42 research outputs found

    Some mechanics challenges and solutions in flexible electronics

    Get PDF
    Flexible electronics is an emerging field with potential applications such as large area flexible displays, thin film solar panels, and smart prosthesis, to name a few. Promising future aside, there are challenges associated with flexible electronics including high deformability requirements, needs for new manufacturing techniques and high performance permeation barriers. This thesis aims to explore possible solutions to address these challenges. First, a thin stiff film patterned with circular holes is proposed as a deformable platform to be used in flexible electronics in either component and circuit level. Second, we explore possible pathways to improve the quality of transfer printing, a nanofabrication technique that can potentially enable roll-to-roll printing of flexible devices. Third, we investigate the failure mechanism of multilayer permeation barriers for flexible electronics and offer an improved design to achieve better mechanical reliability

    Degradation of OLEDs devices: study methods and solutions

    Get PDF
    2014 - 2015The fast technological evolution of our world requests the necessity to make this development sustainable. In order to achieve this goal more relevance has been given to the energy consumption and environmental impact of the modern society consumerism. These considerations point out the need to increase the effort in research of unconventional materials, new concept and system architecture for devices that could help to overcome these challenges. Nanotechnologies offer the best chance for this innovation and particularly organic electronics has been encouraged thanks to new attractive properties and promising applications. Organic materials could be processed in thin film accommodating the issue of preservation of manufacturing energy usage despite from bulky and costly processes of inorganic industries. Fabrication techniques, including spin or spray coating, ink-jet or roll-to-roll printing, can be applied at low-temperature and over large area substrates. This is promising to maximize production throughput and to reduce costs. Also many other advances have been reached in the electronic applications of organic materials such as the spread of light-weight, transparent, flexible and disposable devices. Particularly, in recent years, OLEDs (Organic Light Emitting Diode) have been successful commercialized both for display applications and light source. OLED-based displays are nowadays commercialized both for high performances cell displays and TVs, becoming the major competitors of other well established technologies, like LCD or LED. In imaging application, very often Active Matrix approach is used to supply the display. In recent years the use of another Organic Electronics device, Organic TFT (OTFT), has been exploited as Active Matrix driver with encouraging results. In the last years also OLED-based light source have been introduced on the market. OLED panel technology allows overcoming some of the more critical issues of the preceding technologies such as very low energy conversion, as for incandescence source light, lack of good colors and the use toxic elements as for fluorescent tube and costly manufacturing processes and point light source as for the more recent LED technology. Despite all these amazing results, important drawbacks still remain in the field of organic electronic devices. Low charge carrier mobilities or still expensive manufacturing cost, but the more critical is extreme sensitivity to ambient conditions, temperature, light, and particularly oxygen and moisture, which could degrade their optical and electrical characteristics. This work of thesis sets in this scenario and the aim of this research is the study of degradation phenomena through methodology that leads to the identification of the different mechanisms of degradation involved, responsible of device short life time. The present work has been developed focusing the attention on the various degradation mechanisms in OLED and the possibility to use their nature to develop innovative analysis methods. For this purpose has been exploited both extrinsic and intrinsic degradation. Critical issues in this field are the complexity of the involved phenomena and the relatively recent interest of the scientific community in the intrinsic degradation topic. In the organization of the thesis work, after a brief presentation of the main features of organic electronics, an overview of the basic principles of permeation and typical encapsulation solutions are presented. In particular, specific attention has been paid on barrier requirements and solutions