Long-Range Order Promotes Charge-Transfer Excitations in Donor/Acceptor Co-Crystals

Electronic and optical properties of doped organic semiconductors are dominated by local interactions between donor and acceptor molecules. However, when such systems are in crystalline forms, long-range order competes against short-range couplings. In a first-principles study on three experimentally resolved bulk structures of quaterthiophene doped with (fluorinated) tetracyanoquinodimethane, we demonstrate the crucial role of long-range interactions in donor/acceptor co-crystals. The band structures of the investigated materials exhibit direct band gaps decreasing in size with increasing amount of F atoms in the acceptors. The valence band maximum and conduction band minimum are found at the Brillouin zone boundary and the corresponding wavefunctions are segregated on donor and acceptor molecules, respectively. With the aid of a tight-binding model, we rationalize that the mechanisms responsible for these behaviors, which are ubiquitous in donor/acceptor co-crystals, are driven by long-range interactions. The optical response of the analyzed co-crystals is highly anisotropic. The absorption onset is dominated by an intense resonance, corresponding to a charge-transfer excitation. Long-range interactions are again responsible for this behavior, which enhances the efficiency of the co-crystals for photo-induced charge separation and transport. In addition to these results, our study clarifies that cluster models, accounting only for local interactions, cannot capture the relevant impact of long-range order in donor/acceptor cocrystals.

Automated summary

This study investigates the electronic and optical properties of doped organic semiconductors in crystalline forms, demonstrating the crucial role of long-range interactions in donor/acceptor co-crystals. The band structures show direct band gaps decreasing with increasing amount of F atoms in the acceptors, with the valence band maximum and conduction band minimum located at the Brillouin zone boundary. The optical response of the co-crystals is highly anisotropic, and long-range interactions enhance the efficiency of photo-induced charge separation and transport. Cluster models that only consider local interactions cannot capture the impact of long-range order in donor/acceptor cocrystals.