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.

共轭分子的卤化反应是一种通过诱导效应与长程分子间静电相互作用，调控分子薄膜电子结构的高效策略。将卤化分子与其未改性的原始对应物共混，近期被证实可通过调控混合比例改变共混物的能级，从而实现有机太阳能电池开路电压的精准调节。本研究表明，该效应的主流理论依据并非对所有分子取向均适用。我们针对由并五苯（pentacene）与全氟并五苯（perfluoropentacene）构成的典型共混体系开展了全面的实验与理论分析，以建立分子结构与电子性质之间的内在关联。研究发现，根据基底特性的差异，该共混体系会呈现直立或平躺两种取向的混合堆叠结构基元。在直立取向情形下，共混体系的电离势介于两种纯组分的电离势之间，这与已被广泛认可的分子四极矩平均图景相符。然而针对平躺取向的样品，我们通过实验观测到其电离势低于两种纯组分的电离势，这与上述简单理论框架存在矛盾。基于薄膜原子级结构的静电模拟，能够复现两种分子取向下的复杂实验现象。具体而言，由平躺分子构成的薄膜所展现出的超低电离势，被证实为单层结构的特征：相较于具有相同分子取向的较厚薄膜，四极相互作用可使最高占据分子轨道能级产生约0.4 eV的差值。