presented to protect the devices. These solutions, besides being the way to control extrinsic degradation, are also used to isolate other degradation components becoming a tool for studying the intrinsic degradation in organic devices. It has been also pointed out the importance to evaluate the barrier characteristic in terms of WVTR and here has been presented some solution to evaluate ultra-high barrier permeation. The study has been then centered on the design and development of a Calcium corrosion test for the evaluation of ultra-high barrier. It has been investigated all the aspect of layout and setup to achieve a higher sensitivity level. This measurement method has been employed for the design and optimization of both a glass to glass and Thin Film Encapsulation barrier permeation system. In the first case (glass to glass) the developed encapsulated system has been used in an effective way to estimate lifetime of a simple OLED structure and validate the system performances. Afterwards, the investigations have been focused on intrinsic degradation process. For this purpose the glass to glass encapsulation system has been also used to neglect external agents within the development of an innovative method based on accelerated environmental aging conditions for the study of intrinsic degradation phenomena components. Thus an innovative methodology to study this issue has been proposed and tested on a case study. In order to reach this goal resulted very important to separate extrinsic degradation component from the intrinsic one so in this thesis we worked on the following topics reaching very satisfying results: • reliable evaluation of permeation barriers using the electrical calcium corrosion test • glass to glass and thin film encapsulation permeation barrier system • devices (OLED) lifetime under accelerated aging conditions • intrinsic degradation study: methodology and application Regarding the first topic a measurement system based on Calcium corrosion test has been first studied, designed and developed taking into account every detail leading to a result alteration. Then first a glass to glass and later a thin film encapsulation system has been designed and developed in the same way. The Calcium corrosion test measurement system was then used to measure the barrier performances of the developed encapsulation systems revealing values of WVTR in line with literature results obtained through the selected techniques. Particularly had been detected a WVTR value of 4 10-6 g m-2 day-1 and approximately 10-2 g m-2 day-1 respectively for glass to glass and Thin Film Encapsulation system. Regarding OLED lifetime, using simple devices, experiments were conducted for one OLED-structure processed on glass. The structure used an ITO bottom contact as well as an aluminum top contact. The realized devices after 3000 hours have lost only a 20% in luminance from the initial value validating the performances of the developed encapsulation solution for devices realized on glass substrate in the same time of observation. This result if from one side is very encouraging for lifetime issue on the other side is the key element that allows neglecting external effect during further degradation study. The results achieved on these topics results precious to focus on the last aspect faced during this work. In fact using our encapsulation system and applying different experimental techniques, typically not employed, has been possible to put in light single intrinsic degradation mechanism involved. Particularly has been decided to focus on possible phenomena that can occur during OFF-time periods (without electrically stressing the samples). Experiments have been conducted at different storage conditions on two types of blue OLEDs representing the worst stability case. This study revealed , in condition that allow to neglect other degradation source, that physical aging occurs for both types of devices, leading to irreversible time-dependent luminance loss. The proposed method in this case study (accelerated environmental aging conditions) coupled with other more common used analysis conditions allows to study the degradation topic in a more complete way. [edited by author]Il rapido sviluppo tecnologico dell’era contemporanea richiede la necessità di rendere questo processo evolutivo sostenibile. Per raggiungere questo obiettivo una grande rilevanza è stato assegnata al consumo energetico e all'impatto ambientale del moderno consumismo della nostra società. Queste considerazioni sottolineano la necessità di aumentare gli sforzi nella ricerca di materiali non convenzionali, di nuova concezione e architetture di sistema per i dispositivi che possano aiutare a superare queste sfide. Il campo