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

memory and charge storage effects [3] caused by the same trapping processesdescribed in Section V.The best PPV–nanocrystal LEDs generally exhibit electroluminescenceefficiencies on the order of 0.1 cd/A. This compares to efficiencies in excess of10 cd/A, which can now be achieved with entirely organic LEDs. However,these high efficiencies are the result of a huge amount of research anddevelopment work devoted to organic LEDs, whereas nanocrystal LEDshave not been so extensively studied. There are thus many unexplored routesthat could improve the performance of nanocrystal-based LEDs. Amongthese is the optimization of heterojunction-band offsets to reduce the largebarriers to carrier injection into the nanocrystals, which might be achievedthrough the use of new polymers and of different nanocrystal materials. Inaddition, a method is needed to prepare thin films of nanocrystals that areboth highly luminescent and electronically accessible: a difficult task becauseisolation from the environment is a general precondition for high photoluminescenceefficiency. If accomplished, it may then be possible to utilize thesuperior photostability and continuously tunable emission of inorganicnanocrystals in LEDs. If not, nanocrystal–polymer composites may still finduse as tunable phosphors in other electroluminescent devices [124].B. PhotodiodesPhotovoltaic blends of conjugated polymers and semiconductor nanocrystalscan be fabricated by spin-coating films from solutions containing both thepolymer and nanocrystal components. These blends offer the possibility totune the sensitivity to incident light through both the polymer and nanocrystalcomponents. Furthermore, the band offsets that cause many nanocrystal–conjugated polymer interfaces to perform poorly in LEDs have exactly theopposite effect in photovoltaic applications, where efficient separation, ratherthan generation, of excitons at the polymer–nanocrystal interface is required.As for the case of LEDs, the performance of polymer-composite photovoltaicdevices is known to be extremely sensitive to device structure and blendmorphology [70,125]. The potential to optimize the charge transfer andcharge transport properties of these composites through nanoparticle surfacemodification (Fig. 10) [5], control of particle shape [6], or through incorporationof nanocrystal-binding functionality on the polymer backbone[118,126] is thus particularly promising. Furthermore, the high atomicnumbercontrast between inorganic nanocrystals and conjugated polymers,and the conductive nature of the polymers facilitates electron microscopystudies of these materials [5,127].The optimum morphology for a photovoltaic composite is determinedby two principal requirements (Fig. 21). The first is that the majority (ideallyCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.