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Printable Organic-Inorganic Transparent Semiconductor Devices

Final Report Summary - POINTS (Printable Organic-Inorganic Transparent Semiconductor Devices)

Executive Summary:
Points was set to make break-through advances in printed electronics in terms of new low-temperature printable materials with enhanced performance. This was achieved through combining the best properties of organic and inorganic (metal-oxide) materials into hybrid structures at both the molecular and at structural-interface level. In particular, the project focused on semiconducting and dielectric materials based on (i) molecular precursors, (ii) nanoparticle composites and (iii) hierarchically structured materials. Furthermore, expertise of the consortium in cost-efficient mass fabrication processes was coupled to the materials development to obtain optimum performance and processability. Industrial relevancy of the work was assured by selected demonstrators of industrial interest for ICT applications. Focused materials included IZO, GIZO, IO and GZTO for semiconductor and Al2O3, AlOOH, HfO, and novel high-k oxide/PMMA nanocomposites for insulator. Spin coating as well as inkjet, gravure and flexo printing were utilized for solution processed TFTs and circuits. Sputtering was used as fabrication method for reference devices and in materials selection.

The main results of the project were:
• Several alkoxide precursors were under detailed studies for low-temperature decomposition.
• Nanoparticle synthesis was improved for controlled composition for ternary materials.
• Nanoparticle processing into homogeneously thin, dense, and smooth films required for functional device stacks has been achieved.
• Semiconductor curing was achieved at 180 °C combined with FUV illumination resulting in mobility of the order of 3 cm2/Vs. For 200 °C annealing, the mobility reached 7 cm2/Vs with solution processed semiconductor and dielectric.
• Gravure-printed nanoparticle-based dielectrics were tested in capacitor structures. The results consistently reached a capacitance density of about 100 pF/mm2 and a leakage current density of 1 μA/mm2 as obtained at an electric field of 3 MV/cm. Best project dielectrics reached 1 nA/cm2 leakage current.
• Flexo printed TFTs reached a 4 cm2/Vs mobility performance on plastic.
• Statistical circuit simulation models were developed for the Aplac circuit simulator and those were used in circuit design.
• Simple logic flip-flop circuits, ring oscillators and LED drivers were implemented using the project materials. Those components were used to construct functional card demonstrators that were taken to the LOPE-C 2014 conference exhibition.
• The achievements of the project have resulted in 9 journal articles, 5 conference papers, contributions in 1 book, 1 doctoral dissertation, 2 publications in university series, 57 conference presentations (including the demonstrators exhibited at the LOPE-C 2014 conference) and 2 patent applications.
It is expected that the results will enable and catalyse development of printed electronics within the next couple of years.

For more information, please contact:
• project coordinator: Dr. Ari Alastalo, VTT, ari.alastalo@vtt.fi
• project website: http://points-fp7.eu/
• partners since the beginning of the project: VTT Technical Research Centre of Finland, IISB Fraunhofer Gesellschaft zur Foerderung der angewandten Forschung, FCT-UNL Faculdade de Ciencias e Technologia, Universidade Nova de Lisboa, UCAM University of Cambridge, Multivalent, Promethean Particles, Stora Enso Oyj, Bayer Technology Services GmbH, University Dunarea de Jos of Galati, IGCatalysts

Project Context and Objectives:
Printed electronics is an emerging disruptive technology which has made impressive progress in the last 10 years. In particular, (i) the field-effect mobility of solution-processed, organic TFTs has increased to levels exceeding that of thin-film amorphous silicon TFTs, (ii) the performance of polymer LEDs is surpassing that of fluorescent tubes, and (iii) the efficiency of printed organic solar cells is constantly improving. However, it has also become clear that significant further improvements in performance and reliability of materials and devices are needed to enable meeting real-world application requirements for a range of demanding ICT applications such as ambient intelligent labels, radiofrequency identification tagging, intelligent packaging, integrated sensor systems, etc. A potential approach to achieve these is to not restrict oneself to organic materials but also incorporate inorganic materials. This is because (i) inorganic semiconductors with three-dimensional covalent or ionic bonding tend to exhibit significantly higher charge carrier mobilities than organic semiconductors, (ii) printable inorganic conductors, such as metal nanoparticle inks, achieve much higher conductivities than conducting polymers, and (iii) the dielectric properties and breakdown characteristics of the best inorganic insulators are superior to those of polymer dielectrics.

