Fabrication of high-performance electronic devices using the novel semiconductors is essential for developing future electronics which can be applicable in large-area, flexible and transparent displays, sensors and solar cells. One of the major bottlenecks in the fabrication of high-performance devices is a large interfacial barrier formation at metal/semiconductor interface originated from Schottky barrier and interfacial dipole barrier which causes inefficient charge injection at the interface. Therefore, having a favorable contact at electrode/semiconductor is highly desirable for high-performance devices fabrication. In this dissertation, the fabrication of nanoelectronic devices and investigation of their transport properties using carbon nanotubes (CNTs) and graphene as electrode materials will be shown. I investigated two types of devices using (i) semiconducting CNTs, and (ii) organic semiconductors (OSC). In the first part of this thesis, I will demonstrate the fabrication of high-performance solution-processed highly enriched (99%) semiconducting CNT thin film transistors (s-CNT TFTs) using densely aligned arrays of metallic CNTs (m-CNTs) for source/drain electrodes. From the electronic transport measurements at room temperature, significant improvements of field-effect mobility, on-conductance, transconductance and current on/off ratio for m-CNT/s-CNT devices were found compared to control palladium (Pd contacted s-CNT devices. From the temperature dependent transport investigation, a lower Schottky barrier height for the m-CNT/s-CNT devices was found compared to the devices with control metal electrodes. The enhanced device performance can be attributed to the unique device geometry as well as strong ?- ? interaction at m-CNT/s-CNT interfaces. In addition, I also investigated s-CNT TFTs using reduced graphene oxide (RGO) electrodes. In the second part of my thesis, I will demonstrate high-performance organic field-effect transistors (OFETs) using different types of graphene electrodes. I show that the performance of OFETs with pentacene as OSC and RGO as electrode can be continuously improved by increasing the carbon sp2 fraction of RGO. The carbon sp2 fractions of RGO were varied by controlling the reduction time. When compared to control Pd electrodes, the mobility of the OFETs shows an improvement of ?200% for 61% sp2 fraction RGO, which further improves to ?500% for 80% RGO electrode. Similarly, I show that when the chemical vapor deposition (CVD) graphene film is used as electrodes in fabricating OFET, the better performance is observed in comparison to RGO electrodes. Our study suggests that, in addition to ?-? interaction at graphene/pentacene interface, the tunable electronic properties of graphene as electrode have a significant role in OFETs performance. For a fundamental understanding of the interface, we fabricated short-channel OFETs with sub-100nm channel length using graphene electrode. From the low temperature electronic transport measurements, a lower charge injection barrier was found compared to control metal electrode. The detailed investigations reported in this thesis clearly indicated that the use of CNT and graphene as electrodes can improve the performance of future nanoelectronic devices.

Organic field effect transistors (OFETs) are of significant research interest due to their promising applications in large area, low-cost electronic devices such as flexible displays, sensor arrays, and radio-frequency identification tags. A major bottleneck in fabricating highperformance OFET is the large interfacial barrier between the metal electrodes and organic semiconductors (OSC) which results in an inefficient charge injection. Carbon nanotubes (CNTs) are considered to be a promising electrode material which can address this challenge. In this dissertation, we demonstrate fabrication of high-performance OFETs using aligned array CNT electrodes and investigate the detailed electronic transport properties of the fabricated devices. The OFETs with CNT electrodes show a remarkable enhancement in the device performance such as high mobility, high current on-off ratio, higher cutoff frequency, absence of short channel effect and better charge carrier injection than those OFETs with metal electrodes. From the low temperature transport measurements, we show that the charge injection barrier at CNT/OSC interface is smaller than that of the metal/OSC interface. A transition from direct tunneling to Fowler-Nordheim tunneling observed in CNT/OSC system shows further evidence of low injection barrier. A lower activation energy measured for the OFETs with CNT electrodes gives evidence of lower interfacial trap states. Finally, OFETs are demonstrated by directly growing crystalline organic nanowires on aligned array CNT electrodes. In addition to investigating the interfacial barrier at CNT/OSC interface, we also studied photoconduction mechanism of the CNT and CNT/OSC nanocomposite thin film devices. We found that the photoconduction is due to the exciton dissociations and charge carrier separation caused by a Schottky barrier at the metallic electrode/CNT interface and diffusion of the charge iv carrier through percolating CNT networks. In addition, it is found that photoresponse of the CNT/organic semiconductor can be tuned by changing the weight percentage of CNT into the organic semiconductors.

