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

For many applications, a host material is needed which can not onlyinteract electronically with the nanocrystals but can also transport charge. Itis all the more desirable if this host matrix can be processed into thin films.Conducting and semiconducting polymers have the potential to fill this role,and for that reason, composites of quantum dots and electronically activepolymers have been investigated by a number of researchers.Perhaps the first report of this kind was by Wang and Herron [38], whogrew CdS clusters inside a polyvinylcarbazole (Fig. 9) matrix. Polyvinylcarbazole(PVK) is a hole conductor that is transparent in the visible region, withan absorption edge near 380 nm. The CdS/PVK composites are yellow incolor and have an absorption edge near 440 nm. Upon irradiation with visiblelight, holes are transferred from the CdS to the PVK, imparting photoconductivityto the composite with an action spectrum similar to the CdS absorptionspectrum. The same authors ultimately explored clusters of CdS, PbI 2 ,HgS, InAs, Ga 2 S 3 , and In 2 S 3 , to produce photoconductive films with photoresponsesthrough the near infrared and investigated the dependence ofcharge generation efficiency and residual quantum dot fluorescence on theelectric field applied to the composites [39,40].More recent experiments have provided additional evidence for bothelectron transfer from photoexcited PVK to CdS clusters, as well as holetransfer from excited CdS to PVK [66,67] and have also studied the photorefractiveproperties [34] of the PVK/CdS composite.With the explosion of interest in semiconducting polymers over the lastdecade, a wide range of additional polymer hosts have been synthesized.Among the most prominent and intensely studied families of conjugatedpolymers are those derived from poly( p-phenylenevinylene) (PPV), several ofwhich are shown in Fig. 9. Unlike PVK, these polymers strongly absorbvisible light. Nevertheless, single-component polymer photodiodes generallyexhibit a low efficiency in converting incident photons into electrical charges.This is because the dominant photogenerated species in most conjugatedpolymers is a strongly bound neutral exciton. Because these neutral excitationscan be dissociated at an interface between the polymer and an electronacceptingspecies, charge separation is often facilitated via inclusion of ahigh-electron-affinity substance such as C 60 [39,68] or another polymer witha higher electron affinity [69,70]. Common features of all such successfulcharge-separation-enhancing materials include both an electron affinity highenough to make charge transfer energetically favorable and the ability toFigure 9 Chart showing the structures of several polymers and small moleculesthat exhibit interesting charge transfer behavior when adsorbed onto or blended withCdS and CdSe nanocrystals.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.