  The electronic properties of hybrid organic-inorganic semiconductor
interfaces depend strongly on the alignment of the electronic carrier levels in
the organic/inorganic components. In the present work, we address this energy
level alignment from first principles theory for two paradigmatic
organic-inorganic semiconductor interfaces, the singlet fission materials
tetracene and pentacene on H/Si(111), using all-electron hybrid density
functional theory. For isolated tetracene on H/Si(111), a type I-like
heterojunction (lowest-energy electron and hole states on Si) is found. For
isolated pentacene, the molecular and semiconductor valence band edges are
degenerate. For monolayer films, we show how to construct supercell geometries
with up to 1,192 atoms, which minimize the strain between the inorganic surface
and an organic monolayer film. Based on these models, we predict the formation
of type II heterojunctions (electron states on Si, hole-like states on the
organic species) for both acenes, indicating that charge separation at the
interface between the organic and inorganic components is favored. The paper
discusses the steps needed to find appropriate low-energy interface geometries
for weakly bonded organic molecules and films on inorganic substrates from
first principles, a necessary prerequisite for any computational level
alignment prediction.