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Organic electronics have reached the level of commercialization and are important parts of our daily life. They are integrated into portable computers, cell phones, identification cards, television, cars, etc. The organic thin film transistors (OTFTs) are the most attractive organic electronic elements that have applications as electronic flexible paper, sensors, smart cards, erasable memory devices, RF-ID tags, and in backplanes for OLED displays. Their performance has already exceeded the performance of transistors based on the amorphous silicon (α-Si). Organic thin film phototransistors (OPTs) have attracted significant research attention as functional OTFTs due to the unique structure of OTFTs (three-terminal device) complemented with the light (fourth terminal). The OTFTs structure enables modulation and amplification of the output signal (the drain current) while light gives the functionality and enhances the performance. Compared to organic photodiodes, OPTs have higher sensitivity and lower noise due to the OTFT structure. Additionally, the advantage of OPTs over inorganic PTs lays in a variety of light responsive organic materials that can be used as active channel materials. Accordingly, use of organic compounds enabled OPTs fabrication from solution, melt, and printing, over large areas of plastic substrates with which they are compatible. So far, many researchers have reported high-performance OPTs. Typically, synthesis of the new light receiving/emitting semiconducting materials is the common approach for the OPT development. Another way is to engineer the device structure by introducing new layers with different functionalities. Often, synthesis is costly, complex, lengthy, and not industrially feasible. This thesis focuses on the development of new methods and materials for OPT performance enhancement to avoid lengthy synthesis and fabrication processes. According to the layers in a typical OPT, that is, from the top to the bottom: active channel, channel/gate dielectric interface, and the gate dielectric layer, the thesis has three major focuses: engineering of the active channel for high-performance OPTs using existing small molecule and existing or new dielectric polymeric materials (Chapters 3-5), interface engineering (Chapter 6), and engineering of the gate dielectric layer (Chapter 7). Utilizing blends of a UV-A responsive 2,7-dipentyl[1]benzothieno[3,2-b][1]benzothiophene (C5-BTBT) small molecule semiconductor and various dielectric polymers (polyesters, PMMA, PVAc, PS, and PC) we developed highly photoresponsive and photosensitive OPTs. Furthermore, we designed and synthesized a new polyimide that is soluble, thermally stable, with reduced deep coloration and more importantly with the strong electron withdrawing groups. High-performance and highly photosensitive OPTs were achieved with capabilities of the application as photo memory elements characteristic of fast switching and long retention times of the persistent photocurrent. We demonstrated that by simple channel/dielectric interface modification using organosilanes with different end groups, the drain photocurrent, and hole mobility could be modulated and enhanced under the UV light illumination. In the final part, we demonstrated that both active channel and dielectric layer engineering could synergistically enhance the performance of OPTs for potential fabrication as photo memory elements. This thesis contributed significantly to fundamental knowledge of photoresponsive organic electronic devices and application of OPTs in the area of printed and flexible electronics / Thesis / Doctor of Philosophy (PhD) / Organic electronics have become a part of our daily life since they are integrated into cell phones, computers, TVs, displays, etc. Their advantage is their versatility due to a variety of organic compounds that can be used as semiconductors for the specific applications, low-cost processing methods (solution, printing, and melt) and large-area flexible substrates that can be used for their fabrication. For the same reasons, organic phototransistors are very attractive for modern optoelectronics. Generally, in this study, we developed and demonstrated new strategies of developing an organic phototransistor and enhancing/optimizing its performance. Firstly, we developed semiconducting blends that are responsive to UV-A light when integrated into an organic phototransistor. Secondly, by channel/gate dielectric interface manipulation we demonstrated control over photoelectrical properties of the organic phototransistor and discovered mechanisms of the enhancement. Thirdly, we optimized and developed reliable, high-performance, and highly UV responsive organic phototransistors with potential application as a photo memory element

