Charge Transfer in Poly (3-Hexylthiophene) / Titanium Dioxide Inverse Opals: Effects of Surface Passivation and Donor Concentration

Hybrid photovoltaics are attractive because of their low cost and ability to be applied onto flexible substrates. However defect states can trap charges and are undesirable. We investigate TiO2 inverse opals treated with titanium tetrachloride (TiCl4) and subsequently coated with poly(3-hexylthiophene) (P3HT) to elucidate the effect of surface passivation on the photoinduced charge transfer and polaron dynamics. The chemical, physical and morphological properties were characterized using UV-vis spectroscopy, reflectance microscopy, X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Passivation resulted in increased wall thickness of the inorganic framework and crystallite size. Photoinduced absorption spectroscopy showed enhanced polaron absorptions and reduced polaron lifetimes with increased titanium tetrachloride concentration and reduced concentration of the solution from which the P3HT is cast. The 3D structure presents an opportunity to study the charge transfer within a percolated network

Nanoestructuración y propiedades de superficies de polímeros con aplicaciones en energía

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 21/11/2016The integration of functional polymers in organic electronics has attracted great interest for their potential application in photovoltaics or diodes due to their characteristics such as high chemical tenability, low temperature processing, light weight and durability, among others. The incorporation of polymers into nowadays devices, that tend towards miniaturization, rises several challenges. In general, the macroscopic properties of polymers are closely related to their structure, that is hierarchical, from the nanometer to the millimeter scale. Hence, from a fundamental viewpoint, understanding the effect of the above mentioned miniaturization in the structure and physical phenomena would provide control over the properties of the nanoscaled polymer materials, helping in the design of new potential applications. In this Thesis we have attempted to fulfill the objective of preparing binary systems formed by pairs of functional polymers or pairs of organics materials, and understanding the modification of certain polymer properties when nanostructuring them, mainly in their surfaces. For the preparation of these binary systems we employed different methodologies: direct solution blending (Chapter 3, blends of donor/acceptor organic compounds, Chapter 4, blends of semiconducting and ferroelectric polymers), bilayer structures from semiconducting/ ferroelectric polymers, prepared by sequential spin coating (Chapter 4) and nanostructuring of a ferroelectric polymer in the form of nanospheres to be incorporated in a semiconducting polymer film (Chapter 4). In Chapter 3, the conduction mechanism and the molecular dynamics on a bulk heterojunction formed by a binary blend of donor/acceptor organic compounds have been studied by dielectric spectroscopy. In Chapter 4, the modification of the ferroelectric properties in poly(vinylidene fluoride- trifluoroethylene) copolymers due to nanostructuring and to the combination with a semiconducting polymer have been addressed by Piezoresponse force microscopy. Nanostructured functional polymer surfaces were prepared by laser techniques, mainly Laser Induced Periodic Surface Structures (LIPSS). In Chapter 5, we first report on the creation of LIPSS on a model polymer: polystyrene...La integración de polímeros funcionales en la electrónica orgánica es de gran interés por su potencial aplicación en dispositivos fotovoltaicos y diodos, debido principalmente a características tales como la alta resistencia química, la posibilidad de procesado a baja temperatura, su ligereza y durabilidad, entre otras. La incorporación de polímeros en los dispositivos actuales que tienden a la miniaturización afronta varios retos. En general, las propiedades macroscópicas de polímeros están estrechamente relacionadas con su estructura, que es jerárquica desde la escala de los nanómetros a la milimétrica. Por lo tanto, desde el punto de vista fundamental, comprender el efecto de la mencionada miniaturización en la estructura y los fenómenos físicos de estos polímeros proporcionaría control sobre las propiedades de los materiales poliméricos incorporados en dispositivos ayudando al diseño de nuevas aplicaciones potenciales. En esta Tesis hemos tratado de cumplir con el objetivo de preparar sistemas binarios formados por pares de polímeros funcionales o pares de materiales orgánicos, y comprender las variaciones de ciertas propiedades del polímero cuando se nanoestructura, sobre todo cuando se crean superficies nanoestructuradas. Para la preparación de estos sistemas binarios se emplearon diferentes metodologías: la mezcla directa en solución (Capítulo 3, mezclas de los compuestos orgánicos donadores / aceptores, Capítulo 4, mezclas de polímeros semiconductores y ferroeléctricos), las estructuras de dos capas de polímeros semiconductores / ferroeléctricos, preparadas por ‘spin coating’ secuencial (Capítulo 4) y nanoestructuración de polímero ferroeléctrico en forma de nanoesferas que se incorpora a una película de polímero semiconductor (Capítulo 4). En lo que se refiere al estudio de procesos físicos fundamentales en estos sistemas, en el Capítulo 3, el mecanismo de conducción y la dinámica molecular en una heterounión en volumen formada por la mezcla de compuestos orgánicos donador / aceptor se ha estudiado mediante espectroscopia dieléctrica. En el Capítulo 4, la modificación de las propiedades ferroeléctricas en copolímeros al azar de poli(fluoruro de vinilideno) y poli(trifluoroetileno) debida a nanoestructuración y a la combinación con polímeros semiconductores han sido abordadas por Microscopía de Piezorespuesta...Fac. de Ciencias FísicasTRUEunpu

