Final Report Summary - MINOTOR (Modelling of electronic processes at interfaces in organic-based electronic devices)
Executive summary:
MINOTOR is a three-year project focusing on the theoretical modelling of interfaces involving organic semiconductors for applications in the blooming field of organic electronics. The project gathers ten groups, with academic and industrial partners, including one research group in US (Georgia Institute of Technology). This project was motivated by the fact that many key electronic processes occur at interfaces in organic-based devices such as light-emitting diodes, solar cells or field-effect transistors; this is the case for charge injection, light conversion into charges or recombination of charges into light. The project was methodology oriented by looking at the most appropriate theoretical approaches to describe the interfacial electronic processes and material oriented by establishing guidelines for the choice of the interfacial components expected to improve device performance. We wanted to obtain a unified view by considering in parallel metal / organic interfaces (M / O), organic / organic interfaces (O / O), and inorganic / organic (I / O) interfaces. In the case of M / O interfaces featuring a strong coupling between the molecules and the surface, standard DFT approaches based on LDA or GGA functionals perform well. This has been applied to the study of metals covered by self-assembled monolayers, showing that the work function of electrodes can be significantly tuned by Self-assembled monolayers (SAMs) and that there is a high degree of flexibility in the design of the SAM-forming molecules (by changing the nature of the anchoring group, functional head or intermediate spacer). This also prevails for many organic semiconductors deposited on bare metal surfaces, though the inclusion of dispersion forces is recommended to best describe the contact geometry. In the case of weak coupling (for instance in presence of passivated surfaces), DFT can qualitatively reproduce the existence of Fermi level pinning effects. A quantitative estimate of the pinning levels was reached via a phenomenological model developed in the framework of the project. In the case of O / O interfaces featuring a partial charge transfer between the donor and acceptor moieties, long-range corrected DFT functionals are recommended to properly describe the interfacial charge distribution. In contrast, DFT fails in doing a good job for systems dominated by polarisation effects. However, microelectrostatic (ME) models can be applied to such systems to determine the energy landscape around the interface, and for instance to examine pathways for charge separation in solar cells. The ME model was also used to unravel the mechanism of molecular doping in organic semiconductors. Our work also demonstrated that generating realistic morphologies of the organic / organic interfaces with the help of force field calculations is highly recommended to describe the actual structural disorder which strongly impacts the interfacial electronic processes. In the case of I / O interfaces, the theoretical calculations have shown that the work function of oxide layers (such as ITO or ZnO) can also be tuned by grafting SAM-forming molecules functionalised by carboxylic or phosphonic groups. A proper DFT description of the interfaces is challenging since one needs to properly describe at the same the electronic gap of the materials and the alignment of their frontier electronic levels. The use of a tight-binding DFT formalism appears as an attractive approach to tackle this issue. Many theoretical results have been directly confronted to corresponding UPS / XPS measurements to validate the theoretical procedure and assist the interpretation of the experimental data. Devices have also been fabricated to connect theory and experiment: spintronic devices based on C60 / Fe interfaces, diodes and transistors showing the influence of SAMs on oxide nanorods and contact electrodes on the I / V characteristics, respectively, and solar cells demonstrating the role of the interface morphology on the device performance.
Project context and objectives:
Project context
The MINOTOR project took place at the crossroad between fundamental research on organic based materials science, and the need for a more efficient engineering of devices in energy gathering and conversion. The main paradigm of organic devices is their multi-layer architecture involving materials with the most suitable electronic properties to obtain maximum device efficiency. At this stage, the intrinsic properties of the constitutive materials, like molecular crystals, polymers or inorganic electrodes, are well-known and many studies, both experimental and theoretical, have unravelled the key parameters required for optimal light emission or absorption, charge and energy transport ability. The combination of materials optimising these different aspects should lead to clear processing ways for developing new devices that reach new limits in efficiency.
However, what has been far less considered is the way to ideally combine these materials and to process them together without loss of efficiency. Building multi-layer devices imply multiple interfaces that can strongly affect the device performance. Therefore a proper control of the processes taking place at these interfaces is highly desirable. It is proven experimentally that the electronic and structural properties of two distinct materials in interaction strongly differ with respect to their bulk properties. Electronic and structural reorganisations, generating interface dipoles and band bendings, are affecting the relative position of the electronic levels which, in turns change the entire device performance. Therefore, a fundamental understanding of the factors at the origin of these electronic interface properties is critically needed. So far, only little theoretical and experimental work was performed on the comprehension and the prediction of such properties.
The MINOTOR project proposed to gather top level scientists specialised in multi-scale modelling and experimental characterisation of interfaces to unravel the nature of the interactions between organic semiconductors and organic, metallic and inorganic surfaces and their impact on the entire device performance. This project also aimed at developing an efficient toolbox and assessing the efficiency of theoretical methodologies to investigate and predict the structural and electronic properties of interfaces.
General objectives
The objectives of the project were twofold:
1. The main objective was the development and assessment of a modelling protocol for studying organic-based molecular devices that could be subsequently turned into an efficient engineering tool. This protocol should be able to provide an understanding of interfacial effects, and should have been evaluated and compared with experimental measurements.
2. The second objective of this project was to unravel and evaluate the different effects that might arise at an interface with respect to bulk material properties, and how this should affect the efficiency of these materials when processed in electronic devices. To confirm our understanding in the processes taking place at the interface, we confronted our theoretical results to UPS experiments and device fabrications.
To unravel these two issues, MINOTOR was casted down into three scientific Work packages (WPs), each one corresponding to a given type of interface that is present in organic-based multi-layer devices. The first WP was devoted to the study of metal organic interfaces, and encompasses the deposition of molecular electron donor or acceptor organic molecular layers on different metal surface and the possibility to control electronic properties of metallic electrodes by using self-assembled monolayers. Deposition of organic molecules was also considered for spintronics applications using ferromagnetic metals. The objective of this WP was to understand the impact of the organic layer on the charge injection efficiency in the metal electrode. It also permits to clarify the origin of the pinning level and the influence of the type of interaction (strong vs. weak) between the organic semi-conductor and the metal surface.
