Electron Microscopy Characterization of Pentacene and Perfluoropentacene Grown on Different Substrates

This thesis deals with the study of the morphology, arrangement and orientation of organic semiconductor films by (scanning) transmission electron microscopy ((S)TEM) techniques. The organic semiconductor perfluoropentacene (PFP) as well as the organic heterostructures of pentacene (PEN) and PFP have been investigated. PFP has been grown on graphene substrate, while the organic mixtures formed by PEN and PFP have been deposited with different mixing ratios on two different substrates, i.e. SiO2 and KCl. PFP deposited on graphene exhibits an epitaxial growth in island shapes where the molecules lie flat and parallel to the substrate adopting the so called ‘π-stacked polymorph’. Within this work, the lateral alignment of the PFP molecules with respect to the graphene substrate has been determined. It was found that the long molecular axis of PFP is aligned along the zig-zag direction of the graphene. However, this alignment is not exactly parallel, but exhibits a small offset. Furthermore, the morphology of the PFP islands has been investigated. A characteristic angle around 68° was found between confining edges of PFP islands. The combination of TEM micrographs and electron diffraction patterns has enabled the determination of the planes that run parallel to the confining edges of the islands ‘as seen’ by the electron beam in the two-dimensional projection. From that the possible side facets associated with each confining edge have been suggested. Finally, electron tomography experiments were used to gain insight into the shape of the PFP islands, allowing the 3D reconstruction of them. PEN:PFP blends have been prepared with mixing ratios of [2:1], [1:1] and [1:2] on an inert substrate such as SiO2. Although different phases and morphologies have been observed for each mixture, a mixed phase made out of PEN and PFP which exhibits similar lattice parameters in all cases has been found independently of the mixing ratio. The monocrystalline SAED pattern of the mixed phase has been shown for the first time on this substrate. The diffraction pattern is rather similar to the one of the pure PEN in �0 0 1� direction, suggesting that the crystal structure of the mixed phase is similar to the one of pure PEN. For non-equimolecular blends, the respective pure phase in excess is present apart from the mixed phase. A different morphology was observed for the different PEN:PFP mixing ratios. The equimolecular mixture of PEN and PFP exhibits fiber-like structures consisting of the mixed phase. For the mixture with PFP in excess, some fibers are formed on a background layer. The PFP is contained in the fibers, while the background layer is made out of the mixed phase. For the mixture with PEN in excess, a grainy structure (grain size of 10 nm-60 nm) with contributions of pure PEN and of the mixed phase is detected. PEN:PFP blends with mixing ratios of [2:1] and [1:2] grown on KCl substrates have been investigated too. The mixed phase formed by PEN and PFP is also present and both blends reveal a quite different morphology. The composition, orientation and crystalline details of each phase have been inspected. In the blend with PEN in excess, the mixed phase together with the pure PEN phase are found in a uniform layer formed with domains that are rotated in-plane by 90° towards each other. In contrast, the blend with excess of PFP presents two different arrangements. The majority of the sample exhibits some spicular fibers made out of PFP on a background layer composed by the mixed phase. The other arrangement present to a lesser extent consists of a film of pure PFP lying in direct contact with the KCl substrate. The importance of PFP grown on graphene lies in the relevance of the graphene substrate together with the π-stacked arrangement exhibited by PFP on this substrate. This motif enhances charge carrier mobility along the stacking direction. The knowledge of the relative alignment as well as the faceting are a key information since the physical properties depend on these parameters. Furthermore, considering the role of the organic heterostructures in the development of organic electronic devices, a detailed understanding of the basic arrangement of the organic molecules in the organic blend is a requirement for the development of new organic devices

Using Molecular Design to Influence Intermolecular Interactions

This thesis describes the impact of molecular design on intermolecular interactions. Chapter 2 explores tuning the properties of contorted hexabenzocoronene (HBC) derivatives to improve photovoltaic performance. First, the interaction between contorted HBC derivatives with varying degrees of "bowl" character and fullerenes are explored in solution. Association constants were determined by fluorescence quenching experiments with fullerenes C70, C60, and Phenyl-C61-butyric acid methyl ester (PCBM). NMR titration experiments mimic fluorescence quenching results that suggest that association in solution increases with shape-complementarity between donor and acceptor. Second, efforts towards the synthesis of azulene HBC, an HBC derivative with red-shifted absorption, are discussed. Calculations of this target molecule and a selected intermediate are compared to those of the parent contorted HBC. Finally, an azulene HBC synthetic intermediate is explored as a potential sensor. Chapter 3 presents a study of the single molecule conductance of cobalt chalcogenide clusters. The synthesis of cobalt chalcogenide clusters decorated with a variety of conjugated molecular connectors was developed. Single molecule conductance of these clusters was shown to take place through the molecular connectors, and was tunable by controlling the substitution of the connectors. The tunability of cluster conductance that was demonstrated in the single molecule experiments was shown to extend to thin film experiments in chapter 4. Preliminary investigation into the mechanism of conductance of these films is discussed. In chapter 5, a family of nickel telluride clusters with a variety of ligands is synthesized. The X-ray crystal structures of these clusters are analyzed and insight into how ligand sterics and electronics influence the final cluster structure is discussed

