Les polymères semi-conducteurs semicristallins sont utilisés au sein de diodes électroluminescentes, transistors ou dispositifs photovoltaïques organiques. Ces matériaux peuvent être traités à partir de solutions ou directement à partir de leur état solide et forment des agrégats moléculaires dont la morphologie dicte en grande partie leurs propriétés optoélectroniques. Le poly(3-hexylthiophène) est un des polymères semi-conducteurs les plus étudiés. Lorsque le poids moléculaire (Mw) des chaînes est inférieur à 50 kg/mol, la microstructure est polycristalline et composée de chaînes formant des empilements-π. Lorsque Mw>50 kg/mol, la morphologie est semicristalline et composée de domaines cristallins imbriquées dans une matrice de chaînes amorphes. À partir de techniques de spectroscopie en continu et ultrarapide et appuyé de modèles théoriques, nous démontrons que la cohérence spatiale des excitons dans ce matériau est légèrement anisotrope et dépend de Mw. Ceci nous permet d’approfondir la compréhension de la relation intime entre le couplage inter et intramoléculaire sur la forme spectrale en absorption et photoluminescence. De plus, nous démontrons que les excitations photogénérées directement aux interfaces entre les domaines cristallins et les régions amorphes génèrent des paires de polarons liés qui se recombinent par effet tunnel sur des échelles de temps supérieures à 10ns. Le taux de photoluminescence à long temps de vie provenant de ces paires de charges dépend aussi de Mw et varie entre ∼10% et ∼40% pour les faibles et hauts poids moléculaires respectivement. Nous fournissons un modèle permettant d’expliquer le processus de photogénération des paires de polarons et nous élucidons le rôle de la microstructure sur la dynamique de séparation et recombinaison de ces espèces. / Microstructure plays a crucial role in defining the optoelectrical properties of conjugated polymeric semiconductors which can be used in light harvesting and generating devices such as organic light emitting diodes, field effect transistors or photovoltaic devices. These polymers can be processed from solution or solidstate and form photophysical aggregates, consequently providing a complex network which controls the fate of any photogenerated species. poly(3-hexylthiopene) is one of the most studied polymeric semiconductor. In this material, the molecular weight (Mw) of the polymer governs the microstructure and highly impact the optical and electronic properties. Below Mw≈ 50 kg/mol, the polymer chains forms polycrystalline domains of π-stacked molecules while high Mw (>50 kg/mol) consists of a two-phase morphology of molecularly ordered crystallites that are embedded in amorphous regions. Such morphology provides a bidimensionnal network hosting both neutral excitations, known as Frenkel excitons, and polarons. By means of steady-state and ultrafast spectroscopy experiment and backed up theoretical modeling, we demonstrate that the spatial coherence of such excitations are anisotropic in the lattice and depends on the Mw of the polymer, providing a deep understanding of the interplay between interchain (excitonic) and intrachain coupling in polymer aggregates. Moreover, we show that direct excitation at the interface between molecularly ordered and amorphous regions generates tightlybound charge pairs which decay via quantum tunneling over >10 ns. The yield of delayed photoluminescence arising from the recombination of those charge pairs varies between ∼10% and ∼40% for low and high Mw films respectively. We provide a quantitative model that describes the photogeneration process of those geminate polaron pairs and determine the role of the microstructure in the charge separation and recombination processes.

