Inkjet printing of solutions as precursors of: i) organic semiconducting single crystals on self-assembled monolayers modified substrates and ii) nanoscale-thin dielectric layers.

In the last years inkjet Printing (IJP) has become a very important technology for creating flexible devices for electronics, due to its simplicity, low cost and high precision. In this frame, the European Project i-FLEXIS sought for realizing novel ionizing radiation detectors based on Organic Semiconducting Single Crystals (OSSCs) on flexible substrates, capable of low power operation, flexibility, optical transparency, all at low fabrication costs and high throughput. Chapter 1 – Introduction – the main characteristics of the organic semi-conductors, on which the organic electronics is based, will be indicated and described. In addition, the production methodologies will be illustrated, focusing on Inkjet printing, which will be described in detail. The surface on which the crystals grow has an important role since its chemical and physical characteristics influence two fundamental aspects: i)crystal type; ii)electrical conduction inside an electronic device. The characteristics of the surface can be modified using a chemical approach. This work has been focused on Self Assembled Monolayers (SAMs), used to cover the surface, and optimize its characteristics; these organic molecules have been studied and their properties, functions and applications will be described. At the end the growth via low temperature combustion reaction of a nanolayer of AlOx, used as a dielectric inside of a TFT, will be illustrated. The solution containing the precursors is deposited using ink jet printing. Chapter 2 – Results, Discussions, Materials and Methods – will concern one of the currently main prospected applications of IJP, which is the fabrication of organic electronics devices based on organic semiconductors crystals. The organic molecule involved in this PhD research is the TIPS-pentacene, due to its excellent semiconducting behavior and its capability to detect X-rays. The defects and the heterogeneity of the substrates, in addition to the high evaporation rate (caused by the spreading of the printed drop), promote the formation of little crystals and poly-crystalline domains. This problem can be solved using the printing of a solvo-phobic corral based on fluorinate SAMs, which keeps the drop compact reducing the evaporation rate and creating TIPS single crystals in the range of 1 to 2,5 mm length. The increase of the electrical conduction between the electrodes and the TIPS crystals is made possible by their orientation. The method used to orientate these crystals is geometrical: a corral with a high length versus width ratio is printed, this promotes their growth along the major axe of the corral. The verification of the electrical conduction between the electrodes and the TIPS crystals has been carried out by preliminary tests at the University of Trieste and, subsequently, by tests on the X-ray detection at the University of Bologna, a collaborating group in the frame of the i-FLEXIS project. The printed TIPS single crystals are able to collect charge carriers created by the absorption of ionizing radiation. Chapter 3 – Ink jet printed nano-thin dielectric layers – will be about the development of a printed dielectric layer made by AlOx at the New University of Lisbon, in order to obtain a complete printed device. The dielectric layer has been grown through the chemical reaction of combustion at low temperature, after its printing on a silicon substrate. The ink formulation has been limited by the precursors type and concentration, in addition to the limitations provided by the printer itself. The solution has consisted of a mix of solvents that respect all the required parameters. All the inks have been printed and tested. The best ink obtained from the tests has been adopted as a dielectric layer inside the TFTs, which subsequently have been electrically tested successfully. Chapter 4 – Summary of the Conclusion