PhD thesis - Evans Research Group - University of Wisconsin-Madison

22threshold voltage.The structural properties of polycrystalline pentacene layers can be correlated withcharge carrier mobility in OFETs [3, 7, 8]. A high degree of structural order in organicsemiconductors is required to achieve high field effect mobilities in OFETs. Stronginteractions between π orbitals between molecules found in well-ordered organicsemiconducting films can provide efficient carrier transport pathways in organicsemiconductors. A large number of studies have been focused on devices using thick filmsof organic semiconductors [9-11]. The effect of the grain boundary on the field effectmobility of carriers in polycrystalline organic films has been extensively studied. Inoligothiophene FETs, the mobility of holes increased linearly with grain size, which showsgrain boundaries can act as a bottle neck in transporting charges [12].Thick film devices, however, are not well suited to explore the charge transport layerbecause the layers above the accumulation layer add to the structural complexity of thepentacene layers without shedding any light on charge transport in devices.Chwang et al. studied electrical properties of single grains of sexithiophene as a functionof the total thickness of the semiconductor film [13]. In these devices, the mobility ofcharge carriers was 10 -4 cm 2 /Vs, which is two orders of magnitude lower than typical thickfilm devices [14] and the thickness did not influence the mobility. They demonstrated thatthe sexithiophene FET is contact limited and this results in a large voltage drop at thecontact between gold and the sexithiophene film. Dinelli et al. showed that the mobility ofholes in sexithienyl thin film transistors did not increase with increasing film thicknessafter the 2 nd molecular layer was completed [15]. The saturated mobility was 0.043 cm 2 /Vs.