Polarization-sensitive time-resolved visible-infrared pump-probe experiments demonstrate that one can efficiently generate long-lived charges in donor-acceptor charge transfer complex (CTC) of conjugated polymer doped with fullerene, MEH-PPV/dinitroanthraquinone/C(60). In particular, a strong enhancement of the photoinduced charge generation is observed in the red part of the spectrum, i.e. inside the polymer band gap, which makes the current material attractive for photovoltaic applications. The spectroscopic results indicate that enhanced generation of charges is due to a consecutive photoinduced electron transfer from the polymer to the CTC-acceptor in the first step and then, in the second step, to the fullerene. The LUMO energy difference between the CTC-acceptor and fullerene appears to be a key parameter for efficient charge separation in these ternary systems. The results are also discussed in respect to the charge generation processes in widely used polymer-fullerene blends, where formation of weak CTCs has recently been discovered.

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Polymer solar cells have shown high potential to convert solar energy into electricity in a cost-effective way. One of the basic reasons limiting the polymer solar cell efficiency is insufficient absorption of the solar radiation by the active layer that limits the photocurrent. To increase the photocurrent, one needs low-bandgap materials with strong absorption below 2 eV. In this work, we study two types of low-bandgap materials: ground-state charge-transfer complexes (CTCs) of a conjugated polymer, MEH-PPV (poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene]), and an exohedral metallocomplex of fullerene, (η2-C60)IrH(CO)[(+)DIOP] (IrC60). We demonstrate that the CTC formed between MEH-PPV and conjugated molecules with high electron affinity, namely, 2,4,7-trinitrofluorenone (TNF) and 1,5-dinitroantraquinone (DNAQ), can have strong optical absorption extending down to the near infrared. We have observed that the photoexcited CTC can generate free charges. We also report on optical studies of IrC60 as a possible acceptor for polymer/fullerene solar cells. IrC60 strongly absorbs in the visible spectral range, in particular in the red part, and therefore has a potential for increasing the photocurrent as compared with polymer/methanofullerene solar cells. Our studies of MEH-PPV/IrC60 blended films show that long-lived charges are efficiently generated at MEH-PPV upon photoexcitation of the blend.

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