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Institute of Solid State Physics

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Research

Organic electronics
The dominant semiconductor in electronics is silicon and in high-performance computing applications, silicon has no real competition. There are, however, many applications outside the field of high-performance computing where other materials can outperform silicon. These applications include displays, efficient lighting, solar cells, environmental sensors, and chemical sensors. While organic materials are best known as insulating plastics, they can be modified into conductors or semiconductors and used for electronics. Organics can be thought of as the chameleons of the materials. They can be hard or soft, transparent or opaque, elastic or brittle, conducting or insulating. They can be optimized for their photoreactivity, chemical reactivity, flexibility, or biocompatibility. Organic material can also be processed in inexpensive and energy-efficient ways like stamping, spraying, dip coating, and printing.

Some of the pioneering work on organic conductors and semiconductors was performed more than 30 years ago at the Graz University of Technology by Hartmut Kahlert, Günther Leising, and Franz Stelzer. One of the early successes was the production of the first blue organic light emitting diode (OLED). Since that time, TU Graz has developed into an internationally recognized center for organic electronics.

Organic electronics is one of the areas where Graz University of Technology holds the most patents. To further develop these discoveries, faculty members of the university have been involved in founding the Joanneum Research Institute on Nanostructured Materials and Photonics as well as the NanoTecCenter Weiz GmbH. The R&D activities of these institutes include printing, imprinting and structuring processes and technologies to fabricate transistors, solar cells, OLEDs and integrated sensor devices from organic semiconductors and hybrid materials.

Projects now running in organic electronics are:
  Photovoltaics, Bettina Friedel
  Organic (Opto)electronic devices, Emil List
  X-ray structural determination of organic thin films, Roland Resel
  Computational modeling of organic materials and interfaces, Egbert Zojer

Surface Science
Surface science research in the institute started around 1970 with the construction of a field ion microscope by Prof. Erich Krautz. In 1973 Klaus Rendulic joined the institute and started working on fundamental aspects of adsorption and desorption kinetics. Using molecular beam techniques and laser spectroscopy, the surface science group (K. Rendulic, A. Winkler, M. Leisch) did groundbreaking work on the interaction of hydrogen with metal surfaces.

The current fields of research of the Surface Science group are: organic molecule adsorption, nucleation and thin film growth; correlation between the morphology and structure of ultra-thin organic films and their electrical properties; reactivity of surfaces with respect to heterogeneous catalysis; infra red reflection absorption spectroscopy (IRAS) of thin films on various substrates; and surface imaging (AFM, STM, SEM); and the physical and chemical fundamentals of fibre to fibre bonds in paper.

Projects now running in surface science are:
  Chemical reactions at surfaces, Robert Schennach
Christian Doppler Laboratory of Surface Chemical and Physical Fundamentals of Paper Strength, Robert Schennach
  Organic molecule adsorption, nucleation and thin film growth, Adolf Winkler

Research Projects

FWF - project: Crystal Structure Solution from Thin Organic Films: Indexation & Epitaxially Aligned Crystallites

The solution of crystal structures from thin films is essential in organic thin film technology, since new types of polymorphs arise when a substrate surface is present during the crystallisation process. Such surface induced crystal structures play an important role in organic electronics and in pharmaceutical research, where a considerable enhancement of material performance has been observed. The crystal structure solution from thin films is performed by a combination of experimental and theoretical techniques, combining grazing incidence X-ray diffraction (GIXD) with modelling of the molecular packing. This project will solve two open problems associated with crystal structure solution from thin films. The first problem is associated with the assignment of Laue indices to the Bragg peaks observed by grazing incidence diffraction (the so-called indexation) to determine the lattice constants. There, two components of the scattering vector – the in-plane part, qxy, and the out-of-plane part of the scattering vector, qz – have to be used. Mathematical equations for the fundamentals have to be derived, a numerical solution method for the indexation has to be developed and a refinement process for the lattice constants must be determined. The second problem is associated with biaxially oriented crystals at surfaces, such structures are obtained by epitaxial thin film growth. The collection of a complete diffraction pattern will be performed by a novel experimental approach using GIXD. Rotating the sample around the surface normal and collecting the diffracted intensity during a whole rotation of 360° enables the recording of a complete diffraction pattern even for epitaxially grown crystallites; the diffraction patterns will be collected from large volumes of reciprocal space. Enhancement in the quality of the experimental data in terms of peak position and peak intensities is expected. Crystal structure solution from thin films will be applied to thin films of the molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene, a molecule with outstanding performance in organic electronic devices. The non-symmetric chemical structure of the molecule makes it prone to form numerous polymorphic phases. A systematic variation of crystallisation conditions by solution processing and physical vapour deposition using biaxial crystallisation by directed nucleation methods and by uniaxial and single crystalline surfaces will allow the exploration of the ability of the molecule to form new polymorphic phases. This project will be a considerable contribution to the field of thin film crystallography, since a defined methodology for crystal structure solution from thin films is not fully developed so far.

