Extended donor and acceptor molecules for organic electronics

Die Untersuchung von halbleitenden Materialien auf der Basis von organischen Molekülen stellt ein Gebiet der angewandten Forschung an der Schwelle zur industriellen Nutzung dar. Geringes Gewicht und hohe mechanische Flexibilität ermöglichen völlig neue Produkte, die mit anorganischen Halbleitern nicht zu realisieren sind. Die Herstellung von Bauteilen wie Transistoren, Solarzellen oder Leuchtdioden aus organischen Materialien ist ein komplexes Gebiet, das einer Vielzahl von unterschiedlichen Optimierungen bedarf, um eine konkurrenzfähige Leistung zu erreichen. Die synthetische organische Chemie bietet vielfältige Möglichkeiten, mit maßgeschneiderten Lösungen zum Optimierungsprozess beizutragen. Zum einen können neue aktive Materialien hergestellt werden mit besserer Leistung und leichterer Verarbeitbarkeit. Zum anderen sind Substanzen zugänglich, die z.B. bei der Ladungsträgerinjektion hilfreich sein können.rnIn dieser Arbeit wurde an beiden dieser Fronten gearbeitet. Dabei lag die Entwicklungsstrategie darin, ausgedehnte π-konjugierte Moleküle herzustellen, die entweder besonders elektronenarme Akzeptoren oder elektronenreiche Donoren darstellen. Die genaue Kontrolle der elektronischen Niveaus stellt einen wichtigen Bestandteil dar, um niedrige elektrische Kontaktbarrieren zu Metallen zu erreichen und ausreichend stabile Materialien zu erreichen.rnDer erste Fokus der Arbeiten lag in der Funktionalisierung von Coronen. Dieser PAH stellt einen guten Kompromiss bezüglich seiner Größe dar: Er ist groß genug, um Diffusion in andere Schichten von Bauteilen zu vermeiden, aber nicht zu groß, um Verarbeitung durch Vakuumsublimation zu ermöglichen. Bislang sind praktisch keine Coronen-Derivate in der Literatur beschrieben, weshalb eine neue Synthese entwickelt werden musste, die die Einführung starker Donor- und Akzeptorfunktionalitäten erlaubt. Die photochemische Cyclodehydrierung von substituierten [2.2.2]paracyclophan-trienen stellte sich als hervorragende Möglichkeit heraus, dies zu bewerkstelligen. Es wurde eine Reihe von methoxy-substitutierten Coronenen mit unterschiedlicher Symmetrie hergestellt. Mittels optischer Spektroskopie konnte gezeigt werden, dass Methoxygruppen wenig Einfluss auf die elektronischen Eigenschaften von Coronen haben. Unter Spaltung der Methylether und anschließender Oxidation allerdings sind Coronenketone zugänglich, welche bis zu drei α-Diketongruppen besitzen. Diese Moleküle sind enorm starke Akzeptoren, was durch Cyclovoltammetrie und Vergleich zu anderen Akzeptoren eindrucksvoll gezeigt werden konnte. Die Sublimation dieses Akzeptors auf die Oberfläche von Metallen zeigt einen dramatischen Einfluss auf die Austrittsarbeit dieses Metalls, was zur Herstellung eines ohmschen Kontakts zu organischen Halbleitern von außerordentlichem Nutzen ist. rnDen zweiten Teil der Arbeit bilden Benzodithiophen enthaltende Polymere, die für den Einsatz als aktive Komponente in elektronischen Bauteilen entwickelt wurden. Nach systematischer Strukturoptimierung wurde ein Polymer enthalten, welches in einem Feldeffekt-Transistor auf Standard-Silizium-Substraten Ladungsträger-Mobilitäten über 0,1 cm2/Vs erreicht mit großer Reproduzierbarkeit und ausgezeichneter Transistor-Charakteristik. Es konnte gezeigt werden, dass die durch die Monomergeometrie erzeugte Kurvung des Polymers zu einem optimalen Kompromiss aus Löslichkeit und effektiver Packung darstellt. Auf für industrielle Anwendungen besonders interessanten polymer-basierten Substraten wurde eine noch erheblich bessere Leistung gezeigt. Auf einem PET-Substrat wurden Feldeffekt-Mobilitäten von 0,5 cm2/Vs gemessen mit überzeugenden Reproduzierbarkeit und Stabilität.rnDamit konnte in der Arbeit ein bedeutender Beitrag zur Weiterentwicklung von Materialien für den Einsatz in elektronischen Bauteilen geleistet werden. Die Substanzen versprechen noch erhebliches Potenzial nach intensiver Optimierung und wurden deshalb zum Patent angemeldet.rn

