Polymeric materials are attractive candidates for the fabrication of low cost, large area photovoltaic devices. Controlling the band gap of the electroactive polymer is an essential factor in optimizing the resulting devices. In this dissertation, a methodology for the synthesis of well-defined semiconducting materials with tunable band gaps is described. First, the synthesis, characterization, and computational analysis of a variety of trimers consisting of two 3-hexylthiophene units flanking a central moiety consisting of thiophene, or one of the electron donating monomers isothianaphthene or thieno[3,4,b]thiophene will be described. From this analysis the influences of the electronic and steric structure of the materials will be investigated. Several of these trimers will then be used in the synthesis of well-defined, higher order, oligomers of thiophene and isothianaphthene in varying compositions. Polymerization of these oligomers yields polymers of known sequence allowing the band gap of the polymers to be systematically varied. Finally, preliminary investigations into the development of alternate oligomer core units will be described. The control over the band gap that this method affords will be useful in the optimization of polymeric semiconductor devices.

Charge transfer salts of Tetracyanoquinodimethane (TCNQ) were synthesised and their electrical and magnetic properties were investigated. These salts show unusual electrical and magnetic behaviour in contrast to conventional organic compounds. These salts have crystal structures which in general consist of TCNQ radical ions stacked in chains, isolated from each other by the diamagnetic cations. They are thus regarded as "one-dimensional" electrical and magnetic systems. The ESR spectra of these salts are attributed to triplet excitons showing that the spin-spin and electronelectron correlation effects are important. In the ESR spectra (Chapter III) of some TCNQ salts dipolar splitting is observed confirming the spin-spin interaction. These triplet excitons are regarded as bound electron-hole pairs. The experimentally determined dipolar splitting tensors are presented in Chapter III and the intensity data in Chapter IV. A large number of fine structure lines are observed in the ESR spectra of Pyridinium-TCNQ and 4-Aminopyridinium-TCNQ apart from regular triplet exciton lines (Chapter III). These lines are attributed to the trapping of excitons on an extended formula finit (TCNQ2 )n. In Chapter IV the temperature dependent magnetic susceptibilities are discussed in terms of Heisenberg antiferromagnetism and Pauli paramagnetism. In Chapter V temperature dependent behaviour of electrical conductivity is discussed in terms of an exciton band model, the lattice structure of the salts and one-dimensional lattice consisting of defects giving rise to high and low conducting segments. Low temperature electrical and magnetic phases are discussed (Chapters IV and VII) in terms of a band and hopping mechanisms.In Chapter VI self consistent field calculations are made with reference to the tight binding one electron band theory using simplified Roothaan equations considering CNDO approximations. Theoretical results are related to experimental band gaps, spinspin interactions and charge alteration.

No description available.

<p>InP nanowires with short InAs segments were grown on InP (111)B substrates by Au assisted vapour-liquid-solid growth in a gas source molecular beam epitaxy system. Nanowire crystal structure and morphology were investigated by transmission electron microscopy as a function of temperature, growth rate, and V/III flux ratio. At 370C predominantly kinked nanowires with random morphology and low areal density were observed with a rough parasitic 2D film. At 440C, nanowire density was also reduced but the 2D film growth was smoother and nanowires grew straight without kinking. An optimum temperature of 400C maximized areal density with uniform nanowire morphology. At the optimum temperature of 400C, an increase in V/III flux ratio changed the nanowire morphology from rod-shaped to pencil like indicating increased radial growth. Growth rate did not affect the crystal structure of InP nanowires. For InAs nanowires, changing the growth rate from 1 to 0.5 μm/hr reduced the presence of stacking faults to as low as one per nanowire. Short InAs segments in InP nanowires were found to grow through two mechanisms for nanowires of length L and diameter D. The first mechanism described the supply of In to the growth front via purging of In from the Au droplet where L was proportional to D. The second mechanism involved direct deposition of adatoms on the nanowire sidewall and subsequent diffusion to the growth front where L was proportional to 1/D. For intermediate growth durations, a transition between these two mechanisms was observed. For InP and InAs nanowires, the growth mode was varied from axial to radial through the inclusion of Al to form a core shell structure. Al_xIn_1-xAs(P) shells were grown on InAs cores with Al alloy fractions between 0.53 and 0.2. These nanowires were examined by transmission electron microscopy and it was found, for all values of x in InAs-Al_xIn_1-xP structures, that relaxation had occurred through the introduction of dislocations. For InAs-Al_xIn_1-xAs structures, all values except x=0.2 had relaxed through dislocation formation. A critical thickness model was developed to determine the core-shell coherency limits which confirmed the experimental observation of strain relaxation. The effects of passivation on the electronic transport and the optical properties were examined as a function of structural core-shell passivation and chemical passivation. The mechanisms for the observed improvement in mobility for core-shell versus bare InAs nanowires was due to the reduction in ionized impurity scattering from surface states. Similarly an increase in photoluminescence intensity after ammonium sulfide passivation was explained by the reduction of donor type surface states.</p> / Doctor of Philosophy (PhD)

