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.