Dielectrics for all-additive jet-printed organic electronics (Invited paper)
In an all-additive process we use jet-printing and additive solution processes such as spin-coating to fabricate electronic circuits. This approach employs the smallest number of processing steps and the least amount of materials. It is therefore promising for inexpensive, potentially disposable electronics. Active-matrix pixel circuits for paper-like displays are a promising initial application for our processes since the feature size does not need to be extremely small and the performance requirements for the pixel thin-film-transistors (TFT) are not too strenuous. We use jet-printed nanosilver to define the conductors such as gate and data lines as well as pixel pads. The printing of continuous silver lines depends strongly on the ink and on the surface properties of the substrate. Therefore, strongly hydrophobic surfaces, which are often desired for improving the molecular ordering of the polymer semiconductor, are not suitable for printing continuous data lines and pixel pads in bottom contact TFTs. With carefully chosen polymer gate dielectrics, printed silver lines around 50-60 microns were achieved. The gate dielectric is also critical for the TFT performance and we have investigated several polymer dielectrics which were deposited by spin-coating at a layer thickness around 200-500nm. As the organic semiconductor we used jet-printed polythiophene PQT-12. This process employs many new materials which are combined to build transistors. Therefore, we have measured the transistor properties for various dielectrics and various process conditions. Typical saturation mobilities for our PQT-based TFTs were around 10-2cm2/Vs and mobilities around 10-1cm2/Vs are possible under optimized conditions. The best combination of materials was used to fabricate active matrix backplanes on low-temperature flexible substrates with a pixel size varying from 500 to 1000 microns. The pixel performance was measured on various pixel layouts in order to determine the best pixel design in terms of fast response and low feedthrough voltage. We will also discuss approaches for multilayer pixel structures in which a ‘mushroom metal' layer extends over the data and gate lines to shield their voltage potential. Such designs yield pixels with a high fill-factor and with improved performance for displays and image sensors. In one approach, micromolding is used in combination with jet-printing.
Daniel, J. H. ; Arias, A. C. ; Ready, S. E. ; Krusor, B. S. ; Street, R. A. Dielectrics for all-additive jet-printed organic electronics. (Invited paper) 2007 MRS Spring Meeting; 2007 April 9-13; San Francisco; CA.