Variation of stable silicon isotopes: Analytical developments and applications in Precambrian geochemistry

Die vorliegende Dissertation behandelt die Gesamtgesteinsanalyse stabiler Siliziumisotope mit Hilfe einer „Multi Collector-ICP-MS“. Die Analysen fanden in Kooperation mit dem „Royal Museum for Central Africa“ in Belgien statt. Einer der Schwerpunkte des ersten Kapitels ist die erstmalige Analyse des δ30Si –Wertes an einem konventionellen Nu PlasmaTM „Multi-Collector ICP-MS“ Instrument, durch die Eliminierung der den 30Si “peak” überlagernden 14N16O Interferenz. Die Analyse von δ30Si wurde durch technische Modifikationen der Anlage erreicht, welche eine höherer Massenauflösung ermöglichten. Die sorgsame Charakterisierung eines adäquaten Referenzmaterials ist unabdingbar für die Abschätzung der Genauigkeit einer Messung. Die Bestimmung der „U.S. Geological Survey“ Referenzmaterialien bildet den zweiten Schwerpunkt dieses Kapitales. Die Analyse zweier hawaiianischer Standards (BHVO-1 and BHVO-2), belegt die präzise und genaue δ30Si Bestimmung und bietet Vergleichsdaten als Qualitätskontrolle für andere Labore. Das zweite Kapitel befasst sich mit kombinierter Silizium-/Sauerstoffisotope zur Untersuchung der Entstehung der Silizifizierung vulkanischer Gesteine des „Barberton Greenstone Belt“, Südafrika. Im Gegensatz zu heute, war die Silizifizierung der Oberflächennahen Schichten, einschließlich der „Chert“ Bildung, weitverbreitete Prozesse am präkambrischen Ozeanboden. Diese Horizonte sind Zeugen einer extremen Siliziummobilisierung in der Frühzeit der Erde. Dieses Kapitel behandelt die Analyse von Silizium- und Sauerstoffisotopen an drei unterschiedlichen Gesteinsprofilen mit unterschiedlich stark silizifizierten Basalten und überlagernden geschichteten „Cherts“ der 3.54, 3.45 und 3.33 Mill. Jr. alten Theespruit, Kromberg und Hooggenoeg Formationen. Siliziumisotope, Sauerstoffisotope und die SiO2-Gehalte demonstrieren in allen drei Gesteinsprofilen eine positive Korrelation mit dem Silizifizierungsgrad, jedoch mit unterschiedlichen Steigungen der δ30Si-δ18O-Verhältnisse. Meerwasser wird als Quelle des Siliziums für den Silizifizierungsprozess betrachtet. Berechnungen haben gezeigt, dass eine klassische Wasser-Gestein Wechselwirkung die Siliziumisotopenvariation nicht beeinflussen kann, da die Konzentration von Si im Meerwasser zu gering ist (49 ppm). Die Daten stimmen mit einer Zwei-Endglieder-Komponentenmischung überein, mit Basalt und „Chert“ als jeweilige Endglieder. Unsere gegenwärtigen Daten an den „Cherts“ bestätigen einen Anstieg der Isotopenzusammensetzung über der Zeit. Mögliche Faktoren, die für unterschiedliche Steigungen der δ30Si-δ18O Verhältnisse verantwortlich sein könnten sind Veränderungen in der Meerwasserisotopie, der Wassertemperatur oder sekundäre Alterationseffekte. Das letzte Kapitel beinhaltet potentielle Variationen in der Quellregion archaischer Granitoide: die Si-Isotopen Perspektive. Natriumhaltige Tonalit-Trondhjemit-Granodiorit (TTG) Intrusiva repräsentieren große Anteile der archaischen Kruste. Im Gegensatz dazu ist die heutige Kruste kaliumhaltiger (GMS-Gruppe: Granit-Monzonite-Syenite). Prozesse, die zu dem Wechsel von natriumhaltiger zu kaliumhaltiger Kruste führten sind die Thematik diesen Kapitels. Siliziumisotopenmessungen wurden hier kombiniert mit Haupt- und Spurenelementanalysen an unterschiedlichen Generationen der 3.55 bis 3.10 Mill. Yr. alten TTG und GMS Intrusiva aus dem Arbeitsgebiet. Die δ30Si-Werte in den unterschiedlichen Plutonit Generationen zeigen einen leichten Anstieg der Isotopie mit der Zeit, wobei natriumhaltige Intrusiva die niedrigste Si-Isotopenzusammensetzung aufweisen. Der leichte Anstieg in der Siliziumisotopenzusammensetzung über die Zeit könnte auf unterschiedliche Temperaturbedingungen in der Quellregion der Granitoide hinweisen. Die Entstehung von Na-reichen, leichten d30Si Granitoiden würde demnach bei höheren Temperaturen erfolgen. Die Ähnlichkeit der δ30Si-Werte in archaischen K-reichen Plutoniten und phanerozoischen K-reichen Plutoniten wird ebenfalls deutlich.

