Organic thin films as active materials in field effect transistors and electrochemical sensing

This PhD thesis is focused on Organic Electronics, an emerging field where different disciplines converge to gain insights into the properties of organic materials and their applications. Under the present work different organic materials have been realized and analysed for application both in Organic Field Effect Transistors and electrochemical sensing with Organic Electrochemical Transistors. An overview about Organic Electronic is reported with the most recent advancement of the last years: a state of the art of research about Organic Field Effect Transistors (OFETs) and Organic Electrochemical Transistors (OECTs) is given, with an overview on the emerging Organic Bioelectronics. The main motifs of the research performed are reported along the discussion. In the application of the supersonic molecular beam epitaxy method, thin films of Copper Phthalocyanine have been grown, reaching an unprecedented order in the crystalline structure, as the characterization by Raman spectroscopy and AFM have shown. A modified-pentacene molecule (2,3-CN2-TIPS-Pn films) has been used as active layer for the building of an OFET device, which showed an ambipolar behaviour with balanced electrons and holes mobility on the order of 2⋅10-3cm2/Vs. The charge transport properties of 2,3-CN2-TIPS-Pn films show the effectiveness of TIPS-Pn functionalization with cyano e− withdrawing groups to promote e- transport while maintaining equivalent h− transport. A second OFET device has been realized with tetracene organic thin films deposited on different dielectrics substrates: the devices have been characterized and the mobility measured. For the tetracene film deposited on the polystyrene substrate, we have found a mobility of 2⋅10-1 cm2/Vs, the highest retrieved up to now in literature for tetracene. The molecular structures of all the organic molecules used, have been deeply investigated by means AFM analysis and XRD-advanced algorithm tools. For the films made with the TIPS molecule, the GIXRD analysis revealed a favourable arrangement of the molecules in the TFT channel. The XRD analysis performed on the tetracene films revealed interesting correlation between the mobility of the film and the AFM and structural parameters: in particular the polystyrene film shows the best surface coverage and the highest alpha phase percentage of the molecular structure. New insights into the device physics of OECT have been discovered: in the sensing experiments with OECTs, the role of the gate electrode has been investigated. This clarified the two working principles an OECT can operate (faradaic or non-faradaic mode). We found that an OECT can switch between these two modes of operation simply changing the metal wire acting as gate electrode. In particular the faradaic operational mode lead to the possibility to exploit the transistor as a halide sensor, able to detect Na+ ions in solution with a sensibility up to 10μM. Then the role of electrolyte has been studied with micellar structures, which open unexplored horizons for the application of OECT with a new class of electrolytes. The ability of micelles to dope/dedope efficiently the PEDOT:PSS permitted to investigate the doping process of the polymer, that is one of the main issue today in organic electronics. The modulation signals have been correlated with the surface charge of the micelles, measured by the zeta-potential techniques and the injection of micelles into the polymer structure has been probed by an optical spectroscopy measurement, performed in-situ during the OECT current acquisition. As a consequence of the micelle experiment, bilayer structure, like liposomes, have been tested and detected for the first time with an OECT. Although this experiment is currently in progress, it seems particularly promising, mainly because the opportunity to exploit the ability of liposomes to trap and release drugs in a controlled way. A new nanoparticles-based sensor has been developed, able to detect the presence in solution of iron-oxide magnetic nanoparticles functionalised with different polymeric coatings: we provide the ability of OECTs to detect and monitor selectively, with an appropriate choice of the electrolyte, different nanosystems. We demonstrate an on-line sensing based on OECTs, with an easy sampling/sample preparation, for the detection of functionalized magnetic nanoparticles. OECTs have promising applications in bioelectronics as well as in nanomedicine or neuroscience. They are becoming an ideal platform for both in-vitro and in-vivo biomedical applications, as well as for the development of protocells inside miniaturized electro-chemical laboratory

Microstructure and Electrical Properties of Fe,Cu Substituted (Co,Mn)₃O₄ Thin Films

