Investigation of energy levels due to transition metal impurities in metal-free phthalocyanine

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Abstract
We have determined the energy levels due to transition metal impurities in metal‐free phthalocyanine. These energy levels were computed using the Anderson model and appear as multiple charge states, analogous to the deep states in inorganic systems. Two types of localized states can be classified, a donor‐type appearing in the lower half of the gap and an acceptor type appearing in the upper half of the gap. Only donors occurred when the d‐band energy of the transition metal, ED, was small, for instance in the case Cd, Cr, Fe and Mn. Both donor and acceptor levels coexist at intermediate values of ED, e.g., in the case of Co and Zn. However, when the value of ED is large, then only acceptor levels appear in the gap, e.g., with Cu and Ni. The computed localized energy levels of transition metals in H2Pc agreed well with those estimated from experimental redox potential data of phthalocyanine metal complexes. The implication of the existence of these impurity levels in H2Pc’s is very important as they can have a very profound influence on the electrical and photoelectrical properties of phthalocyanines.