Studio delle proprieta ottiche di film di silicon oxynitrides (sion) per applicazioni fotovoltaiche

Nelle celle solari HIT (Heterojunction Intrinsic Thin layer) attualmente in circolazione il materiale maggiormente utilizzato è sicuramente il silicio, per la sua buona caratteristica di assorbimento e disponibilità in natura. Tuttavia, la struttura amorfa del silicio impiegato come emettitore, limita fortemente la conducibilità e aumenta gli effetti di degradazione legati all'esposizione alla luce. In quest'ottica, nel presente lavoro di tesi, vengono analizzati sottili layers di Silicon Oxynitride ossigenato, depositati in forma nanocristallina presso l'Università di Costanza su substrati in vetro, tramite PECVD (Plasma-Enhanced Chemical Vapor Deposition). Il materiale, che non presenta i difetti intrinseci della forma amorfa, è dal punto di vista delle proprietà fisiche fondamentali ancora poco conosciuto e rappresenta una possibile alternativa agli analoghi campioni in forma amorfa. Le misure e le analisi presentate in questa tesi, svolte presso il Dipartimento di Fisica e Astronomia di Bologna, sono finalizzate ad indagare le principali proprietà ottiche, quali l'energy gap e l'indice di rifrazione dei layers. I risultati ottenuti, espressi in funzione dei parametri di deposizione, mostrano che un aumento della concentrazione di ossigeno in fase di deposito implica un aumento lineare dell'ampiezza dell'energy gap e un calo dell'indice di rifrazione. Anche altri parametri come la potenza di deposito e il tempo di annealing sembrano giocare un ruolo importante sul valore dell'energy gap. I risultati appaiono inoltre essere in buon accordo con quanto ottenuto da studi precedenti su layers simili ma con una maggiore fase amorfa. In conclusione, la possibilità di regolare il valore dell'energy gap e l'indice di rifrazione in base ai parametri di deposizione, unita alle buone prerogative tipiche dei materiali cristallini, conferisce al materiale buone prospettive per applicazioni fotovoltaiche come emettitori in celle ad eterogiunzione o rivestimenti con proprietà antiriflettenti

Thiophene-containing organic semiconducting heteroacenes for electronic applications