Developing Structural Probes for Designed Molecular Architectures

This thesis is concerned with the development of techniques to probe and control molecular morphology in organic semiconductors and investigate structure-property relationships. An angle-dependent polarised Raman technique is developed to probe molecular order and orientation in 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) thin lms, and applied to understand the role of blending with polystyrene on morphology and charge transport properties in inkjet-printed organic field-effect transistors (OFETs). Compared to pristine devices, blending improves film uniformity in terms of device coverage and molecular orientation and increases saturation mobility from [Mathematical formula appears here. To view, please open pdf attachment]. A zone-casting apparatus is developed to systematically control the grain size and molecular orientation of TIPS-pentacene thin films. Processing parameters such as solvent, solution temperature, substrate temperature and casting speed are systematically controlled to study film coverage, grain size, alignment and orientation, and applied to investigate charge transport properties in TIPS-pentacene and 6,13-bis(triethylsilylethynyl) pentacene (TES-pentacene) OFETs. Casting speed and grain orientation strongly influence device performance. TIPS- pentacene shows higher mobilities and grain-boundary-limited transport while TES-pentacene charge transport is limited by crystal orientation. The anisotropic structural and optical properties of TIPS-pentacene and TES-pentacene films are characterised using polarised UV-visible absorption and Raman spectroscopy. Evidence for J- and H-aggregation is found from a packing-induced red-shifted and blue-shifted absorption transition, respectively. Angle-dependent polarised Raman spectroscopy is used to probe the molecular orientation and order in the films. The pentacene long-axis orientation is found to be ±(50-60)° relative to the zone-casting direction for both materials, while TIPS-pentacene shows a higher Raman anisotropy than TES-pentacene. Quantum-chemical studies are performed on a range of conjugated polymer systems. Using simulated structure-property relationships, the effect of side chain placement & positioning, blending with small molecules, thermal annealing and heavy-atom substitution on the opto- electronic and charge transport properties of the polymers are discussed

Morphology and Charge Transport in Polymer Organic Semiconductor Field-Effect Transistors

