In this research, the contents are divided into two sections. In the first section, we investigated carrier transport behavior of organic optoelectronic devices by using space charge limited current(SCLC) method. Firstly, we fabricated a hole-only device (ITO/Spiro-MeOTAD/Al) for Sprio-MeOTAD and the current density¡V voltage(J-V) characteristics of the device was measured. The J-V characteristics of the device do not match with SCLCs very well at high voltage since the number of hole injection was not enough to achieve SCLCs condition. To enhance the injection of hole carrier into the organic layer, a MoO3 buffer layer was inserted between ITO electrode and organic layer. The current density in device with MoO3 buffer layer achieved 5 times enhancement, indicating that the concentration of hole in MoO3 device is increment. Hence, we succeeded in making the J-V characteristics of the hole-only device to match with SCLCs well at high voltage, and the hole mobility of Sprio-MeOTAD estimated by SCLCs was 1.44¡Ñ10-4cm2/Vs. Li salt was also doped into Sprio-MeOTAD as an n-type dopant. We found that Li salt could form hole-traps in Sprio-MeOTAD, which reduced hole carriers in Spiro-MeOTAD. The current density of the device was decreased, and the device could not achieve SCLCs condition at high voltage. In the second section, we use two novel iridium(Ir) complexes to fabricate blue-green emitting devices by solution process. First, we obtained optimum concentration of phosphorescent emitters by controlling of the dopants concentration. Then, we adjusted the thickness of the electron injection layer, hole injection layer, and emission layer to design more suitable device structure. Finally, we succeeded in fabricating blue-green light emitting devices. The maxima luminescence was 37.7cd/m2 and maxima current efficiency was 1.68 cd/A in the Ir complex based devices.

Several results were reviewed on the charge injection into thin conjugated polymer films. The space charge limited tunneling current (SCTC) and space-charge limited current (SCLC) models were used to explain the charge injection and transport in conjugated polymer films such as those used in organic light emitting diodes. On the basis of regional approximation, the effect of the space charge on the current density of electrons tunneling from metal electrodes to the lowest unoccupied molecular orbital of a polymer film is calculated. The space charge is considered to decrease with increasing distance of the injecting electrode. If the space charge occupies only a limited region between the tunneling (short) distance and the collecting electrode, the current (I) is found to be independent on a power law of the applied voltage (V). However, if the space charge occupies all the region between the tunneling distance and the collecting electrode, I is found to vary exponentially on V for lower V values, and that follow approximately the law of Child at high V in region I, the law of Trap filled limited current at middle voltage in region II and the region III show Ohm¡¦s law at lower voltage. The space charge limited tunneling current (SCTC) theory gives the same results of the space charge limited conduction theory when the energy barrier for charge carrier injection is small or when the polymer layer is thick. Under various bias voltages, the charge transport mechanism was analyzed to investigate the effects on the thin conjugated polymer films induced by the parameters such as background concentration, trap-state density and space charge etc... The results show the transport current decrease with increasing the trap-state density and also increase the characteristic temperature parameters. In this study, the regional approximation is applied to calculate the charge injection and current transport mechanism in the organic thin film. The SCTC and SCLC models are compared to the experimental data. It shows that there is a good agreement between theory and experiment, concerning both current magnitude and current versus voltage dependence. It is very important that our model gave a new way to simplify the calculation and to discuss the charge transport of organic light emitting diode.

