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

For samples measured in darkness, the current in response to a fixed-voltagestep is found to decay monotonically with a nonexponential form [33,108].We find that our data [33] consistently follow Kohlrausch’s [109] stretchedexponential relaxation function: t bIðtÞ ¼ I 0 expsalthough others have observed power-law decays under different conditions[108]. Stretched exponential functions are characteristic of many relaxationprocesses in molecular and electronic glasses [110] and have previously beenobserved for various carrier relaxation processes in TiO 2 [85] and CdSe [111]nanocrystals.Our devices exhibit strong history-dependent ‘‘memory’’ effects, asshown in Fig. 17. Several experimental facts need to be accounted for inany description of this phenomenon. First, if allowed to ‘‘rest’’ in the dark atroom temperature, the films will slowly regain their original current–voltagecharacteristics over a period of days. In addition, the original dark conductivitycan be restored by even a very brief exposure to light of energy above thenanocrystal bandgap. Light below the bandgap has no effect, ruling out lightinduceddetrapping. Longer exposure will raise the dark conductivity of a filmto levels that exceed the original dark conductivity levels of the device.Finally, under zero bias, this ‘‘persistent photoconductivity’’ is found toFigure 17 Current–voltage scans showing the effect of device history. Scan A(circles) is the initial scan. Scan B (diamonds) was taken after two subsequent current–voltage sweeps. Scan C (squares) was taken after holding the device at +2 V biasfor 10 min. Scan D (triangles) was taken after illumination with light just above thebandgap.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.