delle nanotecnologie offre le migliori possibilità per rendere attuabile questo tipo di innovazione ed in particolare l’elettronica organica si è messa in luce grazie alle sue interessanti proprietà e promettenti applicazioni. I materiali organici possono essere processati sotto in film sottili mitigando così il problema del consumo di energia di produzione, a differenza dei processi complessi e costosi usati dalle industrie di produzione di materiali inorganici. I dispositivi organici inoltre possono essere realizzati impiegando tecniche di fabbricazione innovative che comprendono il rivestimento a spruzzo, la stampa a getto d'inchiostro o la stampa roll-to-roll; tali tecniche richiedono basse temperature e offrono la possibilità di realizzare dispositivi su larga scala. Anche questo aspetto risulta promettente nell’ottica della massimizzazione del throughput di produzione e della riduzione dei costi. Molti altri vantaggi sono stati raggiunti con l’applicazione all’elettronica di materiali organici come ad esempio la diffusione di dispositivi leggeri, trasparenti, flessibili e smaltibili più facilmente rispetto a quelli classici. In particolare, negli ultimi anni, gli OLED (Organic Light Emitting Diode) hanno avuto un enorme successo commerciale sia per applicazioni nei display sia come sorgente luminosa. I display OLED sono oggi utilizzati sia in schermi ad alte prestazioni per cellulari che per televisori, diventando i principali concorrenti di altre tecnologie consolidate, come LCD o LED. In applicazione di questo tipo, molto spesso l'approccio a matrice attiva è utilizzato per alimentare il display stesso. Negli ultimi anni l'uso di un altro dispositivo elettronico basato su materiali organici, il TFT organico (OTFT), è stato sfruttato per polarizzare circuiti a matrice attiva con risultati incoraggianti. Negli ultimi anni anche sorgenti di luce OLED sono state introdotte sul mercato. Questo tipo di tecnologia infatti permette di superare alcuni dei problemi più critici delle tecnologie precedenti, come la bassa conversione di energia tipica delle lampadine ad incandescenza, la mancanza di buoni colori e l'uso di elementi tossici tipiche dei tubi a fluorescenza e processi produttivi costosi e sorgenti di luce puntiformi tipici della più recente tecnologia LED. Nonostante tutti questi risultati sorprendenti, esistono ancora alcune problematiche aperte nel campo dei dispositivi elettronici organici. La scarsa mobilità dei portatori o i costi di produzione ancora oggi piuttosto elevati ne limitano per ora la diffusione massiccia, ma l’aspetto più critico riguarda l’estrema sensibilità alle condizioni ambientali, temperatura, luce, e in particolare l'ossigeno e l'umidità, che possono modificare sensibilmente le caratteristiche ottiche ed elettriche di questi materiali. Questo lavoro di tesi si sviluppa in questo contesto con lo scopo di studiare i fenomeni di degrado mediante metodologie che portino all'identificazione dei diversi meccanismi di degrado coinvolti, responsabili del breve tempo di vita dei dispositivi. Il presente lavoro è stato sviluppato focalizzando l'attenzione sui vari meccanismi di degrado nei dispositivi OLED e sulla possibilità di utilizzare la loro natura per sviluppare metodi di analisi innovativi. A questo scopo è stato studiato sia il degrado estrinseco che quello intrinseco. Le criticità di questo studio sono la complessità dei fenomeni coinvolti e l’interesse relativamente recente che la comunità scientifica ha rivolto allo studio del degrado intrinseco. Nel lavoro di tesi, dopo una breve presentazione delle caratteristiche principali dell’ elettronica organica, è stata presentata dapprima una panoramica dei principi di base dei processi di permeazione e successivamente tipiche soluzioni di incapsulamento. In particolare, l’attenzione è stata focalizzata sui requisiti di barriera e sulle soluzioni per proteggere i dispositivi. Queste soluzioni, oltre ad essere il modo per controllare la degrado estrinseca, vengono anche utilizzati per isolare altre componenti di degrado diventando uno strumento per studiare il degrado intrinseco in dispositivi organici. È stata poi sottolineata l'importanza di poter valutare le prestazioni di materiali barriera in termini di WVTR e sono state presentate alcune soluzioni per la caratterizzazione di alte barriere alla permeazione di gas. Lo studio è stato poi centrato sulla progettazione e lo sviluppo di un Calcium Test per la valutazione di barriere alla permeazione di vapor d’acqua tenendo in conto tutti gli elementi che potessero garantire un livello maggiore di sensibilità. Questo metodo di misura è stato poi impiegato per la progettazione e ottimizzazione sia di un sistema di incapsulamento rigido su vetro e