Achieving Small Exciton Binding Energies in Small Molecule Acceptors for Organic Solar Cells: Effect of Molecular Packing.

Because of strong exciton binding energy (Eb), an exciton dissociation process and extra energy losses are present in organic solar cells relative to inorganic and perovskite solar cells. Here, we calculated the Eb of a series of small molecule acceptors in solid crystals by a self-consistent quantum mechanics/embedded charge approach. The results show that the Eb values are substantially reduced from the gas phase to solid state because of electronic polarization (mainly from the induction effect of charges). Moreover, in contrast to little changes in the gas phase, the Eb in the solid state can vary significantly, indicating an important molecular packing effect. Remarkably, an extremely weak Eb of 0.04 eV is achieved in a three-dimensional packing crystal, which is comparable to the Eb of organo-lead trihalide perovskites. This work underlines the importance of three-dimensional molecular packing for achieving small Eb and will be helpful in reducing energy losses in organic solar cells.