Triplet formation from the charge-separated state in blends of MDMO-PPV with cyano-containing acceptor polymers

Polymer blends of MDMO-PPV with three different electron accepting cyano-substituted polymers have been investigated with photoinduced absorption (PIA) spectroscopy. In these blends, an efficient photoinduced charge transfer occurs that quenches the photoluminescence and produces a significant photovoltaic effect when the blends are incorporated as active layer into a solar cell configuration. Surprisingly, PIA spectroscopy in the millisecond time domain reveals that in the blends neutral triplet photoexcitations are almost exclusively formed in favor of polaronic charge carriers with yields higher than in the individual polymers. In the blends, the triplet state resides on the component with the lowest optical bandgap. The enhanced triplet generation is rationalized by the recombination of photogenerated charge carriers into a triplet state. This process may occur when the energy of the charge-separated state is higher than that of the triplet state. We demonstrate that this process is likely to occur in polymer solar cells with a high open-circuit voltage.