Nanoscopic studies of conjugated polymer blends by (electric) scanning probe microscopy

Conjugated polymers and conjugated polymer blends have attracted great interest due to their potential applications in biosensors and organic electronics. The sub-100 nm morphology of these materials is known to heavily influence their electromechanical properties and the performance of devices they are part of. Electromechanical properties include charge injection, transport, recombination, and trapping, the phase behavior and the mechanical robustness of polymers and blends. Electrical scanning probe microscopy techniques are ideal tools to measure simultaneously electric (conductivity and surface potential) and dielectric (dielectric constant) properties, surface morphology, and mechanical properties of thin films of conjugated polymers and their blends.rnIn this thesis, I first present a combined topography, Kelvin probe force microscopy (KPFM), and scanning conductive torsion mode microscopy (SCTMM) study on a gold/polystyrene model system. This system is a mimic for conjugated polymer blends where conductive domains (gold nanoparticles) are embedded in a non-conductive matrix (polystyrene film), like for polypyrrole:polystyrene sulfonate (PPy:PSS), and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). I controlled the nanoscale morphology of the model by varying the distribution of gold nanoparticles in the polystyrene films. I studied the influence of different morphologies on the surface potential measured by KPFM and on the conductivity measured by SCTMM. By the knowledge I gained from analyzing the data of the model system I was able to predict the nanostructure of a homemade PPy:PSS blend.rnThe morphologic, electric, and dielectric properties of water based conjugated polymer blends, e.g. PPy:PSS or PEDOT:PSS, are known to be influenced by their water content. These properties also influence the macroscopic performance when the polymer blends are employed in a device. In the second part I therefore present an in situ humidity-dependence study on PPy:PSS films spin-coated and drop-coated on hydrophobic highly ordered pyrolytic graphite substrates by KPFM. I additionally used a particular KPFM mode that detects the second harmonic electrostatic force. With this, I obtained images of dielectric constants of samples. Upon increasing relative humidity, the surface morphology and composition of the films changed. I also observed that relative humidity affected thermally unannealed and annealed PPy:PSS films differently. rnThe conductivity of a conjugated polymer may change once it is embedded in a non-conductive matrix, like for PPy embedded in PSS. To measure the conductivity of single conjugated polymer particles, in the third part, I present a direct method based on microscopic four-point probes. I started with metal core-shell and metal bulk particles as models, and measured their conductivities. The study could be extended to measure conductivity of single PPy particles (core-shell and bulk) with a diameter of a few micrometers.

Morphology and charge transport in conjugated polymers

To assist rational compound design of organic semiconductors, two problems need to be addressed. First, the material morphology has to be known at an atomistic level. Second, with the morphology at hand, an appropriate charge transport model needs to be developed in order to link charge carrier mobility to structure.rnrnThe former can be addressed by generating atomistic morphologies using molecular dynamics simulations. However, the accessible range of time- and length-scales is limited. To overcome these limitations, systematic coarse-graining methods can be used. In the first part of the thesis, the Versatile Object-oriented Toolkit for Coarse-graining Applications is introduced, which provides a platform for the implementation of coarse-graining methods. Tools to perform Boltzmann inversion, iterative Boltzmann inversion, inverse Monte Carlo, and force-matching are available and have been tested on a set of model systems (water, methanol, propane and a single hexane chain). Advantages and problems of each specific method are discussed.rnrnIn partially disordered systems, the second issue is closely connected to constructing appropriate diabatic states between which charge transfer occurs. In the second part of the thesis, the description initially used for small conjugated molecules is extended to conjugated polymers. Here, charge transport is modeled by introducing conjugated segments on which charge carriers are localized. Inter-chain transport is then treated within a high temperature non-adiabatic Marcus theory while an adiabatic rate expression is used for intra-chain transport. The charge dynamics is simulated using the kinetic Monte Carlo method.rnrnThe entire framework is finally employed to establish a relation between the morphology and the charge mobility of the neutral and doped states of polypyrrole, a conjugated polymer. It is shown that for short oligomers, charge carrier mobility is insensitive to the orientational molecular ordering and is determined by the threshold transfer integral which connects percolating clusters of molecules that form interconnected networks. The value of this transfer integral can be related to the radial distribution function. Hence, charge mobility is mainly determined by the local molecular packing and is independent of the global morphology, at least in such a non-crystalline state of a polymer.

