Printing studies with conductive inks and exploration of new conducting polymer compositions

In addition to low cost and high volume, continuous production of devices such as transistors and RFID tags, printable electronics show promise in the fabrication of a multiplicity of sensors, displays, photovoltaic arrays, smart cards, etc. Due to flexibility and insensitivity to substrates, the use of organics in printed electronics has opened up a number of new opportunities in novel applications. In the present work, the process capability of flexography and offset lithography for patterning conductive materials was determined using small scale equipment (rotary letterpress and duplicator respectively). Process parameters including: type of substrate, line widths, line gaps, print thickness, directional effects, etc. were investigated. It was thus shown that the high volume printing processes of offset lithography and flexography can be used to obtain functional printed conductive patterns. In order to have greater control over ink composition and physical characteristics than was afforded by commercially available silver metal filled conductive inks, polyaniline (PANI) was synthesized by interfacial polymerization. Printable flexographic inks were formulated therefrom and a PANI ink was used in the flexographic printing of a working gas sensor. The conductivity of these inks was lower than that of silver filled metallic inks. This mitigated their utility in their utility in the printing of functional RFID antennae. Poly (thiophene-2, 5-diyl) (PT) and its derivatives are perhaps the most extensively studied class of conducting polymers and find applications in a variety of organic electronic devices. In the present work, an unprecedented approached to the synthesis and formulation of solution processible polythiophene (PT) compositions was explored. Conducting composites of polythiophene were synthesized by oxidative coupling of bithiophene, catalyzed by Fe3+ bound to the amphiphilic segment of functional block copolymers. Thus, amphiphilic block copolymers such as polystyrene-b-polyethylene oxide (PS-PEO) and polystyrene-b-polyacrylic acid (PS-PAA) complexed with Fe3+ were utilized as templates in the formation of soluble/redispersible prototype inks. The distribution of the conductive phase is, in principle, determined by the morphology of the block copolymer. The composites were characterized by DSC, UV-vis and IR spectroscopy. PT formed in the presence of these amphiphilic block copolymers was oxidised using suitable doping agents. The compositions however failed to exhibit significant conductivity. A number of challenges must be overcome in order to realize the potential economic benefits of using organic polymers in large scale electronic printing applications. The conductivity of inks based on organic conducting polymers can be increased by increasing the overall volume fraction of the conductive entity. The adhesion of the PANI compositions on various substrates could be improved by addition of a binding agent at a level that does not adversely affect the conductivity of the inks. Opportunities afforded by a post treatment/curing step may be considered and explored. Lastly, the ink formulation parameters and printing process variables should be optimized