A Theoretical Perspective on the Photovoltaic Performance of S,N-Heteroacenes: An Even–Odd Effect on the Charge Separation Dynamics

The electronic structure and optical properties of fused S,N-heteroacenes (SNn, SN5–SN10) have been studied theoretically. The calculations reveal that, bond length alteration approaches zero with increasing number of heterocyclic rings in the conjugated molecules. As a general trend of optical property, the absorption maximum is red-shifted with increasing conjugation length, achieved through increasing the degree of polymerization or by incorporating strong electron withdrawing groups at the two ends of the molecules. However, an even–odd relationship is observed during electronic excitation followed by exciton dissociation. Thus, SN5 and SN9 experience better charge separation than SN6 and SN10, respectively. The theoretical results interpret the experimental finding where SN5 is reported to offer better photocurrent efficiency. To compare the photovoltaic performances of the materials, we compute the rate of charge recombination and charge transfer for the composites consisting of some SNn and a well reputed acceptor PC61BM.