Organic thin-film transistors based on tolyl-substituted oligothiopheneso

Thin films based on tolyl-substituted oligothiophenes 5,5'-bis(4-methylphenyl)-2,2':5',2''-terthiophene (1), 5,5''' bis(4-methylphenyl)-2,2',5',2'',5'',2'''-quaterthiophene (2) and 5,5''''-bis(4-methylphenyl)-2,2',5',2'',5'',2''',5''',2''''-quinqethiophene (3) exhibit hole-transport behaviour in a thin-dilm transistor (TTF) configuration with reasonable mobilities and high current on/off (I-on/I-off) ratios. Powder X-ray diffraction (PXRD) reveals that these films grown by vacuum deposition onto the thermally grown silicon oxide surface of a TFT, are highly crystalline a characteristic that can be attributed to the general tendancy of phenyl groups to promote crystallinity. Atomic force microscopy (AFM) reveals that the films grow layer by layer to form large domains, with some basal domain areas approaching 1000 mum(2). The PXRD and AFM data are consistent with an "end-on" orientations of the molecules on the oxide substrate. Variable-temperature current-voltage (I-V) measurement identified the activation regime for hole transport and revealed shallow level traps in thin films of 1 and 2, and both shallow and deep level traps in thin films of 3. The activation energies for thin films of 1, 2 and 3 were similar with values of E-a = 1.21, 100 and 109 meV, respectively. The corresponding trap densities were N-trap/N-v = 0.012. 0.023 and 0.094, where N-trap is the number of trap states and N-v is the number of conduction states. The hole mobilities for the three compounds were similar (mu similar or equal to 0.03 cm(2) V-1 s(-1)), and the I-on/I-off ratios were comparable with the highest values reported for organic TFTs, with films of 2 approaching I-on/I-off = 10(9) at room temperature.