Diese Studie verfolgt das Konzept der "Oligomer-Ansatz", die von Müllen angesprochen wurde et.al. vor etwa 10 Jahren. Der Schwerpunkt dieser Arbeit war die Synthese, Charakterisierung und Anwendung von halbleitenden konjugierten heteroacenes für organische Elektronik.rnZur weiteren Entwicklung der Familie von schwefelhaltigen Pentacene, zwei Moleküle (Benzo [1,2 - b :4,5-b '] bis [b] benzodithiophene und dithieno-[2,3-d: 2', 3'-d ']-benzo-[1,2-b :4,5-b'] dithiophene)rnfacilely wurden synthetisiert und charakterisiert durch eine Kombination verschiedener Methoden. Die beiden neue Moleküle weisen hervorragende ökologische Stabilität und angewendet OFETs Geräte als p-Kanal-Material. Die Vorversuche gaben Ladungsträgerbeweglichkeiten von 0,1 cm2 V-1 s-1 undrn1,6 cm2 V-1 s-1 bzw. aus den beiden Molekülen.rnAusgelöst durch die Frage "je länger desto besser?", Eine Reihe von neuen heteroheptacenes wurden synthetisiert und intensiv im Hinblick auf ihre feste Struktur, Selbst-assenbly auf der studierte Oberfläche, opto-elektro-Eigenschaften und Eigenschaften des Orbits Grenze. Einer derrnheteroheptacene Moleküle wurden als die aktiven Kanäle in OFET Geräten angewendet. Jedoch in Trotz der mehr verlängert Konjugationslänge die Geräte auf der Basis zeigten heptacenes viel schlimmer Ladungsträgerbeweglichkeiten als die heteropentacenes. Viele Faktoren können Festlegung der endgültigen Leistung der Produkte und der chemischen Struktur ist nur einer von ihnen.rnIn dieser Hinsicht scheint es, dass es auch sinnvoll, den Einfluss der Heteroatome Studie und Alkylsubstitution auf der soliden und elektronischen Strukturen. Daher mehr heteroheptacenes wurden synthetisiert. Abwechslungsreiches in der Anzahl und Art der heteroatomare Brücke,rndiese Oligoazene ausgestellt dramatisch anders feste Struktur und opto-elektronischernEigenschaften. Darüber hinaus wurde eine kombinierte DFT Berechnung der Molekülorbitale dieser heptacenes darauf hingewiesen, dass die Einführung von Stickstoff Brücken wird die π-Orbitale zu destabilisieren, während stabilisieren den Schwefel Brücken sowohl HOMO und LUMO Energien. Dies ist wichtig, wenn man will hoch π verlängert Oligoazene synthetisieren und dabei eine angemessene Stabilität.

Development of an X-ray spectrometric system and feasibility tests of Silicon Drift Detector for medical and space applications

The thesis work concerns X-ray spectrometry for both medical and space applications and is divided into two sections. The first section addresses an X-ray spectrometric system designed to study radiological beams and is devoted to the optimization of diagnostic procedures in medicine. A parametric semi-empirical model capable of efficiently reconstructing diagnostic X-ray spectra in 'middle power' computers was developed and tested. In addition, different silicon diode detectors were tested as real-time detectors in order to provide a real-time evaluation of the spectrum during diagnostic procedures. This project contributes to the field by presenting an improved simulation of a realistic X-ray beam emerging from a common X-ray tube with a complete and detailed spectrum that lends itself to further studies of added filtration, thus providing an optimized beam for different diagnostic applications in medicine. The second section describes the preliminary tests that have been carried out on the first version of an Application Specific Integrated Circuit (ASIC), integrated with large area position-sensitive Silicon Drift Detector (SDD) to be used on board future space missions. This technology has been developed for the ESA project: LOFT (Large Observatory for X-ray Timing), a new medium-class space mission that the European Space Agency has been assessing since February of 2011. The LOFT project was proposed as part of the Cosmic Vision Program (2015-2025).

Film di silicon oxinitride (sion) per applicazioni fotovoltaiche, studio di transizioni ottiche.

