The structural, electronic, and optical properties of a series of organic pi-conjugated polymer, oligomer, or molecular materials of interest for applications in organic electronics are described. For this purpose, quantum-chemical techniques ranging from Density Functional Theory to Hartree-Fock ab initio and semiempirical methods are used to evaluate the charge-transport, charge-transfer, and electroluminescence properties of pi-conjugated organic materials. First, the effect of electronic polarization on the charge-transport parameters of organic semiconductors is discussed. A generalized methodology based on a basis set orthogonalization procedure is developed to determine reliable charge-transport characteristics. The charge-transport parameters of a number of organic semiconductors such as oligoacenes and derivatives are studied with this methodology. Then, triplet emitters, in particular iridium complexes, that achieve high efficiency electroluminescence in organic light-emitting diodes are discussed. The effects of ligand substitution and orientation on the luminescent properties of iridium compounds are investigated in order to develop structure-properties relationships. The emission properties of these complexes are found to be governed by an interplay between metal-to-ligand charge transfer excitations and ligand-centered and/or interligand excitations. The extent of mixing of these various excitations turns out to be highly dependent on the nature of the substituents. Design strategies to shift the emission color towards deep blue are proposed. Finally, several classes of materials acting as hosts for phosphorescent emitters are studied. It is shown that restricting the conjugation length leads to high energy gap host materials suitable for blue phosphorescent emitters.

The present thesis is focused on rational design and synthesis of π-conjugated donoracceptor-donor (D-A-D) type oligomers and D-A type copolymers. Thesis is organized in seven chapters, apart from introduction remaining six chapters are grouped into two parts (A and B). Part A deals with Chapters 2, 3, 4 and Part B contains chapters 5, 6 and 7. A brief discussion on the content of individual chapters is provided below. Chapter 1 discusses the introduction to organic solar cell with operating principles and effect of spinodal decomposition on stability of the devices is presented. The status and literature related to the improvement of life time of the organic solar cells by self-assembly approach has been explored. In addition, design and synthesis of the fluorine substituted π-conjugated organic semiconductors for n-type OFETs and OLED has been discussed. Part A This part of the thesis attempt to address some of the challenges listed below (1) Investigation of miscibility of binary components in bulk heterojunction solar cells through H-bonding approach. (2) Synthesis of new low band gap molecular semiconductors having H-bonding sites. (3) Fabrication of bulk heterojunction solar cell devices using these new molecules and exploring the photovoltaics performance. Chapter 2, donor-acceptor-donor (D-A-D) concept has been employed to design low band gap oligomers named as TTB. Barbiturate functional group has been utilized to explore the concepts of supramolecular chemistry. It is shown that, TTB molecule self-organizes via intermolecular H-bonding between barbituric acid units. Interactions between the oligothiophene subunits were also found to be important, affording nanoribbons that were observed by atomic force and transmission electron microscopy. The applicability of TTB for organic electronic applications was investigated by fabricating organic field-effect transistors (OFETs) and organic photovoltaic device. The crystalline nanoribbons were beneficial in understanding the phase morphology of PCBM and TTB blend. Chapter 3, the self-assemble property of TTB was disrupted by the substitution of methyl group on the nitrogen of the barbituric acid moiety. The optical and electrochemical properties of the new derivative have been investigated by UV-Visible spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. Further investigations on the effect of self-assembly on organic solar cells were carried out by fabricating BHJ and OFET. The results proved that the self-assembly within the donor moieties led to complete phase separation between the donor and acceptor which had an adverse effect on the photovoltaic performance. Chapter 4, the conjugation of TTB was extended by the synthesis of two new copolymers by polymerizing with two oliogothiophene (terthiophene and benzobithiophene) derivatives with different donating strength. The investigation of photophysical and electrochemical properties of copolymers were studied by varying the donating strength. As we increase the donating strength of oligothiophenes, the intramolecular charge transfer band of DA copolymers was red shifted. Further, density functional theory (DFT) calculation of these materials was carried out to get insight into