As an important type of organic semiconductors, organic small molecule crystals have great potential for low-cost applications such as plastic solar cells (PSC), organic light emitting diodes (OLED) and organic field-effect transistors (OFET). Among numerous molecular crystals, 6, 13-Bis(triisopropylsilylethynyl)pentacene (Tips-pentacene) has aroused much attention because it combines good solubility in common solvents and strong π-π stacking from self-assembly. However, the inability to achieve ordered array of Tips-pentacene prevents the fabrication of high-performance organic integrated circuits. In this work, two new fabrication methods to pattern Tips-pentacene micro- and nano-scale single crystals are proposed. Both methods are facilitated by nanofabrication techniques such as nanoimprint and photolithography. In the first method, the surface of a silicon substrate is treated by surfactant coating and Tips-pentacene single crystals are deposited in squared patterns. In the second method, we made an ordered array of Tips-pentacene single crystals confined in Teflon-AF patterns. In both techniques, the effects of solvent type, processing temperature and template pattern size on crystal morphology and size are systematically studied.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1970. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliography.

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In this thesis, I utilize first principles density functional theory (DFT) based calculations to investigate the structure and electronic properties including charge transfer behaviors and work function of two types of materials: pentacene based organic semiconductors and ZnO transparent conducting oxides, with an aim to search for high mobility n-type organic semiconductors and fine tuning work functions of ZnO through surface modifications. Based on DFT calculations of numerous structure combinations, I proposed a pentacene and perfluoro-pentacene alternating hybrid structures as a new type of n-type semiconductor. Based on the DFT calculations and Marcus charge transfer theory analysis, the new structure has high charge mobility and can be a promising new n-type organic semiconductor material. DFT calculations have been used to systematically investigate the effect of surface organic absorbate and surface defects on the work function of ZnO. It was found that increasing surface coverage of organic groups and decreasing surface defects lead to decrease of work functions, in excellent agreement with experimental results. First principles based calculations thus can greatly contribute to the investigating and designing of new electronic materials.

Singlet fission is a multiple-exciton-generation process found in organic materials that could help to enhance the efficiency of future photovoltaic devices, by overcoming the Shockley-Queisser limit. In spite of considerable experimental and theoretical attention, different aspects of the process are still not fully understood. The main reason for this is that singlet fission is characterised by a complex interplay of electronic states, vibrational modes and electrostatic screening effects. In this thesis we employ \emph{ab initio} electronic structure techniques to study the excitations involved in fission in molecular crystals and dimers, using the well-studied pentacene molecule as a reference system. Linear-scaling density functional theory (LS-DFT) is used to model the influence of the crystal environment on charge-transfer (CT) configurations in the pentacene molecular crystal. We derive a general dipole correction scheme that allows us to eliminate finite-size effects from the calculations. We find that CT energies are significantly lowered by the response of the crystal environment, bringing them close to the energies of local excitations. This result lends support to the idea that the photoexcited precursor state to fission has significant CT character, and emphasises the role played by CT configurations in fission in the crystal. Furthermore, we use DFT to parametrise a linear vibronic coupling Hamiltonian of a covalent dimer of pentacene, forming the basis for many-body quantum dynamics calculations of the interplay between electronic and vibrational degrees of freedom. This reveals an interesting role for symmetry in fission in such dimers. Due to their high symmetry, couplings that could enable fission are precluded at the ground-state geometry. However, dynamic symmetry breaking by vibrational modes opens up an efficient pathway for fission, via an avoided crossing mediated by virtual CT configurations. Finally, we explore the influence of different side-groups and solvent environments on fission in pentacene dimers. To this end, we employ DFT with both implicit and explicit solvent models, combined with large-scale calculations to achieve sufficient sampling of solvent-solute configurations.

The desire for low cost electronics has led to a huge increase in research focused on organic materials. These materials are appealing due to their unique electrical and material-processing properties and are rapidly being adopted in old and new electronic applications. To create practical devices requires a further understanding of the charge transport properties of the unique anisotropic molecular crystal structures. This work looks at how doping with the transition-metal element manganese can alter the electronic structure of the organic material pentacene. It has been found that using manganese as a dopant provides novel physical characteristics previously not encountered in organic field effect transistors based on pentacene. These organic thin films were characterized using X-ray absorption spectroscopy and the results compared to computational density functional theory analysis.

