Organic thin-film transistors based on tolyl-substituted oligothiophenes

Siddharth Mohapatra, Brian T. Holmes, Christopher R. Newman, Cathal F. Prendergast, C. Daniel Frisbie, Michael D. Ward

Research output: Contribution to journalArticlepeer-review


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/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/N v = 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/off ratios were comparable with the highest values reported for organic TFTs, with films of 2 approaching Ion/Ioff = 109 at room temperature.

Original languageEnglish (US)
Pages (from-to)605-609
Number of pages5
JournalAdvanced Functional Materials
Issue number6
StatePublished - Jun 2004

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • General Chemistry
  • Condensed Matter Physics
  • General Materials Science
  • Electrochemistry
  • Biomaterials


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