Beyond the Edge: Charge‐Transfer Excitons in Organic Donor‐Acceptor Cocrystals

ABSTRACT Charge‐transfer excitons (CTXs) at organic donor/acceptor interfaces are crucial intermediates for charge separation in photovoltaic devices. While blends used in real‐world devices hamper detailed characterization of CTXs, atomistic models of cocrystals offer powerful alternatives for gaining microscopic insights. In this work, we investigate electronic and optical properties of acene‐perfluoroacene cocrystals (anthracene, tetracene, and pentacene), combining experimental synthesis and characterization with first‐principles calculations based on many‐body theory. We prepare ultrathin cocrystals for polarization‐resolved transmission‐absorption spectroscopy, linking exciton polarization with molecular packing. Complementing this analysis, density‐functional and many‐body perturbation theory reveal complex excitonic landscapes that challenge several common assumptions about CTXs in weakly interacting donor‐acceptor systems. For the studied cocrystals, we demonstrate that such CTXs are not limited to the absorption onset, but also occur at higher energy and produce sharp, intense absorption features. Adopting the intrinsic molecular coordinate system, we categorize the various excitons according to their polarization and show that the transition dipole moment of the lowest energy CTX is not necessarily aligned with the donor‐acceptor stacking axis. We further characterize triplet excitons from first principles, which are only indirectly accessible experimentally. This work provides a deep understanding of CTXs in organic cocrystals, developing a refined conceptual framework that is crucial for future design of environmentally sustainable photoactive materials.