their photophysical properties. Part B This part of the thesis attempt to address some of the challenges listed below (1) Investigation of fluorine substituted organic semiconductos like 2,2’ bithiazole and pheanthroimidazole. (2) Synthesis of pentafluoro phenyl appended derivatives of 2,2’ bithiazole and pheanthroimidazole. (3) Fabrication of OFETs and OLEDs using these new molecules and elucidated the device performance with molecular structure. Chapter 5, pentafluorophenyl appended 2,2’-bithiazole derivatives were synthesized. The single crystal x-ray diffraction studies shows the unusual strong type-II F•••F interactions within the distance of 2.668 Å, at an angle of 89.14° and 174.15°. It also shows the usual type-I F•••F interaction within the distance of 2.825Å, at an angle of 137.38° and 135.93°. Upon bromination type-II Br•••Br interaction was observed and the packing was further stabilized by S•••Br interactions. The conjugation was further extended with different aromatic and heteroaromatic substituents and synthesized the star shaped structure. The band gap as well as the electronic energy levels was tuned by substituting various aromatic and heteroaromatic substituents. These star shaped derivatives shows electron mobilities in the order of 10-4 to 10-3cm2/Vs. Chapter 6, Novel D-A copolymers were synthesized by Stille condensation of electron acceptor fluorinated phenanthroimidazole with electron donors like terthiophene and benzobithiophene. Prior to that insoluble pentafluoro phenyl phenanthroimidazole was Nalkylated in presence of DMF which concurrently resulted in C-F activation of the pentafluoro phenyl moiety. As we increase the donor strength from benzobithiophene to terthiophene the absorbance spectra was red shifted from 446 nm to 482 nm in solution and 455 nm to 484 nm in solid state. The band gap of these copolymers was found to be 2.4 eV for PIBDT and 2.2 eV for PIDHTT from the absorbance spectra. The photoluminescence data shows that these materials are promising for the yellow colour as well as orange colour displays, of narrow wavelength range (FWHM 40 nm for PIBDT and 35 nm for PIDHTT), which can be achieved just by the manipulation of donor moieties in the copolymers. The preliminary electroluminiscence data shows high brightness of 888cd/m2 (orange luminescence) for PIDHTT and 410cd/m2 (yellow luminescence) for PIBDT. Chapter 7, Acenaphtho[1,2-b]quinoxaline based donor–acceptor type low band gap conjugated copolymers were synthesized by Stille coupling reaction with the corresponding oligothiophene derivatives. The optical properties of the copolymers were characterized by ultraviolet-visible spectrometry while the electrochemical properties were determined by cyclic voltammetry. The band gap of these polymers was found to be in the range of 1.8-2.0 eV as calculated from the optical absorption band edge. The intense charge transfer band in absorption spectra shows the significant effect of acceptor in the copolymers. X-ray diffraction measurements show weak π–π stacking interactions between the polymer chains. The OFET devices fabricated using these co-polymers showed dominant p-channel transistor behavior with the highest mobility of 1×10-3cm2/Vs.

This work addresses two substantial problems of organic electronic devices: the controllability and adjustability of performance, and the integration using scalable, high resolution patterning techniques for planar thin-film transistors and novel vertical transistor devices. Both problems are of particular importance for the success of transparent and flexible organic electronics in the future. To begin with, the static behavior in molecular doped organic pin-diodes is investigated. This allows to deduce important diode parameters such as the depletion capacitance, the number of active dopant states, and the breakdown field. Applying this knowledge, organic pin-diodes are designed for ultra-high-frequency applications and a cut-off-frequency of up to 1GHz can be achieved using optimized parameters for device geometry, layer thickness, and dopant concentration. The second part of this work is devoted to organic thin-film transistors, high resolution patterning techniques, as well as novel vertical transistor concepts. In particular, fluorine based photo-lithography, a high resolution patterning technique compatible to organic semiconductors, is introduced fielding the integration of organic thin-film transistors under ambient conditions. However, as it will be shown, horizontal organic thin-film transistors are substantially limited in their performance by charge carrier injection. Hence, down-scaling is inappropriate to enlarge the transconductance of such transistors. To overcome this drawback, a novel vertical thin-film transistor concept with a vertical channel length of ∼50nm is realized using fluorine based photo-lithography. These vertical devices can surpass the performance of planar transistors and hence are prospective candidates for future integration in complex electronic circuits.