The second WP treated organic / organic interfaces between donor and acceptor molecules. The main goal of this WP was to understand the nature of the interactions between the two components and their effect on the energy level alignment at the interface. A deeper knowledge on this issue and on the influence of structural disorder aimed in particular to better apprehend the complex process of charge separation taking place at interfaces in a photovoltaic device. Another goal was to assess the impact of tuning the molecular properties on key parameters like the short circuit current and open circuit voltage of photovoltaic cells. A special emphasis was focused on studying the effect of structure relaxation through the modelling of relaxed interfaces.
The third WP was treating, similarly to the first one, the interface between inorganic semi-conductor and various organic donor or acceptor molecules. The main objective was to understand the impact of the interactions between the semiconductor and the organic material on the barrier injections at the inorganic electrode. The study focused on the addition of intercalating layers, like SAMs, to tune the interfacial electronic properties and hence optimise the injection process. The work also included a more realistic description of the oxide surfaces by introducing defects and adsorbed impurities, and its influence on the electronic properties at the interface.
Finally, the entire knowledge gained during the period of the project was compiled within a cross-fertilisation WP to highlight the common features and methodologies identified for an optimal description of the interfacial properties in a development and engineering process.
Achievements
The main achievement of MINOTOR was to assess and provide a modelling protocol for investigating interfaces in organic-based electronic devices and deepen our understanding of the link between the interfacial properties and the efficiency of the devices. This protocol has been widely applied and assessed in comparison to experiment on the three different types of interface that the project encompassed, and allowed the different partners to provide a clear and exhaustive overview of the different interfacial effects that are playing a role in the design of efficient materials for opto-electronic devices. In particular, and inside the three main WPs, the achievements obtained during the MINOTOR project are the following:
- For metallic-organic interfaces, we have analysed and unravelled the origin of the interfacial dipole moment at the interface between clean metal surfaces such as gold and silver, or more reactive metals like Cu or Ca and different organic semiconductors with varying donor / acceptor character, going from tetrathiafulvalene (TTF) to tetracyanoquinodimethane (TCNQ) and the tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), including also pentacene, perylene-tetracarboxylic-dianhydride (PTCDA) and naphthalene-tetracarboxylic-anhydride (NTCDA), tris(8-hydroxyquinolinato)aluminium (Alq3).
- The influence of SAMs on the work function of metallic surfaces has been studied for alkane-thiol and functionalised alkane-thiol based SAMs deposited on the gold (111) and silver (111) surfaces. In particular, the project focused on the ability to tune the metal work function by changing the nature of the anchored molecules. The influence of the orientation of the molecules, the nature of the anchoring or ending groups, the rate of coverage, as well as the respective contribution of the molecular and interfacial contribution to the shift of the work function have been unravelled.
- A pinning level model has been developed for passivated surfaces that correspond to an interface in which the metal is covered by a thin layer of native oxide. The prediction of the pinning levels for these interfaces has been compared with success to experimental photoelectron spectroscopy measurements.
- Applications to spintronic based devices have also been considered by treating the case of ferromagnetic metals combined with organic semi-conductors, both theoretically and experimentally. In particular the interfaces made of Co-graphite and Fe-C60 have been characterised in view of designing performing spin selective interfaces.
- For organic-organic interfaces, we have set up a complete multi-scale modelling process aiming at defining the nature of the interactions between the two semi-conductors, mainly based on partial charge transfer or on weak polarisation interactions, and the impact of the nature of this interaction on the energy profile of charges in the materials and at the interfaces. In particular, the case of the well-known and largely used bulk heterojunction pentacene-C60 device has been examined and the origin of the charge separation process in this interface has been explained. Other interfaces based on TTF, pentacene, Alq3, oligothiophenes as donor materials, and PTCDA, NTCDA, C60, TCNQ as acceptor materials, have been studied, with a good correlation with UPS measurements for the selected couples for which the experimental data have been obtained.
- The importance of the structure of these organic-organic interfaces has also been assessed by generating relaxed interfaces via force-field calculations and Monte-Carlo algorithms, either based on crystal relaxation or on deposition modelling. The influence of this structure relaxation on the electronic properties at interfaces has been evaluated with our multi-scale model. This has been performed for the pentacene-C60 interface, but also the pentacene-PTCDA, the sexithiophene-C60, and the TTF-TCNQ interfaces. The most important effect of the structural disorder on the electronic properties of O-O interfaces is the generation of new pathways for charge separation at the interface.
- An organic solar cell has been made and characterised by BASF partners, based on a heterojunction between merocyanin and C60 molecules. The whole design of this solar cell has benefited from all the knowledge and the guidelines that the project has accumulated.
- For inorganic-organic interfaces, general methodological schemes were devised for simulating the vapour deposition of organic compounds and providing reliable interface configurations for the quantum chemistry calculations.
- A multi-scale modelling approach has been developed for characterising the electronic processes at I-O interfaces. The theoretical tools have been applied in particular to the theoretical study of SAMs (substituted benzoic acids, 4-terbutyl-pyridin) and small organic molecules (pentacene, TTF, TCNQ and F4-TCNQ ) in contact with polar and non-polar ZnO surfaces, allowing us to predict the shift in the substrate work function and to identify its dependence on chemisorption, physisorption, coverage, and water contamination.
- With a bottom-up approach combining quantum mechanic calculations, atomistic simulations, microelectrostatic calculations and experiments, it was possible to relate the SAM-induced threshold voltage shift in pentacene-based transistors to the chemical nature of the SAM and the variation of the electrostatic landscape with respect to the bare SiO2 substrate.
- UPS measurements of the work function shift were carried out for ZnO and TiO2 surfaces covered with organic semiconductors. In spite of the problems in the production of clean ZnO surfaces, the realisation of working ZnO-based diodes allowed for the comparison and exploitation of results in real devices.
Partners
MINOTOR involved 9 partners over 7 European countries and collaboration with a group in the United States, for a total of 36 researchers during the three years of the project. Each partner is a leading group in their field of research and belongs to reputed universities and research centres. The industrial partner of the project is a major industrial player in the field of plastic electronics.
Project results:
The main scientific and technological results / foregrounds of the project MINOTOR have been described in point 4.1.3 of the final publishable summary (file in attachment).
