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

TL;DR

This study demonstrates that long-range interactions significantly influence the electronic and optical properties of donor/acceptor co-crystals, affecting charge-transfer excitations and band structures, which are crucial for optoelectronic applications.

Contribution

The paper provides a first-principles analysis showing the essential role of long-range order in donor/acceptor co-crystals, challenging local interaction models.

Findings

Long-range interactions lower the band-gap with increased F atoms.

Charge-transfer excitations dominate optical absorption.

Cluster models fail to capture long-range effects.

Abstract

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 form, long-range order competes against short-range couplings. In a first-principles study on three experimentally resolved bulk structures of quaterthiophene doped by (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 wave-functions 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 co-crystals.