The desire for low cost electronics has led to a huge increase in research focused on organic materials. These materials are appealing due to their unique electrical and material-processing properties and are rapidly being adopted in old and new electronic applications. To create practical devices requires a further understanding of the charge transport properties of the unique anisotropic molecular crystal structures. This work looks at how doping with the transition-metal element manganese can alter the electronic structure of the organic material pentacene. It has been found that using manganese as a dopant provides novel physical characteristics previously not encountered in organic field effect transistors based on pentacene. These organic thin films were characterized using X-ray absorption spectroscopy and the results compared to computational density functional theory analysis.

In this work, we have investigated the effects of temperature and optical illumination on organic thin film transistors (OTFTs) with indium-tin-oxide electrode. Experimental results have shown that the turn-on characteristics of the pentacene OTFT with ITO as source/drain electrodes is dependent on environmental temperatures. In addition, it has been found the pentacene OTFT is one kind of photo-sensitive devices. External stimulations can change the I-V characteristics significantly. For example, our pentacene OTFT show a shift of the onset and threshold voltage with illumination, which recovers slowly in the dark. Therefore, except for the application of switch devices, petacene OTFT exhibits a potential application for image sensors.

Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 142-148). Also available on the World Wide Web.

Les transistors organiques sont la base de nombreuses applications de l'électronique organique mais leur rendement électrique ainsi que leur stabilité dans le temps sont encore des verrous technologiques devant être levés. De plus, le phénomène de transport de charges dans ces dispositifs reste une notion encore à l'étude même si différents modèles sont utilisés afin d'expliquer l'effet de champ des transistors organiques. C'est dans ce cadre que s'inscrit cette étude portant sur la caractérisation par spectroscopie d'admittance de jonctions Métal/oxyde/semi conducteur organique/métal. Elle a pour but d'analyser le comportement électrostatique du semi conducteur organique. Les mesures de spectroscopie d'admittance ont été réalisées sur une large bande de fréquence (0.1Hz à 1Mhz) dans laquelle les pertes diélectriques mesurées peuvent être associées à des phénomènes d'orientation (oscillation) de dipôles présents dans la structure. Les réponses en fréquences montrent trois types de comportements dynamiques. A basses fréquences, nous avons observé une diffusion ionique, liée au déplacement d'ions H+ à travers la structure. A hautes fréquences, la réponse est due aux imperfections de l'oxyde. Enfin, aux fréquences intermédiaires, la réponse du semi-conducteur organique est identifiée et attribuée à des dipôles permanents. A partir de ces différentes réponses, un modèle analytique est établi et permet de décrire l'ensemble des réponses dynamiques. Le comportement du semi-conducteur est décrit par la somme d'une fonction de type Debye et de type Cole-Cole. L'analyse des paramètres de ce modèle a permis de mettre en évidence l'influence des dipôles permanents sur la permittivité du film de semi-conducteur organique. Enfin, ce modèle a été validé sur différents échantillons à base de pentacène et a été ensuite appliqué au Poly-3-hexylthiophène. / Organic transistors are vital in many applications of organic electronics but the electrical performance and time stability are technological limitation in order to make this technology reliable. Moreover, in these devices, the charge transport phenomenon has not to be clearly understood even if different models are commonly used to explain the field effect in organic transistors. In this context, this thesis talks about the admittance spectroscopy characterization of metal / oxide / organic semiconductor / metal junctions and analysis the organic semiconductor electrostatic behavior.The admittance spectroscopy measurements were performed on a wide frequency range (0.1Hz to 1MHz) in which the measured dielectric loss may be associated with the orientation phenomenon (as oscillation) of dipoles present in the structure.The frequency responses show three dynamic behaviors. At low frequencies (<10Hz), we observed an ionic diffusion, which is related to the ions movement of H+ through the structure. At high frequencies, (>10kHz) the response is due to defects into the oxide. Finally, at intermediate frequencies, the organic semiconductor response is identified and assigned to the permanent dipoles into the bulk. From these responses, an analytical model is developed and used to describe the dynamic responses. The semi-conductor behavior is described by the sum of a Debye and Cole-Cole function type. The analysis of the model parameters has highlighted the influence of permanent dipoles on the organic semiconductor permittivity. Finally, this model has been agreed on different samples based on pentacene and was applied to the Poly-3-hexylthiophene.