Space-Charge-Limited Currents in Organic Crystals

Electrical conductivity measurements were performed with thin (50 μ) single crystals of p-terphenyl, p-quaterphenyl, and anthracene supplied with aqueous electrodes, one of which was an iodine-iodide solution (acceptor electrode), and the other an iodide solution. The results strongly indicate that the acceptor electrode can form ohmic contact for hole injection into these crystals and that space-charge-limited currents can be drawn through them. The crystals were found to contain hole-trapping states the location-in-energy of which can be approximated by a decreasing exponential distribution above the valence band. The measurements showed that the hole mobility in p-terphenyl is about 3×10−2 cm2/v sec, is independent of the field at least up to about 4×104 v/cm, and that the hole-trap concentration is at least 1013 cm−3. The acceptor electrode used does not form ohmic contact to crystals of naphthalene and diphenyl; an explanation for this is proposed. Some theoretical aspects of ohmic contact formation to organic crystals and space-charge-limited current flow in insulators are also discussed.