Newn-channel organic semiconductors for thin film transistors

This thesis describes the development and electrical characterization of two families of high mobility (>0.2 cm2/Vs) n-channel organic semiconductors for thin film transistor (OTFT) application. In order to achieve CMOS-type technology for organic based electronics, the development of ambipolar and n-channel organic semiconductors is critical. This thesis also presents the first ambipolar OTFT based on a film of a single, small molecule, and in correlation reveals the usefulness of solution electrochemistry as an indicator of solid-state conduction properties. An n-channel material, PTCDI-C₈, shows OTFT performance comparable to pentacene, the benchmark organic semiconductor, and also offers better solubility and more robust film growth. In addition to these contributions to new materials development, this thesis addresses several specific OTFT device issues including contact resistance, threshold voltage instability, and transport mechanism.; This thesis presents a four-probe method for characterizing contact resistance in operating OTFTs. Characterizing the role of contacts eliminates a major variable in OTFT performance, and allows better evaluation of the intrinsic semiconductor properties.; Threshold voltage instability is a major concern for OTFTs as product development progresses. The mechanism of instability is not clear in OTFTs, and has received relatively little attention. This thesis reports threshold voltage shift due to oxygen exposure and gate bias stress in n-channel OTFTs. The observation of oxygen sensitivity at the part-per-billion level is extremely interesting and important for future research efforts.; This thesis presents variable temperature measurements of n-channel OTFTs, and reveals that activated transport dominates at low temperature. The small magnitude of the activation energy is difficult to interpret using traditional hopping-type conduction models; consequently, the OTFT data are interpreted using a multiple trap and release (MTR) model. The MTR model is used to variable temperature and gate voltage related phenomena. Trapping plays a major role in OTFTs, and so it is important to understand the distribution and depth of traps in order to design new n-channel materials with improved OTFT performance. Gate voltage dependent mobility measurements suggest that the commonly used MOSFET derived equations may not be adequate to fully describe OTFTs.