Metal/conjugated polymer film/silicon structures have been fabricated. Current-voltage measurements have been performed with both Pi and Au metal electrodes on n- and p-type Si substrates. The conjugated polymer was regioregular poly (3-octylthiophene) (P3OT). Rectification ratios as high as similar to 10(6) have been observed, with turn on voltages of similar to 1.5 V. Evidence of Schottky barrier formation at the Al/polymer interface, and Fowler-Nordheim tunnelling at the P3OT/p-Si interface is shown. Capacitance-voltage characteristics were obtained in the frequency range 10 Hz to 1 MHz. Strong frequency dispersion is observed which is indicative of the presence of traps. A model is proposed to explain this dispersion which shows good agreement with the experimental results. Preliminary optical results indicate that these diodes may have applications as optical sensors.