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 V^1.5–1.7 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.