Effective Modulation of Exciton Binding Energies in Polymorphs of a Small-Molecule Acceptor for Organic Photovoltaics

The operation mechanisms and energy losses for organic solar cells are essentially determined by the exciton binding energies (E_b) of organic active materials. Because the factor of chemical modifications is precluded, polymorphisms featuring different packing motifs of the same molecular structures provide an ideal platform for revealing the influence of solid-state packing. Herein, we have calculated the E_b values in three different cystal phases of a representative acceptor-donor-acceptor molecular acceptor (IDIC) by the self-consistent quantum mechanics/embedded charge approach. The results show that the differences of mere molecular packing modes can result in a substantial change in E_b of ≤50%, in the range of 0.21-0.34 eV among the three IDIC crystal phases. Moreover, a higher backbone packing dimensionality is found to be beneficial for obtaining a smaller E_b. This indicates that polymorph engineering is an effective way to reduce E_b toward low-energy-loss organic solar cells.