I materiali per applicazioni fotovoltaiche hanno destato un interesse crescente nella comunità scienti�ca negli ultimi decenni. Le celle HIT (Het- erojunction Intrinsic Thin Layer ) sono dispositivi di ultima generazione che hanno raggiunto e�cienza elevata mantenendo bassi i costi di pro- duzione, e impiegano silicio amorfo (a-Si) come strato emettitore per il suo buon assorbimento della luce e silicio cristallino come regione attiva. La struttura amorfa del silicio presenta però una bassa conducibilità, oltre ad e�etti di degradazione che limitano considerevolmente la durevolezza di una cella. Per questo motivo si stanno cercando possibili alternative al silicio amorfo, in particolare strutture multifase e composti di silicio, ossigeno ed azoto. In questo lavoro sono esposti i risultati dell'analisi di sottili lay- er di Silicon Oxynitride ossigenato (SiOx Ny :H), in forma microcristallina, deposti tramite PECVD (P lasma-Enhanced Chemical Vapor Deposition) su vetro presso l'università di Costanza. La forma microcristallina è una distribuzione di agglomerati cristallini dell'ordine di grandezza di un mi- crometro in una matrice di silicio amorfo, e attualmente le sue proprietà ottiche ed elettroniche non sono ancora state studiate in maniera appro- fondita. Nonostante ciò, è invece evidente che la fase microstallina non presenta tanti difetti intrinseci come la forma amorfa e ne è quindi una val- ida alternativa. In questa ottica, si è svolto uno studio sperimentale delle proprietà ottiche di layers in forma microcristallina di SiOx Ny :H, quali la misura del gap energetico. I risultati sperimentali, volti a trovare la dipen- denza delle caratteristiche dai parametri di deposizione dei layers, hanno mostrato una riduzione del gap energetico con la concentrazione di N2 O, uno dei gas precursori utilizzati nella deposizione dei layers in camera di processo. In conclusione si può dire che il μc−SiOx Ny :H ha le buone carat- teristiche tipiche dei semiconduttori cristallini, che unite alla possibilità di regolare il valore del gap energetico in base alle scelte in fase di deposizione, gli conferisce buone prospettive per applicazioni in celle fotovoltaiche, come emettitore in celle ad eterogiunzione.

Silicon Photonics Integrated Circuits for Flexible Optical Systems

This dissertation deals with the design and the characterization of novel reconfigurable silicon-on-insulator (SOI) devices to filter and route optical signals on-chip. Design is carried out through circuit simulations based on basic circuit elements (Building Blocks, BBs) in order to prove the feasibility of an approach allowing to move the design of Photonic Integrated Circuits (PICs) toward the system level. CMOS compatibility and large integration scale make SOI one of the most promising material to realize PICs. The concepts of generic foundry and BB based circuit simulations for the design are emerging as a solution to reduce the costs and increase the circuit complexity. To validate the BB based approach, the development of some of the most important BBs is performed first. A novel tunable coupler is also presented and it is demonstrated to be a valuable alternative to the known solutions. Two novel multi-element PICs are then analysed: a narrow linewidth single mode resonator and a passband filter with widely tunable bandwidth. Extensive circuit simulations are carried out to determine their performance, taking into account fabrication tolerances. The first PIC is based on two Grating Assisted Couplers in a ring resonator (RR) configuration. It is shown that a trade-off between performance, resonance bandwidth and device footprint has to be performed. The device could be employed to realize reconfigurable add-drop de/multiplexers. Sensitivity with respect to fabrication tolerances and spurious effects is however observed. The second PIC is based on an unbalanced Mach-Zehnder interferometer loaded with two RRs. Overall good performance and robustness to fabrication tolerances and nonlinear effects have confirmed its applicability for the realization of flexible optical systems. Simulated and measured devices behaviour is shown to be in agreement thus demonstrating the viability of a BB based approach to the design of complex PICs.

Transport Properties and Novel Sensing Applications of Organic Semiconducting Crystals

