Toward Quantifying the Relation between Exciton Binding Energies and Molecular Packing

Reducing the exciton binding energy E_b of organic photoactive materials is critical to minimize the energy loss and improve the photovoltaic efficiency of organic solar cells. However, the relation between the E_b and molecular packing is not well understood. Herein, the E_b in the crystals of a series of A-D-A type nonfullerene acceptors with different lengths of alkyl side chains has been examined by self-consistent quantum mechanics/embedded charge calculations. The variation of molecular packing induced by the different alkyl chains can have an important impact on the polarization effect of charge carriers and thereby the E_b. More interestingly, the E_b values are found to be linearly increased with the ratio of the void fraction vs the packing coefficient of molecular backbones in the solid crystals. Owing to the smallest ratio, a remarkable low E_b of several tens of meV is achieved for the acceptor with an optimal length of alkyl chains.