‘‘Defect state model’’ and effect of transition metal impurities on metal-free phthalocyanine: Electrical and photoconductive properties

Abstract

A mechanistic study of the effect of transition metal impurities on trap density, electrical and photoelectrical properties of metal-free phthalocyanine was carried out. The experimental results showed that electrical and photoconductive properties of H2Pc were a function of trap density (nt) and concentration of metallic impurities (particularly iron). The results were analyzed in terms of several theoretical models. Both the hydrogenic-type shallow impurity state model and conduction due to bridging of host molecules via iron impurity atoms were incompatible with the results. An excellent description of the observed electrical and photoconductive behavior of H2Pc as a function of trap density and impurity can be given using a defect state model, analogous to deep state in inorganic system. This model correctly predicts the dependence of specific conductivity, σ, of H2Pc on the inverse of trap density (1/nt) at low concentration and on 1/nt2 at high trap concentration. Similarly, the model offers an explanation of the change in the dependence of conductivity under illumination from 1/Ne to 1/Ne1/2 as a function of increased trap density.