In this thesis, a first-principles methodology to investigate the impact of electron-phonon interactions on the charge-carrier mobilities in organic molecular crystals has been developed. Well-known organic materials such as oligoacene and oligothienoacene derivatives were studied in detail. The nature of the intramolecular vibronic coupling in oligoacenes and oligothienoacenes was studied using an approach that combines high-resolution gas-phase photo-electron spectroscopy measurements with first-principles quantum-mechanical calculations. The electron interactions with optical phonons in oligoacene single crystals were investigated using both density functional theory and empirical force field methods. The low-frequency optical modes are found to play a significant role in dictating the temperature dependence of the charge-transport properties in the oligoacene crystals. The microscopic charge-transport parameters in the pentathienoacene, 1,4-diiodobenzene, and 2,6-diiodo-dithieno[3,2-<i>b</i>:2',3'-<i>d</i>]thiophene crystals were also investigated. It was found that the intrinsic charge transport properties in the pentathienoacene crystal might be higher than that in two benchmark high-mobility organic crystals, i.e., pentacene and sexithienyl. For 1,4-diiodobenzene crystal, a detailed quantum-mechanical study indicated that its high mobility is primarily associated with the iodine atoms. In the 2,6-diiododithieno[3,2-<i>b</i>:2',3'-<i>d</i>]thiophene crystal, the main source of electronic interactions were found along the π-stacking direction. For negatively charged carriers, the halogen-functionalized molecular crystals show a very large polaron binding energy, which suggests significantly low charge-transport mobility for electrons.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2018-05-10T14:07:53Z No. of bitstreams: 1 monicacristinamelquiades.pdf: 3336410 bytes, checksum: b0292bcffca349c4979d58c19d8cfc83 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2018-09-03T16:03:10Z (GMT) No. of bitstreams: 1 monicacristinamelquiades.pdf: 3336410 bytes, checksum: b0292bcffca349c4979d58c19d8cfc83 (MD5) / Made available in DSpace on 2018-09-03T16:03:10Z (GMT). No. of bitstreams: 1 monicacristinamelquiades.pdf: 3336410 bytes, checksum: b0292bcffca349c4979d58c19d8cfc83 (MD5) Previous issue date: 2017-10-27 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesse trabalho são apresentados os resultados da produção e caracterização de diodos orgânicos emissores de luz sensível ao infravermelho próximo, OLEDs de conversão ascendente de energia que são conhecidos como OLEDs tipo up-converter. O desempenho da camada sensível ao infravermelho é um fator fundamental para o funcionamento do dispositivo. Neste sentido, moléculas de ftalocianinas apresentam respostas eletrônicas quando são expostas a radiação infravermelha. Na primeira parte do trabalho foram investigadas as propriedades ópticas e elétricas de filmes de ftalocianinas fabricados por evaporação térmica a vácuo. As ftalocianinas estudadas foram: ftalocianina de cobalto, ftalocianina de cobre, ftalocianina de magnésio, ftalocianina de platina e naftalocianina de estanho. Essas moléculas foram caracterizadas usando diferentes técnicas, tais como, espectroscopia de absorção UV-VIS e voltametria cíclica, espectrometria de massa, difratometria de raios-X e medidas elétricas, a fim de se obter suporte para o uso dessas moléculas como camada ativa nos dispositivos up-converters. Entre as ftalocianinas estudadas, a naftalocianina de estanho se mostrou a melhor candidata para atuar como camada sensível para a fabricação dos dispositivos. Porém, OLEDs up-converters usando naftalocianina de estanho como camada sensível ao infravermelho apresentaram baixa resposta a radiação infravermelha e baixa eficiência de conversão. Assim, para tentar solucionar esse problema, foi necessário fabricar alguns OLEDs up-converters usando uma camada de ftalocianina codepositada com fulereno como a camada sensível ao infravermelho. A codeposição consiste em uma técnica em que as moléculas de ftalocianinas e de fulereno são sublimadas simultaneamente. A partir da técnica de codeposição, é possível obter uma blenda com diferentes propriedades ópticas e elétricas. A