Organic thin-film transistors fabricated via pneumatic nozzle printing as direct detectors for ionizing radiations

Organic semiconducting materials have recently proved to be a suitable candidate for the direct detection of X-ray radiation. Despite the low atomic number Z, organic materials can respond to ionizing radiation through the photoconductive gain mechanism. The response is governed by trap states: majority carrier traps directly impact the mobility, reducing the collection of charges at the electrodes; minority carrier traps, on the other hand, increase the output current by inducing the injection of extra majority carriers from the contacts in order to preserve charge neutrality. Employing organic semiconductors allows to take advantage of the electrical properties typical of their inorganic counterpart with additional features pertaining to low-Z materials like organic ones: flexibility, radiation hardness, human-tissue equivalence. In this thesis, organic field-effect transistors based on 6,13-bis-(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) fabricated with the novel Pneumatic Nozzle Printing technique have been characterized with the aim to correlate the sensitivity of the devices to the morphology of the semiconducting layer. In particular, the effect of the substitution of Silicon atoms with Germanium ones in the molecular structure has been investigated through many experimental techniques, including photocurrent spectroscopy. The fabricated devices exhibit sensitivities up to (7.4±0.4)×10^2µC/Gy cm^2, with mobilities reaching 0.25 cm^2/V s. Devices with Ge-substituted molecules exhibit higher sensitivities than their Silicon counterpart, but not as much as theoretically predicted based only on the higher absorption coefficient of Germanium atoms: this contrast has been attributed to a less efficient photoconductive gain mechanism.