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

Figure 15 (a) Current–voltage (I–V) Curves for three different ITO/CdSe/metaldevices. The CdSe layer for each device is composed of a 190-nm-thick film of 3.4-nm-diameter nanocrystals separated by TOPO surfactant. Diamonds indicate the I–Vcurve for the A1 cathode device, squares for the Ca cathode device, and circles for theAu cathode device. The anode was ITO in all cases. (b) Diagram showing the relativepositions of the Fermi levels of the metals used as electrodes in our nanocrystal devicesand the quantum-confined conduction band (QC-CB) and valence-band (QC-VB)edge states. The range indicated is obtained from the upper and lower estimates ofEqs. (15)–(17) as described in Section IV.low-work-function metals Ca and A1 produce high currents at low bias,whereas the high-work-function metal Au and ITO exhibit currents that aresmaller by several orders of magnitude at the same voltage. The currents forthe Ca and A1 devices turn on at the same voltage (despite work functionsdiffering by nearly 1.5 eV), suggesting that they provide equivalent electroninjection capabilities and that the observed current–voltage curves are thuslimited by the bulk properties of the samples. As shown in Fig. 15, theseresults are entirely consistent with both the model of charge injectiondescribed above as well as the EA estimates provided by Eqs. (15)–(17).A second means of introducing carriers into a quantum-dot film is byphotoexcitation. Strictly speaking, this involves photoinduced electron transferfrom one nanocrystal to another, but we include its discussion here, ratherthan in Section IV, because its is essentially a nanocrystal–nanocrystal chargetransfer problem. Using different experimental geometries, both we [33] andLeatherdale et al. [21] have demonstrated that photocurrents in films of CdSefollow a spectral response curve (Fig. 16) that nearly matches the quantumconfinedabsorption spectrum of the nanocrystals. Our experiments measuredphotocurrents through thin (200 nm) films of CdSe sandwiched between ITOand Al electrodes, with active areas of a few square millimeters that wereCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.