and geometric Configuration --- p.2 / Chapter 1.2.2 --- Charge Carries of conjugated polymers --- p.4 / Chapter 1.2.3 --- Polymer Light Emitting Diodes --- p.11 / Chapter 1.2.4 --- Device Fabrication --- p.12 / Chapter 1.2.5 --- Polymeric Luminescent Material Development --- p.18 / Chapter 1.2.6 --- Interface and Surface of PLED --- p.21 / Chapter 1.3 --- Aims of this thesis --- p.22 / References --- p.24 / Chapter Chapter 2 --- Instrumentation --- p.26 / Chapter 2.1 --- X-ray Photoelectron Spectroscopy --- p.26 / Chapter 2.1.1 --- Introduction --- p.26 / Chapter 2.1.2 --- Basic Principles and Theory --- p.28 / Chapter 2.1.3 --- Qualitative Analysis Using XPS --- p.29 / Chapter 2.1.4 --- Angular Effect on XPS --- p.29 / Chapter 2.1.5 --- Chemical Shifts --- p.30 / Chapter 2.1.6 --- Quantitative Analysis using XPS --- p.31 / Chapter 2.1.6.1 --- Survey spectrum --- p.32 / Chapter 2.1.6.2 --- Core level spectrum --- p.32 / Chapter 2.1.6.3 --- Valence band spectrum --- p.33 / Chapter 2.1.7 --- Instrumental Setup for XPS --- p.33 / Chapter 2.2 --- HV physical vapor deposition system with nitrogen glove box --- p.36 / Chapter 2.2.1 --- Nitrogen grove box --- p.38 / Chapter 2.2.2 --- HV physical vapor deposition system --- p.38 / Chapter 2.3 --- L-V-I measurement system --- p.41 / Chapter 2.3.1 --- Keithley 236 source-measure unit --- p.41 / Chapter 2.3.2 --- Photo Research PR-650 photo meter --- p.43 / Chapter 2.3.3 --- Test Environment Chamber --- p.43 / Chapter 2.4 --- a-Step Profilometer --- p.44 / References --- p.45 / Chapter Chapter 3 --- Interface study between MEHPPV: PEG and Aluminum --- p.46 / Chapter 3.1 --- Introduction --- p.46 / Chapter 3.2 --- Sample Preparations --- p.47 / Chapter 3.2.1 --- Si(lll) substrate preparation --- p.47 / Chapter 3.2.2 --- Au sputtering on the clean Si Surface --- p.48 / Chapter 3.2.3 --- Polymer film formation --- p.48 / Chapter 3.3 --- Results and Discussion --- p.49 / Chapter 3.3.1 --- XPS Survey scan ofMEHPPV --- p.51 / Chapter 3.3.2 --- XPS of Cls Core level ofMEHPPV --- p.51 / Chapter 3.3.3 --- XPS ofOls Core level ofMEHPPV --- p.55 / Chapter 3.3.4 --- XPS of A12p Core level ofMEHPPV --- p.59 / Chapter 3.3.5 --- XPS Survey scan of PEG --- p.64 / Chapter 3.3.6 --- XPS of Cls Core level of PEG --- p.64 / Chapter 3.3.7 --- XPS of Ols Core level of PEG --- p.67 / Chapter 3.3.8 --- XPS of A12p Core level of PEG --- p.70 / Chapter 3.3.9 --- XPS survey scan of MEHPPV:PEG(10wt% PEG) --- p.73 / Chapter 3.3.10 --- XPS Cls core level of MEHPPV:PEG(10wt% PEG) --- p.73 / Chapter 3.3.11 --- XPS Ols core level of MEHPPV:PEG(10wt% PEG) --- p.76 / Chapter 3.3.12 --- XPS A1 2p core level of MEHPPV: PEG --- p.80 / Chapter 3.3.13 --- Surface migration of bulk absorbed oxygen --- p.84 / Chapter 3.4 --- Conclusions --- p.84 / Reference --- p.87 / Chapter Chapter 4 --- Efficiency enhancement in polymer light emitting diodes using Crown ether 18-C6 and aluminum cathode --- p.89 / Chapter 4.1 --- Introduction --- p.89 / Chapter 4.2 --- Sample preparation --- p.91 / Chapter 4.2.1 --- The Cleaning of substrate --- p.91 / Chapter 4.2.2 --- PEDOT: PSS film formation --- p.93 / Chapter 4.2.3 --- Emissive polymer layer formation --- p.94 / Chapter 4.2.4 --- Deposition of metal cathode --- p.94 / Chapter 4.2.5 --- Epoxy Encapsulation --- p.95 / Chapter 4.3 --- Results and Discussion --- p.95 / References --- p.101 / Chapter Chapter 5 --- Concluding Remarks and Future Work --- p.102 / Chapter 5.1 --- Concluding Remarks --- p.102 / Chapter 5.2 --- Future Work --- p.103

<p>The discovery of semi-conducting and conducting organic materials has opened new possibilities for electronic devices and systems. Applications, previously unattainable for conventional electronics, have become possible thanks to the development of conjugated polymers. Conjugated polymers that are both ion- and electron conducting, allow for electrochemical doping and de-doping via reversible processes as long as both forms of conduction remain available. Doping causes rearrangement of the -system along the polymer backbone, and creates new states in the optical band gap, resulting in an increased electronic conductivity and also control of the color (electrochromism). Doping can also occur by charge injection at a metal – semiconducting polymer interface. Electrochemical electronic devices and solid state devices based on these two types of doping are now beginning to enter the market.</p><p>This thesis deals with organic based-devices whose working mechanism involves electrolytes. After describing the properties of conjugated polymers, fundamentals on electrolytes (ionic conductivity, types, electric double layer and the electric field distribution) are briefly presented. Thereafter, a short review of the field of organic field effect transistors as well as a description of transistors that are gated via an electrolyte will be reviewed.</p><p>Paper I present a novel technique to visualize the electric field within a two-dimensional electrolyte by applying the electrolyte over an array of electronically isolated islands of electrochromic polymer material on a plastic foil. By observing the color change within each polymer island the direction and the magnitude of the electric field can be measured. This technology has applications in electrolyte evaluation and is also applicable in bio-analytical measurements, including electrophoresis. The focus of paper II lies on gating an organic field effect transistor (OFET) by a polyanionic proton conductor. The large capacitance of the electric double layer (EDL) that is formed at organic semiconductor/polyelectrolyte upon applying a potential to the gate, results in low operation voltages and fast response. This type of transistor that is gated via electric double layer capacitor is called EDLC-OFET. Because an electrolyte is used as a gate insulator, the role of the ionic conductivity of the electrolyte is considered in paper III. The effect on the electronic performance of the transistor is studied as well by varying the humidity level.</p>