Contact resistance effects in organic n-channel thin-film transistors

Organic thin-film transistors (TFTs) have undergone tremendous progress in the past few years. Their great potential in terms of mechanical flexibility, light weight, low-cost and large-area fabrication makes them promising candidates for novel electronic products, such as sensor arrays, radio-frequency identification tags and flexible displays. For the realization of these applications and to improve their performance it is necessary to miniaturize organic TFTs to dimensions of a few micrometers or even less. However, with such aggressive scaling, the device physics becomes more and more important. Especially the contact resistance at and close to the interface between the organic semiconductor and the source/drain metal limits the performance of the organic TFTs at miniaturized dimensions. In this thesis, the contact properties of bottom-gate, top-contact n-channel organic transistors are investigated using the gated four-probe (GFP) technique. The TFTs employ the small-molecule semiconductor N,N'-bis-(1H,1H-heptafluorobutyl)-1,7-dicyano-perylene-3,4:9,10-tetracarboxylic diimide (PDI-FCN2) and are fabricated on flexible plastic substrates. The transistors are air-stable and can be operated at low voltages of about 3 V, owing to a thin hybrid gate dielectric composed of aluminum oxide and a self-assembled molecular monolayer. The GFP method enabled a detailed investigation of the influence of a multitude of factors, including the gate-source and drain-source voltages, the contact length, the temperature and the contact metal, on the contact resistance. The contact properties are found to be very sensitive to the thickness of the organic semiconductor layer. In transistors in which this layer is relatively thin, ohmic contact resistances as small as about 0.6 Ohm cm were measured. In transistors with a relatively thick organic semiconductor layer, device physics becomes more complex and the current-voltage characteristics of the contact become nonlinear. Space-charge limited currents are identified as the probable origin of this nonlinear behavior. The good performance of the PDI-FCN2 transistors allows the realization of simple integrated circuits. Flexible organic complementary ring oscillators are fabricated and signal propagation delays per stage as short as 6 µs are demonstrated at a supply voltage of 3.6 V.