In this work, thin films (~1000 nm) of a pure MnCo2O4 spinel together with its partially substituted derivatives (MnCo1.6Cu0.2Fe0.2O4, MnCo1.6Cu0.4O4, MnCo1.6Fe0.4O4) were prepared by spray pyrolysis and were evaluated for electrical conductivity. Doping by Cu increases the electrical conductivity, whereas doping by Fe decreases the conductivity. For Cu containing samples, rapid grain growth occurs and these samples develop cracks due to a potentially too high thermal expansion coefficient mismatch to the support. Samples doped with both Cu and Fe show high electrical conductivity, normal grain growth and no cracks. By co-doping the Mn, Co spinel with both Cu and Fe, its properties can be tailored to reach a desired thermal expansion coefficient/electrical conductivity value

Understanding Relationships Between Morphology and Charge Transfer States in Organic Photovoltaics

Renewable energy sources are becoming increasingly important as the world's energy demands continue to grow and climate change continues to occur. Photovoltaic power generation has great potential, but most commercial photovoltaic cells have typically been made from single-crystal or polycrystalline silicon which are expensive and energy intensive to produce. Organic semiconductors are one class of solar cell materials that have the potential for low cost due to solution processability, roll-to-roll fabrication, material tunability, and the very small amounts of material required to absorb light compared to crystalline silicon. Organic photovoltaic (OPV) efficiencies have increased rapidly in the past few years and are currently at about 11% power conversion efficiency.To find new pathways to improve OPV performance, a deeper understanding of the relationships between morphology, electronic properties, and OPV performance is required. This work will explore these relationships, focusing on the relationship between morphology and charge transfer states. We focus on a small molecule material system with only pure donor and acceptor domains, unlike the typical bulk heterojunction composed of donor, acceptor, and mixed domains. The advantage of studying this system is a more well-defined interfacial area and simpler morphology that allows us to better characterize charge transfer states at the donor-acceptor interface.We first review the basic principles of OPV operation, fabrication and characterization methods, and charge transfer states. We then use grazing incidence wide angle X-ray scattering to characterize the crystal structures and textures of the polymorphs of the donor and acceptor in our small molecule system, which are important for understanding their blend morphology. Next, we demonstrate a new method to control morphology in this system with the use of a thermally degradable binder polymer. Afterwards, we vary interfacial area and charge transfer state density in this system through processing and characterize it using sub-bandgap external quantum efficiency measurements, allowing us to gain valuable insights about the photophysics of this system.Finally, many of the insights and techniques used to study this system are also useful in studying other systems of organic or solution-procesed semiconductors. We optimize the morphology and performance of OPVs made with novel low-bandgap donor-acceptor copolymers. We then find that ionic photoconductivity plays an important role in the behavior of photodetectors made with solution processed amorphous ZnO and in their interaction with organic semiconductors

Diphenylphenoxy-Thiophene-PDI Dimers as Acceptors for OPV Applications with Open Circuit Voltage Approaching 1 Volt

Two new perylenediimides (PDIs) have been developed for use as electron acceptors in solution-processed bulk heterojunction solar cells. The compounds were designed to exhibit maximal solubility in organic solvents, and reduced aggregation in the solid state. In order to achieve this, diphenylphenoxy groups were used to functionalize a monomeric PDI core, and two PDI dimers were bridged with either one or two thiophene units. In photovoltaic devices prepared using PDI dimers and a monomer in conjunction with PTB7, it was found that the formation of crystalline domains in either the acceptor or donor was completely suppressed. Atomic force microscopy, X-ray diffraction, charge carrier mobility measurements and recombination kinetics studies all suggest that the lack of crystallinity in the active layer induces a significant drop in electron mobility. Significant surface recombination losses associated with a lack of segregation in the material were also identified as a significant loss mechanism. Finally, the monomeric PDI was found to have sub-optimum LUMO energy matching the cathode contact, thus limiting charge carrier extraction. Despite these setbacks, all PDIs produced high open circuit voltages, reaching almost 1 V in one particular caseThis work was supported by the Spanish Ministry of Economy, Industry and Competitiveness (MEIC) (TEC2015-71324-R, CTQ2014-55798-R and TEC2015-71915-REDT (MINECO/FEDER))This work was supported by the Catalan Institution for Research and Advanced Studies (ICREA) (ICREA “Academia Award”, AGAUR 2017 SGR 017SGR1527