Cette thèse rapporte l’étude des propriétés de films transparents conducteurs obtenus à partir de dispersions de nanotubes de carbone. La formulation des dispersions représente une étape clé dans le but d’obtenir des films homogènes avec de bonnes propriétés électro-optiques. Plus particulièrement, la création d’interactions attractives en solution entre les nanotubes de carbone permet d’une part de modifier le comportement rhéologique des dispersions et d’améliorer leur dépôt en couche mince par enduction. D’autre part, les travaux présentent une étude concernant l’influence des interactions sur la structuration du réseau de nanotube de carbone qui constitue les films. Ces changements de structuration sont notamment mis en parallèle avec les propriétés électriques des films selon leur épaisseur. L’utilisation de polymères semi-conducteurs a aussi fait l’objet de travaux expérimentaux pour améliorer la formation et les propriétés des films transparents conducteurs. / This thesis reports the study of the properties of transparent conductive films obtained from carbon nanotube dispersions. The dispersion formulation is a key step in order to obtain uniform films with good opto-electrical properties. In particular, the formation of attractive interactions between dissolved carbon nanotubes allows the modification of the rheological behavior of the dispersions and the improvement of their deposition in thin layer by coating. Also, the influence of the interactions on the carbon nanotube network morphology is presented. The structural changes of the networks are then related to both electrical properties and thickness of the films. Finally, the use of semiconducting polymers was analyzed in order to improve the fabrication and the properties of transparent conductive films.

The rapid development in perovskite solar cells (PSC) has generated a tremendous interest in the photovoltaic community. The power conversion efficiency (PCE) of these devices has increased from 3.8% in 2009 to a recent certified efficiency of over 20% which is mainly the product of the remarkable properties of the perovskite absorber material. One of the most important advances occurred with the replacement of the liquid electrolyte with a solid state hole conductor which enhanced PCE values and improved the device stability. Spiro-OMeTAD (2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)- 9,9'-spirobifluorene) is the most common hole transport material in perovskite solar cells. Nevertheless, the poor conductivity, low charge transport and expensive synthetic procedure and purification have limited its commercialisation. Triphenylamines (TPA) like Spiro-OMeTAD are commonly employed due to the easy oxidation of the nitrogen centre and good charge transport. Other triarylamines have similar properties to Spiro-OMeTAD but are easier to synthesise. The aim of this doctoral thesis is to investigate different types of hole transport materials in perovskite solar cells. Three different series of triphenylamine-based HTM were designed, synthesised, characterised and studied their function in perovskite solar cells. A series of five diacetylide-triphenylamine (DATPA) derivatives (Chapter 3) with different alkyl chain length in the para position was successfully synthesised through a five step synthesis procedure. A range of characterisation techniques was carried out on the molecules including; optical, electrochemical, thermal and computational methods. The results show that the new HTMs have desirable optical and electrochemical properties, with absorption in the UV, a reversible redox property and a suitable highest occupied molecular orbital (HOMO) energy level for hole transport. Perovskite solar cell device performances were studied and discussed in detail. This project studied the effect of varying the alkyl chain length on structurally similar triarylamine-based hole transport materials on their thermal, optical, electrochemical and charge transport properties as well as their molecular packing and solar cell parameters, thus providing insightful information on the design of hole transport materials in the future. The methoxy derivative showed the best semiconductive properties with the highest charge mobility, better interfacial charge transfer properties and highest PCE value (5.63%). The use of p-type semiconducting polymers are advantageous over small molecules because of their simple deposition, low cost and reproducibility. Styrenic triarylamines (Chapter 4) were prepared by the Hartwig-Buchwald coupling followed by their radical polymerization. All monomers and polymers were fully characterised through electrochemical, spectroscopic and computational techniques showing suitable HOMO energy levels and desirable optoelectrochemical properties. The properties and performance of these monomers and polymers as HTMs in perovskite solar cells were compared in terms of their structure. Despite the lower efficiencies, the polymers showed superior reproducibility on each of the device parameters in comparison with the monomers and spiro-OMeTAD. Finally, star-shaped structures combine the advantages of both small molecules, like well-defined structures and physical properties, and polymers such as good thermal stability. Two star-shaped triarylamine-based molecules (Chapter 5) were synthesised, fully characterised and their function as hole-transport materials in perovskite solar cells studied. These materials afford a PCE of 13.63% and high reproducibility and device stability. In total this work provided three series of triarylamine-based hole transport materials for perovskite solar cells application and enabled a comparison of the pros and cons of different design structures: small-molecule, polymeric and star-shaped.