Potential impact:
Summary of MINOTOR potential impact and exploitation of results
The field of organic electronics is about to revolutionise the way we produce light and electricity, to cite just two examples. Compared to the traditional silicon electronics, organic electronics enables production, at lower cost and requiring lower energy, of light, thin, flexible devices: transistors, solar cells, new lighting solutions and extremely sensitive sensors. If a large body of knowledge has been accumulated over the last decade on the bulk properties of conjugated organic materials, the lack of a fundamental understanding of the electronic processes going at interfaces, ubiquitous in organic electronics, has so far prevented a full optimisation of the material combinations and device architectures. The MINOTOR project contributed to the organic electronics field by developing a new theoretical paradigm for the modelling of organic heterojunctions, as well as metal-organic and organic-inorganic interfaces. MINOTOR results impact on socio-economy, environment, European and multisectorial collaborations and human potential, as the fundamental theoretical investigations performed on these interfaces, backed-up by experimental studies, aim at identifying engineering strategies for new applications related to energy production, energy saving, health, environmental monitoring, security, safety and welfare of citizen. These studies are thus expected to open new markets fostering industrialisation, economic growth and generation of jobs in Europe. Though the fundamental nature of the project is unlikely to result in short-term economic impact, it should be stressed that the scientific work performed within MINOTOR should guide future device and materials development activity, thus reducing development costs and shortening the time-to-market of new innovations.
In terms of collaboration: 8 universities, 1 research centre and 1 industrial partner have been involved in MINOTOR. This consortium of about 45 researchers has helped to form a cohesive European research area in the field of interface modelling and paved the way for the participation of several partners in follow up projects in the same area and several bilateral collaborations and business relations between partners from the consortium as well as recruitment of excellent young researchers to sustain research on a high scientific level also in industry.
The expanding activity in organic electronics in Europe and worldwide, both academic and industrial, has heightened the need for trained personnel in this area. This project contributed to the training of significant numbers of PhD students and the development of the careers of postdoctoral researchers in this important interdisciplinary area through partners collaborative work, project meetings, exchange of students as well as specific dedicated actions organised within the MINOTOR project such as the workshop ?Electronic processes at interfaces to organic semi-conductors: From modelling to devices which took place from the 29-31 May 2012, at the University of Mons bringing together 85 participants.
The impacts of MINOTOR touched more specifically three main applications: Organic light-emitting diodes (OLEDs), transistors, and photovoltaic cells that are considerably funded by Information and communication technology (ICT) projects in the Sixth and Seventh Framework Programmes (FP6, FP7). The sustainability of the European competitiveness in this field requires in parallel to device applications a continuous increase in the understanding of fundamentals and in the design of knowledge-based materials. This is what MINOTOR aimed at.
- OLEDs for displays are a mature technology, with a market that is expected to grow to USD 15 billion / year by 2015. That expansion will increase the need for materials with improved performances (with a market around USD 5 billion / year in 2015). The need for designed functional materials simplifying the fabrication processes by reducing the number of organic layers is even clearer for future applications of OLEDs in lighting.
- Organic-based transistors for organic logic and memory devices are an emerging technology, in which European companies (Plastic Logic, Polymer Vision, PolyIC) are the leaders. The use of those transistors as back-planes for active matrix displays, circuits for Radio-frequency identification (RFID), volatile memory devices, or detection elements for sensors is expected to generate a USD 8 billion market by 2015. Such growth also relies on optimised materials and improved device design aimed primarily at optimising the charge injection processes. L
- As for photovoltaic applications, devices based on organic materials will constitute a major progress with respect to the main existing technology (i.e. silicon-based systems), both economically (approximately 1 USD / Wp vs. approximately USD 3.5 / Wp) and environmentally (the amount of CO2 released to fabricate 1 m2 of photovoltaic cells is about 100 times lower when using organic materials). The application of organic photovoltaic systems towards broad markets is still limited by modest efficiency and moderate lifetime. A central issue is to maximise the yield of free carrier generation in the organic blends, thus requiring a deep knowledge of the interfacial properties and hence guidelines to choose the best matching partners.
As described in length in the scientific reports and summarised above, MINOTOR largely succeeded in bringing to completion the main objectives associated to these three different applications, namely by:
1. developing a general modelling scheme for the energy level alignment at metal-organic interfaces, thereby allowing to design strategies for the tuning of electron injection to metallic electrodes;
2. providing a molecular picture of the charge separation process at organic heterojunctions that takes into account the detailed rearrangement of the electronic density at these interfaces; and
3. assessing the electronic processes at organic-inorganic interfaces and how one can take advantage of these to fabricate stable low work function metal-oxide electrodes, e.g. through the formation of self-assembled monolayers. It is worth pointing out that these results obtained from modelling studies have been largely confirmed by experimental investigations performed within MINOTOR and exploited in the fabrication of a new generation of OLEDs, Field-effect transistors (FETs) and Organic photovoltaics (OPVs), namely by the industrial partner BASF but also the research centre IMEC and the academic partner LIN.
Other European companies active in the production of materials (be they polymers or inorganics) for organic electronics (Degussa, Solvay, Bayer, Ciba, Merck, HC Starck, AGFA,?) or in device fabrication (Plastic Logic, PolymerVision, Cambridge Display Technology, Philips, Siemens, OSRAM, Novaled, Konarka) will also likely benefit from the output of MINOTOR, since the modelling protocols for interface design and the main findings of the project are applicable to all possible type of interfaces encountered in organic-based electronic devices. In the value chain for organic electronics, MINOTOR is therefore likely to impact on both upstream (the selection and synthesis of new semiconductors and electrode materials) and downstream (the optimisation of device design and the improvement of their performances) Research and development (R&D) activities.
Summary of MINOTOR main dissemination activities
The scientific work has been widely disseminated through publications in international journals, posters and conference talks. In particular 47 articles have been published and some others are in the pipeline. The list of all articles abstracts and references is available on the public web site. In addition MINOTOR PIs, PHds and Postdocs have contributed to dissemination of MINOTOR activities via invited or contributed talks and posters (more than 120), see Final Periodic Report month 36.