Polymer self-assembly and thin film deposition in supercritical fluids

Patterning of flexible electronic devices using large-area printing techniques is the focus of intense research due to their promise of producing low-cost, light-weight, and flexible devices. The successful integration of advanced materials like semiconductor nanocrystals, carbon nanotubes and polymer semiconductors into microscale electronic devices requires deposition techniques that are robust, scalable, and enable fine patterning. To this end, we have established a deposition technique that leverages the unique solubility properties of supercritical fluids. The technique is the solution-phase analog of physical vapour deposition and allows thin films of a semiconducting polymer to be grown without the need for in-situ chemical reactions. To demonstrate the flexibility of the technique, we demonstrated precise control over the location of material deposition using a combination of photolithography and resistive heating. The versatility of the technique is demonstrated by creating a patterned film on the concave interior of a silicone hemisphere, a substrate that cannot be patterned via any other technique. More generally, the ability to control the deposition of solution processed materials with lithographic accuracy provides the long sought-after bridge between top-down and bottom-up self-assembly. In addition, we investigated the self-assembly of polymers in supercritical fluids by depositing thin films and studying their morphology using polarized optical microscopy and grazing incidence wide angle x-ray scattering. We summarized our observations with a two-step model for film formation. The first step is pre-aggregation in solution whereby the local crystalline order is established, and the solution turbulence can easily disrupt the solution-phase self-assembly. The second step to film formation is the longer length scale organization that is influenced by the chain mobility on the surface. We identified pressure and solvent additive as two powerful tools to facilitate the local crystalline order and longer length scale organization. The work demonstrated key insights necessary to optimizing thin-film morphologies and principles for understanding self-assembly in supercritical fluids that could be applied to self-assembly of materials in other contexts. Finally, we developed a simple empirical model based on classical thermodynamics that highlights the interplay of intermolecular interactions and solvent entropy and describes both the temperature and pressure dependence of polymer solubility in supercritical fluids

Structure Formation during Organic Molecular Beam Deposition

Pure and blended thin films of copper phthalocyanine (CuPc), Buckminster fullerene (C60) and coronene (Cor) molecules were deposited in vacuum onto standard silicon wafers and served as model systems for organic layers, as they are applied in organic photovoltaics (OPVs), organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). The blends were prepared by co-deposition, i.e. simultaneous evaporation of two molecular species. The influence of substrate temperature, deposition rate and mixing on the formation of crystal structures and surface profiles was investigated by various x-ray scattering techniques, as well as by atomic force and scanning electron microscopy. The non-linear formation of surface roughness was observed in real-time during the growth by means of in-situ x-ray reflectivity. Depending on the molecular species, three different growth modes were found: The growth of distinct islands, wetting of the substrate and layer plus island growth. Higher substrate temperatures resulted in larger islands and larger crystalline domains at lower island densities. A similar effect was observed when reducing the deposition rate. Faster diffusion and a lower flux of impinging molecules accounts for the improved molecular self-assembly. Mixing of two molecular species lead to smooth CuPc-C60 blends at room temperature and extremely rough CuPc-C60 blends at 400 K. The domain sizes were significantly reduced in blends and long CuPc needles protruding from the thin film appeared. Although a theoretical description of the structure formation is challenging, the studies have shown that a systematic analysis enables to tailor the physical properties of organic thin films by a suitable choice of growth parameters, which is advantageous for technical applications

