Electrical characterization of the rectifying contact between aluminium and electrodeposited poly(3-methylthiophene)

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Abstract
A detailed investigation both of the DC and of the AC electrical properties of the Schottky barrier formed between aluminium and electrodeposited poly(3-methylthiophene) is reported. The devices show rectification ratios up to 2*104 which can be increased further after post-metal annealing. The reverse characteristics of the devices follow predictions based on the image-force lowering of the Schottky barrier, from which the doping density can be estimated. As the forward voltage increases, the device current is limited by the bulk resistance of the polymer with some evidence for injection limitation at the gold counter-electrode at high bias. In the bulk-limited regime, the device current is thermally activated near room temperature with an activation energy in the range 0.2-0.3 eV. Below about 150 K the device current is almost independent of temperature. Capacitance voltage plots obtained at frequencies well below the device relaxation frequency indicate the presence of two distinct acceptor states. A set of shallow acceptor states are active in forward bias and are believed to determine the bulk conductivity of the polymer. A set of deeper acceptors are active only for very small forward voltages and for all reverse voltages, namely when band banding causes the Fermi energy to cross these states. The density of these deeper states is approximately an order of magnitude greater than that of the shallow states. Evidence is presented also for the influence of fabrication conditions on the formation of an insulating interfacial layer at the rectifying interface. The presence of such a layer leads to inversion at the polymer surface and a modification of the I-V characteristics.