Charge transport multiple trapping

Traditionally, charge-carrier transport in pure and doped a-Se is considered within the framework of the multiple-trapping model [17], and the density-of-state distribution in this material was determined from the temperature dependence of the drift mobility and from xerographic residual measurements [18] and posttransient photocurrent analysis. 

FIGURE 2.2.8 Principle of charge transport limited by multiple trapping and thermal release. 

How can we obtain information about the surface or the interface from this experiment There are two answers. One concerns the mobility n. In an amorphous semiconductor, in which charge transport occurs with multiple trapping, this quantity is the time-dependent drift mobility at t= 1/v, where V is the frequency of the wave. Only a fraction of the carriers in the charge... 

Abstract This topic reviews random walk Monte Carlo simulation models of charge transport in DSSC. The main electrmi transport approaches used are covered. Monte Carlo methods and results are explained, addressing the continuous time random walk model developed for transport in disordered materials in the context of the large number of trap states present in the electron transporting material. Multiple timescale MC models developed to look at the morphology dependence of electron transport are described. The concluding section looks at future applications of these methods and the related MC models for polymer blend cells. 

Electron transport is slow because of the large density of traps in Ti02 that act as a bottleneck for charge transport and has therefore attracted extensive literature. Alternative pictures of electron transport in DSSC studies are hopping and multiple... 

An alternative approach [28, 50-54] is based on the assumption that the density of states can be modelled by a Gaussian distribution. Charge carrier transport occurs via direct hopping between the localized sites. In general, the differences between the two models are too small to be detected experimentally. Since our data can be quantitatively explained within the framework of multiple trapping we will restrict the discussion to this model. 

Fig. 4 Principle of the multiple trapping and thermal release limited charge transport...

Due to the low mobility in organic semiconductors, diffusion transport is also very important for the charge injection process. Therefore, an analytical diffusion controlled charge injection model particularly suited for OLEDs has been developed [33]. This model is based on drift-diffusion and multiple trapping theory. The latter can also be used to describe hopping transport in organic semiconductors [34],... 

In order to analyze the interplay between charge injection and bulk conductivity, one must use specific models for both injection and charge transport in bulk. Here we treat the charge injection as diffusion-controlled and the transport is multiple trapping theory. 

---