Charge transport dispersive

According to the Scher-MontroU model, the dispersive current transient (Fig. 5b) can be analyzed in a double-log plot of log(i) vs log(/). The slope should be —(1 — ct) for t < and —(1 + a) for t > with a sum of the two slopes equal to 2, as shown in Figure 5c. For many years the Scher-MontroU model has been the standard model to use in analyzing dispersive charge transport in polymers. 

Edwards, DA, Charge Transport Through a Spatially Periodic Porous Medium Electrokinetic and Convective Dispersion Phenomena, Philosophical Transactions of the Royal Society of London A 353, 205, 1995. 

Arkhipov VI, lovu MS, Rudenko AI, Shutov SD (1979) Analysis of the dispersive charge transport in vitreous 0.55 AS2S3-O.45 86283. Physica Status Solidi (a) 54 67... 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

All the preceding mechanisms of the carrier packet spread and transit time dispersion imply that charge transport is controlled by traps randomly distributed in both energy and space. This traditional approach completely disregards the occurrence of long-range potential fluctuations. The concept of random potential landscape was used by Tauc [15] and Fritzsche [16] in their models of optical absorption in amorphous semiconductors. The suppressed rate of bimolecular recombination, which is typical for many amorphous materials, can also be explained by a fluctuating potential landscape. 

Charge transport through organic polymeric systems shows some unusual features. When the time of flight experiments are performed in inorganic crystalline solids the charge carriers drift in a sheet without any dispersion (except for the normal diffusion effects). All the carriers exit the sample at a specific time Tt. However a similar experiment with polymer films shows a very dispersive transit (Fig. 5 a) which indicates that only a small fraction of the carriers exit the sample at t = Tt. 

Since the early work of Scher and Montroll, there have been many studies of effects of positional disorder on charge transport in disordered solids (Poliak, 1977, 1977a Marshall, 1978, 1981. 1983 Mclnnes and Butcher, 1980 Schirmacher, 1981 Adler and Silver, 1982). These have lied to the general conclusion that while positional disorder can, under some conditions, give rise to dispersive transport, nondispersive transport is almost always attained after a carrier has executed a very few jumps. Hence, dispersive transport occurs over a small fraction of the thickness or at very low temperatures. This leads to the prediction that a transition from nondispersive to dispersive transport occurs within a single transit time or at increasing times with decreasing temperature. 

Polymeric semiconductors are usually disordered, so that charge transport is dispersive, i.e. there is a spread in the apparent mobility of the carriers. This results from the presence of the distribution of dopant and host energy states... 

When charge transport fails to reach a steady state during the time available, the most likely reason is that the transit time is dominated by the time required to escape from the slowest site(s) that a carrier encounters as it crosses the sample. Furthermore, the distribution of release times is such that the carrier continues to encounter slower and slower sites as it crosses a sample. Transport under such conditions is called dispersive, and has been the subject of much study since a seminal paper by Scher and Montroll [73a-e]. The term dispersive alludes to the wide dispersion in release times and/or the fact that carriers that are injected simultaneously spread out, disperse, to an anomalous extent as they cross the sample. The literature has several examples of studies of this subject in amorphous molecular solids [66b, 73f-h]. Some materials undergo a transition from essentially dispersive transport at low temperatures to essentially nondispersive transport at higher temperatures, and this dispersive-to-nondispersive transition has been the subject of significant attention [73i-p]. 

---