Design und Synthese maßgeschneiderter funktioneller Rylenfarbstoffe

Die vorgelegte Dissertation beschäftigt sich mit der Darstellung und Untersuchung von funktionellen Farbstoffen auf der Basis von Rylendiimiden. Diese Substanzklasse zeichnet sich durch gute Funktionalisierbarkeit, hohe chemische und photochemische Stabilität sowie durch hohe Fluoreszenzquantenausbeuten und Extinktionskoeffizienten aus. Rylendiimide spielen eine bedeutsame Rolle in den modernen Materialwissenschaften. Ein Schwerpunkt der Arbeit liegt auf der Synthese und Untersuchung von neuen Perylendiimiden (PDI) im Hinblick auf eine Anwendung in der organischen Elektronik. Das Substitutionsmuster der PDI hat einen signifikanten Einflussrnauf deren supramolekulares Verhalten und die Leistungsfähigkeit in elektronischen Bauteilen. Durch das Einführen neuer Substituenten konnten weitergehende Erkenntnisse über das supramolekulare Verhalten der PDI gewonnenrnwerden. Multichromophore sind wichtige Modellsysteme zur Untersuchung vonrnEnergietransportprozessen und Einzelphotonenquellen. Daher liegt ein weiterer Schwerpunkt auf der Synthese multichromophorer PDI-Systeme. Neben der Darstellung definierter dendritischer Nanoteilchen auf Basis von Poly(phenylenethinylen)-Dendrimeren beschäftigt sich dieser Teil auch mit der Synthese hochverzweigter ethinylverknüpfter Polymere auf Basis von PDI-Monomeren. Aufgrund ihrer außergewöhnlichen photochemischen Eigenschaften spielen Perylen- und Terrylendiimide eine wichtige Rolle als Fluoreszenzmarker.rnDaher beschäftigt sich ein weiterer Aspekt dieser Arbeit mit der Synthese vonrnFluoreszenzmarkern, die sowohl in Polymerisationsreaktionen als auch inrnbiologischen Systemen Anwendung finden können.

Lipophilic nucleosides: studies toward self-assembled functional materials

Molecular self-assembly takes advantage of supramolecular non-covalent interactions (ionic, hydrophobic, van der Waals, hydrogen and coordination bonds) for the construction of organized and tunable systems. In this field, lipophilic guanosines can represent powerful building blocks thanks to their aggregation proprieties in organic solvents, which can be controlled by addition or removal of cations. For example, potassium ion can template the formation of piled G-quartets structures, while in its absence ribbon-like G aggregates are generated in solution. In this thesis we explored the possibility of using guanosines as scaffolds to direct the construction of ordered and self-assembled architectures, one of the main goals of bottom-up approach in nanotechnology. In Chapter III we will describe Langmuir-Blodgett films obtained from guanosines and other lipophilic nucleosides, revealing the “special” behavior of guanine in comparison with the other nucleobases. In Chapter IV we will report the synthesis of several thiophene-functionalized guanosines and the studies towards their possible use in organic electronics: the pre-programmed organization of terthiophene residues in ribbon aggregates could allow charge conduction through π-π stacked oligothiophene functionalities. The construction and the behavior of some simple electronic nanodevices based on these organized thiopehene-guanosine hybrids has been explored.

