  In organic solar cells, photogenerated singlet excitons form charge transfer
(CT) complexes, which subsequently split into free charge carriers. Here, we
consider the contributions of excess energy and molecular quadrupole moments to
the charge separation process. We investigate charge photogeneration in two
separate bulk heterojunction systems consisting of the polymer donor PTB7-Th
and two non-fullerene acceptors, ITIC and h-ITIC. CT state dissociation in
these donor-acceptor systems is monitored by charge density decay dynamics
obtained from transient absorption experiments. We study the electric field
dependence of charge carrier generation at different excitation energies by
time delayed collection field (TDCF) and sensitive steady-state photocurrent
measurements. Upon excitation below the optical gap free charge carrier
generation becomes less field dependent with increasing photon energy, which
challenges the view of charge photogeneration proceeding through energetically
lowest CT states. We determine the average distance between electron-hole pairs
at the donor-acceptor interface from empirical fits to the TDCF data. The
delocalisation of CT states is larger in PTB7-Th:ITIC, the system with larger
molecular quadrupole moment, indicating the sizeable effect of the
electrostatic potential at the donor-acceptor interface on the dissociation of
CT complexes.