Tuning the Interaction Strength of Inorganic/Organic Interfaces       >> more >>

Interfaces between inorganic materials and organic molecules are highly interesting the viewpoint of fundamental science, interesting since the flexibility of organic chemistry allows systematically tuning the strength of the interaction between the two components. While for unreactive, semiconducting substrates, the charge in the organic material is found to be strongly localized, for weakly reactive, metallic substrates, the charge is found to be completely delocalized. At present unclear, however, is how, e.g., degenerately doped semiconductors, which show quasi-metallic conductivity, fit into this classification.

In this project, we will study by means of first principle calculations (density functional theory and beyond) (a) how the localization of charge is affected for a given interface as the nature and strength of the substrate/adsorbate interaction is gradually modified, and (b) how this affects observables at the interface. To this aim, we will investigate the adsorption of small, conjugated organic molecules on semiconductors with different doping concentrations and metals which reactivity will be modified through alloying.

Defects in Organic Monolayers       >> more >>

Inorganic-organic interfaces present a versatile class of systems, providing the opportunity to achieve intriguing functionalities, e.g. as molecular switches, thermoelectrics, cargo-lifters, memories, or transistors, amongst others. The key to optimizing their functionalities lies in a systematic, fundamental understanding of the geometric and electronic structure of the interface. In many experimental setups or technologically relevant devices, organic materials are deposited as disordered or amorphous material.
Most phenomena occurring at the interface between inorganic electrodes and well-ordered organic materials are reasonably well understood. At the same time, the crucial impact of defects and disorder for e.g., the conduction in organic bulk materials has been recognized. Yet a systematic, assessment from first-principles of the impact of defects for transport properties at the interface is has not yet gained appropriate attention. The reason for this can be, in part, traced back to the high computational effort of jointly describing the defect and the surrounding crystalline layer, as well as to the vast configurational space that gives rise to a potential energy surface (PES) with a huge number of local minima. In the present project, we intent to close this gap and obtain an in-depth understanding of the role of defects and disorder in organic monolayers on various observables, including interface dipole, density of states, and interfacial level alignment.
In close collaboration with experimental partners, we will develop an efficient strategy based on Basin Hopping methods to sample the PES and obtain a set of low-energy structures that are occupied at finite temperature. Coarse-graining the PES by describing the layer as a combination of adsorption structures for isolated molecules that are arranged on a regular meshed imposed on the inorganic substrate allows to track the number of minima and keeps exploring the configurational space tractable. The Basin Hopping will be coupled with an atomistic description via density functional theory to predict the electronic structure at defect-containing interfaces and compare it to ideal, well-ordered structures. For selected model systems, such as the adsorption of the small organic electron acceptor Tetracyanoethene on Cu and ZnO substrates, this will allow answering important question such as: “What is the equilibrium concentration of defect at various temperatures?”, “What are common defect motifs?”, or “How do defects affect the energy-levels of the surrounding material?”.