Ph.DDOCTOR OF PHILOSOPH

Charge separation in organic photovoltaics

Polymere mit Halbleiter-Eigenschaften haben ein großes Anwendungspotential in der organischen Photovoltaik, da sich ihre optischen und elektronischen Eigenschaften über die molekulare Struktur gezielt ändern lassen. Durch die Synthese von Copolymeren mit besonders kleiner optischer Bandlücke (low-bandgap Copolymere) konnte die Absorption von Sonnenlicht weiter in den infraroten Spektralbereich ausgedehnt und somit die Konversion von Sonnenlicht in elektrische Energie deutlich verbessert werden. Diese neuartigen Donor-Akzeptor Materialien basieren auf einer alternierenden Anordnung von elektronen-reichen und -armen Blöcken, die durch elektronische Kopplung neue Energieniveaus mit kleinerer optischer Bandlücke bilden. Ziel dieser Arbeit ist die eingehende Untersuchung der photophysikalischen Eigenschaften dieser weitgehend unerforschten Moleküle. Die ersten drei Kapitel bieten dem Leser eine Einführung in das Forschungsgebiet und in die theoretische Beschreibung konjugierter Polymere, sowie einen Überblick über den aktuellen technischen Stand organischer Photovoltaik. Kapitel 4 gibt eine Zusammenfassung der verwendeten experimentellen und theoretischen Methoden. Der erste Teil der Untersuchung von Donor-Akzeptor Materialien gilt den Photoanregungen und der korrekten Zuordnung ihrer spektralen Signaturen (Kap. 5). Diese ermöglicht eine Zuordnung der spektralen Signaturen zu stark gebundenen, elektrisch neutralen Exzitonen, bzw. leichter zu trennenden Ladungsträgerpaaren mit kleinerer Bindungsenergie, sogenannten Polaronenpaaren. Aufgrund der schwachen elektrischen Abschirmung von Ladungen in organischen Materialen liegen die meisten Photoanregungen als Exzitonen vor. In dieser Hinsicht zeigen spektroskopische Messungen auf Femtosekunden-Zeitskala erstmals den andersartigen Charakter von Donor-Akzeptor Materialien und demonstrieren den großen Einfluss ihrer Struktur auf die Art der erzeugten Photoanregungen. Sie zeigen, dass bei Photoanregungen dieser neuartigen Materialien neben Exzitonen auch ein beträchtlicher Anteil an Polaronenpaaren entsteht. Diese Donor-Akzeptor Materialien weisen einen Polaronenpaar-Anteil von bis zu 24% aller Photoanregungen auf, was dem Dreifachen der Effizienz vergleichbarer Homopolymere entspricht (Kap. 6). Weitere Untersuchungen zeigen außerdem eine erhöhte Erzeugungsrate bei kürzeren Anregungswellenlängen. Dies kann auf eine Korrelation mit einem ausgeprägten Elektronentransfer der involvierten Wellenfunktion zurückgeführt werden, welcher in theoretischen Simulationen deutlich wird (Kap. 7). Zusammenfassend geben die in dieser Arbeit dargestellten Ergebnisse einen detaillierten Einblick in die optischen und elektronischen Eigenschaften von Donor-Akzeptor Copolymeren und den starken Einfluss der molekularen Struktur auf die ersten Schritte der photovoltaischen Stromerzeugung. Zusammenhänge zweier Schlüsselfaktoren für die Effizienzsteigerung zukünftiger organischer Solarzellen mit Materialparametern werden deutlich. Dies sind die Erzeugungseffizienz und die Lebensdauer von Polaronenpaaren und deren Abhängigkeit von der Elektronegativität und der Abstand von Akzeptor- zu benachbarten Donorsegmenten. Weiterhin konnte eine ausgeprägte Polaronenpaar Erzeugung über das ganze Absorptionsspektrum nachgewiesen werden. Diese Erkenntnisse bieten eine große Hilfestellung bei der weiteren Optimierung von Polymeren für Photovoltaik. Außerdem heben sie den wichtigen Beitrag der Ultrakurzzeit Spektroskopie zum grundlegenden Verständnis der Polaronenpaarerzeugung hervor. Mit diesen Mitteln könnte eine Verringerung des Spannungsverlustes möglich werden, der zur Ladungsträgertrennung in organischen Materialien nötig ist.Polymeric semiconductors attract a great deal of attention due to their ability to achieve unique optical and electronic properties by tailoring their chemical structure. This allows synthesizing fully conjugated, novel materials with suitable functionalities for organic electronics. For organic photovoltaics, the development of a novel class of materials significantly improved the infrared light absorption and resulted in a better coverage of the solar spectrum. These novel low-bandgap materials, i.e. donor-acceptor copolymers, are based on an alternating arrangement of electron donating and accepting moieties within their repeating unit. They exhibit an exceptionally enhanced absorption behavior what is attributed to their highly electronic coupling. This work is focused on the physical properties of these novel and largely unexplored materials, with an emphasis on light absorption and charge carrier formation. In the first three chapters a proper introduction to the scientific field, the theoretical background and a brief overview on the state of the art of organic photovoltaics is given. Afterwards, the large variety of employed experimental and theoretical methods will be described in detail in the fourth chapter. A careful study of the photoexcited states and their spectral signatures in donor-acceptor copolymers is the first step of their characterization, as presented in the fifth chapter. This allows the assignment of different spectral features to strongly bound, electrically neutral excitons, or to spatially separated charge carriers with weaker Coulomb interaction, called polaron pairs. Due to the weak dielectric screening, which is typical of most organic materials, strongly bound excitons are the major species of formed photoexcitations. However, polaron pairs promise easier dissociation and thus are of great interest regarding their contribution to photocurrent. Observing photoexcitations on femtosecond timescale by ultrafast spectroscopy reveals for the first time a strong impact of the alternating donor-acceptor units on the evolution of generated photoexcitations, as presented in the sixth chapter. In these novel copolymers, not only excitons are formed by light absorption, but also a substantial amount of weakly bound polaron pairs. The found polaron pair yields in donor-acceptor materials reach up to 24% and are thus about three times higher than for commonly used homopolymers. Further experiments, applying different excitation energies, discovered even increased polaron pair yields when exciting with significant excess energy. A strong correlation between polaron pair yield and a calculated charge transfer character of the involved electron wave functions is found to be the origin of this improved behavior, as detailed in the seventh chapter. Further localization due to spatial restrictions in small donor-acceptor molecules significantly increases this effect and are shown to exhibit even higher polaron pair yields. Altogether, this study offers a detailed view on the optical and electronic properties of donor-acceptor copolymers. The results in this study illustrate the important role of chemical structure on the first steps of photovoltaic action, i.e. the formation of charge carriers and their dissociation. We can then relate two key factors for the success of future photovoltaic devices, namely polaron pair formation and their lifetime, to the material’s chemical structure, i.e. the acceptor electron affinity, its separation to the neighboring donor units and the molecular chain length. Furthermore, a substantial polaron pair yield through the whole absorption spectrum of such materials is demonstrated. These results provide the first platform for rational designing of new materials for organic photovoltaics and place ultrafast spectroscopy as an important tool for understanding the governing factors of polaron pair formation, which is of utmost importance in this context. By that, a successful reduction of the required voltage loss for dissociation and extraction of charge carriers from organic materials might thus become reality in near future

