The rationale behind the acceptor-donor-acceptor chemical design of non-fullerene acceptors

Efficiencies of organic solar cells have seen a rapid increase due to the development of non-fullerene acceptors (NFAs). We explain why excitons dissociation into charge-transfer (CT) states, and the subsequent CT state splitting into charge-separated (CS) states, are efficient in NFAs-based solar cells.
We show that the driving force of the excited-to-CT state transition comes from a stronger dielectric stabilization of charges compared to the localized excited state. CT-to-CS transition is driven by the gradient of the acceptor concentration, which bends the electrostatic potential, helping to overcome the Coulomb binding of the CT state. Both effects depend on the related molecular quadrupole moments and their long-range contribution to the solid-state ionization energies and electron affinities.
The results are supported by simple lattice models, as well as atomistic-level descriptions performed for a number of solar cells based on the polymer donor PCE10 and small molecule NFAs. These include IEICO, IEICO-4F, IEICO-4Cl, O-IDTBR, O-IDTBCN, ITIC, ITIC-4F, ITIC-4Cl. The present study suggests several design rules for NFAs with efficient charge separation in photovoltaic applications.