Tailoring surfaces and interface properties by kinetically activated processes controlled by Supersonic Beam Deposition

This PhD thesis is focused on the study of a critical topic in material science, i.e. the full comprehension of the processes occurring at the interface between organic/inorganic materials, and among them. An innovative experimental approach is here proposed: the Supersonic Molecular Beam Deposition (SuMBD), a technique that exploits the great potentiality of a supersonic molecular flux, the kinetic activation of surface processes. It gives an excellent control of beam properties (Kinetic energy and momentum) that in turn leads to the possibility to control surface and interface properties during the film growth and/or synthesis. For this purpose, three different test case have been analyzed: i) SuMBD ability to control electronic properties in organic films and their correlation with the OFET electronics; ii) the synthesis of carbon based nanostructured materials, such as graphene, by using supersonic fullerene on copper, studying the ability to induce the cage rupture exploiting the mentioned kinetic activation; iii) the functionalization of SiO2 based surfaces by porphyrin partially fluorinated (H2TPP(F)), in particular SiC/SiO2 core-shell nanowires, for applications in biomedical field, and Si(100)/SiO2 planar surface for comparison.Questa tesi di dottorato si focalizza sullo studio di un aspetto fondamentale della scienza dei materiali, quale la comprensione dei processi che avvengono all’interfaccia tra materiali organici/inorganici, e tra loro stessi. Viene proposto un approccio sperimentale innovativo: la Deposizione da Fasci Molecolari Supersonici (SuMBD), una tecnica che sfrutta la peculiarità di un flusso molecolare supersonico, ossia la possibilità di attivare cineticamente processi di superficie. Questo approccio permette un ottimo controllo delle proprietà del fascio (energia cinetica e momento) che a sua volta consente di controllare le proprietà della superficie e dell’interfaccia durante la crescita dei film o durante processi di sintesi. A tal proposito, sono stati analizzati tre casi differenti: i) la capacità della tecnica SuMBD di controllare le proprietà elettroniche di film organici e la loro correlazione con l’elettronica di OFET; ii) la sintesi di materiali nano strutturati a base di carbonio, come il grafene, da fasci supersonici di fullerene su rame, studiando inoltre la capacità di indurre la rottura della gabbia sfruttando la sopra citata attivazione cinetica; iii) la funzionalizzazione di superfici a base di porfirina parzialmente fluorurata (H2TPP(F)), in particolare nanofili costituiti da un cuore centrale di SiC ricoperto da SiO2, per applicazione nella biomedicina, e confronto di questo sistema con una superficie planare di Si(100)/SiO2

Functionalization of Oxidic Semiconductor Surfaces by Covalently Bound Molecular Thin Films

The study was intended to achieve covalent fixation of self-assembled monolayers on metal oxide surfaces, in particular rutile titanium dioxide and zinc oxide (ZnO), and to distinguish the microscopic character of their linkage.The results obtained confirm the importance of a detailed, comprehensive analysis of such experiments. The main achievement of this work is a very profound investigation of the influence of preparation pathways as well as stru cture of supporting substrates on the resulting thin film quality and stability. Many essential issues crucial for the attachment of PA-based SAMs on metal oxide surfaces were addressed for the first time in this work. Obtained results reveal possibilities for further investigations of some phenomena