sia di un sistema di incapsulamento flessibile con la tecnica Thin Film Encapsulation. Nel primo caso (vetro su vetro) il sistema incapsulato sviluppato è stato utilizzato in in modo efficace per stimare il tempo di vita di una semplice struttura OLED e convalidare le prestazioni del sistema stesso. In seguito, le indagini si sono concentrate sul processo di degradazione intrinseca. A questo scopo il sistema di incapsulamento rigido è stato utilizzato anche per trascurare agenti esterni nell'ambito dello sviluppo di un metodo innovativo per lo studio dei vari fenomeni che concorrono al degrado intrinseco basato su condizioni ambientali di invecchiamento accelerato. Infine è stata proposta e testata su un caso studio una metodologia innovativa per studiare questo problema. Per raggiungere questo obiettivo è risultato molto importante separare la componente di degrado estrinseca da quella intrinseca così in questa tesi abbiamo lavorato sui seguenti argomenti raggiungendo risultati molto soddisfacenti: • valutazione attendibile delle barriere di permeazione tramite Calcium test elettrico • sistemi di incapsulamento rigido e flessibile • tempo di vita di dispositivi OLED in condizioni di invecchiamento accelerato • studio del degrado intrinseco: metodologia e applicazioni Per quanto riguarda il primo argomento un sistema di misura basato sul Calcium test elettrico è stato dapprima studiato, progettato e sviluppato tenendo conto ogni dettaglio che potesse portare ad un'alterazione risultato. Successivamente sono stati progettati e sviluppati sia un sistema di incapsulamento rigido su vetro sia un sistema di incapsulamento flessibile. Il sistema di misura tramite Calcium test è stato poi utilizzato per valutare le proprietà barriera dei sistemi di incapsulamento sviluppati rivelando valori di WVTR in linea con i risultati di letteratura ottenuti con le tecniche selezionate. In particolare è stato rilevato un valore di WVTR pari a 4 10-6 g m-2 day-1 e circa 10-2 g m-2 day-1 rispettivamente per il sistema vetro-vetro e quello flessibile. Riguardo al tempo di vita di dispositivi OLED, sono stati realizzati, incapsulati e caraterizzati nel tempo dispositivi su vetro utilizzando semplici strutture. I dispositivi realizzati dopo 3000 ore hanno perso solo 20% di luminanza rispetto al valore iniziale validando le prestazioni della soluzione di incapsulamento rigido sviluppata. Questo risultato se da un lato è molto incoraggiante per il tempo di vita dei dispositivi dall'altra parte rappresenta l'elemento chiave che permette di trascurare gli effetti esterni durante gli ulteriori studi sul degrado. I risultati ottenuti su questi argomenti sono risultati indispensabili per concentrarsi sul degrado intrinseco. Infatti utilizzando il nostro sistema di incapsulamento e l'applicazione di diverse tecniche sperimentali, non convenzionali, è stato possibile mettere in luce un singolo meccanismo di degrado intrinseco. In particolare, è stato deciso di concentrarsi su eventuali fenomeni che possono verificarsi durante i periodi di OFF-time (senza sollecitare elettricamente i campioni). Gli esperimenti sono stati condotti in diverse condizioni di conservazione su due tipi di OLED blu che rappresentano il caso peggiore in termini di stabilità. Questo studio ha rivelato, in condizioni che permettono di trascurare le altre fonti di degrado, che entrambi i tipi di dispositivi soffrono di un degrado di tipo fisico. Il metodo proposto in questo caso studio (condizioni ambientali di invecchiamento accelerato) combinato con altre condizioni di analisi (anche utilizzate comunemente) consente di studiare l'argomento degrado in modo più completo. [a cura dell'autore]XIV n.s

    Encapsulations for Organic Devices and their Evaluation using Calcium Corrosion Tests

    Get PDF
    This work investigates the encapsulation of organic light-emitting diodes (OLEDs) and organic solar cells (OSCs) in order to extend their lifetimes. Despite their unquestioned benefits, such as low material consumption and flexibility, their short lifetime span in ambient atmosphere is a clear disadvantage. For protection purposes, the devices are required to be encapsulated with permeation barriers. An appropriate barrier must have a water vapor transmission rate (WVTR) below 10^(-4) g(H2O) m^(-2) d^(-1) – below a monolayer of water permating through the barrier per day. Thus to design such barriers, a highly sensitive method for their evaluation is the primary requirement. Much fundamental research and setup development is thus performed in this work in order to improve the electrical calcium test to a sufficient level of sensitivity, reliability, and measurement capacity. The electrical calcium test uses a thin film of ignoble calcium