Effects of thermal processing on properties of printed nano-Ag inks for polymer-based TFTs
Additively printed polymer thin-film transistors (TFTs) arrays on plastic substrates can enable low cost displays with new functionality and performance due to their flexibility and low-temperature processing conditions. Here, we discuss some of the challenges arising from the integration of solution processed materials such as printable interconnect metal lines produced from nanoparticle inks. This deposition technique is fundamentally different from the well-understood CVD methods, driving the need for a thorough evaluation of these metal features formed from nano-inks. Inkjet-printed metal particles, initially in the form of a stable dispersion of silver clusters sized between 30 and 50nm in ethylene glycol, sinter rapidly when subjected to thermal cure at homologous temperatures as low as 0.1 (100oC) due to their very high surface area/volume ratio. Upon heating, the solvent is evaporated, and Ag particles come in contact with one another and begin neck formation, or sintering process, forming a continuous conductive percolation network, which enables utilizing these inks as a desirable choice for manufacturing conductive metal features. For example, silver printed features can be used as source-drain electrodes in polymer based TFTs. These devices show TFT mobilities of 0.1 cm2/Vs and exhibit no significant contact resistance. In this report, we present the results of investigation the evolution of morphology, densification and linear shrinkage, and electrical resistivity of thin porous films produced from a nanoparticle ink, as a function of different thermal curing conditions. The resistivity was found to be proportional to the pore sizes and concentration in the formed metallic coating. Porosity, in its turn, is a complex function of time and temperature through the competing sintering and grain growth/crystallization mechanism(s). A phenomenological analytical model, which allows for the prediction of the resulting printed features' conductivity, is constructed based on the evolution of these processes. The model can be utilized as a useful tool in achieving the desired electrical performance through thermal treatment control.
Greer, J. R. ; Arias, A. C. ; Endicott, F .; Daniel, J. H. ; Street, R. A. Effects of thermal processing on properties of printed nano-Ag inks for polymer-based TFTs. Materials Research Society Fall Meeting; 2006 November 27-December 1; Boston; MA.