The present thesis is focused on the study of Organic Semiconducting Single Crystals (OSSCs) and crystalline thin films. In particular solution-grown OSSC, e.g. 4-hdroxycyanobenzene (4HCB) have been characterized in view of their applications as novel sensors of X-rays, gamma-rays, alpha particles radiations and chemical sensors. In the field of ionizing radiation detection, organic semiconductors have been proposed so far mainly as indirect detectors, i.e. as scintillators or as photodiodes. I first study the performance of 4HCB single crystals as direct X-ray detector i.e. the direct photon conversion into an electrical signal, assessing that they can operate at room temperature and in atmosphere, showing a stable and linear response with increasing dose rate. A dedicated study of the collecting electrodes geometry, crystal thickness and interaction volume allowed us to maximize the charge collection efficiency and sensitivity, thus assessing how OSSCs perform at low operating voltages and offer a great potential in the development of novel ionizing radiation sensors. To better understand the processes generating the observed X-ray signal, a comparative study is presented on OSSCs based on several small-molecules: 1,5-dinitronaphthalene (DNN), 1,8-naphthaleneimide (NTI), Rubrene and TIPS-pentacene. In addition, the proof of principle of gamma-rays and alpha particles has been assessed for 4HCB single crystals. I have also carried out an investigation of the electrical response of OSSCs exposed to vapour of volatile molecules, polar and non-polar. The last chapter deals with rubrene, the highest performing molecular crystals for electronic applications. We present an investigation on high quality, millimeter-sized, crystalline thin films (10 – 100 nm thick) realized by exploiting organic molecular beam epitaxy on water-soluble substrates. Space-Charge-Limited Current (SCLC) and photocurrent spectroscopy measurements have been carried out. A thin film transistor was fabricated onto a Cytop® dielectric layer. The FET mobility exceeding 2 cm2/Vs, definitely assess the quality of RUB films.

Nanocrystalline Silicon Based Films for Renewable Energy Applications

The present thesis is focused on the study of innovative Si-based materials for third generation photovoltaics. In particular, silicon oxi-nitride (SiOxNy) thin films and multilayer of Silicon Rich Carbide (SRC)/Si have been characterized in view of their application in photovoltaics. SiOxNy is a promising material for applications in thin-film solar cells as well as for wafer based silicon solar cells, like silicon heterojunction solar cells. However, many issues relevant to the material properties have not been studied yet, such as the role of the deposition condition and precursor gas concentrations on the optical and electronic properties of the films, the composition and structure of the nanocrystals. The results presented in the thesis aim to clarify the effects of annealing and oxygen incorporation within nc-SiOxNy films on its properties in view of the photovoltaic applications. Silicon nano-crystals (Si NCs) embedded in a dielectric matrix were proposed as absorbers in all-Si multi-junction solar cells due to the quantum confinement capability of Si NCs, that allows a better match to the solar spectrum thanks to the size induced tunability of the band gap. Despite the efficient solar radiation absorption capability of this structure, its charge collection and transport properties has still to be fully demonstrated. The results presented in the thesis aim to the understanding of the transport mechanisms at macroscopic and microscopic scale. Experimental results on SiOxNy thin films and SRC/Si multilayers have been obtained at macroscopical and microscopical level using different characterizations techniques, such as Atomic Force Microscopy, Reflection and Transmission measurements, High Resolution Transmission Electron Microscopy, Energy-Dispersive X-ray spectroscopy and Fourier Transform Infrared Spectroscopy. The deep knowledge and improved understanding of the basic physical properties of these quite complex, multi-phase and multi-component systems, made by nanocrystals and amorphous phases, will contribute to improve the efficiency of Si based solar cells.

HCl gas gettering of 3d transition metals for crystalline silicon solar cell concepts

