Characterization of ITO—PPHT—metal contacts (PPHT=poly(3-phenylhydrazone thiophene)) using electrical and capacitance measurements

The electrical properties of thin film sandwich devices (M/PPHT/ITO) have been studied in a quantitative manner. Here, M represents Al or In; PPHT stands for poly(3-phenylhydrazone thiophene); and ITO is indium—tin oxide. These devices were studied by analyzing the current—voltage curves and capacitance measurements at different frequencies and biasing d.c. voltages in air. These devices showed the rectification effect, and forward bias corresponds to a negative voltage at the metal (Al or In) with respect to ITO. The device with In contact forms a Schottky junction at the In/PPHT interface, while the device with Al contact passed much smaller currents under a given bias and failed to behave quantitatively as a Schottky junction like the device with In contact. Various evaluated electrical parameters indicate that the Al—PPHT junction is deteriorated by an emerged interposed zone of Al2O3. The dark capacitance measurements of the devices were studied in various frequency ranges and under different d.c. bias conditions. These measurements reveal that the standard Schottky model cannot be applied to such metal/organic interfaces. Three different contributions to the admittance may be distinguished. A surface charge layer associated with a high capacitive term extends over a few angstroms of the semiconductor. This capacitance is only slightly dependent upon superimposed d.c. voltages. The space charge region next to the surface charge layer extends over approximately 100–400 nm. The corresponding capacity is almost independent of any d.c. imposed voltage. At the same time the resistance of the space charge region is highly dependent on external d.c. voltage. Finally, the rest of the metal—PPHT layer shows properties of bulk materials.