Tuning the electronic properties of SAMs by embedded molecular dipoles

project duration: 15.05.2012-14.05.2015

Covalently-bonded self-assembled monolayers (SAMs) on metals have a wide variety of applications ranging from biology, via lithography, corrosion protection and sensing to organic electronic devices. When such SAMs are used for manipulating the electronic properties of surfaces, they usually contain polar chemical units. Typically, these units form the terminal groups of the SAMs, i.e., they are located at the SAM-ambient interface. This is far from ideal, as then changing the dipolar group also changes many SAM properties like its wetting properties or the growth of subsequently deposited layers. To avoid that, in the present project we studied the potential of SAM-forming molecules in which the polar units are “buried” within the molecular backbones. To understand the fundamental properties of such SAMs, we combined a variety of surface-science experiments (conducted primarily in the group of Michael Zharnikov at the Universität Heidelberg) with state of the art quantum-mechanical and molecular dynamics simulations (performed in the group of Egbert Zojer at Graz University of Technology). In the course of our studies, we were indeed able to realize aromatic SAMs with the desired properties which allowed changes of the work-function of a Au substrate by +/- 0.5 eV depending on the orientation of the embedded dipoles and compared to an apolar reference SAM. In these layers the intrinsic film properties could be rationalized at an atomistic level by means of the simulations. This paved the way for further experiments on mixed SAMs containing molecules with different dipole orientations for which a continuous tuning of induced work-function changes could be realized. On more fundamental grounds the above-mentioned study also showed that through a regular arrangement of embedded dipoles on surfaces one is able to locally shift the electrostatic reference energy within the adsorbates. This can be probed efficiently by x-ray photoemission spectroscopy in conjunction with the simulation of core-level shifts. This paved the way for proposing a novel concept for realizing materials with user-defined electronic properties that relies on collective electrostatic effects for realizing, for example, monolayer quantum-well and quantum-cascade structures. Finally, the peculiar charge transport properties through the above-described embedded-dipole SAMs also provided fundamental insight into the properties of molecular electronic devices.

1. A. Kovalchuk, T. Abu-Husein, D. Fracasso, D. A. Egger, E. Zojer, M. Zharnikov, A. Terfort, and R. C. Chiechi,* “Transition Voltages Respond to Synthetic Reorientation of Embedded Dipoles in Self-Assembled Monolayers”, Chemical Science, published on-line, DOI: 10.1039/C5SC03097H
2. Gernot J. Kraberger, David A. Egger, Egbert Zojer,* “Tuning the electronic structure of graphene through collective electrostatic effects”, Adv. Mater. Interfaces, 1500323 (2015). DOI: 10.1002/admi.201500323.
3. T. Abu-Husein, S. Schuster, D. A. Egger, M. Kind, T. Santowski, A. Wiesner, R. Chiechi, E. Zojer,* A. Terfort,* and M. Zharnikov,* “The Effects of Embedded Dipoles in Aromatic Self-Assembled Monolayers”, Adv. Funct. Mater. 25, 3943 (2015). DOI: 10.1002/adfm.201500899.
4. B. Kretz, D. A. Egger, and E. Zojer,* “A toolbox for controlling quantum states in organic monolayers”, Advanced Science, 1400016 (2015). DOI: 10.1002/advs.201400016.
5. V. Obersteiner, D. A. Egger,* G. Heimel, and E. Zojer,* ”Impact of Collective Electrostatic Effects on Charge Transport through Molecular Monolayers”, J. Phys. Chem. C 118, 22395 (2014); doi: 0.1021/jp5084955. Green OA

FWF project “PRO-CVD: Proton conductive copolymers by initiated Chemical Vapor Deposition” (P26993)

The projects aim at the vapor deposition of proton conductive copolymers to study the proton conductivity as a function of the chemical composition (ratio among hydrophobic and acid components, type or length of chemical groups) and as a function of the polymer physical properties (e.g. swelling in water, effect of crystalline order, ionic channels formation). The ultimate goal will be to understand how the properties of the copolymers affect the proton conductivity and the stability of the material at high temperature and in wet environment. A vapor-based deposition technique, initiated Chemical Vapor Deposition (iCVD) will be used to synthetize the materials, plus spectroscopic ellipsometry and x-ray based techniques for the characterization.

