Excitons at organic semiconductor heterojunctions

Excitons in molecular semiconductors generally show high Coulomb binding energies and exchange energies, of order 0.5 eV, because dielectric screening is low.

These both present challenges and opportunities for the use of such materials in both light-emitting diodes and also in solar cells.

Current designs for organic photovoltaic diodes depend on the ionisation of photogenerated excitons at the heterointerface between electron-accepting

and hole-accepting semiconductors.  Photoinduced electron transfer across the heterointerface can allow efficient solar cell operation.  However, these

excitations can stabilise as coulombically-bound charge-transfer excitons that are not easily separated to fully-separated charge carriers. We have recently

developed an optical ‘pump-push’ technique in which an IR push pulse can separate charge-transfer excitons previously generated in the above bandgap pump pulse.

Large exchange energies allow scope for multiple exciton generation for materials for which the triplet exciton energy is less than one half of the singlet exciton

energy, since this favours energetically the fission of a photogenerated singlet to a pair of triplet excitons. If this process can be used in tandem with a lower

energy gap semiconductor that harvests singlet excitons directly then this may enhance solar energy conversion beyond the single-junction SQ limit. 

This is achieved in pentacene/lead sulphide hybrid solar cell devices