Inkjet-printed Polymer Thin Film Transistors at Epson (Printed Electronics Europe 2005)

 

After a short overview of Seiko Epson Corporation, the Cambridge Research Laboratory of Epson (CRLE) is introduced. One of the research groups at the CRLE is the group of Organic Electronics, which focuses on the evaluation of functional materials for organic Thin Film Transistors (OTFTs), the investigation of the fundamental device physics in OTFTs, and the development of novel device geometries and complementary patterning methods for the inkjet printing of OTFTs.

 

 

Epson aims at developing inkjet printing as a tool for the direct, digital printing of patterns of functional liquids containing insulator, semiconductor, or electrically conducting materials for electronic production.

 

While inkjet printing is a highly energy and material efficient way of additively patterning electronically functional materials, a number of specific issues including the ink interaction with the substrate (wetting, spreading, drying), complementary patterning technologies for increasing the resolution of inkjet printing, and post-deposition treatments such as drying, annealing and sintering have to be addressed when applying inkjet printing to the fabrication of electronic devices.

 

Large format industrial inkjet printers have been developed at Seiko Epson Corporation that allow to print large-area OLEDs and PDPs.

 

 

Our technology is based on a drop-on-demand piezoelectric design, with an adjustable driving waveform, which allows printing both water and solvent-based inks from 180 nozzles simultaneously, with droplet volumes controllable from 2 to 20pl.

 

After an analysis of the mechanism and conditions for droplet ejection, the factors determining the spreading of inkjet droplets on non-absorbent surfaces are discussed.

 

 

Organic materials such as conjugated polymers are ideal candidates for printed electronics, as they can be made to be soluble in a wide range of organic solvents, which allows to inkjet print the semiconductor layer in polymer TFTs. Within this context, specific topography and drying phenomena, and the performance of the resulting TFTs are discussed.

 

Complementary Patterning Techniques for enhancing the resolution of inkjet printing are an important research topic at CRLE, and examples of sub-micron channel length polymer TFTs are presented. Furthermore, the influence of different Source/Drain contact materials on the contact resistance at the interface with the semiconductor is discussed.

 

For display applications, the air-stability of the polymer TFTs, i.e. their ability to retain a large current on/off ratio upon operating the devices in air, is of prime importance. Within this context, we address the air stability of polymer TFTs by discussing the evolution of the conductivity of thin semiconductor films and the changes of the current on-off ratio in polymer TFTs upon exposure to air.

 

Finally, a potential application for polymer TFTs in active matrix backplanes for electrophoretic displays is presented.

 

Organic Electronics Team Leader: Thomas Kugler

Date of birth: 12.11.1961

Nationality: German

 

Seiko Epson is a global corporation based in Japan that is at the forefront of technological revolutions in imaging, robotics, precision machinery and electronics.

 

The Cambridge Research Laboratory of Epson (CRLE) is a division of Epson (UK) Ltd. Its research activities are sponsored and directed by the Corporate R & D section of Seiko Epson Corp, the parent company of Epson (UK) Ltd. in Japan. The laboratory is carrying out research into novel electronic devices and processing techniques of electronic devices in collaboration with the Engineering Department and the Cavendish Laboratory, University of Cambridge. Our aim is to develop displays, integrated circuits and computer memory based on completely new principles, and to promote innovation in electronic device production technology by creating these devices on plastic substrates.