A new form of organic energy storage devices (organic capacitors) is presented in the first part of this dissertation. The storage devices are made out of an organic semiconductor material and charge storage elements from synthesized nanoparticles. The semiconducting polymer is obtained by blending poly (vinyl alcohol) and poly (acrylic acid) in crystal state polymers with a known plasticizer; glycerol or sorbitol. Synthesized nanoparticles namely, zinc-oxide (ZnO), erbium (Er), cadmium sulfide (CdS), palladium (Pd) and silver-platinum (AgPt) were used as charge storage elements in fabrication of metal-insulator-semiconductor (MIS) structure. The organic semiconductor and synthesized nanoparticles are tested to evaluate and characterize their electrical performance and properties. Fabrication of the organic capacitors consisted of layer-by-layer deposition and thermal evaporation of the electrode terminals. Capacitance versus voltage (C-V) measurement tests were carried out to observe hysteresis loops with a window gate that would indicate the charging, discharging and storage characteristics. Experimental investigation of various integrated energy harvesting techniques combined with these organic based novel energy storage devices are performed in the second part of this dissertation. The source of the energy is the wind and is harvested by means of miniature wind turbines and vibrations, using piezoelectric transduction. In both cases, the generated electric charge is stored in these capacitors. The performance of the organic capacitors are evaluated through their comparison with commercial capacitors. The results show that the voltage produced from the two energy harvesters was high enough to store the harvested energy in the organic capacitors. The charge and energy levels of the organic capacitors are also reported. The third part of this dissertation focuses on harvesting energy from a self-induced flutter of a thin composite beam. The composite beam consisted of an MFC patch bonded near the clamped end and placed vertically in the center of a wind tunnel test section. The self sustaining energy harvesting from the unimorph composite beam is exploited. The effects of different operational parameters including the optimum angle of attack, wind speed and load resistance are determined. / Ph. D.

