Measurement of carrier transport and injection in metal-free tetraphenylporphyrin

Carrier transport and charge injection are studied in metal-semiconductor structures employing metal-free tetraphenylporphyrin (H2-TPP) as the organic semiconductor. H2-TPP is deposited on an indium tin oxide (ITO) substrate by means of an apparatus for molecular beam depositions, and aluminum is employed as the top electrode. The ITO/H2-TPP/Al structures thus obtained are investigated by a large-signal capacitance-voltage method, and transport and charge injection are simultaneously measured. At low electrical fields a space-charge limited transport is found, and the device behavior is symmetrical, notwithstanding the different energy barriers at the ITO and Al electrodes. At higher electrical fields the transport regime becomes Schottky-barrier limited, with the asymmetry expected from the energy difference between the two contact metals. The charge injected into the device behaves symmetrically at low fields, and shows a peak at the transition voltages between the space-charge and the Schottky regime, both in the positive and negative bias direction.