Solution-Processable Polymer Photocatalysts for Hydrogen Evolution from Water

Methods of storing renewable energy are urgently required to meet future energy demands. Photocatalytic hydrogen production from water represents an attractive method of storing solar energy for a diverse range of end-use applications. Semiconducting polymers are an emerging class of photocatalysts with eminently tunable structures and properties. However, the insolubility of most polymer photocatalysts limits processability and, therefore, opportunities to optimise the morphology of these materials for photocatalytic applications. Processability was achieved with the introduction of solubilising side-chains, which were systematically varied in order to study their influence on the photocatalytic performance of polymers. It was found that high hydrogen evolution rates could be achieved by incorporating oligo(ethylene glycol) side-chains, which seem to promote interaction with water during photocatalysis as well as affording solubility in common organic solvents. The polymer backbone was also varied to further improve the performance of solution-processable polymer photocatalysts. A fluorene-based polymer, FS-TEG, was prepared that displayed high activity under visible light, with an external quantum efficiency of 10.0% at 420 nm. Polymers were processed into a variety of forms, including photocatalytic films, both free-standing and cast on substrates. The substrate was varied to improve performance, with roughened glass slides found to achieve the highest areal hydrogen evolution rates. Photocatalytic polymers were also cast on planar substrates, which enabled precise control over film formation. Important parameters such as optimum film thicknesses for hydrogen evolution performance were subsequently established. Processability also enabled facile preparation of composites and blends. Incorporation of a narrow band gap dye was shown to triple the hydrogen evolution rate of FS-TEG films while the formation of heterojunctions with inorganic photocatalysts also enhanced performance. The scope for fabricating composites of this kind is boundless and, in the long term, devices capable of overall water splitting that utilise these materials are envisaged