Charge Injection Layers at Inorganic/Organic Interfaces

In organic electronics, charge injection layers (CILs) are commonly added between the inorganic electrode and the active organic material to optimize charge injection (respectively extraction) barriers and exciton lifetimes in organic light emitting devices (OLEDs) or photovoltaic cells (OPVs). Most studies in this field have focused on the effect of the CIL on the effective work function on the substrate. The question how CILs affect the morphology of subsequently deposited organic material and what the effect on eventual charge transfer processes is remains open. Therefore, a density functional theory study based on advanced exchange-correlation functionals (including hybrid and non-local functionals), as well as many-body perturbation theory, such as the GW approach and the random-phase-approximation (RPA), is proposed in which the influence of various CILs on the morphology and electronic levels of the active organic material is analyzed for the example of different combinations of CILs and prototypical organic materials adsorbed on zinc oxide substrates. In collaboration with experimental partners, the mechanisms of bonding and interface dipole formation at technologically relevant interfaces will be investigated.

Defects in Semiconductors

Defects play an important role in determining the electrical and optical properties of semiconductors. Defects are also primarily responsible for degradation effects that influence device reliability. Many defects in semiconductors are still poorly understood. We study defects by Electron Beam Induced Current (EBIC) and Electrically Detected Magnetic Resonance (EDMR).

This research is done in cooperation with Infineon Technologies Austria AG and KAI Kompetenzzentrum Automobil- und Industrie-Elektronik GmbH

Contact: Peter Hadley

Marie Curie Fellowship: Smart multi Stimuli-responsive Supports for controlled cell growth (Three S)

Initiated Chemical Vapor Deposition (iCVD) is used to synthetize the smart stimuli-responsie material and x-ray based reflectivity techniques to control the response. Cells respond differently to substrates with different stiffnesses. We will develop a light-responsive hydrogel, whose water uptake changes with light irradiation, resulting in stiffness change. The structural changes in the film will be monitored by X-ray scattering and the water uptake changes by X-ray reflectivity (XRR) using humidity controlled cells and temperature-controlled stage.

FWF project: Photophysics and Charge Transport in Hybrid Blends of P3AT and beta-SiC Nanocrystals

In this project, we intend to investigate a system using the non-oxidic inorganic wide-band gap semiconductor 3C-SiC (cubic silicon carbide) as the acceptor in an organic poly(3-alkylthiophene) (P3AT) donor matrix. Silicon carbide as acceptor in hybrid cells has been neglected in the past, probably due to its indirect band gap, missing absorption contributions in the visible and expensive production of suitable nanocrystalline material. However, its band energies are suitable to match the HOMO/LUMO of organic donors, which provides a promising outlook for its functionality in hybrid photovoltaics.
Contact: Bettina Friedel

FWF project: Microscopic Inhomogeneities in Solution-Processed Organic Solar Cells

The project investigates microscopic inhomogeneities in organic semiconductors. Especially in organic thin film photovoltaics, devices suffer from considerable spatial variations, which have detrimental effects on the local device physics and thus on the entire device performance. One suspected source of origin therefore is the PEDOT:PSS electrode interlayer. Correlated to its sub-micron colloidal morphology, the basic microscale diode physics of organic solar cells are analysed and the effects on the entire device explained with the model of parallel microdiodes. The expected new insights will not only help understanding of the device behaviour of solar cells comprising PEDOT:PSS interlayers, but shall also allow conclusions to other colloidal systems in use and in future, like graphene or nanowires.
Contact: Bettina Friedel

TUG Initial Funding Program: Conductive Viscose-Composite Fibers for Smart Clothing and Skin Applications

FWF-project: Surface induced phases of molecular crystals: origin and stability

Polymorphism is a widely observed phenomenon in molecular crystals, it describes the appearance of different crystal structures from one type of molecule. The project focuses on one hardly considered origin of polymorphism: the influence of a solid surface to the crystallisation. The formation of surface induced crystal structures will be investigated on a specific series of rod-like conjugated molecules based on benzothieno-benzothiophene. Thin films will be prepared from the monolayer regime up to thick films by different methods ranging from physical vapour deposition under ultra-high vacuum conditions up to solution processing by drop casting and spin coating. Several crystallisation parameters will be varied ranging from the temperature of deposition, the supersaturation during the crystallisation process and the total deposited amount of material. Disordered surfaces will be used for the crystallisation, silicon oxide and polymer surfaces with different roughnesses, surface energies and polarities will be used. The prepared films will be characterised in terms of their structural properties by a variety of methods including surface science techniques but also atomic force microscopy, different methods of x-ray diffraction and infrared absorption spectroscopy. A part of the project is associated with modelling to consider molecular packing including force field calculations and molecular dynamics. The project will answer questions related to the origin and stability of surface induced crystal structures. The variation of the growth conditions will reveal important preparation parameters which cause the formation of a surface induced crystal structure. Studies of monolayer formation will give answers, if a first initial wetting layer is an important prerequisite for the formation of surface induced phases. The experimental investigations together with the modelling will reveal characteristic structural features of surface induced phases. In-situ temperature measurements will give answer if the surface induced phases are thermodynamically stable or if they represent a metastable state originated by growth kinetics.

