Production of high-performance OTFT materials for flexible displays and OTFT devices

Giles Lloyd, Head of Materials Business

March 02, 2020

With the acquisition of the world-leading OTFT materials from Merck – now branded as FlexiOM™ – FlexEnable acquired a fully developed and established product portfolio with applications for flexible displays and OTFT devices. We not only offer a complete package of an industrially-proven flexible electronics technology with process-optimised materials, but also have access to the established supply chain to ensure continued and uninterrupted supply of the FlexiOM™ high-performance organic transistor materials.

In this blog we explain what makes FlexiOM™ materials best-in-class and how they are formulated and optimised to ensure highest quality.

Developing high-performance materials

MoleculesThe nature of the chemical structure of the material directly impacts on the performance. For many years there has been a debate on whether polymer or small molecule structures are better performing. The FlexiOM™ organic semiconductors are polymeric in nature, and for very good technical reasons as we explain below.

Small molecule materials tend to have a higher charge carrier mobility. Advanced formulation technology combined with soluble versions of the small molecules created interesting material technologies, however there still remained critical issues in processing. Soluble small molecules typically have very broad solubility profiles, meaning they can be dissolved in a very broad range of solvents. This effectively restricts the choice of solvent  for the subsequently deposited dielectric layer to fluorinated solvents. These solvents are heavily restricted and controlled in production environments and can add substantially to overall production cost. In addition, the fluorinated materials typically used with fluorinated solvents are very hydrophobic and hence difficult to coat subsequent layers on to.

Small molecule materials also have a crystalline structure. High crystallinity generally results in higher mobility due to the greater structural order within the material. However, it is also a well-known constraint within the large area electronics industry as it can lower the uniformity of the device performance. The same is true in organic semiconductors, high crystallinity produces lower uniformity. This is an essential parameter for high yielding display production.

Polymeric materials solve the issues stated above for organic semiconductor systems. Polymers have narrower solubility profiles thereby allowing good selection of solvents both for the deposition of the semiconductor polymer but also selection of solvents to allow deposition of the dielectric layer on top without dissolving the semiconductor. FlexiOM™ semiconducting polymers are amorphous by nature and hence have uniform electrical performance over large areas, similar to amorphous silicon transistors.

Chemistry of materials

Synthetic chemistry of high performance materials is not a simple task. Typically synthesis requires a large number of steps during production and at each stage purity and yield has to be monitored and optimised to ensure the highest grade of semiconducting material is produced in high quantities. Purity, especially, is critical to the semiconductor and display industry with impurities commonly creating trap states affecting performance and in severe cases, the production yield of devices.

Dielectric synthesis has similar demands, particularly on the purity but also on the physical specification with parameters such as molecular weight of the polymer. Molecular weight can be described as the average length of the chains of the polymer. This is important to ensure the rheology of the formulations is controlled. Rheology is critical to control film formation and thickness in large area production technologies. Control of the length of the chains, through control of the polymerisation, is an additional crucial step in material production.

The synthesis techniques, including polymerisation, for both FlexiOM™ semiconductor and dielectric materials have been heavily optimised and together with the established supply chain form a core part of the product portfolio.

Formulation control

FlexEnable FlexiOM materialsHaving the highest performing semiconducting polymer and dielectric is the first step in a production-compatible technology like FlexEnable’s OTFT platform. Processing these materials in solution form using standard machines such as slot-die coaters and spin coaters is the next step. Formulation technology is therefore a critical tool in the development of production-ready devices. Crucially, as no two coaters in different production lines are the same, the ability to optimise the formulation for each specific line is essential. Formulation specification is typically described by the rheology. This includes parameters such as how viscous the liquid is and how easily it will wet and coat the layer the liquid is deposited on. Once you have a wet coating on your substrate, you then need to control the drying to ensure uniform film formation.

FlexEnable offers customised and optimised formulation products. With FlexiOM™ formulations, we can optimise the composition including solid contents, solvent selection and potentially additives to deliver excellent coating and drying of the formulations.

The FlexiOM™ product portfolio includes world-leading performance in optimised and ready-to-use formulations. This includes FlexiOM™ FE-S500 organic semiconducting polymer, FlexiOM™ FE-D320 low-k dielectric and FlexiOM™ FE-D048X crosslinked dielectric for optimal processing and integration of the OTFT device.

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