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′′′‐quaterthiophene (2) and 5,5′′′′‐bis(4‐methylphenyl)‐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 (Ion/Ioff) 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 Ea = 121, 100, and 109 meV, respectively. The corresponding trap densities were Ntrap/Nv = 0.012, 0.023, and 0.094, where Ntrap is the number of trap states and Nv 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 Ion/Ioff ratios were comparable with the highest values reported for organic TFTs, with films of 2 approaching Ion/Ioff = 109 at room temperature. Oligothiophenes with tolyl endcaps form highly crystalline thin films with large oriented grains (see Figure). These films exhibit respectable p‐type mobilities and exceptionally high Ion/Ioff ratios when used as the transport layer in a field‐effect transistor. The device performance appears to be limited by traps but is remarkably stable upon prolonged exposure to air.