Chemical transformations of the pyrene K-region for functional materials

In dieser Arbeit wurde eine neue Methode zur asymmetrischen Substitution der K-Regionen von Pyren entwickelt, auf welcher das Design und die Synthese von neuartigen, Pyren-basierten funktionalen Materialien beruht. Eine Vielzahl von Substitutionsmustern konnte erfolgreich realisiert werden um die Eigenschaften entsprechend dem Verwendungszweck anzupassen. Der polyzyklische aromatische Kohlenwasserstoff (PAK) Pyren setzt sich aus vier Benzolringen in Form einer planaren Raute mit zwei gegenüberliegenden K-Regionen zusammen. Der synthetische Schlüsselschritt dieser Arbeit ist die chemische Transformation der einen K-Region zu einem α-Diketon und der darauffolgenden selektiven Bromierung der zweiten K-Region. Dieser asymmetrisch funktionalisierte Baustein zeichnet sich durch zwei funktionelle Gruppen mit orthogonaler Reaktivität aus und erweitert dadurch das Arsenal der etablierten Pyren Chemie um eine vielseitig einsetzbare Methode. Aufbauend auf diesem synthetischen Zugang wurden fünf wesentliche Konzepte auf dem Weg zu neuen, von Pyren abgeleiteten Materialen verfolgt: (i) Asymmterische Substitution mit elektronenziehenden versus -schiebenden Gruppen. (ii) Darstellung von Pyrenocyaninen durch Anbindung von Pyren mit einer der K-Regionen an das Phthalocyanin Gerüst zur Ausdehnung des π-Systems. (iii) Einführung von Thiophen an die K-Region um halbleitende Eigenschaften zu erhalten. (iv) Symmetrische Annullierung von PAKs wie Benzodithiophen und Phenanthren an beide K Regionen für cove-reiche und dadurch nicht-planare Strukturen. (v) Verwendung des K-Region-funktionalisierten Pyrens als Synthesebaustein für das Peri-Pentacen. Neben der Synthese wurde die Selbstorganisation in der Festphase und an der flüssig/fest Grenzfläche mittels zweidimensionaler Weitwinkel-Röntgenstreuung (2D WAXS) bzw. Rastertunnelmikroskopie (STM) untersucht. Die halbleitenden Eigenschaften wurden in organischen Feld-Effekt Transistoren (OFETs) charakterisiert.

Molekulare und supramolekulare Eigenschaften von neuen Donor-Akzeptor-Polymeren

Die organische Halbleitertechnologie befindet sich seit Jahrzehnten im Focus des Interesses, da sie eine kostengünstige und umweltverträgliche Alternative zu anorganischen Silizium-basierten Halbleitern darstellt. Die Möglichkeit der gezielten Funktionalisierung von definierten Strukturen durch synthetische Methoden, welche eine große Vielfalt an Materialien ermöglicht, steht dabei besonders im Vordergrund. Die Modifikation von physikalischen Eigenschaften ermöglicht dabei eine stark erleichterte Anpassung für den geplanten Anwendungsbereich. Im Rahmen der vorliegenden Dissertation wurden organische Halbleitermaterialien basierend auf Cyclopenta[2,1-b:3,4-b']dithiophen (CDT) dargestellt und hinsichtlich ihrer strukturellen und elektronischen Eigenschaften untersucht. In Kombination mit Benzo[c][1,2,5]thiadiazol (BTZ) und weiteren Akzeptoren wurden zunächst Donor(D)-Akzeptor(A)-Polymere synthetisiert und Struktur-Eigenschaft-Beziehungen aufgestellt. So konnte ein sehr hochmolekulares Polymer CDT-BTZ-Polymer (Mn = 36 kg mol-1, PDI = 2.6) erhalten werden, welches sich durch eine hohe lamellare Ordnung und eine gemessene Ladungsträgermobilität in FETs von über 5.0 cm2V-1s-1 bei Raumtemperatur auszeichnete; bei niedrigen Temperaturen (240 K) war letztgenannte 6.5 cm2V-1s-1. Aufgrund dieses hohen Ladungstransports und der Abwesenheit niedermolekularer Polymerketten innerhalb des Polymers konnte erstmals eine Messung eines HALL-Effektes bewerkstelligt werden. Dies war der erste Beweis eines Band-artigen Ladungstransportes an einem Polymerhalbleiter. Des Weiteren wurde durch synthetische Veränderung der Grundstruktur des Polymers zu längeren Alkylketten eine anisotrope Anordnung der Polymerketten erreicht und die Ladungsträgermobilität (6.5 cm2V-1s-1 bei Raumtemperatur) weiter gesteigert. Darauf aufbauend wurde der Einfluss von stereoisomeren Seitenketten an CDT-BTZ-Polymeren auf Packungsverhalten, Parametern (Sperrstrom, Einschaltstrom) in FETs und Löslichkeit in organischen Lösungsmitteln untersucht. Durch cis-trans-Isomerisierung der Seitenketten wurde hier eine neue Methode zur Optimierung des Packungsverhaltens von Polymeren in dünnen Filmen und Lösung gefunden. Zuletzt wurden D-π-A-Farbstoffen, welche CDT als Verbrückungseinheit (π) beinhalten, dargestellt. Durch Variation von D und A konnten Struktur-Eigenschaft-Beziehungen in der Anwendung in Solarzellen (Feststoffsolarzellen, Flüssigsolarzellen) gefunden werden. Die Untersuchungen der photoinduzierten Absorption und der Photolumisenzenzquantenausbeute lieferten dabei Erklärungen für physikalische Prozesse wie Ladungsinjektion- und rekombination.

