Achieving gigahertz transit frequencies in low-voltage organic thin-film transistors (TFTs) will require a contact resistance below about 1 Ohm-cm [1,2]. A general approach to reduce the contact resistance in organic devices is to modify the surface of the metal contacts with a chemisorbed interface layer, ostensibly by reducing the nominal injection barrier. Combined with a thin gate dielectric, this approach can enable contact resistances below 30 Ohm-cm and transit frequencies above 10 MHz at low voltages in coplanar organic TFTs [3,4]. However, further reduction of the contact resistance depends strongly on non-idealities of the interface other than the nominal barrier height according to the Schottky-Mott limit. We show a detailed study on the efficacy of interface layers based on various thiols to improve the contact resistance in coplanar dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) TFTs. We compare the contact resistance of multiple sets of TFTs to results from ultraviolet photoelectron spectroscopy measurements and find strong evidence that Fermi-level pinning prevents a significant reduction of the contact resistance below about 100 Ohm-cm in DNTT TFTs. Therefore, we conclude that this approach may not be a generally sufficient method by itself to eliminate the contact resistance in organic TFTs.
Flexible transistor active matrix array is fabricated on PEN substrate using all screen-printed gate, source and drain electrodes. Parylene-C and DNTT act as gate dielectric layer and semiconductor, respectively. The transistor possesses high mobility (0.33 cm2V-1 s-1), large on/off ratio (< 106) and low leakage current (~10 pA). Active matrix array consists of 10×10 transistors were demonstrated. Transistors exhibited average mobility of 0.29 cm2V-1s-1 and on/off ratio larger than 104 in array form. In the transistor array, we achieve 75μm channel length and a size of 2 mm × 2 mm for each element in the array which indicates the current screen-printing method has large potential in large-area circuits and display applications.
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