Charge transport in nickel phthalocyanine crystals. I. Ohmic and space-charge-limited currents in vacuum ambient

Measurements of the temperature dependence of ohmic and space-charge-limited (SCL) bulk currents flowing in the c' direction in crystals of beta -nickel phthalocyanine in vacuum ambient have been used to locate the statistically important energy levels and their state densities in this molecular solid. Two temperature-induced changes in the activation energy E for ohmic conduction have been observed. From these findings the presence is deduced of a discrete electron level 0.43 eV below the conduction band, with a state density of 5*10 25 m -3 , and a hole level 0.64 eV above the valence band, having a state density of 3*10 18 m -3 . The thermal band gap is 1.63 eV. Measurement of the thermoelectric power showed that NiPc is n-type over the temperature range 325-425K, the excess donor concentration being 8*10 17 m -3 . The room temperature electrical conductivity is 1.0*10 -3 Omega -1 m -1 .