Singular Temperatures Connected to Charge Transport Mechanism Transitions in Perylene Bisimides from Steady-State Photocurrent Measurements

Perylene bisimides (PBIs) are n-type semiconducting and photogenerating materials widely used in a variety of optoelectronic devices. Particularly interesting are PBIs that are simultaneously water-soluble and liquid-crystalline (PBI-W+LC) and, thus, attractive for the development of high-performing easily processable applications in biology and “green” organic electronics. In this work, singular temperatures connected to charge transport mechanism transitions in a PBI-W+LC derivative are determined with high accuracy by means of temperature-dependent photocurrent studies. These singular temperatures include not only the ones observed at 60 and 110 °C, corresponding to phase transition temperatures from crystalline to liquid-crystalline (LC) and from LC to the isotropic phase, respectively, as confirmed by differential scanning calorimetry (DSC), but also a transition at 45 °C, not observed by DSC. By analyzing the photocurrent dependence simultaneously on temperature and on light intensity, this transition is interpreted as a change from monomolecular to bimolecular recombination. These results might be useful for other semiconducting photogenerating materials, not necessarily PBIs or even organic semiconductors, which also show transport behavior changes at singular temperatures not connected with structural or phase transitions.

Controlled synthesis of poly(neopentyl p-styrene sulfonate) via reversible addition-fragmentation chain transfer polymerisation

The controlled synthesis of poly(neopentyl p-styrene sulfonate) (PNSS) using RAFT polymerisation has been studied. Selected experimental conditions led to the production of PNSS with variable molecular weights and low dispersities (D{stroke}≤1.50). The controlled synthesis of poly(neopentyl p-styrene sulfonate) (PNSS) using reversible addition-fragmentation chain transfer polymerisation has been studied under a wide range of experimental conditions. PNSS can be used as an organic-soluble, thermally labile precursor for industrially valuable poly(p-styrene sulfonate), widely employed in technologies such as ionic exchange membranes and organic electronics. The suitability of two different chain transfer agents, three solvents, three different monomer concentrations and two different temperatures for the polymerisation of neopentyl p-styrene sulfonate is discussed in terms of the kinetics of the process and characteristics of the final polymer. Production of PNSS with systematically variable molecular weights and low dispersities (D{stroke} ≤1.50 in all cases) has been achieved using 2-azidoethyl 2-(dodecylthiocarbonothioylthio)-2-methylpropionate in anisole at 75°C, with an initial monomer concentration of 4.0molL-1. Finally, a poly(neopentyl p-styrene sulfonate)-b-polybutadiene-b-poly(neopentyl p-styrene sulfonate) (PNSS-b-PBD-b-PNSS) triblock copolymer has been synthesised via azide-alkyne click chemistry. Moreover, subsequent thermolysis of the PNSS moieties generated poly(p-styrene sulfonate) end blocks. This strategy allows the fabrication of amphiphilic copolymer films from single organic solvents without the need for post-deposition chemical treatment.

