Establishing a microscopic model for nonfullerene organic solar cells: Self-accumulation effect of charges

A one-dimensional many-body tight-binding model is established to mimic the charge distribution and dynamics in nonfullerene organic solar cells. Two essential issues are taken into account in the model: the alternating donor and acceptor structure, which is beneficial for the direct generation of charge transfer state, and the local imbalance of the intrinsic electrons and holes. The most remarkable outcome of the model is that, due to the strong Coulomb attractive potential energy, the intrinsic charges in the cells are self-accumulated in a small spatial region and outside the self-accumulation region the charge density vanishes so that the recombination is regarded to be largely suppressed. The photogenerated electrons are subsequently observed to spread freely outside the self-accumulation region, implying that the Coulomb attraction does not matter in the ultrafast charge separation dynamics. These findings enable the understanding of the high performance of emerging nonfullerene cells, and the designing rules of molecules and devices are then comprehensively discussed.