Hybrid solar cells

Solarzellen sind als "alternative Energiequelle" mehr denn je im Fokus von Forschung und Entwicklung. Derzeit basieren praktisch alle kommerziell erhältlichen Module auf klassischen Halbleitermaterialien wie Silizium. Dieses ist in sehr hoher Reinheit und einkristallin verfügbar, woraus sehr gute Materialeigenschaften resultieren. Gewinnung und Reinigung sind allerdings sehr kostenintensiv und energieaufwendig. Insbesondere weist Silizium, im Vergleich zu Farbstoffmolekülen, einen sehr geringen Absorptionskoeffizienten auf. Durch Verwendung von organischen Halbleitern kann daher u.a. die Schichtdicke von Solarzellen drastisch reduziert werden. Neben d\"unnen Schichten verspricht man sich von günstiger Prozessierung erheblich niedrigere Herstellungskosten. Aufgrund anderer Transportmechanismen (Hopping-Transport) zeigen organische Halbleiter eine erheblich niedrigere Ladungsträgermobilität als anorganische Halbleiter (Bandtransport). Zudem ist in organischen Solarzellen nach der Photonenabsorption eine Trennung der noch gebundenen Ladungträger (Exzitonen) nötig. Die Kombination aus organischen und anorganischen Halbleitern für die Photovoltaik wird Hybrid-Solarzellen genannt. Hiervon verspricht man sich die Nutzung der hohen Absorbanz des organischen Materials und der guten Transporteigenschaften der verwendeten anorganischen Halbleiter. Bislang kamen hauptsächlich Polymere zum Einsatz. Wenig Erfahrung gibt es hingegen in der Kombination von anorganischen Halbleitern und kleinen Molekülen mit aromatischen Ringen. Diese zeigen gute optische Eigenschaften. Dies wurde in der vorliegenden Arbeit am Beispiel von Zink(II)-Phthalocyanin (ZnPc) nachgewiesen. Optische Spektroskopie wurde verwendet, um die optischen Konstanten, Schichtdicke und Rauigkeit der Schichten simultan zu bestimmen. Eine organisch-anorganische Grenzfläche innerhalb einer Hybrid-Solarzelle wurde aus ZnPc und Zinkoxid hergestellt und charakterisiert. Hierfür wurde mittels Photelektronenspektroskopie der Verlauf der Bandstruktur innerhalb des Bauelemtes nachvollzogen. Mit Hilfe dieser Methode wurden Abschätzungen für die Leerlaufspannung getroffen und anhand von Strom-Spannungs-Kennlinien überprüft. Die Kennlinien weisen einen sehr geringen Photostrom auf. Die Ursache dafür scheint eine schlechte Exzitonendissoziation zu sein. Hierfür wurden zwei Verbesserungsansätze gewählt. Zum einen wurde die Bandstruktur mittels Dotierung modifiziert, um die Energie zu erhöhen, welche für die Exzitonentrennung zur Verfügung steht. Zum anderen sollte durch Nanodrähte die Distanz zum dissoziationsverursachenden p-n-Übergang verringert werden, um so in die Reichweite der Exzitonen zu gelangen. Anhand von spektral aufgelösten Photostrommessungen konnte die Exzitonendiffusionslänge auf 16 nm bestimmt werden. Eine Steigerung der Effizienz wurde leider nicht erzielt

Site specific characterisation of hydrocracking catalysts using nanoanalytical electron microscopy

During use, carbonaceous material or ‘coke’ can deposit on catalysts resulting in decreased activity and lifetime. In this thesis, the results of investigations into the structure and distribution of coke, on hydrocracking catalysts, are reported. The material consists of zeolite Y, alumina binder as well as tungsten and nickel sulfide. An extensive investigation regarding the preparation of the catalysts for electron microscopy was carried out. It was established that microtoming produced specimen damage and hence regions of porosity, zeolite and alumina binder were difficult to identify. Single beam and dual beam focused ion beam (FIB) milling produced intact specimens and the spatial distribution of the catalysts was maintained, although thinner specimens were obtained using the latter technique. Energy-dispersive X-ray (EDX) mapping identified gallium and platinum as artefacts in specimens that had been prepared by a single beam FIB system. In addition, argon ion beam milling was used and this technique produced large regions of thin material. Energy-filtered transmission electron microscopy (EFTEM) was employed to reveal the distribution of carbon in the catalyst. Carbon was identified on alumina binder, zeolite grains and meso-/macro-pores, although the distribution of carbon was generally not uniform as it is determined by the density and strength of acid sites, geometry of pores and the proximity of metal sulfide crystallites. All of these factors, especially pores size and shape, vary in the catalysts. Coke is thought to consist of polyaromatic hydrocarbons (PAHs). Electron energy-loss spectroscopy (EELS), of selected PAH standards, was performed to obtain the electron energy-loss near edge structure (ELNES) of carbon. In addition, the ELNES of four PAHs was modelled using multiple scatter calculations. EELS of the catalysts revealed that PAHs are present on zeolitic components but ELNES was not identified on the alumina binder. This is possibly because alumina contains larger pores than zeolite Y; therefore larger molecules can diffuse into the alumina structure, which increases the chemical variety of the coke species as the molecules are not sterically impeded

Synthesis of Conjugated Polymers and Small Molecules for Organic Light-Emitting Devices and Photodetectors