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La thèse porte sur les mesures de courants transitoires limités par la charge d'espace dans des films minces organiques (épaisseur < 500 nm). Ce type de films est souvent utilisé dans des applications dans le domaine de l'électronique organique comme couches actives semi-conductrices. Le transport électrique dans ces films dépend en premier lieu du transport des porteurs de charge dans le milieu massif et de leur piégeage, mais aussi de l'efficacité de l'injection des porteurs de charges à partir des électrodes métalliques. L'ensemble est de plus conditionné par le taux de désordre inhérent aux matériaux organiques. L'approche qui consiste à utiliser la mesure de courants transitoires est extrêmement attractive car elle permet en principe de fournir une information sur tous ces aspects à l'issue d'un seul type de mesure. Dans ce cadre, trois contributions principales peuvent être dégagées de la thèse. 1) Tout d'abord, nous avons validé un montage expérimental qui utilise un amplificateur à transfert d'impédance pour la mesure des courants transitoires limités par la charge d'espace. Ce type de montage s'avère supérieur au circuit de pont électrique le plus largement utilisé jusqu'à maintenant car il présente une meilleure sensibilité en courant, une meilleure bande passante, et ne nécessite aucun réglage ni de la symétrie du pont ni de l'ajustement de la taille de l'échantillon. On a pu démontrer que le pic de courant de déplacement initial, qui sature l'amplificateur au tout début de la mesure n'introduit pas d'erreur dans la mesure de la mobilité. 2) Ensuite concernant l'étude plus spécifique du transport dans un dendrimère à base de tri-arylamine, les réponse en courant obtenues expérimentalement se sont avérées en bon accord avec le modèle de déplacement-diffusion. Cependant, la troisième leçon que nous avons apprise est que l'obtention d'un tel accord a nécessité que soient très bien définies les conditions initiales tant de l'expérience que de la simulation et qu'un modèle théorique le plus complet possible de l'échantillon soit considéré. Pour le dendrimère ce modèle a dû prendre en compte l'effet de la barrière au contact et les effets de piégeage. Un accord encore meilleur a été obtenu en intégrant de surcroit les effets de désordre. 3) La complète impossibilité d'obtenir un bon accord sans un modèle physique complet de l'échantillon indique que les paramètres liés au piégeage, à la barrière au contact et à la mobilité peuvent véritablement être ajustés sans aucune ambigüité. Ainsi, une caractérisation électrique complète en cohérence avec la simulation a pu être obtenue à l'issue d'un seul type de mesures. Les résultats obtenus, alliant à la fois amélioration technique et support numérique, témoignent de la grande utilité de cette technique de mesure de courant transitoire limité par la charge d'espace pour caractériser en détails le transport dans les films minces organiques. / The thesis concerns space-charge-limited transient current measurements in thin (le500 nm) organic films. Such films find important applications in organic electronics, where they are referred to as organic semiconductor layers. Electrical transport in such films depends on bulk charge carrier transport and trapping, as well efficiency of charge carrier injection from electrodes. These, are all in turn depend on disorder inherent to organic materials. The transient measurement approach is very attractive, as it can, in principle, deliver information on all these aspects in one single measurement. In the thesis, three main contributions are presented. 1) A transimpedance amplifier based setup for space-charge-limited current transient measurement is validated. This type of setup is superior to the widely used bridge circuit, notably because of better current sensitivity, bandwidth, no need for bridge symmetry and no need for per sample adjustment. It is demonstrated that initial displacement current spike, which saturates the amplifier at the beginning of measurement, does not introduce error in the measurement of mobility. 2) A dendrimer molecule has been investigated. Experimental current responses are shown to be in agreement with the drift-diffusion model. However, obtaining agreement requires well defined initial conditions in experiment as well as in simulation, and also complete theoretical model of the sample. In the case of dendrimer, this model had to take into account both contact barrier and trapping effects. Furthermore, better agreement was obtained when taking disorder effects into account. 3) The impossibility of obtaining any agreement without complete physical model of the sample indicates that trapping, contact barrier and mobility parameters could be fitted without ambiguity. Therefore, complete electrical characterization consistent with simulation can be obtained using the transient technique. The results obtained further increase well known usefulness of transient space-charge-limited current characterization of thin organic films.

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) constitute the central theme of this thesis. Within this theme, three primary areas of focus are pursued. The first focus is the realization of a transparent three-stage ring oscillator with buffered output and an output frequency in the megahertz range. This leads to the possibility of transparent radio frequency applications, such as transparent RFID tags. At the time of its fabrication, this ring oscillator was the fastest oxide electronics ring oscillator reported, with an output frequency of 2.16 MHz, and a time delay per stage of 77 ns. The second focus is to ascertain whether a three-terminal device (i.e., a TFT) is an appropriate structure for conducting space-charge-limited-current (SCLC) measurements. It is found that it is not appropriate to use a diode-tied or gate-biased TFT configuration for conducting a SCLC assessment since square-law theory shows that transistor action alone gives rise to I proportional to V² characteristics, which can easily be mistakenly attributed to a SCLC mechanism. Instead, a floating gate TFT configuration is recommended for accomplishing SCLC assessment of AOS channel layers. The final focus of this work is to describe an assessment procedure appropriate for determining if a dielectric is suitable for use as a TFT gate insulator. This is accomplished by examining the shape of a MIM capacitor's log(J)-ξ curve, where J is the measured current density and ξ is the applied electric field. An appropriate dielectric for use as a TFT gate insulator will have a log(J)-ξ curve that expresses a clear breakover knee, indicating a high-field conduction mechanism dominated by Fowler-Nordheim tunneling. Such a dielectric produces a TFT with a minimal gate leakage which does not track with the drain current in a log(I[subscript D])-V[subscript GS] transfer curve. An inappropriate dielectric for use as a TFT gate insulator will have a log(J)-ξ curve that does not express a clear breakover knee, indicating that the dominate conduction mechanism is defect driven (i.e., pin-hole like shunt paths) and, therefore, the dielectric is leaky. It is shown that experimental log(J)-ξ leakage curves can be accurately simulated using Ohmic, space-charge-limited current (SCLC), and Fowler-Nordheim tunneling conduction mechanisms. / Graduation date: 2010

