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. ...