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Drakopoulou, Sofia, (2021)  - Crescita, morfologia e risposta elettronica dei transistor organici a effetto di campo in stato solido e in elettrolita  - , Tesi di dottorato - (, , Universitą degli studi di Modena e Reggio Emilia ) - pagg. -

Abstract: Organic electronic devices, such as light emitting diodes (OLEDs), field effect transistors (OFETs) and solar cells (OPVs) have reached a technological maturity and, in the case of OLEDs and OPVs, industrial production. Significant advancements in chemical synthesis, materials processing and device engineering have boosted the device performance and reliability. However, several concepts regarding the mechanism of the device operations are still unresolved, especially in OFETs. Charge transport in the organic semiconductors involves different interfaces of the materials and one of the most important questions that people tries to address is how the morphology of the device affects the mechanism of charge transport across the device. Indeed, morphology, molecular and energy disorder, and surface defects can easily influence their performance. There is a compelling quest for understanding the mechanical aspects of the organic thin film nucleation and growth on real test patterns in order to understand the morphology. The main goal of this thesis was to understand the correlation between different growth modes, morphology, and the electrical response of OFETs in solid state operation as well as in an electrolytic environment. Pentacene is the workhorse organic semiconductor that we used throughout this thesis. The motivation is understanding the physics of the pentacene transistors as a function of the semiconductor channel thickness, and it was the core of the EC-Marie Curie project SPM 2.0 that supported my research work. As a new important finding out of this thesis, we discovered and assessed a new anomalous growth of Pentacene thin films vs increase of the thickness, viz. the mass of organic semiconductor in the OFET channel. In this novel growth mode, there is a breakdown to the usually observed growth mode upon rapid roughening, where a layer-by-layer growth at the early stages suddenly evolves into a self-affine mode characterized by growing islands made of terrace stacks. We observed this mode at the lower deposition temperatures, but we discovered that at a precise range of deposition temperature and rate, viz. 80°C and 0.1 A/s, this growth mode is not observed, instead an iteration of wetting/dewetting transition occurs as thickness increases. Its peculiar features consist of the fact that the morphology of the islands as stacks of monomolecular terraces, is retained. However, the morphological parameters, such as correlation lengths and roughness, that we extract from atomic force microscopy (AFM) images exhibit anomalous oscillations with period increasing with thickness. In order to explain the trend of the parameters, we devised an empirical equation that encompasses both self-affine 3D growth and the oscillations typical of wetting/dewetting transition as in the spinodal dewetting phenomena. We then analyzed the electrical characteristics of the OFET operated as solid-state device as well as electrolyte gated devices. The correlation of the transistor parameters with the morphology were analyzed. Experiments using bimodal AFM allowed us to investigate the mechanical properties of conductive and semiconductive thin films. The latter activity was carried out at CSIC-ICMM in Madrid during the secondment at the laboratory of Professor Ricardo Garcia.