and determines the amount of incoming water based on the decrease in its electrical conductance. In order to obtain highly precise results, this work identifies the reaction product (calcium hydroxide) and electrical resistivity of evaporated calcium films ((6.2 +- 0.1) 10^(-6-) Ohm cm). In contrast to a common assumption for the evaluation of calcium tests, calcium is found to corrode laterally inhomogeneous. However, it is shown theoretically and experimentally that this inhomogeneity does not distort the WVTR-measurement. Besides these fundamental investigations, calcium test design problems – as well as their solutions – are shown such as the damaging of an inorganic barrier film by an adjacent calcium sensor. As a result, a powerful and reliable measurement setup has been created. Subsequently, an investigation of a variety of barriers is presented, based on calcium tests, but also on device encapsulation and electroplating into defects: Permeation through evaporated aluminum thin films is found to occur mainly through macroscopic defects (radii > 0.4 μm) characterizable by optical inspection. Barriers made via atomic layer deposition (ALD) show improved performance with increasing layer thickness. Using ALD on foils provides excellent but, thus far, unreliable barriers. Permeation through bare polymer foils as well as sputtered zinc tin oxide (ZTO) increases roughly linear with increasing humidity and the measured WVTRs are highly comparable to reference values. The POLO barrier with a WVTR in the lower 10^(-4) g(H2O) m^(-2) d^(-1)-regime reaches the sensitivity limit of the current calcium test layout. In summary, in-depth investigations on permeation through different barriers are conducted here which reveal basic WVTR-dependencies from process- and climate parameters. Finally, water is identified as the predominant cause for device degradation, reducing the active area. For one type of both OLEDs and OSCs, the amount of water causing a 50% loss in active area (T50- water-uptake) is quantified via a comparative aging experiment involving calcium tests. Further for the case of the OSC, this T50-water-uptake of (20 +- 7) mg(H2O) m^(-2) is shown to be independent of climate conditions. As a result, the previously unspecific request for an aimed device lifetime can now be translated into a specific requirement for the permeation barrier: a water vapor transmission rate. Regarding the field of encapsulation, this work improves an essential measurement technique, characterizes a variety of permeation barriers, and investigates degradation of devices by ambient gases. The encapsulation field still poses several open questions. This work, however, strengthens the belief that organic devices will outlive them.:1 Introduction 2 Fundamentals 2.1 Organic Semiconductors 2.2 Organic Solar Cells 2.3 Organic Light-Emitting Diodes 2.4 Humidity, Evaporation, and Condensation 2.5 Principles of Permeation 3 State of the Art in Barrier Production and Evaluation 3.1 Barrier Technologies 3.2 Permeation Measurement Techniques 4 Experimental 4.1 Description of the As-Delivered Substrates 4.2 Treatment of Substrates 4.3 Deposition of Calcium Tests and Devices by Thermal Evaporation 4.4 Permeation Barriers by Atomic Layer Deposition 4.5 Defect Evaluation by Electrodeposition 5 Calcium for Permeation Tests Properties and Corrosion Behavior 5.1 Electrical Conductance and Optical Transmission 5.2 Corrosion Product 5.3 Laterally Inhomogeneous Calcium Corrosion 5.4 Implications for Optical and Electrical Calcium Corrosion Tests 6 Electrical Calcium Test 6.1 Measurement Setup 6.2 Calcium Test Layout 6.3 Comparability with Other Methods – OE-A Round Robin 6.4 Limitations and Future Prospects of the Electrical Calcium Test 6.5 Setup and Layout – Conclusions 7 Barrier Investigation 7.1 Thermally Evaporated Aluminum as Thin Film Encapsulation 7.2 ZnSnO (Magnetron Sputtered) on Polymer Foil 7.3 Al2O3 (ALD) on Polymer Substrate and as Thin Film Encapsulation 7.4 Summary and Conclusions for the Investigated Barriers 8 Encapsulation and Lifetime of Devices 8.1 Phenomenology of Device Degradation in Ambient Atmosphere 8.2 OLED Degradation Investigated by Calcium Tests 8.3 OSC Degradation Investigated by Calcium Tests 8.4 Discussion 8.5 Conclusions 9 Conclusions and Future Prospects Bibliography Acknowledgements Statement of AuthorshipDiese Arbeit untersucht die Verkapselung organischer Leuchtdioden (OLEDs) und organischer Solarzellen (OSCs), um ihre Lebensdauer zu verlängern. Trotz unbestrittener Vorteile wie geringer Materialaufwand und mechanische Flexibilität stellt die kurze Lebensdauer dieser Bauteile an Luft einen deutlichen Nachteil dar. Um sie zu schützen, müssen sie mit Permeationsbarrieren