Understanding the fundamentals of Organic semiconductors is crucial aspect towards the technological applications. Conjugated polymers have shown many interesting physical properties. Especially the electronic and optical properties of these materials have great impact on the daily life. Much work has been devoted to gain the knowledge on the electrical and photo physical properties of these materials. Despite the large number of studies in fabrication and characterisation on these devices some of the fundamental properties like charge transport, carrier generation and its control by doping are not well accomplished. The Thesis consists of 6 chapters. First chapter is a brief introduction on various properties of semiconducting polymers. Different charge transport models and their basic mechanisms are discussed. Chapter 2 discusses the synthesis, device making and experimental methods used to characterise the polymer devices. Chapter 3 is focused on transport properties in polypyrrole devices and its variation with different experimental conditions. Chapter 4 is aimed to understand the anomalies in the current-voltage characteristics appearing in some of the thiophene based devices. In Chapter 5, the impedance measurement technique is used to characterise the poly (3-hexylthiophene) devices and the outcomes are presented and chapter 6 summarises all the experimental results obtained in this thesis work and presents some future aspects and directions. Chapter 1: Some of the basic properties and recent advancements in the field of organic semiconductors are discussed in this chapter. Organic semiconductor devices based on conjugated polymers are now becoming alternatives to inorganic semiconductors in many fields. Mobility of these conjugated polymers can be increased by adding dopants and also by choosing appropriate metal contacts for charge injection and extraction. The complexity of the metal-polymer interfaces can be better understood by varying the carrier density and studying their transport properties with various experimental tools. Chapter 2: The polymer films prepared in this study are electrochemically deposited on to various conducting substrates. The doping and de-doping of the carriers is done by passing a current and reversing its polarity for different time intervals. Device structures for the measurements are obtained by making a top contact on top of the polymer layer. The current-voltage (I-V) and impedance measurements are carried out in metal-polymer-metal geometry. Temperature dependent studies down to 10 K were performed in a continuous flow cryostat to understand the role of temperature in transport studies. Impedance and light measurements are also carried out in the same geometry. Chapter 3: Transport measurements on polypyrrole devices have shown a space-charge limited (SCLC) conduction, which is also known as bulk property of the materials. I-V curves displayed non-ohmic behaviour at higher voltages and by varying the carrier density the devices show a transition from trap controlled SCLC to trap free/trap filled SCLC. Traps distribution and energies are estimated from the temperature dependent I-V measurements. Poole-Frenkel behaviour, i.e. field dependent mobility is observed in all the polypyrrole samples. The zero field mobility follows Arrhenius behaviour at higher temperatures. Also the temperature variation of mobility displays field dependent and field independent regimes in fully doped and lightly de-doped polypyrrole films. A zero-bias anomaly is observed as the field goes to zero value below 50 K, due to coulomb-blockade transport. Capacitance measurements have shown pseudo inductive behaviour at higher bias, which is also connected with trap-filling regime of PPy devices. Chapter 4: Current-Voltage anomalies are observed in polythiophene (PTh) and poly (3-methylthiophene) [P3MeT] based devices. The origin of this anomaly is not straight forward in polymer devices, so we investigated it in detail. We propose this is a property specific to the above two materials from various experimental studies. The anomalous behaviour appears when the bias is swept from negative to positive keeping the substrate deposited with polymer as anode. The magnitude of peak to valley current ratio (PVCR) which characterises the merit of device can be varied more than two orders of magnitude by varying the carrier density and as well as by varying scan rate. Since the trap states are also one of the reasons for the origin of this anomaly the rate of filling of these states can be helpful in tuning the magnitude of PVCR. Photo generated carriers in the above devices also help in tuning and controlling the magnitude of anomaly, which can make this device as a suitable candidate for opto-electronic studies. Different conductive substrates such as indium tin oxide, platinum, gold and stainless steel are used for deposition of the above polymers. Top contacts (gold, silver and aluminium) have been also varied to understand the origin of this anomaly. Anomalies are observed with all these different substrates and different top contacts. Finally impedance measurements have shown an elongated tail in the Cole-Cole plot in the region of NDR. Chapter 5: Impedance measurements on poly (3-hexylthiophene) devices have shown different relaxation mechanism by varying the doping concentration. For moderately doped devices the relaxation mechanism is classical Debye-type, whereas for highly de-doped samples the relaxation time of the carriers is distributed. Charge transport parameters such as contact resistance, mobility and conduction mechanism details can be obtained by identifying and fitting the data to the equivalent circuit model. The relaxation time of the carriers can give rough estimation of mobility and capacitance. The capacitance variation with applied bias gives the nature of conduction mechanism in the devices. If the capacitance variation is unaffected by the applied bias the transport is bulk limited, if it changes significantly the transport can be considered as either contact limited or depletion layer controlled. Current-Voltage measurements also show that Schottky behaviour is present in all the devices. The rectification ratio varies with doping concentration; at one optimum doping concentration the rectification is very high. I-V measurements on P3MeT devices with varying carrier density have shown a transition in the conduction mechanism from SCLC to contact limited. In the devices with less carrier density the contact limited mechanism is dominating at lower bias voltage and as the bias increases the bulk limited transport takes over. This highlights the role of carrier density in the transport mechanism. Chapter 6: The conclusions from all the works presented in the thesis are summarised in this chapter. Some of the future directions works are presented.

Monitoring the working environment of laboratories and industries for pollutants is of primary concern to ensure the healthiness of working personnel. Semiconducting metal oxides (SMOs) are sensitive to the gas ambience and can be tuned for sensing purpose. As SMOs are not selective, an array of sensors with differential selectivity may resolve to great extent. The objective of the thesis is to understand the physicochemical properties and gas sensing characteristics of Cr1-xFexNbO4. Applying principal component analysis to the sensor response data either for selection of features or for differentiation of analysts is also of concern. Preparation of Cr1-xFexNbO4, phase characterization, lattice parameters estimation, morphological and micro chemical analysis (SEM & EDX), electrical characterization by direct current (DC & AC) in the temperature range of 423 K to 573 K, weighted magnetic moment of iron and chromium deduced from susceptibility measurements, spin nature of iron and surface compositions of different valences of chromium and iron deduced from X-ray photoelectron spectroscopy of are presented. The wide dynamic range hydrogen sensing characteristics of CrNbO4 bulk pellets at different temperatures along with the cross-sensitivity towards NH3, NOx(NO+NO2) and PG (petroleum gas) are investigated. The preparation of Cr1-xFexNbO4 thick and thin films by screen-printing and PLD are also presented. The thick films are tested at different temperatures towards hydrogen. The n-type or p-type nature of thick films towards hydrogen with varying iron concentration in Cr1-xFexNbO4 is reported. The thin films are characterized for phase formation, morphology by XRD, SEM and AFM. XPS performed surface characterization. Electrical resistance measurements at different temperatures and preliminary experiments on hydrogen sensing are presented. The probable hydrogen sensing mechanism of CrNbO4 was revealed by X-ray photoelectron spectroscopy. The experimentally observed reduction in metal ion oxidation states upon interacting with hydrogen is best illustrated by Kröger Vink notation. Principal component analysis was applied for three different types of studies: i) The fit parameters of the transient response of CrNbO4 thick films towards hydrogen are analyzed for finding out the better feature for calibration, ii) Different thick films of CrNbO4, Cr0.5Fe0.5NbO4 and FeNbO4 operated at various temperatures for testing H2 and VOCs are analyzed for redundancy in sensor behaviour and iii) Cr0.8Fe0.2NbO4 thick films are studied for sensing H2, NH3 and their mixtures and usefulness of PCA in resolving them in PC-space. In addition, H2 and VOCs are tested at different temperatures and redundancy in temperature is deduced to construct a sensor array with a minimum number of sensors. Finally, a sensor array consisting of Cr0.8Fe0.2NbO4 thick films, operating at different temperatures is built, and qualitative discrimination of analysts in PC-space is demonstrated. Finally, the major findings of the present investigations and suggestions for future aspects of experimentation are provided

