  Efficiencies of organic solar cells have practically doubled since the
development of non-fullerene acceptors (NFAs). However, generic chemical design
rules for donor-NFA combinations are still needed. Such rules are proposed by
analyzing inhomogeneous electrostatic fields at the donor-acceptor interface.
It is shown that an acceptor-donor-acceptor molecular architecture, and
molecular alignment parallel to the interface, results in energy level bending
that destabilizes the charge transfer state, thus promoting its dissociation
into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs
including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is
suggested that the molecular quadrupole moment of ca 75 Debye A balances the
losses in the open circuit voltage and gains in charge generation efficiency.