verkapselt werden. Eine geeignete Barriere zeichnet sich durch eine Wasserpermeationsrate (WVTR) unterhalb von 10^(-4) g(H2O) m^(-2) d^(-1) aus – weniger als eine Monolage Wasser pro Tag. Folglich wird zur Entwicklung einer solchen Barriere primär eine äußerst empflindliche Methode zu ihrer Vermessung benötigt. Um für den elektrischen Calcium-Test ein hinreichendes Maß an Messgenauigkeit, Zuverlässigkeit und Probendurchsatz zu erzielen, werden in dieser Arbeit Grundlagenuntersuchungen sowie die Entwicklung des Messaufbaus umfassend behandelt. Der elektrische Calcium-Test bestimmt die Menge eindringenden Wassers anhand der Leitfähigkeitsabnahme einer dünnen Schicht Calcium – eines unedlen Metalls. Um eine hohe Genauigkeit zu erlangen, werden das Reaktionsprodukt (Calciumhydroxid) und der spezifische Widerstand ((6,2 +- 0,1) 10^(-6) Ohm cm) aufgedampfter Calcium-Filme bestimmt. Entgegen einer für die Auswertung von Calcium-Tests üblichen Annahme wird für Calcium ein lateral inhomogenes Korrosionsverhalten festgestellt. Allerdings kann theoretisch und experimentell nachgewiesen werden, dass hierdurch die WVTR-Messung nicht verfälscht wird. Neben diesen Grundlagenuntersuchungen werden Design-Probleme des Calcium-Tests und deren Lösung vorgestellt, z. B. die Schädigung der anorganischen Barriere durch direkten Kontakt mit dem Calcium-Sensor. Im Ergebnis ist damit ein ebenso leistungsstarker wie zuverlässiger Messaufbau entwickelt worden. Im nächsten Schritt wird die Untersuchung einer Vielzahl von Barrieren mithilfe von Calcium-Tests, aber auch Bauteil-Verkapselung und galvanischer Abscheidung in Defekten, vorgestellt: Die Permeation durch aufgedampfte Aluminium-Dünnfilme geschieht demnach im Wesentlichen durch Makro-Defekte (Radien > 0,4 μm), die einer optischen Charakterisierung zugänglich sind. Barrieren, die durch Atomlagenabscheidung (ALD) hergestellt werden, verbessern sich mit steigender Schichtdicke, wobei solche Schichten auf Folien ausgezeichnete – aber bisher unzuverlässige – Permeationsbarrieren darstellen. Sowohl für einfache Polymerfolien als auch für gesputterte Zink-Zinn-Oxid-Barrieren (ZTO) werden zum einen gute Übereinstimmungen der gemessenen WVTR mit Vergleichswerten erzielt, zum anderen wächst in beiden Fällen die WVTR grob linear mit der anliegenden Luftfeuchte. Die POLO-Barriere mit einer WVTR im unteren 10^(-4) g(H2O) m^(-2) d^(-1)-Bereich erreicht die Messgrenze des aktuellen Messaufbaus. Kurzgesagt, es werden tiefgehende Untersuchungen zur Permeation durch verschiedene Barrieren durchgeführt, die grundlegende Zusammenhänge zwischen WVTR und Prozess-/Klimabedingungen beleuchten. Schließlich wird Wasser, das die aktive Fläche reduziert, als die vorrangige Degradationsursache identifiziert. Für je eine Sorte OLEDs und OSCs wird mittels eines vergleichenden (gegenüber Calcium-Tests) Alterungsexperiments dieWassermenge bestimmt, die die aktive Fläche um 50% verringert (T50-Wasser-Aufnahme). Für die OSC wird zudem gezeigt, dass die T50-Wasser-Aufnahme von (20 +- 7) mg(H2O) m^(-2) unabhängig von den Klimabedingungen ist. Folglich kann die zuvor unspezifische Forderung nach einer angestrebten Lebensdauer nun in eine konkrete Anforderung an die Barriere übersetzt werden: eine Wasserpermeationsrate. Mit Blick auf das Feld der Verkapselung verbessert diese Arbeit eine wichtige Messmethode, charakterisiert eine Vielzahl an Permeationsbarrieren und untersucht die Bauteilalterung durch Lufteinwirkung. Auch wenn das das Forschungsfeld der Verkapselungen nach wie vor eine Reihe offener Fragen aufweist, so bestärkt diese Arbeit doch in der Hoffnung, dass die organischen Bauteile selbige überdauern werden.:1 Introduction 2 Fundamentals 2.1 Organic Semiconductors 2.2 Organic Solar Cells 2.3 Organic Light-Emitting Diodes 2.4 Humidity, Evaporation, and Condensation 2.5 Principles of Permeation 3 State of the Art in Barrier Production and Evaluation 3.1 Barrier Technologies 3.2 Permeation Measurement Techniques 4 Experimental 4.1 Description of the As-Delivered Substrates 4.2 Treatment of Substrates 4.3 Deposition of Calcium Tests and Devices by Thermal Evaporation 4.4 Permeation Barriers by Atomic Layer Deposition 4.5 Defect Evaluation by Electrodeposition 5 Calcium for Permeation Tests Properties and Corrosion Behavior 5.1 Electrical Conductance and Optical Transmission 5.2 Corrosion Product 5.3 Laterally Inhomogeneous Calcium Corrosion 5.4 Implications for Optical and Electrical Calcium Corrosion Tests 6 Electrical Calcium Test 6.1 Measurement Setup 6.2 Calcium Test Layout 6.3 Comparability with Other Methods – OE-A Round Robin 6.4 Limitations and Future Prospects of the Electrical Calcium Test 6.5 Setup and Layout – Conclusions 7 Barrier Investigation 7.1 Thermally Evaporated Aluminum as Thin Film Encapsulation 7.2 ZnSnO (Magnetron Sputtered) on Polymer Foil 7.3 Al2O3 (ALD) on Polymer Substrate and as Thin Film Encapsulation 7.4 Summary and Conclusions for the Investigated Barriers 8 Encapsulation and Lifetime of Devices 8.1 Phenomenology of Device Degradation in Ambient Atmosphere 8.2 OLED Degradation Investigated by Calcium Tests 8.3 OSC Degradation Investigated by Calcium Tests 8.4 Discussion 8.5 Conclusions 9 Conclusions and Future Prospects Bibliography Acknowledgements Statement of Authorshi