Self-assembled monolayers and surface-initiated polymer, or polymer brushes, have attracted attention as they form dense layers with much higher structural order than bulk or solution polymers. Another field of research which has emerged over the last two decades is the field of organic and polymer electronics. In this field molecular order and surface modification are of major influence on the device performance, hence that both self-assembled monolayers as polymer brushes have been investigated to find applications in organic electronic devices. After an introduction into the field self-assembled monolayers, polymer brushes and organic electronics, the first part of this thesis focusses on three applications of surface modification techniques for applications in devices. Alignment of the active material is crucial for high mobilities in organic electronics. Chapter 2 discusses the synthesis of a liquid crystalline surface-initiated polymer and its application to induce strong homeotropic alignment. The alignment is homogeneous over large areas and can be patterned by combining the polymerization with soft lithographic techniques. Mobilities of organic electronic materials can also be strongly influenced by dopants in the material. In field-effect transistors the positioning of the dopant is thought to be crucial, as the conductance predominantly takes place in only a small channel near the dielectric interface. In chapter 3 dopant functionalized monolayers and polymer brushes are presented which enable the localized deposition of dopants in the channel of organic transistors. It is shown that the mobility of charges and hence the device performance is affected by the introduction of this dopant layer. Polymer brushes have been suggested for the fabrication of highly ordered semiconducting polymers. In chapter 4 the use of a thiophene functionalized polymer brush is shown, that can be used as a template for the subsequent growth of highly conjugated surface grafted polythiophene layers. Thick polythiophene layers are obtained, that are low in roughness and show photoluminescence and polychromism upon doping. The second part (chapter 5 and 6) of this thesis presents new techniques for surface polymerizations. It is attractive to investigate reduction of reactor volume for polymer brush growth. Chapter 5 discusses a method to achieve volume reduction by back-filling the superfluous volume with beads. It is found that this influences the polymerization kinetics significantly. The combined advantages of less volume and enhanced reaction speeds enable reduction of the total amount of monomer needed by up to 90%. Chapter 6 presents a controlled way to convert initiators for atom transfer radical polymerization into initiators for nitroxide mediated polymerization. In this way mixed polymer brushes and block co-polymer brushes become accessible. This combination makes it an attractive tool to fabricate complex polymer architectures. The technologies used in this thesis show that the synthesis of polymer brushes enable the fabrication of complex architectures without the wastes normally associated with surface-initiated polymers. Combined with several functionalized polymer brushes with properties that enhance order, influence mobility or serve as template for the growth of surface attached conjugated polymers this shows the high potential for the application of surface-initiated polymers in organic electronics.

I) Introduction p-Conjugated polymers inherently combine electronic properties of inorganic semiconductor crystals and material characteristics of organic plastics due to their special molecular design. This unique combination has led to developing new unconventional optoelectronic technologies and, further, resulted in the evolution of semiconducting polymers (SCPs) as fundamental components for novel electronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) and organic solar cells (OSCs).[1–5] Moreover, the material flexibility, capability for thin-film formation, and solution processibility additionally allow utilizing modern printing technologies for the large-scale fabrication of flexible, light-weight organic electronics. This especially enables to significantly increase the production speed and, moreover, to drastically reduce the costs per unit.[6, 7] In particular, transistors are the most important elements in modern functional electronic devices because of acting as electronic switches in logic circuits or in displays to control pixels. However, due to molecular arrangement and interactions, the electronic performance of SCPs cannot compete with the one of monocrystalline silicon which is used in state-of-the-art high-performance microtechnology.