The Binding Energy of Charge-Transfer Excitons Localized at Polymeric Semiconductor Heterojunctions

We address the binding energy of charge-transfer excitons at organic semiconductor heterojunctions by investigating a polymer blend where the energy of the intramolecular singlet exciton is just sufficient to create separated charge pairs, placing the system at the threshold for photovoltaic operation. At 10 K, we report long-lived photoluminescence arising from charge recombination and triplet-exciton bimolecular annihilation. Both mechanisms regenerate singlet excitons in the electron acceptor, but we demonstrate that charge recombination dominates singlet regeneration dynamics on <= 300 ns time scales. This occurs by tunnelling of separated electron and holes across heterojunctions. The separated charge pairs are therefore degenerate with repopulated singlet states. From the difference of the charge-transfer and intrachain exciton emission energies, we determine that the binding energy of charge-transfer excitons with respect to bulk charge separation is >= 250 meV. Directed charge flow away from the heterojunction would avoid formation of strongly bound charge-transfer excitons, that act as traps and limit current generation in organic solar cells.