Graphene modified indium tin oxide electrodes for organic solar cells

In this thesis, we explore the use of graphene incorporated onto indium tin oxide (G/ITO) as a structural template to modify the orientation of copper phthalocyanine (CuPc) molecules for organic photovoltaic (OPV) device applications. We also investigate the effectiveness of 2,3,5,65 tetrafluoro57,7,8,85tetracyanoquinodimethane (F45TCNQ) as a work function modifier for G/ITO without compromising the templating properties of graphene. Photoemission spectroscopy (PES) is employed to assess the electronic properties at the anode5CuPc interface, while X5ray diffraction (XRD) and near5edge X5ray absorption fine structure (NEXAFS) are used to determine the molecular orientation of CuPc. OPV devices are fabricated to attempt to correlate the observations at the microscopic level with the macroscopic device performance. First, we investigate the electronic properties of CuPc deposited on G/ITO and ITO using PES. While the interaction between CuPc molecules and ITO and G/ITO is similar, the hole injection barrier (HIB) is ~0.9 eV for CuPc/G/ITO as compared to 0.5 eV for CuPc/ITO. Therefore, further modification of G/ITO to reduce the HIB is required. The XRD spectrum of CuPc molecules deposited onto graphene grown on copper foil (G/Cu) verifies that graphene is an effective structural template, causing CuPc molecules to ‘lie’ on the substrate. NEXAFS data shows that the orientation of CuPc molecules changes from ‘standing’ on ITO to ‘tilted’ on G/ITO. Next, the effectiveness of F45TCNQ deposited on ITO and G/ITO as a work function modifier is assessed. A thin layer of F45TCNQ is able to increase the substrate work function to ~5 eV, which is close to the ionization potential of CuPc molecules. This suggests that barrierless extraction of holes from CuPc into F45TCNQ modified ITO or G/ITO may be possible. F45TCNQ molecules are found to be predominantly tilted on G/ITO, suggesting that the templating property of graphene may be propagated through F45TCNQ molecules. CuPc molecules deposited onto F45 TCNQ/G/ITO attain a ‘lying’ configuration, confirming that the templating property of graphene is preserved despite the inclusion of a layer of F45TCNQ. The HIB is dramatically reduced to ~0.2 eV for CuPc/F45TCNQ/G/ITO, and ~0.1 eV for CuPc/F45TCNQ/ITO. Optical absorption of templated CuPc molecules over the visible range is enhanced by over 40% as compared to the non5templated molecules. Therefore, the structure of F45TCNQ/G/ITO appears to be a potential anode design to improve OPV device performance. Our test cells however do not show an improvement in OPV parameters due to the poor quality of transferred graphene, and the high series resistance in our unoptimized OPV device. Finally, the diffusion of F45TCNQ through a CuPc film is studied using time5of5flight secondary ion mass spectrometry (TOF5SIMS). The F5 depth profiles establish that a higher quantity of F45 TCNQ molecules diffuse into CuPc on the G/ITO sample. This is attributed to the weaker interfacial adhesion between F45TCNQ and graphene, and the crystallinity of the templated CuPc film. The quantity of diffused F45TCNQ in the G/ITO sample is only about 0.2 mol%. At this dopant concentration, the conductivity of the film should increase; thus doping of the whole organic film may be favourable for OPV devices.Open Acces

Electron Microscopical Investigations of Organic Solar Cells

Diese Arbeit behandelt die Analyse der Nanomorphologie von organischen Solarzellen, welchen einen großen Einfluss auf die Effizienz der Zelle hat. Ein breites Spektrum elektronenmikroskopischer Techniken wurde angewendet um die Verteilung von Donator- und Akzeptordomänen in der Absorberschicht abzubilden. Die Ergebnisse trugen zum fundamentalen Verständnis der Zusammenhänge zwischen Nanomorphologie, Herstellungsparametern, optoelektronischen Eigenschaften und Effizienz der Solarzellen bei

Performance enhancement of organic photovoltaic cells through nanostructuring and molecular doping