Molecular crystals on polar / non-polar surfaces

The crystallization of molecules on surfaces depends strongly on the interaction of the molecules with the substrate surface. To get an idea about the influence of electrostatic interactions at the crystallization behavior of organic molecules, a specific combination of molecules and substrates are chosen. The two molecules - ternaphtalene as a non-polar molecule and septithiophene as a weakly polar molecule – are deposited on a polar substrate (Galliumnitride) and a nonpolar substrate (Aluminiumoxide). The Bachelorwork deals with the characterization of the crystallographic properties of thin organic films prepared on these two surfaces. The experimental method is x-ray diffraction pole figure technique and specular x-ray diffraction.
The work is in close collaboration with the Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz.
In case of interest please contact Roland Resel.

NAWI grant: Conducting Cellulose Fiber Networks for OPV Applications

Electronics based on bio-degradable materials is becoming increasingly attractive. Paper, a composite sheet of pressed cellulose fibers, has been considered for substrate in flexible thin film electronics, but so far only with a high density of fillers or with coatings to smoothen the surface and prevent short circuits. In the present study, filler-free paper is used, where each individual cellulose fiber is integrated carrier of an entire diode architecture. Thus shorts are hindered and the photosensitive surface area is additionally increased. Electrical conductivity is achieved by adsorption of silver nanowires to the cellulose fiber surface subsequent to paper formation. Electronic properties of the conductive paper and diodes are investigated, along with surface chemistry and micromorphology.
Contact: Bettina Friedel

Marie-Curie ITN: THINFACE

The project comprising 5 university partners, 1 research organization, and two full as well as four associated industrial partners deals with the development and optimization of next generation energy-saving and generating devices. The focus lies on organic/inorganic hybrid systems spanning the full range from advanced quantum-modelling to device fabrication and testing.

TUG Initial Funding Program: Dynamics of SiC-NC:Polymer Blends for Hybrid PV

Silicon carbide (SiC) is a valuable interesting candidate for hybrid photovoltaics. In this project, the stability of dispersed SiC-NCs in polymer solutions and the effects on thin film formation are investigated.
Contact: Bettina Friedel

