  Control of the band-edge offsets at heterojunctions between organic
semiconductors allows efficient operation of either photovoltaic or
light-emitting diodes. We investigate systems where the exciton is marginally
stable against charge separation, and show via E-field-dependent time-resolved
photoluminescence spectroscopy that excitons that have undergone charge
separation at a heterojunction can be efficiently regenerated. This is because
the charge transfer produces a geminate electron-hole pair (separation
2.2-3.1nm) which may collapse into an exciplex and then endothermically
(E=100-200meV) back-transfer towards the exciton.
