Organic gate dielectrics for tetracene field effect transistors

Over the last three decades, thin films of organic semiconductors (OS) have been the object of intense research. These films can be used in a wide variety of new-generation optoelectronic devices, such as Organic Light Emitting Diodes (OLED), Organic Field Effect Transistors (OFET) and photovoltaic cells. Recently, vacuum sublimed tetracene films were used to realize the first Organic Light Emitting Field Effect Transistor (OLEFET), which integrates in a single device the current modulation function of a FET with the light generation capability of a LED.;The demonstration of OLEFETs is not straightforward. First of all, an efficient integration of optical and electronic functionalities requires the use of a semiconductor with both efficient electroluminescence and good charge transport properties. Secondly, an ambipolar charge transport has to be achieved to produce high performance OLEFETs. Within this context, controlling the dielectric substrate surface chemistry has proven to be an efficient strategy, since it contributes to avoid the suppression of the electron transport induced by the electronic trap states at the dielectric/semiconductor interface. At the same time, the modification of the chemical and physical nature of the dielectric substrate influences the morphology/structure of the organic thin-films, in turn influencing the final device performance.;In this work, polycrystalline tetracene thin films - to be incorporated in OLEFETs - were vacuum sublimed on different organic dielectric substrates, including polymers (parylene C, polymethylmethacrylate, polystyrene) and self-assembled monolayers of hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The scope of the work was indeed to shed light on the role of the organic dielectric surface in influencing the charge transport properties of tetracene OLEFETs.;The tetracene deposition rate was 3.5 A/s, the substrates were kept at room temperature and the pressure inside the vacuum chamber was 2.5x10 -6 Torr.;The growth process was studied from sub-monolayer to complete coverage by means of ex-situ Atomic Force Microscopy (AFM). The nucleation density, the grain size and the connectivity between the grains were observed to be strongly dependent on the physical and chemical properties of the dielectric substrate surface.;Thin-film transistors, in the bottom gate -- top contact configuration, were fabricated to explore charge transport and electroluminescence in tetracene films vacuum sublimed on the different organic gate dielectrics. It was found that the use of the organic dielectric layers leads to considerably different values of the charge carrier mobility, ranging between 1-10-2 and 2-10-1 Cm2V-1s-1, the highest mobility being observed in the case of polystyrene.;The natural extension of this project is to establish a sound correlation between charge transport, light emission and film morphology/structure in organic thin films for applications in OLEFETs.