camada codepositada apresentou uma redução significativa na mobilidade de buracos, o que levou a um aumento da eficiência de conversão do dispositivo, evidenciando o uso destes materiais para a fabricação de OLEDs sensíveis ao infravermelho para aplicações em sistemas de visão noturna. / In this work the results of the production and characterization of Organic Light Emitting Diodes with near-infrared sensitive layer up-converter are presented. The infrared sensitive layer performance is a key factor for the operation of the device. Phthalocyanine molecules have an electronic response when they are exposed to the infrared radiation. In the first part of the work the optical and electrical properties of phthalocyanine films manufactured by vacuum thermal evaporation were investigated. The phthalocyanines studied were: cobalt phthalocyanine, copper phthalocyanine, magnesium phthalocyanine platinum phthalocyanine and tin naphthalocyanine. These molecules were characterized using different techniques, such as UV-VIS absorption spectroscopy and cyclic voltammetry, mass spectrometry, X-ray diffractometry and electrical measurements, in order to obtain support for the use of these molecules as active layer of up-converter devices. Among the studied phthalocyanines, tin naphthalocyanine was the best candidate to act as a sensitive layer for the manufacture of the devices. However, OLEDs up-converters using tin naphthalocyanine as an infrared sensitive layer showed low response to infrared radiation and low conversion efficiency. Thus, to solve this problem, it was necessary to manufacture OLEDs up-converters using a phthalocyanine layer co-deposited with fullerene as infrared sensitive layer. Co deposition consists of a technique in which the phthalocyanine and fullerene molecules are simultaneously sublimated. From the co-deposition technique, it is possible to obtain a blend with different optical and electrical properties. The co-deposited layer showed a significant reduction in hole mobility, which led to an increase in the conversion efficiency of the device, evidencing the use of these materials for the manufacture of infrared sensitive OLEDs for applications in night vision systems.

Organic semiconductors have gained a lot of interest due to their ease of processing, low-cost and inherent mechanical flexibility. Although most of the research has been on their electronic and optical properties, knowledge of the thermal properties is important in the design of electronic devices as well. Our group has used ac-calorimetric techniques to measure both in-plane and transverse thermal conductivities of a variety of organic semiconductors including small-molecule crystals and polymer blends. For layered crystals composed of molecules with planar backbones and silylethynyl (or germylethynyl) sidegroups projecting between the layers, very high interplanar thermal conductivities have been observed, presumably implying that heat flows between layers mostly via interactions between librations on these sidegoups. Since most organic semiconducting devices require materials in thin film rather than bulk crystal form, I have focused on using the “3ω- technique” to measure the thermal resistances of thin films of this class of organic semiconductors, including bis(triisopropylsilylethynyl) pentacene (TIPS-pn), bis(triethylsilylethynyl) anthradithiophene (TES-ADT), and difluoro bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT). For each material, several films of different thicknesses have been measured to separate the effects of intrinsic thermal conductivity from interface thermal resistance. For sublimed films of TIPS-pn and diF-TES-ADT, with thicknesses ranging from less than 100 nm to greater than 4 μm, the thermal conductivities are similar to those of polymers and over an order of magnitude smaller than those of single crystals, presumably reflecting the large reduction in phonon mean-free path due to disorder in the films. On the other hand, the thermal resistances of thin (≤ 205 nm) crystalline films of TES-ADT, prepared by vapor-annealing of spin-cast films, are dominated by their interface resistances, possibly due to dewetting of the film from the substrate during the annealing process.

