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

all) of the optically excited electron–hole pairs should diffuse to an interfaceand experience charge separation instead of recombining. The second requirementis that once separated, the electrons and holes should be efficientlyextracted from the device with minimal losses to recombination. In mostdevices, a large fraction of the incident light is absorbed by the polymercomponent, even at high nanocrystal concentrations. Because the typicalexciton diffusion length in a conjugated polymer is on the order of 10 nm, thefirst condition implies that a large-area distributed interface is required so thatno exciton is formed farther than one diffusion length from an interface. Thesecond condition, however, requires that a continuous path to the appropriateelectrode be readily accessible from every segment of the distributedinterface. These morphological considerations are summarized in Fig. 21.The performance of early MEH–PPV/CdSe nanocrystal photovoltaicdevices as a function of composition serves to illustrate the dual importance ofcharge separation and charge transport [5]. Although the charge generationrate (as monitored by photoluminescence quenching) plateaued at lowernanocrystal concentrations, device efficiency continued to improve at higherconcentrations. This can be explained by the growth of the aggregateddomains of pyridine-treated nanocrystals to provide more effective electrontransport pathways at the higher nanocrystal concentrations (Fig. 10). Thesedevices operated with short-circuit quantum efficiencies of up to 5% andpower conversion efficiencies of f0.25% at 514 nm [5]. Although chargegeneration in a MEH–PPV/CdSe composites is very efficient, the short-circuitquantum efficiencies of the composites are far from 100%. Indeed, we notethat similar efficiencies are obtained in devices containing only nanocrystals[33] (although the low open-circuit voltages in these devices preclude photovoltaicapplications). This suggests that in the nanocrystal-polymer devices,recombination losses due to inefficient transport are high, perhaps due tocharge trapping at ‘‘dead ends’’ within the phase-separated morphology ofthe blends.Consistent with this hypothesis, Huynh et al. demonstrated polymer–nanocrystal composites with improved efficiencies by using blends of anisotropicnanocrystal rods and the conjugated polymer poly(3-hexylthiophene)(P3HT) [6]. The external quantum efficiencies of these devices were 16% andpower conversion efficiencies were 2% under 514 nm illumination. Theimproved efficiencies of these devices can be attributed to enhanced carriertransport in both the P3HT phases and the nanocrystal phases, as well asimproved device morphology. First, P3HT has a significantly higher holemobility than MEH–PPV. In addition, the long nanorods tend to align themselvesend to end, thus reducing the number of interparticle hops required foran electron to cover a fixed distance. The 3-fold increase in quantum efficiencyand 10-fold increase in power conversion was achieved by using rods with onlyCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.