[5, 8] Nonetheless, intensive and continuing efforts of scientists focused on improving the performance of OFETs, with the special focus on the charge carrier mobility, by optimizing the polymer structure, processing conditions and OFET device architecture. By this, it was possible to identify crucial relationships between polymer structure, optoelectronic properties, microstructure, and OFET performance.[8] Nowadays, the interdisciplinary scientific success is represented by high-performance SCPs with charge carrier mobilities exceeding the value of amorphous silicon.[3, 9] However, further research is essential to enable developing the next generation of electronic devices for application in healthcare, safety technology, transportation, and communication. II) Objective and Results Within the scope of this doctoral thesis, current high-performance p-conjugated SCPs should be studied comprehensively to improve the present understanding about the interdependency between molecular structure, material properties and charge transport. Therefore, the extensive research approaches focused on different key aspects of high charge carrier mobility polymers for organic transistors. The performed investigations comprised the impact of, first, novel design concepts, second, precise structural modifications and, third, synthetic and processing conditions and led to the major findings listed below. 1. The design concept of tuning the p-conjugation length allows to gradually modulate physical material properties and demonstrates that a strong localization of frontier molecular orbitals in combination with a high degree of thin-film ordering can provide a favorable platform for charge transport in p-conjugated semiconducting polymers.[1] 2. The replacement of thiophene units with thiazoles in naphthalene diimide-based p- conjugated polymers allows to increase interchain interactions and to lower frontier molecular orbitals. This compensates the potentially detrimental enhancement of backbone torsion and drives the charge transport to unipolar electron transport, whereas mobility values are partially comparable with those of the respective thiophene containing analogs. 3. p-Conjugated diketopyrrolo[3,4-c]pyrrole-based copolymers can be synthesized within fifteen minutes what, in combination with avoiding aqueous washings and optimizing processing conditions, allowed an increase in morphological and energetic order and, thus, improved the charge transport properties significantly. III) Conclusion The key findings of this doctoral thesis provide new significant insights into important aspects of designing, synthesizing and processing high charge carrier mobility polymers. By this, they can guide future research to further improve the performance of organic electronic devices - decisive for driving the development and fabrication of smart, functional and wearable next-generation electronics. References [1] T. Erdmann, S. Fabiano, B. Milián-Medina, D. Hanifi, Z. Chen, M. Berggren, J. Gierschner, A. Salleo, A. Kiriy, B. Voit, A. Facchetti, Advanced Materials 2016, 28 (41), 9169–9174, DOI:10.1002/adma.201602923. [2] Y. Karpov, T. Erdmann, I. Raguzin, M. Al-Hussein, M. Binner, U. Lappan, M. Stamm, K. L. Gerasimov, T. Beryozkina, V. Bakulev, D. V. Anokhin, D. A. Ivanov, F. Günther, S. Gemming, G. Seifert, B. Voit, R. Di Pietro, A. Kiriy, Advanced Materials 2016, 28 (28), 6003–6010, DOI:10.1002/adma.201506295. [3] A. Facchetti, Chemistry of Materials 2011, 23 (3), 733–758, DOI:10.1021/cm102419z. [4] A. J. Heeger, Chemical Society Reviews 2010, 39, 2354–2371, DOI:10.1039/B914956M. [5] H. Klauk, Chemical Society Reviews 2010, 39, 2643–2666, DOI:10.1039/B909902F. [6] S. G. Bucella, A. Luzio, E. Gann, L. Thomsen, C. R. McNeill, G. Pace, A. Perinot, Z. Chen, A. Facchetti, M. Caironi, Nature Communications 2015, 6, 8394, DOI:10.1038/ncomms9394. [7] H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E. P. Woo, Science 2000, 290 (5499), 2123–2126, DOI:10.1126/science.290.5499.2123. [8] D. Venkateshvaran, M. Nikolka, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A. J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. McCulloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne, H. Sirringhaus, Nature 2014, 515 (7527), 384–388, DOI:10.1038/nature13854. [9] S. Holliday, J. E. Donaghey, I. McCulloch, Chemistry of Materials 2014, 26 (1), 647–663, DOI: 10.1021/cm402421p.

