Molecular order in organic-based field-effect transistors

Various conjugated materials have been used in the literature for fabricating organic-based thin-film transistors. According to the mechanism describing charge transport, these materials can be categorized into two main classes: one class is amorphous materials, e.g. conjugated polymers; the other is structurally ordered materials, e.g., conjugated oligomers. In conjugated polymers, a direct relationship exists involving conductivity, doping level and mobility, which explains the actual impossibility of achieving materials possessing both a high mobility and a low conductivity. On the other hand, charge transport in conjugated oligomers occurs following a trap-limited mechanism, which allows a distinct control of mobility and conductivity. Carrier mobility in films of conjugated oligomers can be improved by increasing long range order, which allows the lowering of the concentration of the grain boundaries. Films of oligomers with liquid crystal-like structure can thus be easily realized and present a mobility close to that of an oligomer single crystal. Conductivity of oligomers can be decreased by controlling the purity of these materials. High mobility and low conductivity values, required for fabricating efficient thin-film transistors, can thus be realized with conjugated oligomers, allowing the fabrication of organic thin-film transistors presenting characteristics close to those of amorphous-silicon based ones.