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

siderably larger than might be estimated from the results of electron transfertheory as described in Section II. Furthermore, the sample-to-sample variationsin activation energy are larger than any size-dependent trend that maybe present or that may be predicted by Eqs. (2)–(5). For these reasons, weconclude that, at least at temperatures between 180 and 300 K, transport inthe samples studied to date has been dominated by the effects of trapping anddisorder. In the future, studies of more ordered films, samples with narrowersize distributions, or samples with improved surface passivation might allowsize-dependent trends to be identified.VI.NANOCRYSTAL-BASED DEVICESAs has been noted in the previous sections of this chapter, an understanding ofcharge transfer and charge transport is fundamentally important to therational design of nanocrystal-based optoelectronic devices. In this section,we briefly review the construction and operation of various thin-film lightemittingand photovoltaic devices based around II–VI semiconductor nanocrystals.The nearly universal geometry employed for these devices is shown inFig. 1 and is identical to that used for the charge transport studies described inSection V. The thin-film ‘‘active layer’’ between the metal electrodes can bedeposited by a variety of methods, including spin-coating, drop-casting, andelectrochemical deposition, as well as by controlled layer-by-layer selfassembly.Many of these devices also incorporate an organic semiconductoras some part of the active layer.A. Light-Emitting DiodesNanocrystal light-emitting diodes (LEDs) have been fabricated in a variety ofconfigurations (Fig. 2). These include nanocrystal–polymer bilayer heterojunctions[1,3,18], nanocrystal–polymer intermixed composites [2,117–119],close-packed nanocrystal films [116,120], and even self-assembled stacks ofnanocrystal and organic monolayers [4,121,122]. Although some of thesedevices exhibit broad, and even white electroluminescence spectra [116,117,121,122], we focus on those device preparations which yield well-definedelectroluminescence peaks which can be controlled by adjusting the size of thenanocrystals used to fabricate the device.This quantum-confined electroluminescence arises from the radiativerecombination of excitons formed by the injection of electrons and holes intothe quantum dot or from exciton transfer to the dot from the host material.For this reason, the electroluminescence spectrum of a nanocrystal LEDgenerally resembles the solution photoluminescence spectrum of the nano-Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.