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

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Abstract
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*1025 m-3, and a hole level 0.64 eV above the valence band, having a state density of 3*1018 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*1017 m-3. The room temperature electrical conductivity is 1.0*10-3 Omega -1 m-1.