Organic semiconductors have witnessed a prolific boom for their potential in the manufacturing of lightweight, flexible, and even biocompatible electronics. One of the fields of research that has yet to benefit from organic semiconductors is high temperature electronics. The lightweight nature and robust processability is attractive for applications such as aerospace engineering, which require high temperature stability, but little has been reported on taking such a leap because charge transport is temperature dependent and commonly unstable at elevated temperatures in organics. Historically, mechanistic studies have been bound to low temperature regimes where structural disorders are minimal in most materials. Discussed here is a blending approach to render semiconducting polymer thin films thermally stable in unprecedented operation temperature ranges for organic materials. We found that by utilizing highly rigid host materials, semiconducting polymer domains could be confined, thus improving their molecular and microstructural ordering, and a thermally stable charge transport could be realized up to 220°C. With this blending approach, all-plastic high temperature electronics that are extremely stable could also be demonstrated. In efforts to establish a universal route towards forming thermally stable semiconducting blends, we found that the molecular weight of conjugated polymer plays a crucial role on the miscibility of the blends. Finally, we found that the choice of the host matrix ought to consider the charge trapping nature of the insulator.<br>

I) Introduction p-Conjugated polymers inherently combine electronic properties of inorganic semiconductor crystals and material characteristics of organic plastics due to their special molecular design. This unique combination has led to developing new unconventional optoelectronic technologies and, further, resulted in the evolution of semiconducting polymers (SCPs) as fundamental components for novel electronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) and organic solar cells (OSCs).[1–5] Moreover, the material flexibility, capability for thin-film formation, and solution processibility additionally allow utilizing modern printing technologies for the large-scale fabrication of flexible, light-weight organic electronics. This especially enables to significantly increase the production speed and, moreover, to drastically reduce the costs per unit.[6, 7] In particular, transistors are the most important elements in modern functional electronic devices because of acting as electronic switches in logic circuits or in displays to control pixels. However, due to molecular arrangement and interactions, the electronic performance of SCPs cannot compete with the one of monocrystalline silicon which is used in state-of-the-art high-performance microtechnology.[5, 8] Nonetheless, intensive and continuing efforts of scientists focused on improving the performance of OFETs, with the special focus on the charge carrier mobility, by optimizing the polymer structure, processing conditions and OFET device architecture. By this, it was possible to identify crucial relationships between polymer structure, optoelectronic properties, microstructure, and OFET performance.[8] Nowadays, the interdisciplinary scientific success is represented by high-performance SCPs with charge carrier mobilities exceeding the value of amorphous silicon.[3, 9] However, further research is essential to enable developing the next generation of electronic devices for application in healthcare, safety technology, transportation, and communication. II) Objective and Results Within the scope of this doctoral thesis, current high-performance p-conjugated SCPs should be studied comprehensively to improve the present understanding about the interdependency between molecular structure, material properties and charge transport. Therefore, the extensive research approaches focused on different key aspects of high charge carrier mobility polymers for organic transistors. The performed investigations comprised the impact of, first, novel design concepts, second, precise structural modifications and, third, synthetic and processing conditions and led to the major findings listed below. 1. The design concept of tuning the p-conjugation length allows to gradually modulate physical material properties and demonstrates that a strong localization of frontier molecular orbitals in combination with a high degree of thin-film ordering can provide a favorable platform for charge transport in p-conjugated semiconducting polymers.[1] 2. The replacement of thiophene units with thiazoles in naphthalene diimide-based p- conjugated polymers allows to increase interchain interactions and to lower frontier molecular orbitals. This compensates the potentially detrimental enhancement of backbone torsion and drives the charge transport to unipolar electron transport, whereas mobility values are partially comparable with those of the respective thiophene containing analogs. 