Ionization in organic thin films: Electrostatic potential, electronic polarization, and dopants in pentacene films

Abstract

Ionization processes in thin films are central to organic electronics. The ionization potential $I(p)$ or electron affinity $A(p)$ of any molecule $p$ depends on the electronic polarization of the surrounding molecules and on electrostatic interactions $W(p)$ that are evaluated in films using the potential ${\ensuremath{\Phi}}^{(g)}($r$)$ due to gas-phase charge densities. $W(p)$ is combined with a self-consistent treatment of electronic polarization to obtain $I(p)$ and $A(p)$ using molecular quantum theory and the film's structure. $I(p)$ and $A(p)$ are not additive but contain cross terms in electronic polarization and electrostatics. The procedure accounts quantitatively for $I(p)$ of pentacene and perfluoropentacene films with standing molecules in bilayers or lying molecules in monolayers. Surface or subsurface dopants in pentacene films are modeled as ion pairs with Coulomb interactions between a fixed anion and an adjacent cation. Variations of ${\ensuremath{\Phi}}^{(g)}($r$)$ due to an ion pair modulate $I(p)$ and $A(p)$ locally and rationalize observed changes for tunneling into occupied and unoccupied pentacene states, respectively. As in molecular exciton theory, intermolecular overlap is neglected in the computation of $I(p)$ or $A(p)$. Electrostatic interactions are conveniently quantified by ${\ensuremath{\Phi}}^{(g)}(0)$ at the center of molecules.