Oxford spin-out develops new display production method

Traditionally, many electronic devices such as displays have been manufactured using vacuum and vapour deposition-based processes. While these have proved sufficient to meet the needs of today’s marketplace, many companies are now developing solution-based manufacturing processing techniques that will offer some distinct advantages.

Not only will these upcoming processing methods be simpler and more cost effective than existing techniques, they will also enable manufacturers to build electronic systems such as flexible displays from organic thin film transistors.

Inorganic thin-film transistors have already been widely used in liquid-crystal televisions, where they are deployed to drive pixels on the display. Such thin-film transistors are made by depositing thin films of an active layer of semiconductor material, as well as a dielectric layer and metallic contacts over a supporting rigid substrate.

But while next-generation displays will also use arrays of such transistors to drive individual pixel elements, they will be constructed from organic semiconductor materials deposited onto a flexible substrate, rather than a rigid one, using solution processing manufacturing techniques instead.

Manufacturing such devices is challenging. Because the substrate upon which the organic thin films are manufactured is flexible, it is important to ensure the stack of materials that form the transistor layers do not separate in a process of delamination as the display is put through cyclic stresses.

Now, as the result of a project called Customised Adhesion of Inter-layers for Plastic Electronics (CAIPE), chemists at Oxford Advanced Surfaces (a spin-out from the Chemistry Department at Oxford University) have helped to develop a new proprietary wet chemical process manufacturing technology that claims to have overcome the interlayer delamination issue.

It is important to ensure the stack of materials that form the transistor layers do not separate

Assisted by funding from the Technology Strategy Board, the company worked in a consortium with engineers at both OMIC, the Organic Materials Innovation Centre at the University of Manchester Electronics Consortium, and PETEC, the Printable Electronics Technology Centre, which designs, develops and prototypes printable electronic devices.

While the researchers at OMIC developed the organic thin-film material for the transistors, the chemists at Oxford Advanced Surfaces developed a proprietary means to chemically modify specific surfaces used to construct the structure transistor to effectively ’bond’ key layers of the transistor to ensure that delamination did not occur.

The engineers at PETEC then developed a manufacturing process using both the organic material supplied by OMEC and Oxford Advanced Surfaces reactive adhesive technology.

Jon-Paul Griffiths, co-founder of Oxford Advanced Surfaces, believes that the technology his company has developed is one that could change the face of the electronics industry. ’Without using our surface modification chemistry, you would not get any adhesion at all between certain layers of the transistor, and without that it would be impossible to create a stable product such as a OTFT in high volumes,’ he said.

Developed from work carried out at Oxford University’s Department of Chemistry by Griffiths and his colleague Mark Moloney, Oxford Advanced Surfaces’ process itself first involves coating a base material with a proprietary chemical agent called an adhesion promoter, a process that is then followed by curing with heat, UV or other suitable energy sources.

In the case of earlier chemical compounds developed by the company, these adhesion promoters comprised a compound with a structure with a functional tail covalently linked to a reactive head that, once coated onto a material, could modify the material so that it could be bonded onto others.

In the case of the compound developed under the CAIPE programme, the company took the idea one step further, producing a multi-headed chemical compound that could adhere effectively to two disparate material surfaces within the transistor structure. ’While one reactive head group modifies one substrate in the transistor stack, another reactive head group modifies the other,’ said Dr Griffiths.

In the manufacture of an organic thin-film transistor, there are many such interfaces between the materials that comprise each individual transistor. Dr Griffiths admits that the adhesion promoter his company has developed is not intended to ensure that all such layers adhere, but only the interlayer where de-lamination presents a problem.

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