Printed electronics and the use of organic materials in printed devices can be considered as one of the ways to deal with dynamic changes in the environment. These new technologies make it possible to use materials prepared from renewable sources in an environmentally sound way at the expense of metallic materials, often based on rare earths, such as conductive transparent ITO (Indium Tin Oxide).
The use of organic materials suitable for the functional layers of specific printed electronics devices is linked to the need to optimise the material, including the composition of the print formulation, to suit the given layout of the printed device, including the end user’s requirements for quality parameters.
Thus, in MADRAS project, funded within the Horizon 2020 program, focused on the field of Organic and Large-Area Electronics (OLAE), it is necessary to solve a suitable material arrangement for an organic photodetectors-based biometric sensor. For the case of study, the organic photodetector requires three printed layers acting as active layer, conductive transparent electrode and a Hole Transport Layer (HTL), printed on top of a TFT structure for fingerprint recognition.
In the case of a conductive layer formed by conductive polymers, the targeted desired parameter is a conductivity of 500 to 900 S/cm. At the same time, the material must show good adhesion to the preceding layer, transparency and stability during at least 6 months. In view of the printing of further layers with water-based inks, we might consider that it will be advantageous to formulate organic solvent-based conductive polymer inks.
The HTL layer, on the other hand, must have a charge mobility of 0.7 cm2/V.s as a minimum quality parameter. The organic material must be incorporated into a printing formulation, allowing the formation of layers with long-term stability and very good adhesion to other functional layers of the resulting device.
The Centre for Organic Chemistry Ltd. (COC) has vast experience in the synthesis of conductive polymers based on poly(3,4-ethylenedioxythiophene) (PEDOT), including the synthesis of alternative starting monomers. One of the directions of developing conductive polymers in COC is the modification of the doping group, the carrier of negative ions. In a commercial configuration, polystyrene sulfonate (PSS) is used as standard.
As part of the project solution, COC examines the influence of the ratio of PEDOT and PSS on the charge mobility properties of the printed layers. At the same time new dopants are tested, which can significantly affect the resulting electronic properties of the investigated layers. Hybrid systems are also tested in two ways: one where PEDOT is polymerised in the presence of Tungsten oxide (WO3) nanoparticles or carbon nanostructures, so that a homogeneous layer is formed on the surface of these particles, and another where both parts of the system are connected by conductive or ionic bonds.
The materials are being investigated in collaboration with the University of Pardubice (UPA), which performs formulation of printing inks and printability verifications. The electrical properties (conductivity, charge mobility) and adhesion to the specific photoactive layer or substrate are measured. Most promising samples are further tested at Eurecat for the development of organic photodetectors.
Figure 1 Samples of printing formulation, tested printed layers and sensor structures based on PEDOT / PSS
The preparation of conductive polymers and their printing formulations is currently being optimised. COC is ready to subsequently perform up scaling both on a laboratory scale in volumes of up to 6 Litres, and subsequently in conditions of glass pilot plant up to a volume of 100 Litres and subsequently, in conditions of regional pilot plant, with a volume of up to 600L.
Figure 2 Optimisation and up-scaling of organic materials synthesis at COC facilities
COC is an industrial research organisation that provides a wide range of services in the transfer of new innovative knowledge to the industrial sphere. As part of research projects funded within the EU, by the national government or private entities, it provides research into new materials in the field of printed and flexible electronics, photoactive materials, materials for battery construction and functional coatings. The company is fully equipped for the optimisation of synthetic procedures in laboratory conditions and their subsequent verification in the conditions of glass and regular pilot plant. At the same time, it has the necessary background for the analytical characterisation of synthetic processes and the purity of the resulting products. As part of the service provided, COC is able to ensure cooperation in the transfer of technology to an industrial scale by being able to ensure low-tonnage production of products during the product marketing stage. Subsequently, a final technological regulation suitable for the operating conditions of the final industrial implementer is developed.
In addition, COC is developing applications for its own materials, in particular hybrid conductive materials modified with conductive polymers and photoactive organic substances capable of generating reactive oxygen species based on organic colorants. COC has a long experience on to the application of functional materials and the possibility of their fixation in a polymer matrix so as to prevent their migration and, at the same time, preserve their functional properties.
Dr. Lubomir Kubac, Executive Head of COC since 2009:
- Long-term experience in the field of industrial research and design of new technologies
- Expert experience in the field of new functional modifications of conductive polymers and their fixation on solid carriers modified as dopants of conductive polymers
- Experience in the field of research project management and subsequent transfer of research results to the industrial scale
- Design of realizations of functional colorants
- More than 20 technologies implemented on an operational and pilot scale