Thesis describes recent advances in the synthesis of donor-acceptor conjugated copolymers and their efficient doping via molecular p-dopants.

Thesis describes recent advances in the synthesis of donor-acceptor conjugated copolymers and their efficient doping via molecular p-dopants.:Chapter I Preface Motivation and Goals Outline 7 Chapter II 8 State of the Art & Characterization Techniques 8 2.1. General Introduction 8 2.1.1. Concept of Conjugated Polymers 9 2.1.2. Electronic Conduction and Necessity of Doping in Conjugated Polymers 11 2.1.3. Solubility and Processing. 14 2.2. Doping 17 2.2.1. Concept of Doping in Conjugated Polymers 17 2.2.2. Morphological Changes of the Material upon Doping. Conductivity. 20 2.2.3. State-of-the-art p-dopants. 23 2.3. Synthetic Strategies for the Design of (Semi)conducting Polymers 28 2.3.1. A Concise Review: from Polyacetylene till Modern DA Polymers 28 2.3.2. Synthetic Routes to Conjugated Polymers 31 2.3.3. Step-growth vs Chain-growth 34 2.3.4. Benchmark solution-processable Polymers 38 2.4. Characterization techniques 41 2.4.1. Conductivity Measurements 41 2.4.2. Electrochemical Voltammetry 42 2.4.3. Uv-vis-near-infrared 44 2.4.4. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. 44 2.4.5. Morphological studies. 45 2.4.6. Electron Spin Resonance Spectroscopy. 46 Chapter III 48 Results & Discussion 48 3.1. Diketopyrrolopyrrole-Based Copolymers 50 3.1.1. Motivation 50 3.1.2. Results and Discussion 51 3.1.4. Summary 89 3.2. Naphthalene Diimide-based Copolymer 90 3.2.1. Motivation 90 3.2.2. Results and Discussion 92 3.2.4. Summary 105 3.3. Isoindigo-Based Copolymers 107 3.3.1. Motivation 107 3.3.2. Results and Discussion 108 3.3.4. Summary 119 Summary & Conclusions 120 Outlook 123 Chapter IV 125 Experimental Part 125 4.1. General Methods and Instrumentation 125 4.2. Synthesis 129 4.2.1. Synthesis of diketopyrrolopyrrole copolymer. 129 4.2.2. Synthesis of electron-conducting polymer (PNDIT2) 132 4.2.3. Synthesis of polyisoIndigo 132 4.2.3. Synthesis of Dopants 135 4.3. Cyclic voltammetry measurements 136 4.4. GIWAX data. 143 4.5. Films preparation 145 References 147 Table of Abbreviations 159 List of Publications 161 Acknowledgements 162 Appendix 163

