Energy level alignment between 9-phosphonoanthracene self-assembled monolayers and pentacene

The alignment of molecular energy levels between a self-assembled monolayer of 9-phosphonoanthracene formed on silicon dioxide and pentacene has been studied using photoelectron spectroscopies. The semiconducting band gap of pentacene was found to be nested within that of the monolayer, resulting in a 1.3±0.1 eV barrier for hole injection from pentacene into the monolayer. The corresponding barrier to electrons, estimated from the adiabatic highest occupied molecular orbital/lowest unoccupied molecular orbital gaps of anthracene and pentacene, is 0.3±0.2 eV. Thus, the monolayer presents a significant energetic barrier to hole injection from a pentacene overlayer, but only a small to moderate barrier to electrons.