Les performances des modulateurs de lumières à commande optique (OASLMs) à base de cristaux liquides (CLs) dépendent fortement des propriétés de la couche photosensible. Afin de concilier transparence, résolution latérale et production à bas coûts, les semi-conducteurs organiques apparaissent comme des candidats idéaux. Nous avons choisi d'utiliser un mélange P3HT:PCBM comme couche photosensible. Nos résultats ont montré que les cristaux liquides se réorientaient en fonction de l'intensité lumineuse seule et sans tension appliquée. Des mesures complémentaires indiquent que l'effet photovoltaïque est à l'origine de ce phénomène. Ce type de dispositif nous permet de moduler spatialement l'orientation des CLs et démontre son potentiel dans des applications liées à l'holographie. Un second type de dispositif intégrant des couches d'interfaces de PEIE et de PEDOT:PSS nous permet de contrôler l'orientation des CLs et donne de nouvelles pistes permettant de fabriquer des OASLMs autonomes. / The performances of liquid crystals (LCs) based optically addressed Spatial Light Modulators (OASLMs) strongly depends on the photosensitive layer properties. To accommodate device transparency, lateral resolution and low cost production, organic semiconductors appear as the ideal candidates. We chose to use a P3HT: PCBM blend as the photosensitive layer. Our results showed that the liquid crystals reorient according to the luminous intensity alone and without external power supply. Additional measurements indicate that the photovoltaic effect is at the origin of this phenomenon. This type of device allowed spatial modulation of the LCs orientation and demonstrates its potential in holographic applications. A second type of device integrating interfacial layers of PEIE and PEDOT: PSS allowed us to control the orientation of the LCs and gives promising routes towards the design of self-sustainable OASLMs.

In the last 25 years, organic or "plastic" solar cells have gained commercial interest as a light-weight, flexible, colorful, and potentially low-cost technology for direct solar energy conversion into electrical power. Currently, organic solar cells with a maximum power conversion effciency (PCE) of 12% can compete with classical silicon technology under certain conditions. In particular, a variety of strongly absorbing organic molecules is available, enabling custom-built organic solar cells for versatile applications. In order to improve the PCE, the charge carrier mobility in organic thin films must be improved. The transport characterization of the relevant materials is usually done in neat layers for simplicity. However, the active layer of highly efficient organic solar cells comprises a bulk heterojunction (BHJ) of a donor and an acceptor component necessary for effective charge carrier generation from photo-generated excitons. In the literature, the transport properties of such blend layers are hardly studied. In this work, the transport properties of typical BHJ layers are investigated using space-charge limited currents (SCLC), conductivity, impedance spectroscopy (IS), and thermally stimulated currents (TSC) in order to model the transport with numerical drift-diffusion simulations. Firstly, the influence of an exponential density of trap states on the thickness dependence of SCLCs in devices with Ohmic injection contacts is investigated by simulations. Then, the results are applied to SCLC and conductivity measurements of electron- and hole-only devices of ZnPc:C60 at different mixing ratios. Particularly, the field and charge carrier density