Organic-on-inorganic semiconductor contact barrier diodes. I. Theory with applications to organic thin films and prototype devices

Abstract

We discuss the properties of organic-on-inorganic (OI) semiconductor contact barrier diodes. A model for charge transport is developed which suggests that thermionic emission over the organic/inorganic contact barrier dominates at low current densities, whereas space-charge effects dominate transport through the organic layer at high current densities. The effects of charge trapping in the organic layer are also considered. This model is applied to OI diodes using thin films of the prototypical aromatic compound; 3,4,9,10-perylenetetracarboxylic dianhydride, (PTCDA) vapor-deposited onto n- and p-Si substrates. Several electrical and optical properties of PTCDA are investigated to provide a basis for analyzing the OI diodes. Both ohmic and space-charge-limited transport are observed in the PTCDA. We discuss mobility, transient response, and photoresponse of the thin-film organic material. Also described are the general properties of organic-on-inorganic contact barrier diodes which employ PTCDA and related compounds on either p- or n-Si substrates. Comparisons between diode performance and the theory are made. The contact barrier diodes exhibit high breakdown voltages (≤230 V) and reverse dark currents limited by generation and recombination of carriers in the Si bulk. From the forward current-voltage characteristics, apparent OI contact barriers of φBp=(0.75±0.02) eV and φBn=(0. 61±0.01) eV are formed with p- and n-Si substrates, respectively. The resulting diodes are superior, in many respects, to conventional Schottky diodes due to enhanced contact barriers and reduced edge effects.