Design, synthesis and thermal behaviour of a series of well-defined clickable and triggerable sulfonate polymers

In the printing industry, the exploitation of triggerable materials that can have their surface properties altered on application of a post-deposition external stimulus has been crucial for the production of robust layers and patterns. To this end, herein, a series of clickable poly(R-alkyl p-styrene sulfonate) homopolymers, with systematically varied thermally-labile protecting groups, has been synthesised via reversible addition-fragmentation chain transfer (RAFT) polymerisation. The polymer range has been designed to offer varied post-deposition thermal treatment to switch them from hydrophobic to hydrophilic. Suitable RAFT conditions have been identified to produce well-defined homopolymers (Đ, Mw/Mn < 1.11 in all cases) at high monomer conversions (>80% for all but one monomer) with controllable molar mass. Poly(p-styrene sulfonate) with an isobutyl protecting group has been shown to be the most readily thermolysed polymer that remains stable at room temperature, and was thus investigated further by incorporation into a diblock copolymer, P3HT-b-PiBSS, by click chemistry. The strategy for preparation of thermal modifiable block copolymers exploiting R-protected p-styrene sulfonates and azide-alkyne click chemistry presented herein allows the design of new, roll-to-roll processable materials for potential application in the printing industry, particularly organic electronics.

Développement de la polymérisation par (hétéro)arylation directe pour l’électronique organique

L’électronique organique suscite un intérêt grandissant en recherche grâce aux nouvelles possibilités qu’elle offre pour faciliter l’intégration de dispositifs électroniques dans nos vies. Grâce à elle, il est possible d’envisager des produits légers, flexibles et peu coûteux à produire. Les classes majeures de dispositifs étudiées sont les cellules photovoltaïques organiques (CPO) et les transistors organiques à effet de champ (TOEC). Dans les dernières années, une attention particulière a été portée sur les méthodes de polymérisation des matériaux organiques entrant dans la fabrication de ces dispositifs. La polymérisation par (hétéro)arylation directe (PHAD) catalysée au Pd offre une synthèse sans dérivé organométallique utilisant simplement un lien C-H aromatique, ce qui facilite la purification, diminue le nombre d’étapes et rend possible la production de matériaux à plus faible coût. De plus, la PHAD permet la préparation de matériaux qui était difficile, voire impossible, à obtenir auparavant. Cependant, l’inconvénient majeur de la PHAD reste sa limitation à certaines classes de polymères possédant des monomères ayant des positions bloquées favorisant qu’une seule paire de liaisons C-H. Dans le cadre de ces travaux de doctorat, l’objectif général est d’étudier la polymérisation par PHAD afin d’accéder à des classes de monomères qui n’étaient pas envisageables auparavant et à étendre l’application de cet outil dans le domaine des polymères conjugués. Plus spécifiquement, nous avons étudié l’utilisation de groupements protecteurs et partants sur des unités de benzodithiophènes et de bithiophène-silylés. Suivant ces résultats, nos travaux ont porté sur la polymérisation de dérivés de bithiophènes avec des bromo(aryle)s, une classe de polymères fréquemment utilisée en électronique organique mais qui était jugée impossible à polymériser par PHAD auparavant. Cette étude a montré l’importance de contrôler la PHAD afin d’obtenir le polymère souhaité. Finalement, nous avons étudié l’effet du système catalytique sur le taux de β−ramifications lors de la synthèse de polymères à base de thiophènes. Dans cette dernière étude, nous avons démontré l’importance d’utiliser des outils de caractérisation adéquats afin de confirmer la qualité des polymères obtenus.