dependence of the mobility is evaluated, suggesting that the hole transport is dominated by exponential tail states acting as trapping sites. For comparison, transport in DCV5T-Me33:C60, which shows better PCEs in solar cells, is shown not to be dominated by traps. Furthermore, a temperature-dependent IS analysis of weakly p-doped ZnPc:C60 (1:1) blend reveals the energy-resolved distribution of occupied states, containing a Gaussian trap state as well as exponential tail states. The obtained results can be considered a basis for the characterization of trap states in organic solar cells. Moreover, the precise knowledge of the transport-relevant trap states is shown to facilitate modeling of complete devices, constituting a basis for predictive simulations of optimized device structures. / Organische oder "Plastik"-Solarzellen haben in den letzten 25 Jahren eine rasante Entwicklung durchlaufen. Kommerziell sind sie vor allem wegen ihres geringen Gewichts, Biegsamkeit, Farbigkeit und potentiell geringen Herstellungskosten interessant, was zukünftig auf spezielle Anwendungen zugeschnittene Solarzellen ermöglichen wird. Die Leistungseffzienz von 12% ist dabei unter günstigen Bedingungen bereits mit klassischer Siliziumtechnologie konkurrenzfähig. Um die Effzienz weiter zu steigern und damit die Wirtschaftlichkeit zu erhöhen, muss vor allem die Ladungsträgerbeweglichkeit verbessert werden. In organischen Solarzellen werden typischerweise Donator-Akzeptor-Mischschichten verwendet, die für die effziente Generation freier Ladungsträger aus photo-induzierten Exzitonen verantwortlich sind. Obwohl solche Mischschichten typisch für organische Solarzellen sind, werden Transportuntersuchungen der relevanten Materialien der Einfachheit halber meist in ungemischten Schichten durchgeführt. In der vorliegenden Arbeit wird der Ladungstransport in Donator-Akzeptor-Mischschichten mithilfe raumladungsbegrenzter Ströme (space-charge limited currents, SCLCs), Leitfähigkeit, Impedanzspektroskopie (IS) und thermisch-generierter Ströme (thermally stimulated currents, TSC) untersucht und mit numerischen Drift-Diffusions-Simulationen modelliert. Zunächst wird mittels Simulation der Einfluss exponentiell verteilter Fallenzustände auf das schichtdickenabhängige SCLC-Verhalten unipolarer Bauelemente mit Ohmschen Kontakten untersucht. Die Erkenntnisse werden dann auf Elektronen- und Lochtransport in ZnPc:C60-Mischschichten mit verschiedenen Mischverhältnissen angewendet. Dabei wird die Beweglichkeit als Funktion von elektrischem Feld und Ladungsträgerdichte dargestellt, um SCLC- und Leitfähigkeitsmessungen zu erklären, was mit einer exponentiellen Fallenverteilung gelingt. Zum Vergleich werden dieselben Untersuchungen in DCV2-5T-Me33:C60, dem effizientesten der bekannten Solarzellenmaterialien dieser Art, wiederholt, ohne Anzeichen für fallendominierten Transport. Des weiteren werden erstmals schwach p-dotierte ZnPc:C60-Mischschichten mit temperaturabhängiger IS untersucht, um direkt die Dichte besetzter Lochfallenzustände zu bestimmen. Dabei werden wiederum exponentielle Fallenzustände sowie eine Gaußförmige Falle beobachtet. Insgesamt tragen die über Fallenzustände in Mischschichten gewonnenen Erkenntnisse zum Verständnis von Transportprozessen bei und bilden damit eine Grundlage für die systematische Identifizierung von Fallenzuständen in Solarzellen. Außerdem wird gezeigt, dass die genaue Beschreibung der transportrelevanten Fallenzustände die Modellierung von Bauelementen ermöglicht, auf deren Grundlage zukünftig optimierte Probenstrukturen vorhergesagt werden können.

