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Organic Thin‐Film Transistors Based on Tolyl‐Substituted Oligothiophenes
Author(s) -
Mohapatra S.,
Holmes B. T.,
Newman C. R.,
Prendergast C. F.,
Frisbie C. D.,
Ward M. D.
Publication year - 2004
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.200400034
Subject(s) - materials science , thin film transistor , crystallinity , thin film , substrate (aquarium) , layer (electronics) , oxide , threshold voltage , crystallography , analytical chemistry (journal) , transistor , nanotechnology , composite material , voltage , organic chemistry , oceanography , chemistry , geology , metallurgy , physics , quantum mechanics
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 ( 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 μm 2 . 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 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.