Other dissemination activities include:
1) Success story project: 228424 MINOTOR (01 June 2009 till 31 May 2012). Modelling of electronic processes at interfaces in organic-based electronic devices (FP7). The MINOTOR project was targeted as a success story?by the European Commission. The following summary was provided by the coordinators.
What did the project achieve?
MINOTOR aimed to develop a multi-scale modelling approach ranging from the atomistic to the mesoscopic scale to model the processes taking place at the interfaces in electronic devices based on organic semiconductors, namely solar cells, field-effect transistors, and light-emitting diodes. These devices rely on the use of multi-layered structures and the main challenge is to understand how key parameters at the interfaces between each layer combine with the bulk of the individual materials to define the device performances. MINOTOR integrates the know-how from leading research groups in Europe and the United States in the field of plastic electronics for a better understanding of this challenging thematic. We have developed in MINOTOR new modelling approaches to investigate electronic and structural properties at organic-organic, organic-metal, and organic-inorganic interfaces, in close conjunction with corresponding experimental measurements.
In practice, MINOTOR has unravelled the various interfacial effects that can affect the intrinsic performance of a device; the electronic structure at the interface differs significantly from the bulk material, and the modelling work has highlighted the different contributions to this discontinuity. We have for instance disentangled the origin of the electronic level alignments at organic / organic interfaces that impact the short-circuit current and the open circuit voltage of solar cell devices, two key quantities that directly impact the efficiency of those cells. We have also shown that by modifying the surface of electrodes, we can tune the charge or spin injection barriers in devices such as OLEDs. Finally, the influence of lattice mismatches at the interface between two crystalline organic semi-conductors on the interfacial electronic properties was also quantified.
How?
MINOTOR brought together expertise from European academic laboratories and one industry which are leaders in their research field. Multi-scale chemistry and physics research approaches, based on both modelling and experimental techniques to identify each key point in the research challenges, were used to understand the modification of the electronic properties of materials at an interface and the impact of the different types of interfaces (interface between an electrode and a semiconductor, between two semiconductors) on the final characteristics of the device.
Why does this matter?
The challenge of producing cheap and efficient electronic devices based on organic (plastic) materials is a very attractive alternative to get reliable, economic, and sustainable sources of energy. Plastic solar cells are promising systems to replace conventional silicon based solar cells, in particular due to their low production cost, their sustainability and the recycling possibilities; however, their efficiency is still too low (below 10 %) for being a really affordable replacement solution to conventional devices. Due to the multi-layered structure of such devices composed of thin films or nano-particles, the control of the surface properties becomes crucial to optimise the device efficiency. In addition, interfaces are at the junction of two worlds: the world of materials of the physicists and the world of molecules of the chemists. This merging implies the definition of a new paradigm and hence the development of new computational methods to take into accounts the specificities of the two fields.
Who is involved?
MINOTOR involved 9 partners from 7 European countries and 1 in the United States, for a total of 40 persons, including principal investigators, dedicated researchers (funded by MINOTOR) and one part-time administrative assistant. The partners are leading groups in their field of research and belong to reputed universities and research centres, and the industrial partner of the project is a major industrial player in the field of plastic electronics.
The partners involved were: University of Mons (Belgium) (coordinators), IMEC research centre (Belgium), BASF (Germany), University of Twente (Netherlands), University of Linköping (Sweden), Karlsruhe Institute of Technology (Germany), University of Bordeaux (France), University of Madrid (Spain), University of Bologna (Italy), and Georgia Institute of Technology (United States) as associate partner.
What is the European added value? (criterion added by the cabinet)
The MINOTOR project, through the support of the European Union, gathered expertise from different groups in different countries, as well as a very large European Industrial actor in the field of plastic electronics.
Globally, this project took place in the context of a world-level challenge, which is the quest for sustainable and recyclable sources of energy at a reasonable cost. This is one of the objectives of plastic-based electronics. To this extent, the MINOTOR project is in the core of the most challenging research endeavours for the European countries. This project contributed to spur world-class research in Europe through the investigation of a central aspect (i.e. interfaces) in the development of efficient organic electronic devices. It brought together high-level researchers who contributed to the development of the research field with 42 international scientific publications. In addition, within the framework of the project, an international workshop will be organised in May 2012 to bring together world-leading scientists in the field of interfaces for the further development of interdisciplinary and international collaborations.
How much money has the EU invested in this?
The EU invested EUR 3 million in this project, which has run over three years, from 1 June 2009 to 31 May 2012.
For more information:
1. http://www.materianova.be/MINOTOR/
2. Jérôme Cornil (the coordinator) has been invited for an oral talk to industrial technologies 2012 (in the context of the industrial application Aarhus integrating nano, materials and productions, 19-21 June, Aarhus, Denmark). All information for this conference are detailed via the following link: http://industrialtechnologies2012.eu
3. The basic concepts of organic electronic and the interactions between light and matter have been presented to secondary school students to stimulate their curiosity and, hopefully, encourage them to choose scientific studies.
4. An article explaining the project MINOTOR electronic goes organic has been published by insight publishers Ltd., ISSN 2040-7335, edited by William Davis (projects issue 28, published May 2012). Summary: Organic electronics is a rapidly developing new field that involves using organic / carbon-based) materials to build electronic circuitry complementary to microelectronics based on mineral semiconductors such as silicon. David Beljonne, Jérôme Cornil and Roberto Lazzaroni explain how a new project called MINOTOR is helping to advance our understanding of the topic.
5. The project has been promoted by an international Workshop on 'Electronic processes at interfaces to organic semiconductors: from modelling to devices' that has been organised at the University of Mons (Belgium), 29-31 May 2012. For this occasion, the conference, and hence the project, was advertise to several academic laboratories and research centres at international level. A short video to promote the conference and the project was also performed and will be available soon.
The programme was organised as follows:
- There were 10 oral presentations of the work realised during the three years of MINOTOR presented by the PI of each group (talk of 30 minutes).
- We have also invited 7 speakers outside MINOTOR (talk of 30 minutes).
- There were 8 external contributed talks (talk of 20 minutes).
- A poster session has been organised with a total of 31 posters.
- The information for logistics and registration to the workshop has been provided on the MINOTOR website.
- A booklet with the agenda, the abstract of all talks and the list of participants has been distributed to all participants.
- The total number of participants was 85.