L'objectif de ce travail de recherche est l'élaboration et l'optimisation de transistors à effet de champ organiques de type N (OTFTs). Des transistors en structure Bottom Gate Bottom Contact sont fabriqués à basse température T<120°C. Trois différentes molécules organiques conductrices d'électrons, déposées par évaporation thermiques, sont utilisées pour la couche active. Les OTFTs à base de la première molécule à corps LPP présentent de faibles mobilités à effet de champ de l'ordre de 10-5cm2/V.s. L'étude d'optimisation menée sur les conditions de dépôt de cette dernière n'a pas permis d'améliorer ses performances électriques. L'étude de stabilité électrique ''Gate Bias Stress'' a mis en évidence les instabilités de cette molécule. Les OTFTs à base des deux dérivés indénofluorènes (IF) possèdent des mobilités plus importantes. Dans les conditions optimales la molécule IF(CN2)2 méta permet d'atteindre une mobilité d'effet de champ µFE=2.1x10-4 cm2/V, alors que la molécule IF(CN2)2 para permet d'obtenir des mobilités µFE=1x10-2cm2/V.s après recuit. L'étude de stabilité électrique a mis en évidence une meilleure stabilité des OTFTs à base de IF(CN2)2 para. Une étude des phénomènes de transport de charges est menée pour les deux types de molécules. Les OTFTs de type N réalisés sont utilisés pour la réalisation d'un circuit logique de type inverseur pseudo-CMOS. Finalement, ce procédé basse température nous a permis de réaliser des OTFTs sur substrat flexible. / The main goal of this present work consists in the fabrication and optimization of N type organic field effect transistors. Bottom Gate Bottom Contact transistors are performed at low temperature T<120°C. Three different electro-deficient organic molecules are thermally evaporated and used as active layer. OTFTs based on LPP core molecule present low field effect mobility around 10-5cm2/V.s. The optimization study investigated on deposition parameters of this molecule on OTFTs performances does not allow improving this mobility. Moreover gate bias stress measurements reveal important instabilities related to this molecule. Indenfluorene derivatives core (IF) based OTFTs show better performances. Field effect mobility µFE=2.1x10-4 cm2/V is reached using IF(CN2)2 meta in optimized deposition conditions and µFE=1x10-2 cm2/V.s is obtained using IF(CN2)2 para after annealing treatment. The investigated gate bias stress study highlights the good electrical stability of IF(CN2)2 para based OTFTs. Temperature measurements allow us studying the charge transport phenomenon in these indenofluorene derivatives. Fabricated N-type OTFTs are used to perform a first electronic circuit that consists in a logic gate (invertor).Finally this low temperature process led us to achieve OTFTs devices on flexible substrates (PEN).

When a temperature differential is applied across a semiconductor, a thermal voltage develops across it in response. The ratio of this thermal voltage to the applied temperature differential is the Seebeck coefficient, a transport coefficient that complements measurements of electrical and thermal conductivity. The physical interpretation of the Seebeck coefficient is the entropy per charge carrier divided by its charge and is hence a direct measurement of the carrier entropy in the solid state. This PhD thesis has three major outcomes. The first major outcome is a demonstration of how the Seebeck coefficient can be used as a tool to quantify the role of energetic disorder in organic semiconductors. To this end, a microfabricated chip was designed to perform accurate measurements of the Seebeck coefficient within the channel of the active layer in a field-effect transistor (FET). When measured within an FET, the Seebeck coefficient can be modulated using the gate electrode. The extent to which the Seebeck coefficient is modulated gives a clear idea of charge carrier trapping and the distribution of the density of states within the organic semiconductor. The second major outcome of this work is the observation that organic semiconducting polymers show Seebeck coefficients that are temperature independent and strongly gate voltage modulated. The extent to which the Seebeck coefficient is modulated in the polymer PBTTT is found to be larger than that in the polymer IDTBT. Taken together with conventional charge transport measurements on IDTBT, the voltage modulated Seebeck coefficient confirms the existence of a vanishingly small energetic disorder in this material. In the third and final outcome of this thesis, the magnitude of the Seebeck coefficient is shown to be larger for organic small molecules as compared to organic polymers. The basis for this is not yet clear. There are reports that such an observation is substantiated through a larger contribution from vibrational entropy that adds to the so called entropy-of-mixing contribution so as to boost the magnitude of the Seebeck coefficient in organic small molecules. As of now, this remains an open question and is a potential starting point for future work. The practical implications of this PhD thesis lie in building cost-effective and environmentally friendly waste-heat to useful energy converters based on organic polymers. The efficiency of heat to energy conversion by organic polymers tends to be higher than that for conventional semiconductors owing to the presence of narrow bands in organic polymer semiconductors.