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

decay with stretched exponential kinetics, characteristic of persistent photoconductivityin many materials [112,113].We have simulated this behavior with a model of space-charge-limitedcurrent in the presence of a fixed number of deep-trap sites and found that thecalculations are able to describe the essential features of the experimental data[33]. In the space-charge limit, the field at any point in the device is stronglymodified by the presence of the injected charge, whereas the total amount ofcharge is fixed by the applied bias. In our simplified model, with a fixednumber of deep traps, the effective mobility [Eq. (11)] of the carriers isgradually reduced as more and more of them fall into deep traps. When trapdensities are comparable to the space-charge density, the trapping rate beginsto fall as the trap sites become occupied, leading to a decreasing rate oftrapping and a nonexponential decay of the current with time as in thedepletive trap model [110,114]. Exposure to above-bandgap light creates freecarriers that can move to neutralize the trapped space charge, accounting forthe restored conductivity. The persistent photoconductivity can be accountedfor if the hole mobility is assumed to be much lower than the electronmobility, so that positive charge slowly builds up under irradiation, [115].The presence of positive charge allows the maintenance of a higher density ofnegative space charge and hence a higher current. This conductivity thendecays as the positive charge is swept out of the device, or neutralized throughrecombination.Finally, the assumption of space-charge-limited currents allowed us tocalculate the electron mobility in these films. We found a large sample-to-samplevariation, but the mobilities fell in the range of f10 4 –10 6 cm 2 /V 1 /s 1for the ‘‘untrapped’’ electrons in our samples. Because the space-charge limitrepresents the maximum single-carrier current that can be passed through anydevice, these values represent good lower limits for the electron mobility inCdSe quantum-dot solids, even in the event that the devices were not trulyoperating in the space-charge-limited regime.Both additional measurements and more sophisticated modeling of thetransport and relaxation phenomena are required. Introduction of a distributionof traps in which carriers slowly relax to intrinsically deeper, less mobilesites or in which they are gradually localized through disorder-induced‘‘trapping,’’ as in a Coulomb glass, would both follow naturally from thedata. These possibilities are perhaps more realistic than assuming a fixeddensity of deep traps, as we have done, but the work to date provides a neededstarting point.The temperature dependence of the conductivity often provides valuableinsight into the microscopic physics behind carrier transport in a materialsystem. Although the time and exposure dependence of our current–voltage curves complicate the interpretation of the temperature data, weCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.