Tunneling at Organic/Metal Interfaces in Oligomer-Based Thin-Film Transistors
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formance of sexithiophene-based FETs has been improved by a factor of nearly 50 by controlling the molecular ordering in the evaporated film, from a disordered three-dimensional structure to a wellordered two-dimensional organization where all the molecules stack along a packing axis nearly parallel to the substrate surface. 3 This approach, first demonstrated on oligothiophenes, has been recently confirmed in the case of pentacene.4 Transistors also have been realized from single crystals of oligothiophene, which showed mobility values only slightly higher, by about one order of magnitude, than those observed in highly ordered films.5 These results suggest that the field-effect mobilities actually obtained with ordered oligomer films in the range of 0.1-0.5 c m 2 V " V , almost reached the limit—of the order of 1-5 cm 2 V"V—which can be expected for perfectly ordered crystalline materials. Such a limit can be deduced from careful time-of-flight mobility measurements carried out on single crystals of condensed aromatic hydrocarbons.6 These considerations point out the problems needing to be faced to improve organic semiconductor-based devices. Among them, one can consider (1) the lack of solution processability of oligomerbased semiconductors, which does not yet allow the full use of the advantages expected from organic materials, and (2) the problem raised by the efficiency of
charge injection at the contact between the metal electrode and the organic semiconducting layer, a very general problem for all organic-based devices. These two questions will be considered in the following sections. Toward Solution-Processable Organic Semiconductors The solution processability—that is, solubility—of oligothiophenesriT,where n is the length of the oligimer chain, is known to largely decrease with increasing n, originating from strong intermolecular interactions between the IT systems. Thus it is important to determine whether or not the field-effect mobility in oligothiophenes remains in the same range of values, whatever the conjugation length is. In this regard, analysis of literature data shows that many research groups have reported a sharp decrease of mobility with n, though the origin is still unexplained. Various research groups have shown that sexithiophene (6T) presents a charge-carrier mobility27"10 of about 0.02 cm2V V whereas mobility values as low as 10~7 cm2V " : s~' have been reported for quaterthiophene (4T).711 This behavior is at variance with what has been observed for a long time on another deeply studied family of molecular crystals—the polyacenes—where time-of-flight mobility at room temperature is found to remain inside the range 0.1-1 cm2V 's \ whatever the length of the molecule, from naphtacene to tetracene.6 Thus we first carried out—as described in the following discussion—a rigorous analysis of the dependence of the chargetransport properties of oligothiophenes as a function of their chain length n, using high-purity 4T and quinquethiophene (5T). Field-effect transistors fabricated from the
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