Die vorliegende Arbeit beschäftigt sich mit der Leistungssteigerung organischer Solarzellen durch Änderung der Geometrie an der Donor-Akzeptor Grenzfläche und dem Einstellen der elektronischen Eigenschaften von Grenzflächen durch molekulares p-Dotieren. Kristalline und gleichmäßige Nanosäulen aus dem organischen Halbleiter Pentazen wurden durch glancing angle deposition (GLAD) hergestellt, die einen ineinandergreifenden Heteroübergang zu Methanofulleren [6,6]-Phenyl-C61-Butansäure Methylester (PCBM) als Akzeptor ermöglichten. Die Kurzschlussspannung der nanosäulenbasierten Solarzellen war signifikant erhöht im Vergleich zu planaren Heteroübergängen zwischen denselben Materialien. Die Leistungssteigerung der Solarzellen konnte maßgebend der vergrößerten Grenzfläche zugewiesen werden, wegen des verringerten Einflusses der kurzen Exciton Diffusionslänge. Molekulares p-Dotieren mit Tetrafluorotetracyanoquinodimethan (F4TCNQ) als Dotand in polyfuranbasierten Solarzellen wurde für verschiede Dotierkonzentrationen untersucht. Ultraviolettphotoelektronenspektroskopie wurde verwendet, um die Veränderungen der Energieniveaus mit zunehmender Dotierkonzentration zu analysieren, welche zu einer Vergrößerung der 0,2 V Kurzschlussspannung auf bis zu 0,4 V führte. Nach Kombination dieser Beobachtung mit Ergebnissen an dotierten Polymerfilmen, insbesondere bezüglich deren Morphologie und Absorptionsverhalten, wurde vorgeschlagen, dass ein resultierender Dipol an der Donor-Akzeptorgrenzfläche präsent ist. Zusammenfassend zeigt die vorliegende Arbeit das Potential sowohl der GLAD Technik als auch des molekularen, elektrischen Dotierens für die Leistungsverbesserung organischer Solarzellen.The present work mainly focuses on improving the performance of OPVCs by tailoring the donor-acceptor interface geometry and by tuning the electrical properties of interfaces with p-type molecular doping. Crystalline and uniform nanocolumns of pentacene (PEN) and diindenoperylene (DIP) were fabricated by glancing angle deposition (GLAD), forming an interdigitated donor/acceptor heterojunction with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and/or fullerene as the electron acceptor. The short circuit current of nanocolumn-based OPVCs increased significantly compared to planar heterojunction OPVCs made from the same materials. The performance improvement of OPVCs had been verified to be contributed decisively by the donor-acceptor interface area enlargement because of reduced impact of short exciton diffusion length in organic materials. P-type molecular doping as applied in polyfuran (PF) based OPVCs was investigated by using tetrafluorotetracyanoquinodimethane (F4-TCNQ) as the dopant for various doping ratios. Ultraviolet photoelectron spectroscopy (UPS) was applied to analyze the energy level shift with increasing doping ratio leading to the enlargement of the open circuit voltage in OPVCs, from 0.2 V to close to 0.4 V. Combining this observation with the results of doped polymer films, their morphology and absorption behavior, a net dipole pointing towards the donor material at the donor-acceptor interface of OPVCs is proposed. Overall, this work demonstrates the potential of both the GLAD technique and molecular electrical doping for improving the performance of OPVCs

Preparation and Characterization of thin films of organic semiconductors and their heterostructures