Charge accumulation and transport in degenerately doped semiconducting polymers with mixed ionic and electronic conductivity

This thesis is part of the fields of Material Physics and Organic Electronics and aims to determine the charge carrier density and mobility in the hydrated conducting polymer–polyelectrolyte blend PEDOT:PSS. This kind of material combines electronic semiconductor functionality with selective ionic transport, biocompatibility and electrochemical stability in water. This advantageous material properties combination makes PEDOT:PSS a unique material to build organic electrochemical transistors (OECTs), which have relevant application as amplifying transducers for bioelectronic signals. In order to measure charge carrier density and mobility, an innovative 4-wire, contact independent characterization technique was introduced, the electrolyte-gated van der Pauw (EgVDP) method, which was combined with electrochemical impedance spectroscopy. The technique was applied to macroscopic thin film samples and micro-structured PEDOT:PSS thin film devices fabricated using photolithography. The EgVDP method revealed to be effective for the measurements of holes’ mobility in hydrated PEDOT:PSS thin films, which resulted to be <μ>=(0.67±0.02) cm^2/(V*s). By comparing this result with 2-point-probe measurements, we found that contact resistance effects led to a mobility overestimation in the latter. Ion accumulation at the drain contact creates a gate-dependent potential barrier and is discussed as a probable reason for the overestimation in 2-point-probe measurements. The measured charge transport properties of PEDOT:PSS were analyzed in the framework of an extended drift-diffusion model. The extended model fits well also to the non-linear response in the transport characterization and results suggest a Gaussian DOS for PEDOT:PSS. The PEDOT:PSS-electrolyte interface capacitance resulted to be voltage-independent, confirming the hypothesis of its morphological origin, related to the separation between the electronic (PEDOT) and ionic (PSS) phases in the blend.

Electrochemistry of polycyclic aromatic molecules for advanced carbon nanostructures: luminescence and transistor applications

Polycyclic aromatic hydrocarbons (PAHs) are a large class of π-conjugated organic molecules with fused aromatic rings, which can be considered as fragments of 2D-graphene and have been extensively studied for their unique optical and electronic properties. The aim of this study is to understand the complex electrochemical behaviour of planar, curved, and heteroatom doped polycyclic aromatic molecules, particularly focusing on the oxidative coupling of their radical cations and the electrochemically induced cyclodehydrogenation reactions. In the first part of this thesis, the class of PAHs and aromatic nanostructures are introduced, and the reactivity of electrogenerated species is discussed, focusing on the electrochemical approach for the synthesis of extended π-conjugated structures. Subsequently, the electrochemical properties and reactivity of electrogenerated radical ions of planar and curved polyaromatics are correlated to their structures. In the third chapter, electrochemical cyclodehydrogenation of hexaphenylbenzene is used to prepare self-assembled hexabenzocoronene, directly deposited on an interdigitated electrode, which was characterised as organic electrochemical transistor. In the fourth chapter, the electrochemical behaviour of a family of azapyrene derivatives has been carefully investigated together with the electrogenerated chemiluminescence (ECL), both by ion-annihilation and co-reactant methods. Two structural azapyrene isomers with different nitrogen positions are thoroughly discussed in terms of redox and ECL properties. Interestingly, the ECL of only one of them showed a double emission with excimer formation. A detailed mechanism is discussed for the ECL by co-reactant benzoyl peroxide, to rationalise the different ECL behaviours of the two isomers on the basis of their topologically modulated electronic properties. In conclusion, the different electrochemical behaviours of PAHs were shown, focussing on the chemical reactivity of the electrogenerated species and taking advantage of it for important processes spanning from unconventional synthesis methods for carbon nanostructures to the exploitation of self-assembled nanostructured systems in organic electronics, to novel organic emitters in ECL.

