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

A one-dimensional many-body 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 and the local imbalance of electron and hole densities. The alternating structure is beneficial for the direct generation of charge transfer state which enhances the local imbalance of charges. 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. 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 the Coulomb attraction does not matter in the charge separation dynamics. These findings give rise to appealing understandings of the high performance of emerging nonfullerene cells.