Production cost and environmental impact are the two major concerns that are related to the conventional optoelectronic devices. It is desirable for the modern semiconductors that they are free of toxic/costly metals, they can be processed with low-cost solution-based methods, and their optical, electronic, and mechanical properties can be easily tuned depending on the target application. In this thesis, a range of different conjugated polymers and small molecules are designed and synthesized as semiconductors for organic light-emitting diodes (OLEDs), light-emitting electrochemical cells (LECs), and organic photodetectors (OPDs).In organic light-emitting devices, the emissive molecule is commonly mixed with a charge transporting host matrix, which can be either a small molecule or a conjugated polymer. The latter is beneficial since it does not require deposition of the emitter and matrix components in high vacuum and high temperature conditions. The polymeric materials can be dissolved and printed on a substrate of any desired size and production scale, at room temperature, and even under ambient air. The specific wavelength range of near-infrared (NIR) at λ >700 nm is of interest for a wide range of applications spanning from optical communication to biosensing. However, the low energy of NIR range poses challenges for the materials design, in terms of emission efficiency and light intensity, which are further addressed in this thesis, allowing the fabrication of high-performance NIR-OLEDs and NIR-LECs.For photodetectors, absorption of a wide spectrum of light is beneficial in biosensing and imaging applications. Low noise and fast charge extraction are necessary for the detection of light at high speeds even at low intensities. These aspects are studied in this thesis by designing new polymers with different absorption, charge transport, and morphological properties in the photoactive layer. Two polymers enabled the fabrication of visible (red) OPDs with a low dark current (the main constituent in the noise), high detectivity, and high photoresponse speed

Organic Hole Transport Materials: Properties and Interface Formation

This dissertation is related to organic semiconductor (OSC) materials for organic light emitting diode (OLED) applications investigated together with our project partner Merck KGaA. One often stated advantage of OSCs is the chemical flexibility to easily synthesize new molecules. A deeper understanding of the connection between molecular structure and device characteristics could lead to faster material screening and development. As contribution to this goal, the here presented work analyzes and discusses the properties of hole transport materials (HTMs) and especially their electronic interface properties. Indium tin oxide (ITO) substrates, a commercially available p-dopant, CPTCFA, a literature-known HTM, m-MTDATA, a commercial HTM from Merck KGaA, HTM-B, and aluminum are characterized as thin films by photoelectron spectroscopy (PES) to obtain the materials’ electronic properties. Ultraviolet, visible, near- and mid-infrared absorption spectroscopy on pure and p-doped HTM thin films correlated with density functional theory calculations provides insight into the doping mechanism. For CPTCFA:m-MTDATA, an integer charge transfer is observed. However, this is not the case for CPTCFA:HTM-B, suggesting the formation of a charge transfer complex in this case. The electronic properties at ITO | (p-)HTM hetero- and p-HTM | HTM homointerfaces are studied by PES in step-by-step deposition experiments. A novel density of states-based model for fitting the PES data is presented. This model is able to reproduce the classically obtained results at the heterointerfaces while providing more details and accurate electrical potential distributions. More importantly the model allows for the analysis of the homointerfaces and reveals an unexpected space charge region in the p-doped HTM layer. After the model is advanced, an increased number of states in the energy gap for the molecules of the undoped layer right at the interface is predicted. The interfaces between Al back-contacts and HTM as well as p-doped HTM layers are investigated. These interfaces are relevant for hole-only devices which are used to study effects on less complex device structures. It is shown that Al tends to diffuse into the organic thin film where it reacts with the dopant molecule, redopes the material, and strongly changes the electronic properties. This could be a problem, as this HTM | Al back-contact interface appears to have a strong influence on the properties of hole-only devices, potentially leading to conclusions which do not hold for full devices, where this interface does not exist. Finally, a low excitation energy electron emission (termed L4E) effect is observed. Free electrons are emitted by shining an ultraviolet LED on m-MTDATA thin films even though the photon energy is lower than the ionization potential of the material. The effect is most likely related to a triplet-triplet-annihilation mechanism and an application as room temperature electron source is discussed

Single-crystal growth of organic semiconductors and organic electronic applications