    Device engineering of organic field-effect transistors toward complementary circuits

    Get PDF
    Organic complementary circuits are attracting significant attention due to their high power efficiency and operation robustness, driven by the demands for low-cost, large-area and flexible devices. Previous demonstrations of organic complementary circuits often show high operating voltage, small noise margins, low dc gain, and electrical instability such as hysteresis and threshold voltage shifts. There are two obstacles to developing organic complementary circuits: the lack of high-performance n-channel OFET devices, and the processing difficulty of integrating both n- and p-channel organic field-effect transistors (OFETs) on the same substrate. The operating characteristics of OFETs are often governed by the boundary conditions imposed by the device architecture, such as interfaces and contacts instead of the properties of the semiconductor material. Therefore, the performance of OFETs is often limited if either of the essential interfaces or contacts next to the semiconductor and the channel are not optimized. This dissertation presents research work performed on OFETs and OFET-based complementary inverters in an attempt to address some of these knowledge issues. The objective is to develop high-performance OFETs, with a focus on n-channel OFETs through interface engineering both at the interface between the organic semiconductor and the source/drain electrodes, and at the interface between the organic semiconductor and gate dielectric. Through interface engineering, both p- and n-channel high-performance low-voltage OFETs are realized with high mobilities, low threshold voltages, low subthreshold slopes, and high on/off current ratios. Optimization at the gate dielectric/semiconductor also gives OFET devices excellent reproducibility and good electrical stability under multiple test cycles and continuous electrical stress. Finally, with the interfaces and contacts optimized for both p- and n-channel charge transport, the integration of n- and p-channel OFETs with comparable performance are demonstrated in complementary inverters. The research achieves inverters with a high-gain, a low operation voltage, good electrical stability (absence of hysteresis), and a high switching-speed. A preliminary study of the encapsulation of OFETs and inverters with an additional protective layer is also presented to validate the practicality of organic devices containing air-sensitive n-channel transport.Ph.D.Committee Chair: Kippelen, Bernard; Committee Member: Brand, Oliver; Committee Member: Graham, Samuel; Committee Member: Rohatgi, Ajeet; Committee Member: Shen, Shyh-Chian

    Metrology and Characterisation of Defects in Thin-Film Barrier Layers Employed in Flexible Photovoltaic Modules

    Get PDF
    Flexible thin-film photovoltaic (PV) modules based on copper indium gallium selenide (CIGS) materials are one of the most recent developments in the renewable energy field, and the latest films have efficiencies at or beyond the level of Si-based rigid PV modules. Whilst these films offer significant advantages in terms of mass and the possibility of building-integrated photovoltaic (BIPV) applications, they are at present highly susceptible to long term environmental degradation as a result of water vapour transmission through the protective encapsulation layer to the active (absorber) layer. To maintain the PV module flexibility and to reduce or eliminate the water vapour permeability, the PV encapsulation includes a barrier layer of amorphous aluminium oxide (Al2O3) material of a few nanometres thickness deposited on a planarised polyethylene naphthalate (PEN) substrate. The highly conformal barrier layer of the Al2O3 is produced by atomic layer deposition (ALD) methods using roll-to-roll (R2R) technology. Nevertheless, water vapour permeation is still facilitated by the presence of micro and nano-scale defects generated during the deposition processes of the barrier material, which results in decreased cell efficiency and reduced unit longevity. The state of the art surface metrology technologies including: optical microscopy, white light scanning