La conception de cellules solaires organiques de type hétérojonction en volume a été proposée pour la première fois en 1990. Ces dispositifs sont composés d’un mélange de polymères conjugués, donneurs d’électrons, et de fullerènes, accepteur d’électrons, et ont pour la première fois permis d’atteindre un rendement de conversion énergétique significatif (de l’ordre de 2%) avec des semi-conducteurs organiques. Dans ce contexte, cette thèse a porté sur l'étude approfondie d’une série de molécules donneurs d’électrons de forme d’haltère, dont le groupement planaire est l’unité triazatruxène (TAT) et le cœur déficient en électrons le thienopyrroledione (TPD). Les molécules de cette série se différencient par la nature des chaînes alkyles, attachées à l’unité centrale et aux unités TAT. Plus précisément, la relation entre la nature des chaînes latérales et les propriétés moléculaires et thermiques de ces molécules en forme d’haltère ont été étudiées en détail. L'impact des chaînes alkyles sur la morphologie en film mince à l’échelle nanométrique a également été étudié. Afin de mieux comprendre l’influence de la microstructure des films minces (constitués soit uniquement des molécules donneuses soit de mélanges molécules/fullerènes), le transport de charge dans le plan du film et perpendiculairement au plan ont été mesurées en fonction de la phase (amorphe, cristalline, …) du matériau. Des cellules solaires BHJ en mélange avec le dérivé de fullerène ont également été réalisées. / The prospective conception of electron-donor/electron-acceptor (D/A) bulk heterojunction solar cells was first reported in 1990s, which blended the semiconducting polymer with fullerene derivatives, enhancing the power conversion efficiency. Since then, interests on this domain has been increasing continuously, and the efficiencies of BHJ solar cells have been increased dramatically. In this context, this thesis focuses on the study of a series of dumbbell-shaped small molecule donors, based on a highly planar unit called triazatruxene. The only difference between those molecules is the side-chains attached to central units and TAT units. As a consequence, the relationship between side chains nature and optoelectronic and structural properties of our TAT-based dumbbell-shaped molecular architecture will be investigated in detail. The impact of the alkyl chains on the molecular and thin film properties was also studied, with a particular emphasis put on microstructure and charge transport aspects. In-plane and out-of-plane charge carrier transport, with pure molecules and blend with fullerene, are measured in different systems. BHJ solar cells in blend with fullerene derivatives were also realized.

