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, poly-morphisms 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.