List of websites: http://www.materianova.be/MINOTOR/
MINOTOR is a three-year project focusing on the theoretical modelling of interfaces involving organic semiconductors for applications in the blooming field of organic electronics. The project gathers ten groups, with academic and industrial partners, including one research group in US (Georgia Institute of Technology). This project was motivated by the fact that many key electronic processes occur at interfaces in organic-based devices such as light-emitting diodes, solar cells or field-effect transistors; this is the case for charge injection, light conversion into charges or recombination of charges into light. The project was methodology oriented by looking at the most appropriate theoretical approaches to describe the interfacial electronic processes and material oriented by establishing guidelines for the choice of the interfacial components expected to improve device performance. We wanted to obtain a unified view by considering in parallel metal / organic interfaces (M / O), organic / organic interfaces (O / O), and inorganic / organic (I / O) interfaces. In the case of M / O interfaces featuring a strong coupling between the molecules and the surface, standard DFT approaches based on LDA or GGA functionals perform well. This has been applied to the study of metals covered by self-assembled monolayers, showing that the work function of electrodes can be significantly tuned by Self-assembled monolayers (SAMs) and that there is a high degree of flexibility in the design of the SAM-forming molecules (by changing the nature of the anchoring group, functional head or intermediate spacer). This also prevails for many organic semiconductors deposited on bare metal surfaces, though the inclusion of dispersion forces is recommended to best describe the contact geometry. In the case of weak coupling (for instance in presence of passivated surfaces), DFT can qualitatively reproduce the existence of Fermi level pinning effects. A quantitative estimate of the pinning levels was reached via a phenomenological model developed in the framework of the project. In the case of O / O interfaces featuring a partial charge transfer between the donor and acceptor moieties, long-range corrected DFT functionals are recommended to properly describe the interfacial charge distribution. In contrast, DFT fails in doing a good job for systems dominated by polarisation effects. However, microelectrostatic (ME) models can be applied to such systems to determine the energy landscape around the interface, and for instance to examine pathways for charge separation in solar cells. The ME model was also used to unravel the mechanism of molecular doping in organic semiconductors. Our work also demonstrated that generating realistic morphologies of the organic / organic interfaces with the help of force field calculations is highly recommended to describe the actual structural disorder which strongly impacts the interfacial electronic processes. In the case of I / O interfaces, the theoretical calculations have shown that the work function of oxide layers (such as ITO or ZnO) can also be tuned by grafting SAM-forming molecules functionalised by carboxylic or phosphonic groups. A proper DFT description of the interfaces is challenging since one needs to properly describe at the same the electronic gap of the materials and the alignment of their frontier electronic levels. The use of a tight-binding DFT formalism appears as an attractive approach to tackle this issue. Many theoretical results have been directly confronted to corresponding UPS / XPS measurements to validate the theoretical procedure and assist the interpretation of the experimental data. Devices have also been fabricated to connect theory and experiment: spintronic devices based on C60 / Fe interfaces, diodes and transistors showing the influence of SAMs on oxide nanorods and contact electrodes on the I / V characteristics, respectively, and solar cells demonstrating the role of the interface morphology on the device performance.
Project context and objectives:
Project context
The MINOTOR project took place at the crossroad between fundamental research on organic based materials science, and the need for a more efficient engineering of devices in energy gathering and conversion. The main paradigm of organic devices is their multi-layer architecture involving materials with the most suitable electronic properties to obtain maximum device efficiency. At this stage, the intrinsic properties of the constitutive materials, like molecular crystals, polymers or inorganic electrodes, are well-known and many studies, both experimental and theoretical, have unravelled the key parameters required for optimal light emission or absorption, charge and energy transport ability. The combination of materials optimising these different aspects should lead to clear processing ways for developing new devices that reach new limits in efficiency.
However, what has been far less considered is the way to ideally combine these materials and to process them together without loss of efficiency. Building multi-layer devices imply multiple interfaces that can strongly affect the device performance. Therefore a proper control of the processes taking place at these interfaces is highly desirable. It is proven experimentally that the electronic and structural properties of two distinct materials in interaction strongly differ with respect to their bulk properties. Electronic and structural reorganisations, generating interface dipoles and band bendings, are affecting the relative position of the electronic levels which, in turns change the entire device performance. Therefore, a fundamental understanding of the factors at the origin of these electronic interface properties is critically needed. So far, only little theoretical and experimental work was performed on the comprehension and the prediction of such properties.
The MINOTOR project proposed to gather top level scientists specialised in multi-scale modelling and experimental characterisation of interfaces to unravel the nature of the interactions between organic semiconductors and organic, metallic and inorganic surfaces and their impact on the entire device performance. This project also aimed at developing an efficient toolbox and assessing the efficiency of theoretical methodologies to investigate and predict the structural and electronic properties of interfaces.
General objectives
The objectives of the project were twofold:
1. The main objective was the development and assessment of a modelling protocol for studying organic-based molecular devices that could be subsequently turned into an efficient engineering tool. This protocol should be able to provide an understanding of interfacial effects, and should have been evaluated and compared with experimental measurements.
2. The second objective of this project was to unravel and evaluate the different effects that might arise at an interface with respect to bulk material properties, and how this should affect the efficiency of these materials when processed in electronic devices. To confirm our understanding in the processes taking place at the interface, we confronted our theoretical results to UPS experiments and device fabrications.
To unravel these two issues, MINOTOR was casted down into three scientific Work packages (WPs), each one corresponding to a given type of interface that is present in organic-based multi-layer devices. The first WP was devoted to the study of metal organic interfaces, and encompasses the deposition of molecular electron donor or acceptor organic molecular layers on different metal surface and the possibility to control electronic properties of metallic electrodes by using self-assembled monolayers. Deposition of organic molecules was also considered for spintronics applications using ferromagnetic metals. The objective of this WP was to understand the impact of the organic layer on the charge injection efficiency in the metal electrode. It also permits to clarify the origin of the pinning level and the influence of the type of interaction (strong vs. weak) between the organic semi-conductor and the metal surface.
