Organic thin-film transistors based on tolyl-substituted oligothiophenes

Thin films based on the 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‴-quaterthiphene (2) and 5,5″″-bis(4-methylpheny)-2, 2′:5′,2″:5″,2‴:5‴,2″″- quinqethiophene(3) exhibit hole-transport behavior in a thin-film transistor (TFT) 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 tendency 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 μm². The PXRD and AFM data are consistent with an "end-on" orientation of the molecules on the oxide substrate. Variable-temperature current-voltage (I-V) measurements 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 = 121, 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 (μ = 0.03 cm² V^-1 s^-1), and the I_on/_off ratios were comparable with the highest values reported for organic TFTs, with films of 2 approaching I_on/I_off = 10⁹ at room temperature.