interferometry (WLSI), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were extensively deployed in this project as offline surface characterisation methods to characterise the water vapour barrier layer defects, which are postulated to be directly responsible for the water vapour ingress. Areal surface texture parameters analysis based on wolf pruning, area pruning and segmentation analysis methods as defined in ISO 25178-2; allow the efficient separation of small insignificant defects from significant defects. The presence of both large and small defects is then correlated with the barrier films functionality as measured on typical sets of Al2O3 ALD films using a standard MOCON® (quantitative gas permeation) test. The investigation results of the initial analysis finishes by drawing conclusions based on the analysis of the water vapour transmission rate (WVTR), defects size, density and distribution, where it is confirmed that small numbers of large defects have more influence on the deterioration of the barrier films functionality than large numbers of small defects. This result was then used to provide the basis for developing a roll-to-roll in process metrology device for quality control of flexible PV barrier films. Furthermore, a theoretical model approach was developed in this thesis based on the water vapour diffusion theory to determine the cut- off level between large significant defects and small insignificant defects. The results of the model would seem to reveal that, in order to build up in process, non-contact optical defect detection system for R2R barrier films, the critical spatial resolution required for defect detection need not be less than 3 μm laterally and 3Sq nm (Sq= root mean square surface roughness deviation of non-defective sample area) per field of view (FOV) vertically. Any defect that has dimensions less than this appears to have a significantly lower effect on the PV barrier properties and functionality. In this study, the surface topography analysis results and the theoretical model approach outcomes, both provide the basis for developing a R2R in process metrology device for PV barrier films defect detection. Eventually, the work in this thesis reports on the deployment of new (novel) in-line interferometric optical sensors based on wavelength scanning interferometry (WSI) designed to measure and catalogue the PV barrier films defects where they are present. The sensors have built-in environmental vibration compensation and are being deployed on a demonstrator system at a R2R production facility in the UK

    Single molecule electronics:a systematic approach to study the properties of single porphyrin molecules

    Get PDF
    Single molecule electronics: Een systematische aanpak om de eigenschappen van enkele porfyrine moleculen te bestuderen Porfyrinen zijn zeer interessante en belangrijke complexe organische moleculen, die bestudeerd worden in prototypische moleculaire elektronische apparaten. We waren geïnteresseerd om het geleidingsgedrag van porfyrine moleculen op het single molecule-niveau te bestuderen. We hebben de vereiste trans-amine en thiol gefunctionaliseerde porfyrinen gesynthetiseerd en vervolgens hun binding met goud nanodeeltjes als model oppervlak voor nano-elektroden bestudeerd om te leren over hun interactiesterkte en -dynamiek. Van een reeks gefunctionaliseerde porfyrinen die werd ontwikkeld voor deze studie, toonde de verbinding met vier amino functionaliteiten de hoogste bindingsinteractie met gouden nanodeeltjes. We namen fotogeïnduceerde ladingsoverdracht waar van porphyrins naar quantum dots in hybride systemen, tijdens het onderzoek naar de bindingsinteractie van porphyrinen met PbSe quantum dots. Dit resultaat geeft aan dat deze porfyrinen goede kandidaten zijn voor single molecule elektronische studies. Tot slot hebben we ons gericht op het single molecule elektronische gedrag van porphyrinen met platina electromigration electrodes en mechanisch regelbare breekjunctie technieken. Porphyrinen met amino (-NH2) en thiol (-SH) verankering groepen waren respectievelijk gebonden aan de platina-elektroden (verkregen met electromigration) en goud-elektroden (MCBJ) We onderscheiden moleculair transport van achtergrond-tunneling conductances in platina electromigration techniek. Een verandering in geleiding werd waargenomen, wanneer porfyrine moleculen met en zonder thiol opzeggingen, vrije base, Zn-en Ru-porphyrins aanwezig in MCBJs waren. Onze porfyrinederivaten liet zowel vlak en loodrecht gesondeerd configuraties. Deze studies vergroten ons begrip van metalen - molecuul interacties en elektronen transport in een enkel molecuul, wat cruciaal is voor de verdere ontwikkeling op nanoschaal elektronica.
    corecore