Printed and Low Temperature Processed Oxide Electronics with Superior Electrical Performance and Mechanical Reliability
In line with the forthcoming industrial revolution, printed and flexible electronics is a rapidly developing research area that involves wearable and consumer electronics to be produced in large quantities and be connected via Internet of Things (IoT). In relation to solution processed/printed electronics, oxide semiconductors have developed into a serious contender alongside the organic alternatives, which lead to the plastic electronics excitements in early days, a couple of decades ago. Oxides are increasingly chosen for printed electronics technologies because they are abundant, inexpensive, nontoxic, demonstrate way superior environmental/thermal reliability, and most importantly, possess excellent electronic transport properties. However, on the downside, there are also major challenges, such as high process temperatures, thereby making them not suitable for low-cost, flexible polymer substrates. In addition, their limited mechanical strain tolerance (typically <1%) lower their selection possibilities in typical flexible electronic applications. On the other hand, printed electronics do suffer from a major roadblock, irrespective of the chosen semiconductor technology, which stems from the limited printing resolution that dictates the minimum channel lengths and therefore the maximum operation frequency that can be achieved. In the present thesis, efforts have been made to look for solutions of some of the above-stated bottlenecks in the printed oxide electronics domain. First, an inorganic-organic composite semiconductor precursor ink has been developed; the printed and thermally cured semiconductor film of the composite material can maintain the excellent carrier mobility of the oxide semiconductors, even when they are in near-equal amounts. On the other hand, the presence of large quantity of organics ensures its superior strain tolerance and unaltered transistor properties for bending fatigue tests vi performed down to 1.5 mm bending radius. Next, devices made out of oxide nanoparticles have also been investigated, here the curing temperature can be as low as 100 °C, and therefore, such devices may very well be fabricated on paper or polyethylene terephthalate (PET) substrates. Here, our proposed approach offers a general recipe for low temperature curable nanodispersions/nanoinks; in this case, aromatic surfactants have been used to stabilize the nanoparticles that sublimates near room temperature. In this regard, stable nanoinks from In2O3 nanoparticles are developed using an aromatic compound thymol as the stabilizer; a quick heating at 100 °C ensures its complete removal from the semiconductor film/particle surfaces. High performance field-effect transistors (FETs) have been fabricated and characterized. Next, printable conducting inks are also the essential elements to print passive components in every electronic device; they also have a widespread use in various other application domains, such as transparent touch sensors, heaters, antistatic coating, antenna etc. Here, a precisely controlled synthesis of a mixed-phase of Ag-Ag2O nanoparticles and further a nanoink has been carried out that demonstrated low curing temperature of 80 °C to 120 °C, alongside excellent strain tolerance of layers printed on standard photo-paper substrates. Next, attempts have been made to circumvent the printing resolution limits that at present hover around tens of micrometers range and are three orders of magnitude larger than the dimensions typically used in Si-technology. In this regard, an innovative device architecture that allows easy printing of edge-FETs with near vertical electronic transport through the printed semiconductor layer has been proposed; this can lower the effective channel lengths down to few tens of nanometers, as in this case the thickness of the printed layer becomes the dimension of interest. The FETs fabricated on PET substrate demonstrate high current saturation, On-Off ratio etc., vii however, more interestingly, the depletion-mode inverters have demonstrated a signal gain >200 and switching frequency >300 kHz. In addition, taking advantage of the vertical device geometry a tensile strain tolerance up to 5% has been demonstrated.