Charge-transfer states in conjugated polymer/fullerene blends: Below-gap weakly bound excitons for polymer photovoltaics

We report on the observation of a charge-transfer state forming at the molecular interface between a conjugated polymer and a fullerene based electron acceptor. Electron hole recombination in this state results in a luminescent transition at 840 nm, energetically separated from the polymer emission. This transition can be directly photoexcited by tuning the excitation energy below the conjugated polymer bandgap, demonstrating that the charge-transfer state originates from a ground-state interaction. By electric field induced quenching of the photoluminescence, we determine a binding energy of 130 meV for excitons in the charge-transfer state.