In this fourth article on printing technology for printed electronics, we look specifically at technologies for making Thin Film Transistor Circuits TFTCs using soluble semiconducting inks. That has meant polymers but IBM is making progress in developing inks employing precursors of monomers - specifically oligomers - and these have better electronic properties including carrier mobility. It described this success in the IDTechEx conference Printed Electronics in New Orleans in December 2004.
A wide variety of conventional and new printing technologies are being used or tested for the printing of electronic circuits of various types. However, the varied nature of the inks available to create such circuits and the diversity of circuits required means that no one printing technology will ever be used for all applications. Indeed, the evolution of new conducting, semiconducting, dielectric, battery and other inks with new physical characteristics means that the printing technologies chosen or adapted will be constantly changing for some time to come.
Progress with printing techniques for TFTCs largely revolves around getting shorter channel length ie source drain distance reduced to increase the maximum frequency possible. This opens up more applications in many areas such as telephony and RFID. This progress can be seen by reviewing the summary presented by Avecia in 2002 below. However, there is also need for dielectric layers, passivation layers and conducting layers. Not all are ready in terms of either inks or processes.
For example, OrganicID, which intends to be the first with a commercial TFTC that is printed with polymer semiconductors on flexible substrates is using conductive polymer for some but not all conductors because this so called PEDOT has many magnitudes worse conductance than metal films. Indeed some initial TFTCs will have vacuum deposited aluminium conductors to start with - no a printing process at all. Connecting electrodes and antennas will often be screen printed initially - a slow and wasteful process but it gives the necessary thickness ie conductance with conventional silver inks.
Here IJ means ink jet. Today, the proponents of ink jetted TFTCs such as Plastic Logic and Xerox/ PARC see progress towards one micron resolution. Those developing the high speed reel to reel processes on the right see progress towards sub ten micron resolution. Indeed PolyIC demonstrated a working 13.56 MHz RFID tag at the IDTechEx conference Smart Labels Europe in late 2004.