La structure chimique des semiconducteurs organiques utilisés dans les cellules photovoltaïques à base d’hétérojonction en volume peut fortement influencer les performances du dispositif final. Pour cette raison, une meilleure compréhension des relations structure-propriétés demeure cruciale pour l’amélioration des performances. Dans ce contexte, cette thèse fait état d'études approfondies du transport des charges, de la morphologie et des propriétés photovoltaïques sur de nouveaux copolymères à faible bande interdite. En premier lieu, l'impact de la position des chaînes alkyles sur les propriétés opto-électroniques et morphologiques a été étudié sur une famille de polymères. Les mesures du transport de charges ont montré que la planéité du squelette du copolymère influe sur l’évolution de la mobilité des charges avec la concentration de porteurs libres. Ce comportement suggère que le désordre énergétique électronique est fortement impacté par les angles de torsion intramoléculaire le long de la chaîne conjuguée. Un second copolymère à base d'unités accepteur de [2,1,3] thiadiazole pyridique, dont les niveaux d’énergie des orbitales frontières sont optimales pour l’application photovoltaïque, a ensuite été étudié. Les performances obtenues en cellule photovoltaïque sont très inférieures aux attentes. Des analyses de la morphologie et du transport de charge ont révélé que l’orientation des lamelles cristallines est défavorable au transport perpendiculaire au film organique et empêche ainsi une bonne extraction des charges photo-générées. Enfin, les propriétés opto-électroniques et photovoltaïques de copolymères fluorés ont été étudiées. Dans ce cas, les atomes de fluor favorisent la formation de lamelles orientées favorablement pour le transport. Ces bonnes propriétés nous ont permis d'atteindre un rendement de conversion de puissance de 9,8% avec une simple hétérojonction polymère:fullerène. / The chemical structure of organic semiconductors that are utilized in bulk heterojunction photovoltaic cells may strongly influence the final device performances. Thus, better understanding the structure-property relationships still remains a major task towards high efficiency. Within this framework, this thesis reports in-depth material investigations including charge transport, morphology and photovoltaic studies on various novel low band-gap copolymers. First, the impact of alkyl side chains on the opto-electronic and morphological properties has been studied on a series of polymers. Detailed charge transport investigations showed that a planar conjugated polymer backbone leads to a weak dependence of the charge carrier mobility on the carrier concentration. This observation points out that the intra-molecular torsion angle contributes significantly to the electronic energy disorder. Solar cells using another novel copolymer based on pyridal[2,1,3]thiadiazole acceptor unit have been studied in detail next. Despite the almost ideal frontier molecular orbital energy levels, this copolymer did not perform in solar cells as good as expected. A combined investigation of the thin film microstructure and transport properties showed that the polymers self-assemble into a lamellar structure with polymer chains being oriented preferentially “edge-on”, thus hindering the out-of-plane hole transport and leading to poor charge extraction. Finally, the impact of fluorine atoms in fluorinated polymers on the opto-electronic and photovoltaic properties has been investigated. In this case, the presence of both flat-lying and standing lamellae enabled efficient charge transport in all three directions. As a consequence, good charge extraction was possible and allowed us to achieve a maximum power conversion efficiency of 9.8%.

The thesis is focused on the electrodeposition of tantalum from ionic solution 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl) imide, ([BMP]Tf2N) at 200°C and description of related transfer and kinetic phenomena on the interface between the ionic solution and working electrode. Furthermore, the electrodeposition of tantalum layers in different ionic solutions containing tantalum salts and especially in ([BMP]Tf2N) is introduced. In the last part of the thesis, results of cyclic voltammetry, surface analysis SEM and elemental analysis EDX are discussed. MIS (metal-insulator-semiconductor) structure was determined comprising porous anodic alumina as the insulator and semiconductive TaxOy on the top of the insulator. The electric conductivity of the MIS structures was studied by using amper-volt IV and CV characteristics. The results of the cyclic voltametry, impedance spectroscopy and chronoamperometry are discussed.

<p dir="ltr">High-power microwave (HPM) devices are generally vacuum-based devices that transform electron beam energy into microwaves with peak powers above 100 MW from 1-300 GHz. Solid-state HPM devices provide more compactness and greater reliability while consuming less power. Nonlinear transmission lines (NLTLs) provide a solid-state alternative to HPM generation by sharpening the input pulses from a pulse forming network to create output oscillations.</p><p dir="ltr">The first section of this dissertation evaluates and explores the feasibility of using nonlinear composites containing ferroelectric (e.g., Ba_2/3Sr_1/3TiO₃, BST) and/or ferromagnetic (e.g., Ni_1/2Zn_1/2Fe₂O₄, NZF) inclusions in a linear polymer host (polydimethylsiloxane, PDMS) to tune NLTL properties for HPM applications. Appropriately modelling and designing NLTLs using nonlinear composites require accurately characterizing their linear and nonlinear electromagnetic properties. We first studied the electromagnetic properties of the composites using theoretical, numerical, and experimental approaches. Incorporating these composite models and characterizations into NLTL simulations will be discussed.</p><p dir="ltr">Vacuum-based HPM devices, such as magnetrons and crossed-field amplifiers, generally operate in the space-charge-limited region, which corresponds to the maximum current possible for insertion into the device. This motivated studying the space-charge-limited current and electron flow in a two-dimensional (2D) planar diode with various crossed-magnetic fields using particle-in-cell (PIC) simulations. For non-magnetically insulated diodes (electrons emitted from the cathode can reach the anode), analytical and/or semi-empirical solutions are derived for electrons with nonzero monoenergetic initial velocity that agree well with PIC simulations. For magnetically insulated conditions, we developed new metrics using simulations and analytic theories to assess electron cycloidal and Brillouin flow to understand the implications of increasing injection current for 2D diodes. These analyses provide details on the operation of these devices at high currents, particularly virtual cathode operation, that may elucidate behavior near their limits of operation.</p>