FWF: Organic Thin Film Transistors as Chemical Sensors

Project summary: The goal of the project is to develop novel types of organic thin-film transistors (OTFT) containing a functionalized interfacial layers between the gate dielectric and the active layer to control the device-characteristics. In particular, we will focus on chemically reactive layers, which are suitable for chemical sensing, translating the presence of an analyte into a strong modification of threshold voltage and source-drain current of the transistor. This will allow the demonstration of new concepts for chemical or photochemical probes (to measure the total exposure of the device utilizing irreversible reactions) and sensors (to determine the current level of the analyte using reversible reactions). In preliminary tests, we have already realized devices with a covalently bound silane-based interfacial layer, in which exposure to NH3 shifts the threshold-voltage by up to 70V ! The main task of the project will be the realization of OTFT based sensors, and, most importantly, to understand the physical and chemical details of the involved processes. To achieve the latter, we will apply a multitude of analytical techniques: In addition to an in depth electrical characterisation of the devices, these include numerous surface-sensitive techniques to investigate layer thickness, structure, and morphology as well as the chemical composition of the layers. To complement the experiments, we will also perform standard quantum-mechanical calculations on suitable model systems. As interfacial layers, we will apply (i) covalently bound functional molecules having suitable docking groups (trichlorosilane or trialkoxysilane) groups to link to the substrate (i.e., the SiOx dielectric) and bearing also chemically reactive or photosensitive end groups; for these layers we have gathered significant experience during the past months, especially regarding their application in OTFTs. (ii) Spin-cast polymers (as an additional insulating layer of the dielectric on top of SiOx) bearing the same functional sensing units as the molecules mentioned above. Here, the sensing functionality will be either included directly during synthesis (by our partners), or added a posteriori through surface reactions on the spin-cast films. (iii) Langmuir-Blodgett (LB) type mono- and multilayers of analogous materials; the main potential of these materials is that they allow for well controlled self-assembly processes and a full control over the layer thickness (enabling, e.g., the fabrication of well defined multilayer structures). As reactive functional groups, we will apply analyte-docking groups like (e.g., sulfochloride, crown-ethers and non-protic bases) or photo-isomerisable/cleavable units. The proposed project aims at forming a bridge between the two big national research clusters on organic materials currently under way in Austria: The ISOTEC project of the Austrian Nanotechnology Initiative and the NFN “Interface controlled and functionalized organic films“. With a thematic position between the two clusters (interface controlled organic sensors), this project will help linking those activities to generate additional synergetic effects and simultaneously strongly benefit from numerous collaborations, which are detailed in this proposal.

FWF: Computational Nanotechnology

The fields of nanotechnology and organic semiconducting materials are of enormous interest both from a scientific as well as from a technological point of view. The present project aims at linking those two areas by investigating possibilities for tuning the electronic properties of organic/inorganic interfaces making use of covalently bound self-assembled monolayers (SAMs). These form central building blocks in numerous nanoscopic devices and new functionalities can be expected making use of the huge variety of conjugated organic compounds. The investigated aspects are of significant relevance for all types of organic electronic or optoelectronic devices as well as for the nascent field of single-molecule electronics. The work will focus on a computational approach based on quantum-mechanical electronic-structure calculations using so called slab-geometries. Additionally, it will rely on very close collaboration with numerous national as well as international collaboration partners engaged in experimental investigations. Two aspects of particular interest will be how tuning the chemical structure of the adsoprbed molecules affects the alignment between the electronic states inside the semi-conducting SAM and the metal and how SAMs can be used to tune the work functions of metal electrodes. The central topics will be • to develop general relationships between the chemical structure of the molecules comprising a SAM and the resulting modifications of the properties of the metal/organic interface; here, going beyond our previous research, we will study SAMs with varying polarizabilities of their backbones, SAMs consisting of quinoidal molecules, and in particular the impact of the surrounding molecules, e.g., in mixed monolayers. • We will gain a profound understanding how the detailed nature of the substrate surface affects the properties of the metal/SAM interface; beyond elucidating the detailed bonding chemistry of common docking groups on various metals, we will study the impact of the substrate morphology (including the role of ad-atoms, surface vacancies, and disorder). • Finally, we aim at understanding the electronic properties of organic semiconducting layers grown on top of SAMs bonded to metal substrates. Such multi-layer systems are of particular importance for practical applications. The eventual goal of this research is to propose a versatile toolbox for tuning the properties of metal/organic interfaces, which is based on the gained fundamental insight generated within the current project. The latter is highly multidisciplinary at the borderline between semiconductor physics, computational physics, organic chemistry and advanced materials design and this combination of different disciplines will help boosting the generated added value. contact: Egbert Zojer

Christian Doppler Laboratory for Surface Science Investigations on Paper Strength      >> more >>

In this new CD-Laboratory the strength of fiber – fiber bonds in paper will be investigated. The surface morphology as well as the surface chemistry will be investigated using a collaborative approach. The laboratory head is Prof. Robert Schennach from the Institute of Solid State Physics, Graz University of Technology. Close collaboration with Prof. Wolfgang Bauer from the Institute of Paper Pulp and Fiber Technology, Graz University of Technology and with Prof. Christian Teichert from the Institute of Physics, University of Leoben will enable a simultaneous investigation of the fiber morphology and the surface and interface chemistry. The industrial partners are Mondi Packaging in Frantschach, Lenzing AG and Kelheim Fibres.

Contact: Robert Schennach