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

Abstract

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 840nm, 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 130meV for excitons in the charge-transfer state.