Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

induced electron transfer by monitoring the subgap absorptions characteristicof positive charges (polarons) on the polymer backbone. Consistent with thefluorescence quenching data, the only long-lived photoinduced species weobserved in the DHeO–CN–PPV/CdSe composite was the neutral tripletexciton in the polymer (which has an infrared absorption to a higher-lyingtriplet state). However, in MEH–PPV/nanocrystal composites, we were ableto observe the low- and high-energy absorption signatures characteristic oflong-lived positive polarons on the polymer (Fig. 12a). Because parts of theMEH-PPV polaron absorption occur in the same region as absorptions due tothe polymer triplet exciton, we characterized the absorption spectrum as afunction of temperature and pump modulation frequency at various wavelengths.Whereas the triplet lifetime was very sensitive to temperature, thepolaron lifetime was only weakly temperature dependent (Fig. 12b). Thisallowed us to resolve the contributions to the PIA spectra from the polaronand triplet species, as well as to demonstrate that triplet and polaron excitationscould coexist in the composites. This observation was consistent withthe strong, but not complete, fluorescence quenching in the composites, andboth effects were interpreted in the context of the aggregated blend morphology(Fig. 10). Because the polymer–nanocrystal phase separation occurs onlength scales comparable to the exciton diffusion range in MEH–PPV, someexcitons will decay radiatively or undergo intersystem crossing to form tripletstates, before they have a chance to reach an interface where they mightbe dissociated.By varying the pump modulation frequency, we were able to measurethe lifetime of the charge-separated state, which was found to span a distributionof recombination times from microseconds to milliseconds. The recombinationprocess is therefore much slower than the initial charge-separationprocess. We will see in Section V that recombination is an importantloss mechanism in polymer–nanocrystal photovoltaic devices. Measurementof the recombination time gives a rough estimate of the timescale on whichcharges must be removed from a photovoltaic device in order to avoidrecombination.The timescale on which the initial charge-separation event takes place insemiconducting polymer–quantum dot composites is not yet accuratelyknown, and time-resolved studies are needed to address this issue. Clearly,electron transfer is fast enough to compete with radiative recombination inthese polymers, but different kinetics would be expected depending onwhether exciton diffusion to the nanocrystal–polymer interface or the intrinsiccharge transfer step is the rate-limiting factor. There is also the possibilitythat a fraction of the charges could be generated by hole transfer from thenanocrystal to the polymer following transfer of the entire exciton from thepolymer to the nanocrystals via Fo¨rster transfer. The partial concentration-Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.