Trapping/detrapping processes

Figure 3.22 (a) The main processes determining the collection efficiency of electron transport in DSSCs. (b) Schematic drawing of the trapping/detrapping process. Electrons in trap states are thermally excited to the conduction band (CB) where they are mobile and can undergo transport towards the conducting substrate or recombination to the electrolyte or oxidised dyes... 

The transient current, derivable from equation 1, is given in equations 2 and 3 where T is the transit time and I is the absorbed photon flux. The parameter a can be further derived as equation 4 (4), where Tis the absolute temperature and is the distribution width (in units of kT) of a series of exponential traps. In this context, the carrier mobdity is governed by trapping and detrapping processes at these sites. 

The decay of a small population that serves as a probe of the kinetics of the system has therefore been characterized for any recombination rate law and steady state condition. More generally, the decay may require a number of sequential processes coupled to recombination, for example when prior detrapping of localized carriers is required, as we discuss later on. The free carrier lifetime Tf in (61) does not yet consider the trapping-detrapping dynamics, but just the charge... 

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