| The performance of organic field-effect transistors (OFETs) is determined by both carrier injection and transport in organic semiconductors.; To improve injection in bottom contact OFET devices, self-assembled monolayers (SAMs) of octadecanethiol (ODT), 2-mercapto-benzothiazole (BZT), or 5-mercapto-2,2 '-bithienyl (BTT) were made on Au electrodes, followed by evaporation of pentacene. The devices with the SAMs showed better transistor performance, and apparent carrier mobility in the devices with electron-rich BTT SAMs was 10 times higher than in the devices without SAMs. BTT SAM devices were also better than ODT and BZT SAM devices. This increased performance is attributed to a combination of better adhesion between pentacene and the electrodes and to matching the energy levels of the BTT SAMs to the semiconducting pentacene.; Novel organic semiconductors with a pi-stacked structure were synthesized to understand structural and transport properties of pi-stacked materials. Variable-temperature X-ray diffraction patterns showed that a cooligomer of 4,4'-dinonyl-2,2'-bithiazole and 3,4-ethylenedioxythiophene (NTZN) had a crystal-crystal transition at 110°C. Variable-temperature UV-Vis spectra showed that NTZN had less exciton coupling in the high temperature phase as shown by loss of fine structure. A mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V·s in single-crystal transistors. The pi-stacked structure, which provides more efficient orbital overlap and thereby facilitates carrier transport, is responsible for this high mobility.; On the other hand, the mobility in thin-film devices is affected by film morphology. Oligofluorene-thiophene derivatives with dodecyl side chains were prepared to study the effect of the side chains on the film morphology. The interactions between the side chains promote two-dimensional crystal growth, leading to large and well-interconnected grains.; Poly(phenyleneethynylene) with end-capping thiol groups (PPE-SH) was synthesized to explore 'intra'molecular transport in one molecular wire, which is connected between two electrodes. PPE-SH was self-assembled onto a device with nano-gap Au structures. SEM images of the device before and after the self assembly showed that polymer chains lay flat on the Au electrodes. This may be due to highly polarizable pi electron clouds of conjugated cores, which substantially induce dispersion force attraction towards polarizable Au surfaces. |