The second WP treated organic / organic interfaces between donor and acceptor molecules. The main goal of this WP was to understand the nature of the interactions between the two components and their effect on the energy level alignment at the interface. A deeper knowledge on this issue and on the influence of structural disorder aimed in particular to better apprehend the complex process of charge separation taking place at interfaces in a photovoltaic device. Another goal was to assess the impact of tuning the molecular properties on key parameters like the short circuit current and open circuit voltage of photovoltaic cells. A special emphasis was focused on studying the effect of structure relaxation through the modelling of relaxed interfaces.
The third WP was treating, similarly to the first one, the interface between inorganic semi-conductor and various organic donor or acceptor molecules. The main objective was to understand the impact of the interactions between the semiconductor and the organic material on the barrier injections at the inorganic electrode. The study focused on the addition of intercalating layers, like SAMs, to tune the interfacial electronic properties and hence optimise the injection process. The work also included a more realistic description of the oxide surfaces by introducing defects and adsorbed impurities, and its influence on the electronic properties at the interface.
Finally, the entire knowledge gained during the period of the project was compiled within a cross-fertilisation WP to highlight the common features and methodologies identified for an optimal description of the interfacial properties in a development and engineering process.
Achievements
The main achievement of MINOTOR was to assess and provide a modelling protocol for investigating interfaces in organic-based electronic devices and deepen our understanding of the link between the interfacial properties and the efficiency of the devices. This protocol has been widely applied and assessed in comparison to experiment on the three different types of interface that the project encompassed, and allowed the different partners to provide a clear and exhaustive overview of the different interfacial effects that are playing a role in the design of efficient materials for opto-electronic devices. In particular, and inside the three main WPs, the achievements obtained during the MINOTOR project are the following:
- For metallic-organic interfaces, we have analysed and unravelled the origin of the interfacial dipole moment at the interface between clean metal surfaces such as gold and silver, or more reactive metals like Cu or Ca and different organic semiconductors with varying donor / acceptor character, going from tetrathiafulvalene (TTF) to tetracyanoquinodimethane (TCNQ) and the tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), including also pentacene, perylene-tetracarboxylic-dianhydride (PTCDA) and naphthalene-tetracarboxylic-anhydride (NTCDA), tris(8-hydroxyquinolinato)aluminium (Alq3).
- The influence of SAMs on the work function of metallic surfaces has been studied for alkane-thiol and functionalised alkane-thiol based SAMs deposited on the gold (111) and silver (111) surfaces. In particular, the project focused on the ability to tune the metal work function by changing the nature of the anchored molecules. The influence of the orientation of the molecules, the nature of the anchoring or ending groups, the rate of coverage, as well as the respective contribution of the molecular and interfacial contribution to the shift of the work function have been unravelled.
- A pinning level model has been developed for passivated surfaces that correspond to an interface in which the metal is covered by a thin layer of native oxide. The prediction of the pinning levels for these interfaces has been compared with success to experimental photoelectron spectroscopy measurements.
- Applications to spintronic based devices have also been considered by treating the case of ferromagnetic metals combined with organic semi-conductors, both theoretically and experimentally. In particular the interfaces made of Co-graphite and Fe-C60 have been characterised in view of designing performing spin selective interfaces.
- For organic-organic interfaces, we have set up a complete multi-scale modelling process aiming at defining the nature of the interactions between the two semi-conductors, mainly based on partial charge transfer or on weak polarisation interactions, and the impact of the nature of this interaction on the energy profile of charges in the materials and at the interfaces. In particular, the case of the well-known and largely used bulk heterojunction pentacene-C60 device has been examined and the origin of the charge separation process in this interface has been explained. Other interfaces based on TTF, pentacene, Alq3, oligothiophenes as donor materials, and PTCDA, NTCDA, C60, TCNQ as acceptor materials, have been studied, with a good correlation with UPS measurements for the selected couples for which the experimental data have been obtained.
- The importance of the structure of these organic-organic interfaces has also been assessed by generating relaxed interfaces via force-field calculations and Monte-Carlo algorithms, either based on crystal relaxation or on deposition modelling. The influence of this structure relaxation on the electronic properties at interfaces has been evaluated with our multi-scale model. This has been performed for the pentacene-C60 interface, but also the pentacene-PTCDA, the sexithiophene-C60, and the TTF-TCNQ interfaces. The most important effect of the structural disorder on the electronic properties of O-O interfaces is the generation of new pathways for charge separation at the interface.
- An organic solar cell has been made and characterised by BASF partners, based on a heterojunction between merocyanin and C60 molecules. The whole design of this solar cell has benefited from all the knowledge and the guidelines that the project has accumulated.
- For inorganic-organic interfaces, general methodological schemes were devised for simulating the vapour deposition of organic compounds and providing reliable interface configurations for the quantum chemistry calculations.
- A multi-scale modelling approach has been developed for characterising the electronic processes at I-O interfaces. The theoretical tools have been applied in particular to the theoretical study of SAMs (substituted benzoic acids, 4-terbutyl-pyridin) and small organic molecules (pentacene, TTF, TCNQ and F4-TCNQ ) in contact with polar and non-polar ZnO surfaces, allowing us to predict the shift in the substrate work function and to identify its dependence on chemisorption, physisorption, coverage, and water contamination.
- With a bottom-up approach combining quantum mechanic calculations, atomistic simulations, microelectrostatic calculations and experiments, it was possible to relate the SAM-induced threshold voltage shift in pentacene-based transistors to the chemical nature of the SAM and the variation of the electrostatic landscape with respect to the bare SiO2 substrate.
- UPS measurements of the work function shift were carried out for ZnO and TiO2 surfaces covered with organic semiconductors. In spite of the problems in the production of clean ZnO surfaces, the realisation of working ZnO-based diodes allowed for the comparison and exploitation of results in real devices.
Partners
MINOTOR involved 9 partners over 7 European countries and collaboration with a group in the United States, for a total of 36 researchers during the three years of the project. Each partner is a leading group in their field of research and belongs to reputed universities and research centres. The industrial partner of the project is a major industrial player in the field of plastic electronics.
Project results:
The main scientific and technological results / foregrounds of the project MINOTOR have been described in point 4.1.3 of the final publishable summary (file in attachment).