Polycyclic aromatic hydrocarbons composed of benzenoid rings fused in a linear fashion comprise the class of compounds known as acenes. The structures containing three to six ring fusions are brightly colored and possess band gaps and charge transport efficiencies sufficient for semiconductor applications. These molecules have been investigated throughout the past several decades to assess their optoelectronic properties. The absorption, emission and charge transport properties of this series of molecules has been studied extensively to elucidate structure-property relationships. A wide variety of analogous molecules, incorporating heterocycles in place of benzenoid rings, demonstrate similar properties to the parent compounds and have likewise been investigated. Functionalization of acene compounds by placement of groups around the molecule affects the way in which molecules interact in the solid state, in addition to the energetics of the molecule. The use of electron donating or electron withdrawing groups affects the frontier molecular orbitals and thus affects the optical and electronic gaps of the molecules. The use of bulky side groups such as alkylsilylethynyl groups allows for crystal engineering of molecular aggregates, and changing the volume and dimensions of the alkylsilyl groups affects the intermolecular interactions and thus changes the packing motif. In chapter 2, a series of tetracene and pentacene molecules with strongly electron withdrawing groups is described. The investigation focuses on the change in energetics of the frontier molecular orbitals between the base acene and the nitrile and dicyanovinyl derivatives as well as the differences between the pentacene and tetracene molecules. The differences in close packing motifs through use of bulky alkylsilylethynyl groups is also discussed in relation to electron acceptor material design and bulk heterojunction organic photovoltaic characteristics. Chapter 3 focuses on molecular acceptor and donor molecules for bulk heterojunction organic photovoltaics based on anthrathiophene and benzo[1,2-b:4,5-b’]dithiophene central units like literature molecules containing fluorene and dithieno[2,3-b:2’,3’-d]silole cores. The synthetic strategies of developing reduced symmetry benzo[1,2-b:4,5-b’]dithiophene to study the effect of substitution around the central unit is also described. The optical and electronic properties of the donors and acceptors are described along with the performance and characteristics of devices employing these molecules. The final two data chapters focus on new nitrogen containing polycyclic hydrocarbons containing indolizine and (2.2.2) cyclazine units. The optical, electronic and other physical properties of these molecules are explored, in addition to the synthetic strategies for incorporating the indolizine and cyclazine units. By use of alkylsilylethynyl groups, crystal engineering was investigated for the benzo[2,3-b:5,6-b’]diindolizine chromophore described in chapter 4 to target the 2-D “brick-work” packing motif for application in field effect transistor devices. Optical and electronic properties of the cyclazine end-capped acene molecules described in chapter 5 were investigated and described in relation to the base acene molecules. In both cases, density functional theory calculations were conducted to better understand unexpected optical properties of these molecules, which are like the linear acene series despite the non-linear attachment.

A process has been developed to make the dielectric layer for organic thin-film transistors (OTFTs) in a roll-to-roll vacuum web coater environment. This dielectric layer combined with an organic semiconductor layer and metal layer deposited in vacuum allows a solvent-free process to make organic/inorganic multilayer structures for thin-film electronic devices on a flexible substrate at, potentially, high speed. The polymeric gate dielectric layers were fabricated by flash evaporation of acrylic monomers onto a polymer film with pre-patterned metal gates followed by radiation curing by electron beam, ultra-violent light (UV) or plasma. With a non-polar dielectric surface, charge carrier mobility (μ) of 1 cm²-V^-1s^-1; on/off curren ratio of 10⁸, sub-threshold swing (SS) of 0.3 V/decade and saturated output curve were routinely achieved in dinaphtho-[2,3-b:2'3'-f]thieno[3,2-b]thiophene (DNTT) transistors with dielectric layer of tripropylene glycol diacrylate (TPGDA) of ~400 nm. Apart from the TPGDA, monomer formulas including 1,6-Hexanediol diacrylate (HDDA) as well as several commercial acrylic resins have been used to make the dielectric layer. The highest areal capacitance of 41nF-cm^-2 was achieved with a pin-hole free film of less than 100 nm made of an acrylate mixture resin. A non-polar dielectric surface treatment layer has been developed based on flash evaporation of lauryl acrylate and HDDA mixture. The transistors with the buffer layer showed constant performance and a mobility fivefold greater than those of untreated samples. The effect of humidity, oxygen, and light during switching cycles of both pentacene and DNTT transistors were studied. Water and oxygen/illumination had a distinct effect on both pentacene and DNTT transistors. Oxygen leads to acceptor-like charge traps under illumination, which shifted the turn-on voltage (V_to) to more positive values. In contrast, water in transistors gave rise to donor-like charge traps, which shifted the V_to and the threshold voltage (V_T) more negatively. The DNTT devices showed good stability in dry air without encapsulation, while pentacene transistors degraded with either repeating measurement or long term storage. A DNTT transistor with a PS-coated TPGDA dielectric layer showed stable drain current (I_d) of ~105A under bias stress of the gate voltage (em>V_g) of -20V and the drain voltage (em>V_d) of -20V for at least 144 hours. The V_to shift after the stress was less than 5 V and was recoverable when the device was kept in dry air for a few days. Possible reasons for the V_to shift have been discussed.

