Electronic structure and correlations in pristine and potassium doped Cu-Phthalocyanine molecular crystals

TL;DR

This study uses ab initio calculations to explore how potassium doping alters the electronic structure and magnetic properties of Cu-phthalocyanine crystals, revealing increased conductivity and complex electron interactions.

Contribution

It provides the first detailed ab initio analysis of potassium intercalation effects on CuPc's electronic structure and magnetic behavior, highlighting the emergence of metallic states.

Findings

Potassium doping induces a transition from insulator to metal.

Doping introduces electrons into Pc-ring e_g states, maintaining local magnetic moments.

Large bandwidth promotes charge delocalization, counteracting localization effects.

Abstract

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 $\rm K_2CuPc$ the additional electrons that are introduced by potassium are fully transferred to the $e_g$ 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 $e_g$ 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 $\rm K_2CuPc$ 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, $\rm K_{2.75}{CuPc}$, 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.