Potential impact:
Summary of MINOTOR potential impact and exploitation of results
The field of organic electronics is about to revolutionise the way we produce light and electricity, to cite just two examples. Compared to the traditional silicon electronics, organic electronics enables production, at lower cost and requiring lower energy, of light, thin, flexible devices: transistors, solar cells, new lighting solutions and extremely sensitive sensors. If a large body of knowledge has been accumulated over the last decade on the bulk properties of conjugated organic materials, the lack of a fundamental understanding of the electronic processes going at interfaces, ubiquitous in organic electronics, has so far prevented a full optimisation of the material combinations and device architectures. The MINOTOR project contributed to the organic electronics field by developing a new theoretical paradigm for the modelling of organic heterojunctions, as well as metal-organic and organic-inorganic interfaces. MINOTOR results impact on socio-economy, environment, European and multisectorial collaborations and human potential, as the fundamental theoretical investigations performed on these interfaces, backed-up by experimental studies, aim at identifying engineering strategies for new applications related to energy production, energy saving, health, environmental monitoring, security, safety and welfare of citizen. These studies are thus expected to open new markets fostering industrialisation, economic growth and generation of jobs in Europe. Though the fundamental nature of the project is unlikely to result in short-term economic impact, it should be stressed that the scientific work performed within MINOTOR should guide future device and materials development activity, thus reducing development costs and shortening the time-to-market of new innovations.
In terms of collaboration: 8 universities, 1 research centre and 1 industrial partner have been involved in MINOTOR. This consortium of about 45 researchers has helped to form a cohesive European research area in the field of interface modelling and paved the way for the participation of several partners in follow up projects in the same area and several bilateral collaborations and business relations between partners from the consortium as well as recruitment of excellent young researchers to sustain research on a high scientific level also in industry.
The expanding activity in organic electronics in Europe and worldwide, both academic and industrial, has heightened the need for trained personnel in this area. This project contributed to the training of significant numbers of PhD students and the development of the careers of postdoctoral researchers in this important interdisciplinary area through partners collaborative work, project meetings, exchange of students as well as specific dedicated actions organised within the MINOTOR project such as the workshop ?Electronic processes at interfaces to organic semi-conductors: From modelling to devices which took place from the 29-31 May 2012, at the University of Mons bringing together 85 participants.
The impacts of MINOTOR touched more specifically three main applications: Organic light-emitting diodes (OLEDs), transistors, and photovoltaic cells that are considerably funded by Information and communication technology (ICT) projects in the Sixth and Seventh Framework Programmes (FP6, FP7). The sustainability of the European competitiveness in this field requires in parallel to device applications a continuous increase in the understanding of fundamentals and in the design of knowledge-based materials. This is what MINOTOR aimed at.
- OLEDs for displays are a mature technology, with a market that is expected to grow to USD 15 billion / year by 2015. That expansion will increase the need for materials with improved performances (with a market around USD 5 billion / year in 2015). The need for designed functional materials simplifying the fabrication processes by reducing the number of organic layers is even clearer for future applications of OLEDs in lighting.
- Organic-based transistors for organic logic and memory devices are an emerging technology, in which European companies (Plastic Logic, Polymer Vision, PolyIC) are the leaders. The use of those transistors as back-planes for active matrix displays, circuits for Radio-frequency identification (RFID), volatile memory devices, or detection elements for sensors is expected to generate a USD 8 billion market by 2015. Such growth also relies on optimised materials and improved device design aimed primarily at optimising the charge injection processes. L
- As for photovoltaic applications, devices based on organic materials will constitute a major progress with respect to the main existing technology (i.e. silicon-based systems), both economically (approximately 1 USD / Wp vs. approximately USD 3.5 / Wp) and environmentally (the amount of CO2 released to fabricate 1 m2 of photovoltaic cells is about 100 times lower when using organic materials). The application of organic photovoltaic systems towards broad markets is still limited by modest efficiency and moderate lifetime. A central issue is to maximise the yield of free carrier generation in the organic blends, thus requiring a deep knowledge of the interfacial properties and hence guidelines to choose the best matching partners.
As described in length in the scientific reports and summarised above, MINOTOR largely succeeded in bringing to completion the main objectives associated to these three different applications, namely by:
1. developing a general modelling scheme for the energy level alignment at metal-organic interfaces, thereby allowing to design strategies for the tuning of electron injection to metallic electrodes;
2. providing a molecular picture of the charge separation process at organic heterojunctions that takes into account the detailed rearrangement of the electronic density at these interfaces; and
3. assessing the electronic processes at organic-inorganic interfaces and how one can take advantage of these to fabricate stable low work function metal-oxide electrodes, e.g. through the formation of self-assembled monolayers. It is worth pointing out that these results obtained from modelling studies have been largely confirmed by experimental investigations performed within MINOTOR and exploited in the fabrication of a new generation of OLEDs, Field-effect transistors (FETs) and Organic photovoltaics (OPVs), namely by the industrial partner BASF but also the research centre IMEC and the academic partner LIN.
Other European companies active in the production of materials (be they polymers or inorganics) for organic electronics (Degussa, Solvay, Bayer, Ciba, Merck, HC Starck, AGFA,?) or in device fabrication (Plastic Logic, PolymerVision, Cambridge Display Technology, Philips, Siemens, OSRAM, Novaled, Konarka) will also likely benefit from the output of MINOTOR, since the modelling protocols for interface design and the main findings of the project are applicable to all possible type of interfaces encountered in organic-based electronic devices. In the value chain for organic electronics, MINOTOR is therefore likely to impact on both upstream (the selection and synthesis of new semiconductors and electrode materials) and downstream (the optimisation of device design and the improvement of their performances) Research and development (R&D) activities.
Summary of MINOTOR main dissemination activities
The scientific work has been widely disseminated through publications in international journals, posters and conference talks. In particular 47 articles have been published and some others are in the pipeline. The list of all articles abstracts and references is available on the public web site. In addition MINOTOR PIs, PHds and Postdocs have contributed to dissemination of MINOTOR activities via invited or contributed talks and posters (more than 120), see Final Periodic Report month 36.
Other dissemination activities include:
1) Success story project: 228424 MINOTOR (01 June 2009 till 31 May 2012). Modelling of electronic processes at interfaces in organic-based electronic devices (FP7). The MINOTOR project was targeted as a success story?by the European Commission. The following summary was provided by the coordinators.