3. p-Conjugated diketopyrrolo[3,4-c]pyrrole-based copolymers can be synthesized within fifteen minutes what, in combination with avoiding aqueous washings and optimizing processing conditions, allowed an increase in morphological and energetic order and, thus, improved the charge transport properties significantly. III) Conclusion The key findings of this doctoral thesis provide new significant insights into important aspects of designing, synthesizing and processing high charge carrier mobility polymers. By this, they can guide future research to further improve the performance of organic electronic devices - decisive for driving the development and fabrication of smart, functional and wearable next-generation electronics. References [1] T. Erdmann, S. Fabiano, B. Milián-Medina, D. Hanifi, Z. Chen, M. Berggren, J. Gierschner, A. Salleo, A. Kiriy, B. Voit, A. Facchetti, Advanced Materials 2016, 28 (41), 9169–9174, DOI:10.1002/adma.201602923. [2] Y. Karpov, T. Erdmann, I. Raguzin, M. Al-Hussein, M. Binner, U. Lappan, M. Stamm, K. L. Gerasimov, T. Beryozkina, V. Bakulev, D. V. Anokhin, D. A. Ivanov, F. Günther, S. Gemming, G. Seifert, B. Voit, R. Di Pietro, A. Kiriy, Advanced Materials 2016, 28 (28), 6003–6010, DOI:10.1002/adma.201506295. [3] A. Facchetti, Chemistry of Materials 2011, 23 (3), 733–758, DOI:10.1021/cm102419z. [4] A. J. Heeger, Chemical Society Reviews 2010, 39, 2354–2371, DOI:10.1039/B914956M. [5] H. Klauk, Chemical Society Reviews 2010, 39, 2643–2666, DOI:10.1039/B909902F. [6] S. G. Bucella, A. Luzio, E. Gann, L. Thomsen, C. R. McNeill, G. Pace, A. Perinot, Z. Chen, A. Facchetti, M. Caironi, Nature Communications 2015, 6, 8394, DOI:10.1038/ncomms9394. [7] H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E. P. Woo, Science 2000, 290 (5499), 2123–2126, DOI:10.1126/science.290.5499.2123. [8] D. Venkateshvaran, M. Nikolka, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A. J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. McCulloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne, H. Sirringhaus, Nature 2014, 515 (7527), 384–388, DOI:10.1038/nature13854. [9] S. Holliday, J. E. Donaghey, I. McCulloch, Chemistry of Materials 2014, 26 (1), 647–663, DOI: 10.1021/cm402421p.

Esta tese de doutorado é o resultado de um estudo dos mecanismos de operação de dispositivos poliméricos emissores de luz, com um particular enfoque nos processos de injeção e transporte de portadores de carga em polímeros derivados do poli(p-fenileno vinileno), PPV. Para tanto, se fez necessário o domínio de todas as etapas de produção e caracterização, desde a síntese química dos polímeros até a fabricação propriamente dita dos dispositivos, as quais são brevemente descritas neste trabalho. Basicamente, dois tipos diferentes de dispositivos foram caracterizados: diodos poliméricos emissores de luz (PLEDs), nos quais a camada ativa é composta por um filme fino (100-500 nm de espessura) do polímero eletroluminescente puro, e células eletroquímicas emissoras de luz (LECs), compostas por blendas do polímero conjugado com um eletrólito polimérico condutor iônico. As propriedades ópticas dos dispositivos foram analisadas através dos espectros de absorção óptica na região do ultravioleta/visível e de emissão (foto e eletroluminescência) enquanto as propriedades de injeção e de transporte de carga foram exploradas através de medidas elétricas de corrente-voltagem (I-V), espectroscopia de impedância no domínio da freqüência (condutividade ac), espectroscopia de fotocorrente e ressonância magnética detectada eletricamente (EDMR). A influência de parâmetros como a estrutura dos dispositivos, os metais utilizados como eletrodos e a temperatura permitiram uma análise mais detalhada de alguns modelos teóricos utilizados na interpretação dos resultados experimentais, fornecendo, dessa forma, um maior conhecimento das propriedades físicas dos materiais estudados. / This PhD thesis is an extensive study of the operation mechanisms of polymeric light-emitting devices, with a particular focus on the injection and transport properties of charge-carriers in poly(p-phenylene vinylene), PPV, derivatives. Therefore, it was necessary to dominate all the processes of fabrication and characterization of such devices, from the chemical synthesis of the polymers to the device fabrication, which are briefly described along this work. Two different kinds of devices were studied: polymeric ligh-emitting diodes (PLEDs) composed by a single thin layer (100-500 nm thick) of the pure electroluminescent polymer, and light-emitting electrochemical cells (LECs), which active layers are formed by a blend of the conjugated polymer and an ionic conductive polymeric electrolyte. The optical properties of the devices were analyzed by optical absorption in the ultraviolet-visible range and emission (photo- and electroluminescence) spectra. The charge injection and transport properties were studied by electrical measurements like current-voltage (I-V) curves, impedance spectroscopy in the frequency domain (ac conductivity), photocurrent spectroscopy and electrically detected magnetic resonance (EDMR). The influence of parameters like the device structure, the electrodes work function and the temperature allowed a detailed analysis of some theoretical models commonly used in the interpretation of the experimental results, providing more information about the physical properties of the studied conjugated polymers.

