We investigate the changes in the electronic structure of copper phthalocyanine (CuPc) crystals that is caused by intercalation with potassium. This is done by means of {\it ab initio} LSDA and LSDA+U calculations of the electronic structure of these molecular crystals. Pristine CuPc is found to be an insulator with local magnetic moments and a Pc-derived valence band with a width of 0.32 eV. In the intercalated compound K2CuPc the additional electrons that are introduced by potassium are fully transferred to the eg states of the Pc-ring. A molecular low spin state results, preserving, however, the local magnetic moment on the copper ions. The degeneracy of the eg levels is split by a crystal field that quenches the orbital degeneracy and gives rise to a band splitting of 110 meV. Molecular electronic Coulomb interactions enhance this splitting in K2CuPc to a charge gap of 1.4 eV. The bandwidth of the conduction band is 0.56 eV, which is surprisingly large for a molecular solid. This is line with the experimentally observation that the system with additional potassium doping, K2.75CuPc, is a metal as the unusually large bandwidth combined with the substantial carrier concentration acts against localization and polaron formation, while strongly promoting the delocalization of the charge carriers.
@article{arxiv.cond-mat/0609405,
title = {Electronic structure and correlations in pristine and potassium doped Cu-Phthalocyanine molecular crystals},
author = {Gianluca Giovannetti and Geert Brocks and Jeroen van den Brink},
journal= {arXiv preprint arXiv:cond-mat/0609405},
year = {2009}
}