Ab initio electronic structure and correlations in pristine and potassium-doped molecular crystals of copper phthalocyanine

We investigate the effect that potassium intercalation has on the electronic structure of copper phthalocyanine (CuPc) molecular crystals by means of ab initio density functional calculations. Pristine CuPc (in its alpha and beta structures) is found to be an insulator containing local magnetic moments due to the partially filled Cu d shells of the molecules. The valence band is built out of molecular Pc-ring states with e(g) symmetry and has a width of 0.38/0.32 eV in the alpha/beta polymorph. When intercalated to form K2CuPc, two electrons are added to the Pc-ring states of each molecule. A molecular low spin state results, preserving the local magnetic moment on the copper ions. The degeneracy of the molecular e(g) levels is lifted by a crystal field, resulting in a splitting of 52 meV between occupied and empty bands. Electronic correlation effects enhance the charge gap of K2CuPc far beyond this splitting; it is 1.4 eV. The conduction band width is 0.56 eV, which is surprisingly large for a molecular solid. This finding is in line with the observed metallicity of K2.75CuPc, indicating that in this compound the large bandwidth combined with a substantial carrier concentration prevents polaron localization.