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 $\mathrm{Cu}d$ shells of the molecules. The valence band is built out of molecular Pc-ring states with $e_g$ symmetry and has a width of $0.38∕0.32\mathrm{eV}$ in the $\alpha ∕\beta$ polymorph. When intercalated to form ${\mathrm{K}}_2\mathrm{Cu}\mathrm{Pc}$, 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 $52\mathrm{meV}$ between occupied and empty bands. Electronic correlation effects enhance the charge gap of ${\mathrm{K}}_2\mathrm{Cu}\mathrm{Pc}$ far beyond this splitting; it is $1.4\mathrm{eV}$. The conduction band width is $0.56\mathrm{eV}$, which is surprisingly large for a molecular solid. This finding is in line with the observed metallicity of ${\mathrm{K}}_{2.75}\mathrm{Cu}\mathrm{Pc}$, indicating that in this compound the large bandwidth combined with a substantial carrier concentration prevents polaron localization.

• Received 16 May 2007

DOI:https://doi.org/10.1103/PhysRevB.77.035133