Thin Film and Single Crystal Organic
Heterostructure Field Effect Transistors based
on Perylene Diimide compounds
Creators: name: Di Girolamo, Flavia Viola
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2011

description: Organic semiconductors are carbon-based compounds which offer different advantages respect to their inorganic couterparts, such as the realization of flexible and large area devices, but the electrical properties and the stability are poorer, even if nowadays comparable with amorphous silicon. Several attempts have been made to improve the performances of organic semiconductors; a successful approach in this direction resides in the realization of devices based on junctions between a p-type and a n-type organic semiconductor in which an increase of the density of the majority charge carriers is realized at the interface in both the semiconductors (accumulation heterojunctions).
    This approach leads to several advantages, since the electrical properties arising at the interface as consequence of the energy bands alignment can be exploited for innovative applications. Doping, ambipolar operation, improvement of the morphological and structural properties of evaporated thin films are some examples. An increase of the device performances has been experimentally verified, with an improvement of conductivity (even mobility in some cases) in field effect transistors and of quantum efficiency in light emitting devices.
    This PhD thesis deals with the study of Organic Heterostructure Field-Effect Transistors, basically realized using two organic semiconductors forming an accumulation junction as the active channel in field effect devices. Organic semiconductors both in the form of thin films and single crystals have been considered. Concerning the thin film heterostructures, this thesis has been developed in Naples, at the Department of Physics of the University Federico II and at the CNR-SPIN, while the single crystal growth and the fabrication of the related micrometric scale devices have been led at the Departement de la Matiére Condensée (DPMC) of the University of Geneva.
    In the following, the thesis outline is reported. The first chapter provides some general definitions and concepts which represent the theoretical basis of the experimental results reported in the following chapters. A fundamental step in the realization of thin film heterostructures is the choice of the most suitable n-type semiconductor. Electrical instability and charge trapping phenomena represented an issue that has limited until recently the realization of n-type devices and of p-n heterojunctions, which represent the building block for a wide class of devices. The second chapter will be consequently devoted to the results obtained from the study and the comparison of three different n-type organic semiconductors belonging to the class of perylene diimides oligomers.
    The third chapter is instead dedicated to the experimental results obtained of thin film heterostructure field-effect transistor. The vacuum evaporation and the fabrication procedure have been carefully optimized and the obtained devices have been morphologically, structurally and electrically characterized. The electrical characterization of these devices exhibits several interesting features, such as evidences of a charge transfer phenomenon at the interface, a dependence of the electrical properties on the thickness and an unexpected decrease of the drain source current for increasing negative gates.
    The last chapter is dedicated to Schottky gated single crystal heterostructures. In the device configuration the heterostructure is formed by PDIF-CN₂ and rubrene single crystals; rubrene is laminated on a chromium stripe, with which it forms a Schottky barrier. Transfer curves indicates band-like behavior down to less than 150 K, with the electron mobility remaining as high as 1 cm²/Vs at 50 K in the best devices. Furthermore, the charge carrier density at V_{G} = 0 exhibits a linear temperature dependence. The results can be explained using a simple model accounting for the alignment of the rubrene HOMO band with the bottom of the LUMO band of PDIF-CN₂.