Les phénomènes de diffusion cohérente non-linéaire ont été récemment proposés en alternatives à la fluorescence comme processus de marquage en microscopie multiphotonique. Les matériaux couramment appliqués dans ce contexte buttent toutefois sur une limite inférieure en taille déterminée par le seuil de détection de signaux faibles en optique non-linéaire. Aucun des efforts récents en détection en génération de second-harmonique (GSH), qui est le processus non-linéaire d’ordre le plus bas, n’a permis de descendre à ce jour au-dessous d’une barrière en taille de 40nm même en ayant recours aux techniques de détection les plus sensibles telles que le comptage de photons uniques. Les nanoparticules (NPs) restent ainsi dans la famille des nano-diffuseurs de “grande“ taille. Il apparaît toutefois possible de déplacer de façon significative cette limite inférieure vers les plus petites tailles en substituant aux isolants diélectriques ou aux semi-conducteurs à grands gaps des particules quantiques (PQs) à base de semi-conducteurs à gaps directs.Dans ce travail, un nouveau type de nanosondes hautement non-linéaires a été conçu et développé de façon à franchir cette barrière de taille minimale pour atteindre l’échelle de nanoparticules uniques. Nous considérons ainsi l’excitation résonnante à deux photons de nanoparticules quantiques individuelles à base de CdTe (de la famille des “zinc-blendes”) d’un diamètre d’environ 12.5nm, qui fournissent une émission cohérente efficace par GSH jusqu’à hauteur de 105 comptages de photons par seconde. Elles présentent de plus l’avantage d’une remarquable sensibilité à l’orientation de leur réseau cristallin octupolaire.De plus, il a été démontré que les effets de confinement quantique déterminent fortement les caractéristiques de la susceptibilité non-linéaire du second-ordre χ(2). La caractérisation quantitative du χ(2) des PQs, en particulier leur dispersion spectrale et leur dépendance en taille est menée par spectroscopie de particules uniques ainsi qu’en moyenne d’ensemble par diffusion Hyper-Rayleigh (HRS). Nous fournissons en particulier la preuve que sous certaines conditions, le χ(2) de structures à base de semi-conducteurs en mode de confinement quantique peut très largement dépasser sa valeur en milieu massif. De plus, un nouveau type de PQs hybridant des semi-conducteurs en géométries de type “bâtonnet sur sphère” (BS) a été développé sur la base de composantes cristallines de symétries différentes, afin d’augmenter leur non-linéarité quadratique effective, tout en maintenant leur taille dans un régime proche d’un fort confinement quantique. Le nouveau tenseur hybride complexe χ(2) est analysé en terme d’interférence des susceptibilités constitutives, en prenant en compte les différentes formes et symétries associées aux composantes octupolaires et dipolaires.Il en résulte pour de telles structures une exaltation significative du χ(2), qui excède celle des PQs à constituant unique compte tenu du couplage entre matériaux non-linéaires et d’un temps de décohérence plus long, que nous attribuons à un effet de separation de charge photo-induit. / Nonlinear coherent scattering phenomena from single nanoparticles have been recently proposed as alternative processes for fluorescence in multiphoton microscopy staining. Commonly applied nanoscale materials, however, have reached a certain limit in size dependent detection efficiency of weak nonlinear optical signals. None of the recent efforts in detection of second-harmonic generation (SHG), the lowest order nonlinear process, have been able to cross a ~40 nm size barrier for nanoparticles (NPs), thus remaining at the level of “large” nanoscatterers, even when resorting to the most sensitive detection techniques such as single-photon counting technology. As we realize now, this size limitation can be significantly lowered when replacing dielectric insulators or wide gap semiconductors by direct-gap semiconducting quantum dots (QDs). Herein, a new type of highly nonlinear nanoprobes is engineered in order to surpass above mentioned size barrier at the single nanoparticle scale. We consider two-photon resonant excitation in individual zinc-blende CdTe QDs of about 12.5 nm diameter, which provide efficient coherent SHG radiation, as high as 105 Hz, furthermore exhibiting remarkable sensitivity to spatial orientation of their octupolar crystalline lattice. Moreover, quantum confinement effects have been found to strongly contribute to the second-order nonlinear optical susceptibility χ(2) features. Quantitative characterization of the χ(2) of QDs by way of their spectral dispersion and size dependence is therefore undertaken by single particle spectroscopy and ensemble Hyper-Rayleigh Scattering (HRS) studies. We prove that under appropriate conditions, χ(2) of quantum confined semiconducting structures can significantly exceed that of bulk. Furthermore, a novel type of semiconducting hybrid rod-on-dot (RD) QDs is developed by building up on crystalline moieties of different symmetries, in order to increase their effective quadratic nonlinearity while maintaining their size close to a strong quantum confinement regime. The new complex hybrid χ(2) tensor is analyzed by interfering the susceptibilities from each component, considering different shape and point group symmetries associated to octupolar and dipolar crystalline structures. Significant SHG enhancement is consequently observed, exceeding that of mono-compound QDs, due to a coupling between two nonlinear materials and slower decoherence, which we attribute to the induced spatial charge separation upon photoexcitation.