Metal oxides is a class of inorganic materials that has recently attracted significant attention for use in thin film transistors (TFTs), conductive electrodes, capacitors, optoelectronics, sensors, and electrochromic devices. Through use of vacuum-based sputtering techniques at room temperature, a great variety of conductive or wide-band-gap semiconducting materials with binary, ternary or quaternary composition, such as ZnO, ZnSnO, SnGaZnO, InSnO, InGaO and InGaZnO, are accessible. Since for these materials, the electron conduction band is formed by overlap of the metal cations' spherical s-orbitals, they can exhibit excellent electronic properties for non-crystalline films. For conductors, Al-doped ZnO (AZO) and F-doped SnO2 are potential candidates for transparent materials to replace indium tin oxide (ITO) for which cost efficiency is limited by the shortage of indium. Recent results for insulating inorganic-organic composites have demonstrated relative dielectric constants over 100 thereby outperforming organic dielectrics.

To make use of the superior properties of inorganic materials while retaining the processing benefits of organic materials (compatibility with low-temperature solution processing and printing on plastic/paper substrates as well as good adhesion properties) hybrid organic-inorganic materials are essential. To achieve sufficient solution processability, the inorganic elements need to be attached to organic ligands and/or mixed with organic binders that either are eliminated during thin film processing, e.g. by thermal, laser, UV or electrical annealing or remain in the structure for enhanced operation. The objective of this project was to develop low-temperature solution processing approaches to a broad range of high-performance hybrid organic-inorganic metal-oxide materials. We also explored applications of these new materials in device structures relevant for ICT applications. In particular, we focused on the active semiconducting and gate dielectric layers of TFTs. What is needed in the field is a broad range of oxide materials that can be processed at low temperatures (< 180°C) to be compatible with common plastic substrates and that provide tailor-made controllable electronic properties (mobility, carrier concentration, interface states, permittivity) for optimization of device performance and stability. Points was set to make an improvement in this respect.

The Points achievements were valued on the basis of selected devices and application-relevant circuit test structures for ICT usage. Although the focus of the project was fully on ICT applications, we would like to emphasize that the materials developed in this project can potentially also be applied widely outside the applications targeted in POINTS. Such other application areas for the project outcomes are in displays, lighting and photovoltaics to implement the active light-emitting/absorbing layers, charge-transport layers or transparent electrodes.

The project had the following objectives:
• Develop high-performance solution-processable organic-inorganic metal-oxide semiconductor and dielectric materials. The main performance characteristics of special attention to be monitored during the project included:
o Semiconductor charge carrier mobility to exceed 10 cm2/Vs.
o Dielectric leakage current under 1nA/mm2 at electric fields of 3MV/cm.
o Low temperature processability at 150°C - 180°C.
o Bias stress stability of ΔVT < 3V after 105 s.
o Relative dielectric constant over 10.
• The material approaches included:
o Molecular precursor routes to metal oxides.
o Use of metal-oxide-based polymer-encapsulated nanoparticles dispersed in a liquid.
o Organic ligand/polymer encapsulation of the semiconducting inorganic nanoparticles.
• Develop and upscale new formulations of printing inks for oxide-based semiconductors and insulators.
• For preparation of reference devices and to guide the materials selection, RF sputtering was used.
• Validate and optimize the developed materials for the processing steps that are needed for low-cost mass production of the project target applications on paper and plastic substrates.
o Pre-processing such as substrate treatments
o Printing in sheet-fed and roll-to-roll compatible processes
o curing (oven, laser, UV, electrical)
o post-processing such as lamination
• Develop high-performance printed thin-film transistors (TFT) utilizing the new materials.
• Develop circuit-level models for the components to be available for further work in the field.
• Proof the industrial relevancy through application-specific circuit prototyping such as:
o Elementary logic devices such as inverters and flip-flops.
o Ring oscillator
o Switching multiplexer for memory addressing.
• Disseminate results into public knowledge and IPR, for example, through scientific articles and patents, through European and international conferences and workshops, through participation in industry associations such as the OE-A, using The POINTS project web site.

Project Results:
Plese see the report in pdf format

Potential Impact:
Plese see the report in pdf format

List of Websites:
The project contact information is as follows:
• project coordinator: Dr. Ari Alastalo, VTT, ari.alastalo@vtt.fi
• project website: http://points-fp7.eu/
• partners since the beginning of the project: VTT Technical Research Centre of Finland, IISB Fraunhofer Gesellschaft zur Foerderung der angewandten Forschung, FCT-UNL Faculdade de Ciencias e Technologia, Universidade Nova de Lisboa, UCAM University of Cambridge, Multivalent, Promethean Particles, Stora Enso Oyj, Bayer Technology Services GmbH, University Dunarea de Jos of Galati, IGCatalysts