Electrostatic Interactions Shape Molecular Organization and Electronic Structure of Organic Semiconductor Blends

Halogenation of conjugated
molecules represents a powerful approach
to tune the electronic structure of molecular thin films through inductive
effects and long-range intermolecular electrostatic interactions.
The mixing of halogenated molecules with their pristine counterparts
has recently proven successful in altering the blend’s energy
levels to adjust the open-circuit voltage of organic solar cells by
the mixing ratio. Here, we show that the prevailing rationale for
this effect is not equally valid for different molecular orientations.
We provide a comprehensive experimental and theoretical analysis of
the prototypical blend formed by pentacene and perfluoropentacene
to relate structure with electronic properties. We find a mixed-stack
structural motif in standing and lying orientations depending on the
substrate nature. In the standing orientation, the ionization potential
lies in between the values of the pure components, in line with the
established picture of averaged molecular quadrupole moments. For
the lying orientation, however, we experimentally observe an ionization
potential lower than both pristine values, which seems at odds with
this simple rationale. Electrostatic simulations based on the knowledge
of the atomistic structure of the films capture the complex experimental
scenario for both orientations. In particular, the ultralow ionization
potential of films formed by lying molecules is identified as a signature
of the monolayer structure, where quadrupolar interactions are responsible
for a difference of ca. 0.4 eV in the highest occupied molecular orbital
energy as compared to thicker films with the same molecular orientation.