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 2.2–3.1 nm) which may collapse into an exciplex and then endothermically ($E_A=100–200\mathrm{meV}$) back transfer towards the exciton.

• Received 16 October 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.247402