Electronic, optical, and charge transport properties of A-π-A electron acceptors for organic solar cells: Impact of anti-aromatic π structures

Abstract

Organic solar cells based on acceptor-π-acceptor (A-π-A) electron acceptors have attracted intensive attention due to their increasing and record power conversion efficiencies. To date, almost all of the reported A-π-A electron acceptors are based on aromatic π structures. Here, we have investigated the impact of anti-aromatization of the π-bridges on the optoelectronic properties of A-π-A electron acceptors by (time-dependent) density functional theory. Our calculations show that besides the frontier molecular orbitals corresponding to the aromatic π-bridge based acceptors (“aromatic” acceptors), additional and unique occupied and unoccupied frontier orbitals are found for the acceptors based on the anti-aromatic π-bridges (“anti-aromatic” acceptors). Moreover, by tuning isomeric structures of the π-bridges (e.g., fusion orientations or linking positions of thiophene moieties), the optical excitation energies for the transition between the additional occupied and unoccupied levels turn to be close to or substantially lower with respect to those for the transition between the “aromatic” frontier orbitals. The optical absorption of the “anti-aromatic” acceptors is thus either stronger or broader than the “aromatic” acceptors. Finally, the reorganization energies for electron transport are tunable and dependent on the π-bridge structures. These results indicate a great potential of “anti-aromatic” electron acceptors in organic photovoltaics.