Printed electronics for ubiquitous computing applications

Dissertação para obtenção do Grau de Doutor em Química

Paper for printed electronics and functionality

Mass-produced paper electronics (large area organic printed electronics on paper-based substrates, “throw-away electronics”) has the potential to introduce the use of flexible electronic applications in everyday life. While paper manufacturing and printing have a long history, they were not developed with electronic applications in mind. Modifications to paper substrates and printing processes are required in order to obtain working electronic devices. This should be done while maintaining the high throughput of conventional printing techniques and the low cost and recyclability of paper. An understanding of the interactions between the functional materials, the printing process and the substrate are required for successful manufacturing of advanced devices on paper. Based on the understanding, a recyclable, multilayer-coated paper-based substrate that combines adequate barrier and printability properties for printed electronics and sensor applications was developed in this work. In this multilayer structure, a thin top-coating consisting of mineral pigments is coated on top of a dispersion-coated barrier layer. The top-coating provides well-controlled sorption properties through controlled thickness and porosity, thus enabling optimizing the printability of functional materials. The penetration of ink solvents and functional materials stops at the barrier layer, which not only improves the performance of the functional material but also eliminates potential fiber swelling and de-bonding that can occur when the solvents are allowed to penetrate into the base paper. The multi-layer coated paper under consideration in the current work consists of a pre-coating and a smoothing layer on which the barrier layer is deposited. Coated fine paper may also be used directly as basepaper, ensuring a smooth base for the barrier layer. The top layer is thin and smooth consisting of mineral pigments such as kaolin, precipitated calcium carbonate, silica or blends of these. All the materials in the coating structure have been chosen in order to maintain the recyclability and sustainability of the substrate. The substrate can be coated in steps, sequentially layer by layer, which requires detailed understanding and tuning of the wetting properties and topography of the barrier layer versus the surface tension of the top-coating. A cost competitive method for industrial scale production is the curtain coating technique allowing extremely thin top-coatings to be applied simultaneously with a closed and sealed barrier layer. The understanding of the interactions between functional materials formulated and applied on paper as inks, makes it possible to create a paper-based substrate that can be used to manufacture printed electronics-based devices and sensors on paper. The multitude of functional materials and their complex interactions make it challenging to draw general conclusions in this topic area. Inevitably, the results become partially specific to the device chosen and the materials needed in its manufacturing. Based on the results, it is clear that for inks based on dissolved or small size functional materials, a barrier layer is beneficial and ensures the functionality of the printed material in a device. The required active barrier life time depends on the solvents or analytes used and their volatility. High aspect ratio mineral pigments, which create tortuous pathways and physical barriers within the barrier layer limit the penetration of solvents used in functional inks. The surface pore volume and pore size can be optimized for a given printing process and ink through a choice of pigment type and coating layer thickness. However, when manufacturing multilayer functional devices, such as transistors, which consist of several printed layers, compromises have to be made. E.g., while a thick and porous top-coating is preferable for printing of source and drain electrodes with a silver particle ink, a thinner and less absorbing surface is required to form a functional semiconducting layer. With the multilayer coating structure concept developed in this work, it was possible to make the paper substrate suitable for printed functionality. The possibility of printing functional devices, such as transistors, sensors and pixels in a roll-to-roll process on paper is demonstrated which may enable introducing paper for use in disposable “onetime use” or “throwaway” electronics and sensors, such as lab-on-strip devices for various analyses, consumer packages equipped with product quality sensors or remote tracking devices.

Printable organic and inorganic materials for flexible electrochemical devices

Portuguese Science Foundation - project Electra PTDC/CTM/099124/2008 and the PhD grant SFRH/BD/45224. financial support: Professor E. Fortunato’s ERC 2008 Advanced Grant (INVISIBLE contract number 228144), “APPLE” FP7-NMP-2010-SME/262782-2 and “SMARTEC” FP7-ICT-2009.3.9/258203