Esta tese de doutorado é o resultado de um estudo dos mecanismos de operação de dispositivos poliméricos emissores de luz, com um particular enfoque nos processos de injeção e transporte de portadores de carga em polímeros derivados do poli(p-fenileno vinileno), PPV. Para tanto, se fez necessário o domínio de todas as etapas de produção e caracterização, desde a síntese química dos polímeros até a fabricação propriamente dita dos dispositivos, as quais são brevemente descritas neste trabalho. Basicamente, dois tipos diferentes de dispositivos foram caracterizados: diodos poliméricos emissores de luz (PLEDs), nos quais a camada ativa é composta por um filme fino (100-500 nm de espessura) do polímero eletroluminescente puro, e células eletroquímicas emissoras de luz (LECs), compostas por blendas do polímero conjugado com um eletrólito polimérico condutor iônico. As propriedades ópticas dos dispositivos foram analisadas através dos espectros de absorção óptica na região do ultravioleta/visível e de emissão (foto e eletroluminescência) enquanto as propriedades de injeção e de transporte de carga foram exploradas através de medidas elétricas de corrente-voltagem (I-V), espectroscopia de impedância no domínio da freqüência (condutividade ac), espectroscopia de fotocorrente e ressonância magnética detectada eletricamente (EDMR). A influência de parâmetros como a estrutura dos dispositivos, os metais utilizados como eletrodos e a temperatura permitiram uma análise mais detalhada de alguns modelos teóricos utilizados na interpretação dos resultados experimentais, fornecendo, dessa forma, um maior conhecimento das propriedades físicas dos materiais estudados. / This PhD thesis is an extensive study of the operation mechanisms of polymeric light-emitting devices, with a particular focus on the injection and transport properties of charge-carriers in poly(p-phenylene vinylene), PPV, derivatives. Therefore, it was necessary to dominate all the processes of fabrication and characterization of such devices, from the chemical synthesis of the polymers to the device fabrication, which are briefly described along this work. Two different kinds of devices were studied: polymeric ligh-emitting diodes (PLEDs) composed by a single thin layer (100-500 nm thick) of the pure electroluminescent polymer, and light-emitting electrochemical cells (LECs), which active layers are formed by a blend of the conjugated polymer and an ionic conductive polymeric electrolyte. The optical properties of the devices were analyzed by optical absorption in the ultraviolet-visible range and emission (photo- and electroluminescence) spectra. The charge injection and transport properties were studied by electrical measurements like current-voltage (I-V) curves, impedance spectroscopy in the frequency domain (ac conductivity), photocurrent spectroscopy and electrically detected magnetic resonance (EDMR). The influence of parameters like the device structure, the electrodes work function and the temperature allowed a detailed analysis of some theoretical models commonly used in the interpretation of the experimental results, providing more information about the physical properties of the studied conjugated polymers.

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.