Le développement des transistors organiques, ces dernières années, a permis une nette amélioration de leurs performances et de leur stabilité. Ceci a été possible, notamment, grâce à une meilleure compréhension des mécanismes régissant le transport de charges dans ces dispositifs. Cependant, certains phénomènes restent encore à éclaircir, en particulier au niveau de l’interface entre le semi-conducteur et le diélectrique. Le piégeage des porteurs de charges qui est une des principales causes de perturbations du transport de charges dans les transistors organiques, en est un. Cette thèse se propose donc, d’étudier ce phénomène dans des transistors à base de pentacène.Les groupements polaires, et plus particulièrement les groupements hydroxyles, présents à l’interface entre l’isolant et le semi-conducteur, sont les principaux responsables du piégeage des porteurs de charges dans les transistors organiques. Afin de limiter leur présence, une technologie basée sur l’emploi d’une couche interfaciale diélectrique passivante, pauvre en groupements hydroxyles, à base de fluorure de calcium, a été mise en place. L’influence de cette couche sur le comportement de transistors à base de pentacène a été étudiée, de même que le vieillissement de ces dispositifs sous différentes conditions de stockage (sous vide et à l’air) et sous contrainte électrique.Ainsi, il a été mis en évidence qu’une couche de fluorure de calcium d’une épaisseur trop importante (de l’ordre de 5 nm) modifie la morphologie de la couche de pentacène, ce qui se traduit par une quasi-disparition du transport de charges dans le pentacène en configuration de transistor à effet de champ. Les études de vieillissement ont montré que sous l’effet de la couche interfaciale de CaF2, même d’une très fine épaisseur (de quelques nanomètres), une quantité plus importante d’humidité est présente dans la couche de pentacène, probablement à cause de la nature hygroscopique du fluorure de calcium. / These recent years, Organic Field-Effect Transistor (OFET) development has significantly improved it performances and it stability. This was made possible, through a better understanding of the mechanisms governing charge transport in these devices. However, some phenomena remain unclear, in particular, at the interface between the semiconductor and the dielectric. Charge carrier trapping which is one of the main causes of charge transport disturbance in organic transistors, is one of them. So, this work aims to investigate such phenomena in pentacene-based transistors.Polar groups and particularly, hydroxyl groups, located at the insulator-semiconductor interface, are the main sources of charge carriers trapping in OFET. To prevent their presence, an OFET fabrication technology based on a passivating dielectric, poor of hydroxyl groups, calcium fluoride-based interfacial layer has been developed. Effect of this layer on pentacene-based transistors operation has been studied, as well as these devices aging under different storage atmosphere (in vacuum and in air) and under electrical stress.Thus, it has been highlighted that an interfacial layer of calcium fluoride with a too high thickness (around 5 nm) changes pentacene layer morphology which results in a quasi-disappearance of charge transport in pentacene in OFET configuration. Aging studies showed that under the effect of CaF2 interfacial layer, even with a very thin thickness (a few nanometers), a greater quantity of moisture is induced in pentacene layer probably due to the hygroscopic nature of calcium fluoride.

[pt] Neste trabalho são apresentados os resultados da produção e caracterização de novos dispositivos orgânicos eletroluminescentes (OLEDs), que podem ser divididos idealmente em três grupos. O primeiro composto pelos OLEDs que utilizam os distirilbenzenos (DSBs) como camada eletroluminescente. Os DSBs são sistemas utilizados em química supramolecular como pontes para transferência de carga. No segundo colocamos os OLEDs baseados em 2; 6 - bis(dietanolamina) - 4; 8 - dipiperidinopirimida (5; 4-d) pirimidina (DIP), que é uma droga amplamente conhecida e utilizada no tratamento de doenças cardiovasculares, que por possuir uma intensa fluorescência despertou a curiosidade para investigarmos a possibilidade da sua utilização com camada eletroluminescente. A utilização do DIP como camada eletroluminescente resultou em um OLED com a maior luminância dos grupos estudados, 1500 cd/m2. O terceiro grupo é formado por OLEDs que utilizam moléculas de calixarenos ([Al . 1]3+ e [Zn . 1]2+ onde 1 = 8- oxiquinolinacalix[4]areno) como camada transportadora de elétrons e eletroluminescente. Foram estudados dois sistemas calixarenos, o primeiro sistema é coordenado com alumínio (calix[Al]3+) e o segundo coordenado com zinco (calix[Zn]2+). O sistema calix[Al]3+ apresentou uma interessante propriedade de cooperação entre o anel central calix e um grupamento quinolina coordenado com Al. Utilizando este mecanismo de sinergia produzimos um OLED sintonizável, o qual varia o pico de sua banda de eletroluminescência de 510 nm pra 470 nm em função da tensão aplicada. Desta forma a cor da luz emitida pelo dispositivo pode ser variada com continuidade do azul (X=0,26 : Y=0,33 coordenadas CIE) até o verde claro (X=0,18 : Y=0,25).Todos os sistemas orgânicos foram analisados de um ponto de vista térmico através da analise de calorimetria diferencial de varredura (DSC), com