In order to reduce the costs of crystalline silicon solar cells, low-cost silicon materials like upgraded metallurgical grade (UMG) silicon are investigated for the application in the photovoltaic (PV) industry. Conventional high-purity silicon is made by cost-intensive methods, based on the so-called Siemens process, which uses the reaction to form chlorosilanes and subsequent several distillation steps before the deposition of high-purity silicon on slim high-purity silicon rods. UMG silicon in contrast is gained from metallurgical silicon by a rather inexpensive physicochemical purification (e.g., acid leaching and/or segregation). However, this type of silicon usually contains much higher concentrations of impurities, especially 3d transition metals like Ti, Fe, and Cu. These metals are extremely detrimental in the electrically active part of silicon solar cells, as they form recombination centers for charge carriers in the silicon band gap. This is why simple purification techniques like gettering, which can be applied between or during solar cell process steps, will play an important role for such low-cost silicon materials. Gettering in general describes a process, whereby impurities are moved to a place or turned into a state, where they are less detrimental to the solar cell. Hydrogen chloride (HCl) gas gettering in particular is a promising simple and cheap gettering technique, which is based on the reaction of HCl gas with transition metals to form volatile metal chloride species at high temperatures.rnThe aim of this thesis was to find the optimum process parameters for HCl gas gettering of 3d transition metals in low-cost silicon to improve the cell efficiency of solar cells for two different cell concepts, the standard wafer cell concept and the epitaxial wafer equivalent (EpiWE) cell concept. Whereas the former is based on a wafer which is the electrically active part of the solar cell, the latter uses an electrically inactive low-cost silicon substrate with an active layer of epitaxially grown silicon on top. Low-cost silicon materials with different impurity grades were used for HCl gas gettering experiments with the variation of process parameters like the temperature, the gettering time, and the HCl gas concentration. Subsequently, the multicrystalline silicon neighboring wafers with and without gettering were compared by element analysis techniques like neutron activation analysis (NAA). It was demonstrated that HCl gas gettering is an effective purification technique for silicon wafers, which is able to reduce some 3d transition metal concentrations by over 90%. Solar cells were processed for both concepts which could demonstrate a significant increase of the solar cell efficiency by HCl gas gettering. The efficiency of EpiWE cells could be increased by HCl gas gettering by approximately 25% relative to cells without gettering. First process simulations were performed based on a simple model for HCl gas gettering processes, which could be used to make qualitative predictions.

Development of synthetic methods of silicon nanocrystals functionalized with photoactive molecules

The thesis is focused on the development of a method for the synthesis of silicon nanocrystals with different sizes, narrow size distribution, good optical properties and stability in air. The resulting silicon nanocrystals have been covalently functionalized with different chromophores with the aim to exploit the new electronic and chemical properties that emerge from the interaction between silicon nanocrystal surface and ligands. The purpose is to use these chromophores as light harvesting antennae, increasing the optical absorption of silicon nanocrystals. Functionalized silicon nanocrystals have been characterized with different analytical techniques leading to a good knowledge of optical properties of semiconductor quantum dots.

Characterization of integrated Bragg gratings in silicon-on-insulator

Silicon-on-insulator (SOI) is rapidly emerging as a very promising material platform for integrated photonics. As it combines the potential for optoelectronic integration with the low-cost and large volume manufacturing capabilities and they are already accumulate a huge amount of applications in areas like sensing, quantum optics, optical telecommunications and metrology. One of the main limitations of current technology is that waveguide propagation losses are still much higher than in standard glass-based platform because of many reasons such as bends, surface roughness and the very strong optical confinement provided by SOI. Such high loss prevents the fabrication of efficient optical resonators and complex devices severely limiting the current potential of the SOI platform. The project in the first part deals with the simple waveguides loss problem and trying to link that with the polarization problem and the loss based on Fabry-Perot Technique. The second part of the thesis deals with the Bragg Grating characterization from again the point of view of the polarization effect which leads to a better stop-band use filters. To a better comprehension a brief review on the basics of the SOI and the integrated Bragg grating ends up with the fabrication techniques and some of its applications will be presented in both parts, until the end of both the third and the fourth chapters to some results which hopefully make its precedent explanations easier to deal with.

Characterization of X-ray detectors based on organic semiconducting single crystals

Negli anni recenti, lo sviluppo dell’elettronica organica ha condotto all’impiego di materiali organici alla base di numerosi dispositivi elettronici, quali i diodi ad emissione di luce, i transistor ad effetto di campo, le celle solari e i rivelatori di radiazione. Riguardo quest’ultimi, gli studi riportati in letteratura si riferiscono per la maggiore a dispositivi basati su materiali organici a film sottile, che tuttavia presentano problemi relativi ad instabilità e degradazione. Come verrà illustrato, l’impiego di singoli cristalli organici come materiali alla base di questi dispositivi permette il superamento delle principali limitazioni che caratterizzano i rivelatori basati su film sottili. In questa attività sperimentale, dispositivi basati su cristalli organici semiconduttori verranno caratterizzati in base alle principali figure di merito dei rivelatori. Tra i campioni testati, alcuni dispositivi basati su singoli cristalli di 6,13-bis (triisopropylsilylethynyl)-pentacene (TIPS-Pentacene) e 5,6,11,12-tetraphenyltetracene (Rubrene) hanno mostrato interessanti proprietà e sono stati quindi maggiormente studiati.