This thesis describes single crystal growth of organic semiconductors and their applications in organic electronics. In Chapter 2, a self-assembled monolayer of C60 derivatives with five carboxylic acids was employed to make C60 thin-film transistors. The highly-crystalline C60 monolayer enabled more ordered deposition of C60 thin film, which resulted in higher electrical characteristics when the thin film was used in field-effect transistors (FET). The C60 film with the monolayer was characterized by IR, XPS, and FET measurements. In Chapter 3, organic single crystal FET of rubrene was studied. Graphene was used as an organic electrode instead of conventional gold film. Graphene is a sp2-hybridized carbon crystal which has high conductivity with negligible thickness. These factors are advantageous in organic FET as the graphene-organic interface has no Schottky barrier and structural contact resistance. The FET of rubrene showed much higher hole transport mobility compared to conventional FET using gold electrodes. In Chapter 4, morphology dependence of dibenzotetrathienocoronene (DBTTC) crystal growth on different substrates was studied. Crystal growth of DBTTC on a single-layer graphene surface shows vertical growth of nanowire crystals not observed with other substrates. GIXD was measured to confirm this crystal morphology. Chapter 5 discusses organic solar cells composed of hexabenzocoronene (HBC) and C60 and their improvement by utilizing the co-crystallization effect of these two molecules. In Chapter 6, HBC was fluorinated using two different ring-closing reactions, and their energy difference as well as conformation change was investigated

Grain boundaries in ultrathin organic semiconductors

Organic semiconductors stand out over their inorganic counterparts, since they can be processed in thinner films and at lower energies, onto nearly any surface, and with tunable optical and electrical properties for specific purposes. These unique properties of organic semiconductors allow to save material, energy, space and cost and make them ideal for applications in customer-specific end-products. Although organic semiconductors have been implemented in semiconductor devices such as organic solar-cells, organic light-emitting diodes, organic field-effect transistors or sensors for years, these devices still show inferior performances compared to inorganic devices, especially in terms of reduced mobility, efficiency, reproducibility and stability. It is widely accepted that grain boundaries in organic semiconductors are one of the main responsibles for these drawbacks, since they act as trapping, recombination and/or degradation sites. However, why and how grain boundaries emerge (structurally and energetically), and which properties of grain boundaries mainly influence the device performance is still under investigation. Since addressing these questions will help to control charge transport in organic semiconductors and improve device performance, this work presents a fundamental investigation of grain boundaries in monolayer-thin films of an organic small molecule. These films stand out due to high crystallinity and atomically smoothness across grain boundaries. This, as well as their thinness, allows to characterize single grains and grain boundaries at the location where charge transport takes place in organic field-effect transistors, namely at the semiconductor-insulator interface. By Kelvin probe force microscopy (KPFM) grain boundaries are found as a first result to act as energy barriers or valleys, and different thin-film application techniques are presented resulting in films in which either a specific type of grain boundary predominates, or in films where barriers and valleys coexist. While it is particularly advantageous for future experiments to be able to control the existence of different types of grain boundaries in organic materials, the films with both types prove the fundamental difference between energy barriers and valleys. KPFM measurements not only allow a qualitative differentiation of barriers and valleys, but also a quantitative description of „grain boundary heights“. Valley depths and barrier heights can both be decreased by increasing the charge-carrier density in the organic semiconductor-film. However, they only vanish at charge-carrier densities above the typical operating regime of organic solar-cells and organic light-emitting diodes, which underlines the relevance of investigating charge transport at grain boundaries. Consequently, time-resolved KPFM measurements are conducted to investigate the trapping and detrapping mechanisms at grain boundaries and other local impurities, as well as their influence on global device parameters. While valleys trap charge carriers in deep traps, barriers backscatter electrons, but also indicate an increased trap-state density at the organic-semiconductor interface, thereby leading to a stronger reduction of charge transport than valleys. Valleys, on the contrary, are found to mainly define the global device parameters such as the turn-on and threshold voltage or the qualitative behavior of hysteresis. This finding underlines the need to be able to control not only the grain-boundary density in organic semiconductors, but also their type and absolute height. However, since it is challenging to control the emergence and electric properties of grain boundaries in organic semiconductors by experimental methods, an alternative experiment is presented with the aim to manipulate charge transport across grain boundaries by illumination with far-infrared light. It is assumed that photons from this light source are absorbed, leading to the excitation of charge carriers out of valleys or across barriers and thus to a measurable photocurrent. This photocurrent can be measured energy-resolved by using a modified Fourier transform infrared spectrometer, which allows to detect and characterize grain boundaries even in bulk-like materials. Finally, charge transport in a novel metal-organic framework is investigated directionally, globally and locally, to put the role of grain boundaries in organic semiconductors into a context. It is found that in this special material grain boundaries do not play an as important role as the stacking direction of single planes of the metal-organic framework. To summarize, the findings of this work lead toward controlling the properties of grain boundaries in organic semiconductors and their role in organic semiconductor devices such as field-effect transistors, organic solar-cells or organic light-emitting diodes