a qual determinamos sua temperatura de transição vítrea (Tg). Com o auxilio de medidas eletroquímicas, estabelecemos um procedimento experimental simples e rápido para determinar os níveis energéticos HOMO (highest occupied molecular orbital) dos materiais orgânicos estudados na forma de filmes. Enfim, através da caracterização elétrica foi possível determinar o tipo de injeção de portadores que governa os dispositivos bem como o modelo que controla o transporte de cargas no interior dos OLEDs. / [en] In this work the results of the production and characterization of new organic electroluminescent devices (OLEDs) are presented and discussed. The work can be ideally divided in three parts. The first is composed by the OLEDs that use the distyrilbenzene (DSBs) compounds as electroluminescent layer. The DSBs are systems used in supramolecular Chemistry as bridges for load transfer. This is the first time that they are used in small molecule OLEDs. The second part deals with OLEDs based on dipyridamole (2; 6-bis(diethanolamino) - 4; 8 - dipiperidinopyrimido (5; 4-d)pyrimidine) (DIP) molecule, which is a very well known anti-platelet drug thoroughly used in the treatment of hearth diseases that for possessing an intense fluorescence woke up our curiosity in order to investigate the possibility of its use as electroluminescent layer. The use of DIP as electroluminescent layer resulted in a particular OLED with the highest luminance of the investigated devices, about 1500 cd/m2. Finally, the third part involves OLEDs that use calixarenes molecules [Al . 1]3+ and [Zn . 1] 2+ (1 = 8- oxyquinolinecalix[4]arene) as electroluminescent and electron transporting layer. In this work were studied two calixarenes systems: the first system is coordinated with aluminum (calix[Al]3+) and the second one is coordinated with zinc (calix[Zn]2+). The first one presented an interesting cooperation property among the central calix ring and the quinolina group coordinated with the Al metal. Using this synergic mechanism we were able to produce a tunable OLED, which electroluminescent emission varies continuously from 510 nm to 446 nm as a function of the applied bias voltage. All the organic compounds used in this work were analyzed from a thermal point of view through a Differential Scanning Calorimetry (DSC), in order to determine their transition glass temperature (Tg). By using electrochemistry measurements it was possible to establish a systematic and simple experimental procedure to determine the HOMO (highest occupied molecular orbital) energy levels of the organic materials studied in the form of films. Finally, through the electrical characterization it was possible to determine the type of carrier injection that governs the fabricated devices as well as the model that controls the carrier transport inside the OLEDs.

It has been found that non-magnetic organic semiconductors can show some magnetic responses in low magnetic field (<100 >mT). When applying magnetic field, the electroluminescence, electrical current, photocurrent, and photoluminescence could change with magnetic field, which are called magnetic field effects. Magnetic field effects are generated through spin-dependent process affected by the internal magnetic interaction. In nonmagnetic materials, hyperfine interaction has been supposed to dominantly affect the spin-dependent process recently. But the conclusion was made in weak spin-orbital coupling organic semiconductor. The hyperfine interaction might not be the main reason responsible for magnetic field effects in strong spin-orbital coupling materials. Therefore, the study of magnetic field effects in strong spin-orbital coupling organic semiconductor is important to get a whole view of the origin of the magnetic field effects in nonmagnetic organic semiconductors. This dissertation will clarify the generation mechanism of magnetic field effect in nonmagnetic organic semiconductors and further explore how the strong spin-orbital coupling affecting the magnetic field effect. It has been found the intermolecular excited states are important inter-median for magnetic field effects. The change of intersystem crossing at intermolecular excited states will change the singlet/triplet ratio and further generate magnetic field effects through different recombination and dissociation properties of singlet and triplet intermolecular excited states. Both the energy transfer effect coupled spin orbital coupling and energy transfer effect free spin orbital-coupling are discussed in the dissertation. The tuning of the magnetic field effect by adjusting the spin-orbital coupling is also established through distance effect and interface effect. It has been found that changing inter-molecular spin-orbital coupling is a critical factor to generate magnetic field effects in organic semiconductors. And the sensitivity of different magnetic field effects to strong spin-orbital coupling strength is depending on the final product. The internal magnetic interaction can be hyperfine interaction, spin orbital coupling and spin-spin interaction between electrons. The hyperfine interaction and spin orbital coupling are important in nonmagnetic organic semiconductors. But the electron spin-spin interaction is important in magnetic organic semiconductors. The magnetocurrent for magnetic and nonmagnetic organic semiconductors at different temperature has been compared.