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

dispersing CdSe nanocrystal chromophores into a matrix of the hole-conductingpolymer polyvinylcarbazole (PVK) along with the small-moleculeelectron transporter t-Bu-PBD [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole] were found to give electroluminescence (EL) quantumefficiencies of only 0.0005% [2]. The addition of the nanocrystals did notinfluence the current-voltage behavior of these devices, and it was concludedthat charge was injected into the PVK and PBD components, with thenanocrystals serving as trapping and recombination centers. The low ELefficiency of these devices can thus be attributed to the energetics of the CdSe–polymer interface. Although electron transfer from PBD to CdSe should beenergetically favorable (the electron affinities are 2.6 eV and f4.4 eV, respectively),hole transfer from PVK to the CdSe is not (the ionizationpotentials are 5.3 eV and f6.7 eV for PVK and CdSe). Therefore, in therandomly intermixed morphology of such a device, it is likely that mostelectrons and holes pass through the film without forming an exciton on thenanocrystals.Other semiconducting polymers and device morphologies have alsobeen utilized. For instance, PPV has been used by several groups to fabricatehybrid nanocrystal–polymer LEDs [1,3,18,118,121,122]. Section IV discussedextensive evidence for charge separation at PPV–CdSe interfaces. Justas exciton dissociation is energetically favorable, one would expect excitonformation at such an interface to be unfavorable. However, the use of bilayerstructures similar to that shown in Fig. 2a can give reasonably efficientluminescence from the nanocrystals. In this type of device, holes and electronsbuild up on opposite sides of the interface between the polymer and thenanocrystal layers. Holes are then injected over the barrier into the nanocrystallayer, where the chance of recombining with an electron to form anexciton is high. The increased field due to the charge buildup may alsoenhance carrier transport across the interface. Once an exciton is formed on ananocrystal, it may either decay radiatively, decay nonradiatively, or undergocharge separation at the interface, leaving the hole back on the polymer.Although charge separation at the interface might be energetically favorable,we have shown (see Sect. IV) that the presence of a TOPO surfactant layer onCdSe nanocrystals is sufficient to reduce the rate of charge transfer considerably[5,27], and we believe that the presence of some surface barrier to rapidcharge transfer is important in allowing radiative exciton decay in nanocrystal–polymerLEDs.Bilayer LEDs with a PPV layer at the anode and a plain TOPO-coatedCdSe nanocrystal layer at the cathode have been reported with externalquantum efficiencies from 0.01% [1] up to 0.1% [18]. An approximateenergy level diagram for such a device is shown in Fig. 20. The PPV has alower ionization potential than the nanocrystals, which facilitate holeCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.