ELECTRICAL CHARACTERIZATION OF THE RECTIFYING CONTACT BETWEEN ALUMINUM AND ELECTRODEPOSITED POLY(3-METHYLTHIOPHENE)

A detailed investigation both of the DC and of the AC electrical prope rties of the Schottky barrier formed between aluminium and electrodepo sited poly(3-methylthiophene) is reported. The devices show rectificat ion ratios up to 2 x 10(4) which can be increased further after post-m etal annealing. The reverse characteristics of the devices follow pred ictions based on the image-force lowering of the Schottky barrier, fro m which the doping density can be estimated, As the forward voltage in creases, the device current is limited by the bulk resistance of the p olymer with some evidence for injection limitation at the gold counter -electrode at high bias. In the bulk-limited regime, the device curren t is thermally activated near room temperature with an activation ener gy in the range 0.2-0.3 eV. Below about 150 K the device current is al most independent of temperature. Capacitance-voltage plots obtained at frequencies well below the device relaxation frequency indicate the p resence of two distinct acceptor states, A set of shallow acceptor sta tes are active in forward bias and are believed to determine the bulk conductivity of the polymer. A set of deeper accepters 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 d ensity of these deeper states is approximately an order of magnitude g reater than that of the shallow states. Evidence is presented also for the influence of fabrication conditions on the formation of an insula ting interfacial layer at the rectifying interface. The presence of su ch a layer leads to inversion at the polymer surface and a modificatio n of the I-V characteristics.