Looking for silicene: studies of silicon deposition on metallic and semiconductor substrates

Nel presente lavoro espongo i risultati degli esperimenti svolti durante la mia internship all’Institut des NanoSciences de Paris (INSP), presso l’Università Pierre et Marie Curie (Paris VI), nel team "Phisico-Chimie et Dynamique des Surfaces", sotto la supervisione del Dott. Geoffroy Prévot. L’elaborato è stato redatto e in- tegrato sotto la guida del Dott. Pasquini, del dipartimento di Fisica e Astronomia dell’Università di Bologna. La tesi s’inserisce nel campo di ricerca del silicene, i.e. l’allotropo bidimensionale del silicio. Il cosidetto free-standing silicene è stato predetto teoricamente nel 2009 utilizzando calcoli di Density Functional Theory, e da allora ha stimolato un’intensa ricerca per la sua realizzazione sperimentale. La sua struttura elettronica lo rende particolarmente adatto per eventuali appli- cazioni tecnologiche e sperimentali, mentre lo studio delle sue proprietà è di grande interesse per la scienza di base. Nel capitolo 1 presento innanzitutto la struttura del silicene e le proprietà previste dagli studi pubblicati nella letteratura scientifica. In seguito espongo alcuni dei risultati sperimentali ottenuti negli ultimi anni, in quanto utili per un paragone con i risultati ottenuti durante l’internship. Nel capitolo 2 presento le tecniche sperimentali che ho utilizzato per effettuare le misure. Molto tempo è stato investito per ottenere una certa dimistichezza con gli apparati in modo da svolgere gli esperimenti in maniera autonoma. Il capitolo 3 è dedicato alla discussione e analisi dei risultati delle misure, che sono presentati in relazione ad alcune considerazioni esposte nel primo capitolo. Infine le conclusioni riassumono brevemente quanto ottenuto dall’analisi dati. A partire da queste considerazioni propongo alcuni esperimenti che potrebbero ulteriormente contribuire alla ricerca del silicene. I risultati ottenuti su Ag(111) sono contenuti in un articolo accettato da Physical Review B.

Novel materials for direct X-ray detectors based on semiconducting organic polymers

Conventional inorganic materials for x-ray radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, me- chanical sti ffness, lack of tissue-equivalence and toxicity. Semiconducting organic polymers represent an alternative and have been employed as di- rect photoconversion material in organic diodes. In contrast to inorganic detector materials, polymers allow low-cost and large area fabrication by sol- vent based methods. In addition their processing is compliant with fexible low-temperature substrates. Flexible and large-area detectors are needed for dosimetry in medical radiotherapy and security applications. The objective of my thesis is to achieve optimized organic polymer diodes for fexible, di- rect x-ray detectors. To this end polymer diodes based on two different semi- conducting polymers, polyvinylcarbazole (PVK) and poly(9,9-dioctyluorene) (PFO) have been fabricated. The diodes show state-of-the-art rectifying be- haviour and hole transport mobilities comparable to reference materials. In order to improve the X-ray stopping power, high-Z nanoparticle Bi2O3 or WO3 where added to realize a polymer-nanoparticle composite with opti- mized properities. X-ray detector characterization resulted in sensitivties of up to 14 uC/Gy/cm2 for PVK when diodes were operated in reverse. Addition of nanoparticles could further improve the performance and a maximum sensitivy of 19 uC/Gy/cm2 was obtained for the PFO diodes. Compared to the pure PFO diode this corresponds to a five-fold increase and thus highlights the potentiality of nanoparticles for polymer detector design. In- terestingly the pure polymer diodes showed an order of magnitude increase in sensitivity when operated in forward regime. The increase was attributed to a different detection mechanism based on the modulation of the diodes conductivity.