Inkjet printing and IR sintering optimization for flexible electronic applications

The main focus of this thesis work was to develop mass manufacturing compatible fabrication processes for flexible electronic devices, using paper either as an active layer or as a substrate. Combining common printing techniques with low temperature sintering processes was the key to develop and optimize the production of such simple devices on flexible substrates, mainly paper. The use of paper as a flexible substrate has highly attractive characteristics, being its low cost, wide availability and environmental friendliness, some of the more appealing. Together, they make paper substrates exceptionally interesting for printed electronics, when compared with more traditional substrates, such as polymers. Nevertheless, functional printing of simple components or complex devices on paper is extremely challenging, mainly due to the porosity of the substrate or low heat resistance. Aditionally, commercial inks are developed mainly for non-porous substrates, as polymers, and need a specific range of sintering time and temperatures, to be functional. That makes them immensely defiant to use in porous substrates or be compatible with mass manu-facturing processes, where cost and speed are main restrictions in the industry. In this thesis work, inkjet printing and sintering optimization parameters have been tuned in order to obtain good conductivity on various commercial paper substrates. Multicoated paper Lumi silk from StoraEnso was chosen as the optimal substrate for printed electronics. This was mainly because of its smoothness, which promoted good line uniformity, non-disruptive and less ink penetration into the substrate, without the need for additional coatings. More, this substrate was able to endure slightly higher sintering temperatures than the other paper substrates, for the same period of sintering time. The use of photonic sintering methods reduces drastically the sintering time, allowing a more localized heating, for a very short, sometimes almost instantaneous, period of time. Infrared sintering was then the sintering method of choice, allowing fast sintering of silver layers, under 10 minutes. It is compatible with roll-to-roll fabrication, enabling low-cost mass manufacturing of devices. We demonstrate various devices fabricated by inkjet printing techniques: an OFF/ON switch with semiconducting active layer, a capacitive humidity sensor with a response time of about 5 minutes, and a paper microfluidic device with conductivity detection for protein analysis.