Some properties of evaporable organic semiconductors

Ghalami Bavil Olyaee, Hassan (2013) Some properties of evaporable organic semiconductors. (PhD thesis), Kingston University, .


5,12-Dihydro-quinoxalino [2,3-b ]phenazine (L5H2), 7,16-dihydro-5,7,9,14,16,18-hexaazaheptacene (L7H2) and 7,20-dihydro-5,7,9,11,16,18,20,22-octaazanonacene (L9H2) could have potential applications in molecular electronics. This project involved synthesising them and investigating their Nuclear magnetic resonance (NMR), ultraviolet-visible (UV-visible), Infrared Spectroscopy (IR) and X-ray Diffraction (XRD). It was mainly concerned with a comparative study of electrical and electronic properties of vacuum evaporated undoped and doped thin films of L5H2, L7H2 and L9H2. The vacuum-evaporated thin films of L5H2, L7H2 and L9H2 gave high-quality amorphous thin films on conductive (ITO) glass arid silicon dioxide (Si02) substrates. Schottky diode devices based on undoped and doped L5H2, L7H2 and L9H2 thin films have been successfully fabricated using a range of metal contacts (AI, Au, Cu and Zn) at room temperature after and annealing. The electronic device characteristics were mathematically modelled to determine the Schottky barrier heights at the various metal-semiconductor interfaces. The measurements of Schottky barrier heights were carried out using conventional Current-Voltage (I-V) and Capacitance-Voltage (C-V) techniques. The measured Schottky barrier heights for L5H2, L7H2 and L9H2 thin films, were largely independent of metal work function, which suggests that the Fermi levels in these devices are pinned at mid gap by a high density of states at the interfaces. Results are also presented for organic field effect transistors (OFETs) based on thin films of L5H2 and L7H2. For the first time, OFET devices based on L7H2 thin fihns have been reported. The device characteristics for these OFETs based on L7H2 are compared to device characteristics for similarly-fabricated device structures based on L5H2 thin films. The OFET characteristics were investigated to determine the field-effect charge carrier mobilities and threshold voltages for both L5H2 and L7H2 devices. The effects of thin film morphology, channel length and dielectric thickness on OFET device characteristics have also been considered. The research work presented here clearly shows that OFETs based on L5H2 thin film are possible, and it is likely that their operational device characteristics could be improved immensely by better molecular organisation in the thin film layers, which would result in higher charge carrier mobilities, and better charge injection at the source/drain metal contacts.

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