What did the project achieve?
MINOTOR aimed to develop a multi-scale modelling approach ranging from the atomistic to the mesoscopic scale to model the processes taking place at the interfaces in electronic devices based on organic semiconductors, namely solar cells, field-effect transistors, and light-emitting diodes. These devices rely on the use of multi-layered structures and the main challenge is to understand how key parameters at the interfaces between each layer combine with the bulk of the individual materials to define the device performances. MINOTOR integrates the know-how from leading research groups in Europe and the United States in the field of plastic electronics for a better understanding of this challenging thematic. We have developed in MINOTOR new modelling approaches to investigate electronic and structural properties at organic-organic, organic-metal, and organic-inorganic interfaces, in close conjunction with corresponding experimental measurements.
In practice, MINOTOR has unravelled the various interfacial effects that can affect the intrinsic performance of a device; the electronic structure at the interface differs significantly from the bulk material, and the modelling work has highlighted the different contributions to this discontinuity. We have for instance disentangled the origin of the electronic level alignments at organic / organic interfaces that impact the short-circuit current and the open circuit voltage of solar cell devices, two key quantities that directly impact the efficiency of those cells. We have also shown that by modifying the surface of electrodes, we can tune the charge or spin injection barriers in devices such as OLEDs. Finally, the influence of lattice mismatches at the interface between two crystalline organic semi-conductors on the interfacial electronic properties was also quantified.
How?
MINOTOR brought together expertise from European academic laboratories and one industry which are leaders in their research field. Multi-scale chemistry and physics research approaches, based on both modelling and experimental techniques to identify each key point in the research challenges, were used to understand the modification of the electronic properties of materials at an interface and the impact of the different types of interfaces (interface between an electrode and a semiconductor, between two semiconductors) on the final characteristics of the device.
Why does this matter?
The challenge of producing cheap and efficient electronic devices based on organic (plastic) materials is a very attractive alternative to get reliable, economic, and sustainable sources of energy. Plastic solar cells are promising systems to replace conventional silicon based solar cells, in particular due to their low production cost, their sustainability and the recycling possibilities; however, their efficiency is still too low (below 10 %) for being a really affordable replacement solution to conventional devices. Due to the multi-layered structure of such devices composed of thin films or nano-particles, the control of the surface properties becomes crucial to optimise the device efficiency. In addition, interfaces are at the junction of two worlds: the world of materials of the physicists and the world of molecules of the chemists. This merging implies the definition of a new paradigm and hence the development of new computational methods to take into accounts the specificities of the two fields.
Who is involved?
MINOTOR involved 9 partners from 7 European countries and 1 in the United States, for a total of 40 persons, including principal investigators, dedicated researchers (funded by MINOTOR) and one part-time administrative assistant. The partners are leading groups in their field of research and belong to reputed universities and research centres, and the industrial partner of the project is a major industrial player in the field of plastic electronics.
The partners involved were: University of Mons (Belgium) (coordinators), IMEC research centre (Belgium), BASF (Germany), University of Twente (Netherlands), University of Linköping (Sweden), Karlsruhe Institute of Technology (Germany), University of Bordeaux (France), University of Madrid (Spain), University of Bologna (Italy), and Georgia Institute of Technology (United States) as associate partner.
What is the European added value? (criterion added by the cabinet)
The MINOTOR project, through the support of the European Union, gathered expertise from different groups in different countries, as well as a very large European Industrial actor in the field of plastic electronics.
Globally, this project took place in the context of a world-level challenge, which is the quest for sustainable and recyclable sources of energy at a reasonable cost. This is one of the objectives of plastic-based electronics. To this extent, the MINOTOR project is in the core of the most challenging research endeavours for the European countries. This project contributed to spur world-class research in Europe through the investigation of a central aspect (i.e. interfaces) in the development of efficient organic electronic devices. It brought together high-level researchers who contributed to the development of the research field with 42 international scientific publications. In addition, within the framework of the project, an international workshop will be organised in May 2012 to bring together world-leading scientists in the field of interfaces for the further development of interdisciplinary and international collaborations.
How much money has the EU invested in this?
The EU invested EUR 3 million in this project, which has run over three years, from 1 June 2009 to 31 May 2012.
For more information:
1. http://www.materianova.be/MINOTOR/
2. Jérôme Cornil (the coordinator) has been invited for an oral talk to industrial technologies 2012 (in the context of the industrial application Aarhus integrating nano, materials and productions, 19-21 June, Aarhus, Denmark). All information for this conference are detailed via the following link: http://industrialtechnologies2012.eu
3. The basic concepts of organic electronic and the interactions between light and matter have been presented to secondary school students to stimulate their curiosity and, hopefully, encourage them to choose scientific studies.
4. An article explaining the project MINOTOR electronic goes organic has been published by insight publishers Ltd., ISSN 2040-7335, edited by William Davis (projects issue 28, published May 2012). Summary: Organic electronics is a rapidly developing new field that involves using organic / carbon-based) materials to build electronic circuitry complementary to microelectronics based on mineral semiconductors such as silicon. David Beljonne, Jérôme Cornil and Roberto Lazzaroni explain how a new project called MINOTOR is helping to advance our understanding of the topic.
5. The project has been promoted by an international Workshop on 'Electronic processes at interfaces to organic semiconductors: from modelling to devices' that has been organised at the University of Mons (Belgium), 29-31 May 2012. For this occasion, the conference, and hence the project, was advertise to several academic laboratories and research centres at international level. A short video to promote the conference and the project was also performed and will be available soon.
The programme was organised as follows:
- There were 10 oral presentations of the work realised during the three years of MINOTOR presented by the PI of each group (talk of 30 minutes).
- We have also invited 7 speakers outside MINOTOR (talk of 30 minutes).
- There were 8 external contributed talks (talk of 20 minutes).
- A poster session has been organised with a total of 31 posters.
- The information for logistics and registration to the workshop has been provided on the MINOTOR website.
- A booklet with the agenda, the abstract of all talks and the list of participants has been distributed to all participants.
- The total number of participants was 85.
List of websites: http://www.materianova.be/MINOTOR/
