Investigation of Bulk Currents in Metal-Free Phthalocyanine Crystals

Abstract

The bulk current in many single crystals of metal-free phthalocyanine has been found to exhibit Ohmic behavior up to fields of 104 V/cm and square-law dependence on voltage for higher fields. Photocurrents in these crystals were Ohmic over the entire range. If one interprets these results as space-charge-limited currents (SCLC), trap densities of 1012 to 1014/cm3 are found from the temperature behavior of the I-V characteristic. From the transition between Ohmic and square-law regions, the concentration of free carriers was calculated to be approximately 106 to 107/cm3 in fairly good agreement with that found from Hall measurements. Samples were measured which had both dark and photo currents which varied as V at low fields, but as V1.5–1.7 at higher fields. These observations are interpreted qualitatively in terms of a model in which a layer of higher resistivity than the bulk extends from the contact into the bulk.