Diese Arbeit ist eine Zusammenfassung der Studien zur Präparation und spektroskopischen Charakterisierung von Dünnfilmen des organischen Halbleitermoleküls Perfluoropentacen (PFP) und Pentacenetetrone (P-TET), sowie von Heterostrukturen von PFP und Pentacen (PEN) und von PEN und Buckminster-Fulleren (C60). Durch die Kombination verschiedener Messtechniken wurden die Morphologie und Struktur der Dünnfilme analysiert und der Effekt von Veränderungen des Präparationsprozesses wie zum Beispiel der Variation der Diffusionslängen der Moleküle durch Einstellung der Substrattemperatur auf die Dünnfilme untersucht. Durch die gezielte Auswahl von Substraten verschiedener geometrischer und chemischer Eigenschaften wurden molekulare Dünnfilme in verschiedener, hochgeordneter Ausrichtung hergestellt. Dies ermöglichte die detaillierte polarisationsaufgelöste spektroskopische Vermessung dieser Dünnfilme, wodurch die anisotropen elektronischen, morphologischen und Vibrationseigenschaften der organischen Halbleitermoleküle in kristalliner Form bestimmt werden konnten. Weiterhin wurde der Einfluss der nanoskopischen Qualität des Trägersubstrates auf die resultierenden Dünnfilm-Eigenschaften für die Kombination von hochorientiertem pyrolitischen Graphit (HOPG) als Substrat und PFP und P-TET als Adsorbate untersucht. Hierbei zeigte sich, dass die schwache, aber effiziente Wechselwirkung zwischen dem Graphit-Substrat und dem Adsorbat in einer planaren Adsorptionsgeometrie und großer Kristallitgröße der Acene auf HOPG resultiert. Dies ist speziell interessant, da es spektroskopischen Zugang zu Dünnfilmen in liegender molekularer Orientierung ermöglicht, ohne dass die Moleküle durch starke Wechselwirkung mit dem Substrat chemisch verändert werden, wie es oft bei Kontakt mit Metalloberflächen der Fall ist. Durch Variation der Substratqualität wurde festgestellt, dass bereits mikroskopische Fehlstellen im Substrat diesen Effekt unterbinden, sodass die Substratqualität als kritischer Parameter für die Strukturbildung in molekularen Dünnfilmen identifiziert wurde. Im Falle der Deposition von PFP auf HOPG wurde eine neuartige Kristallphase (Polymorphismus) von PFP entdeckt, in der die PFP-Moleküle relativ zueinander parallel stapeln statt das typische Herringbone-Muster einzunehmen. Weiterhin wurden die PFP-Metall-Grenzflächen an den Metallen Gold, Silber und Kupfer studiert. Da diese Grenzflächen von entscheidender Bedeutung für die Effizienz realer Bauteile sind, ist ihr Verständnis und die Stabilität dieser Grenzfläche von großer Bedeutung für die Weiterentwicklung organischer elektronischer Bauteile. Es zeigte sich hierbei, dass die für PFP postulierte Stabilität gegenüber katalytischen Prozessen weitaus schwächer ist als vorhergesagt. Als Konsequenz treten an Grenzflächen mit reaktiven Silber- und Kupferoberflächen bei Zufuhr von thermischer Energie signifikante Veränderungen der strukturellen und elektronischen Eigenschaften auf, die bei hohen Temperaturen zu einer vollständigen Dissoziation des Moleküls führen. Zudem wurden Studien durchgeführt, die zu einem erweiterten Verständnis in der Strukturbildung und Wechselwirkung organischer Moleküle miteinander beitragen sollen. Solche organische Heterostrukturen sind von großer Bedeutung, da eine Vielzahl prototypialer elektronischer Bauelemente, wie beispielsweise organischer Solarzellen oder ambipolarer organischer Feldeffekttransistoren, auf Kombinationen mehrerer Komponenten zurückgreifen. Da die Effizienz dieser Bauteile wiederum kritisch von der elektronischen Wechselwirkung, der Durchmischung und relativen Anordnung der Komponenten zueinander bestimmt wird, sind die Ergebnisse dieser Arbeit, in der anhand geeigneter Modellsysteme eben solche Zusammenhänge untersucht wurden, von großer Bedeutung. Als Modellsysteme wurden Heterostrukturen von PFP und PEN sowie von PEN und C60 untersucht. Aufgrund der hohen strukturellen und elektronischen Kompatibilität tritt kristalline molekulare Durchmischung von PEN und PFP auf. Es wurde in dieser Arbeit nachgewiesen, dass die vorhergesagte effektive Quadruopol-Wechselwirkung beider Komponenten zu elektronischer Wechselwirkung und erhöhter thermischer Stabilität gegenüber den Einzelkomponenten führt. Darauf aufbauend wurden verschiedene Präparationsmethoden zu ihrer Herstellung verglichen. Zudem wurden detaillierte Erkenntnisse über die Auswirkungen der Durchmischung auf die elektronischen Eigenschaften gewonnen. Obwohl Heterostrukturen von PEN und C60 dagegen molekularer Entmischung unterliegen, beeinflussen sie sich dennoch in ihrer Nanostruktur. Es wurde gezeigt, dass durch Einstellung der Diffusionslängen der Fullerene C60-Nanostrukturen unterschiedlicher Dimensionalität mit gezielter Anlagerung an PEN-Molekularstufen hergestellt werden können. Dies bietet die Möglichkeit zur Fabrikation vergrabener molekularer Nanostrukturen, die spektroskopisch adressiert werden können