Charge transient trapping

In the following, we present the results of charge transient spectroscopy performed on the bottom contacted pentacene OFETs, a variant of DLTS where the current transient is integrated, yielding a charge transient [43, 44]. In combination with capacitance DLTS, this technique can also provide information on the depth profile of the trap distribution [45]. 

If the density of states (DOS) of the trap states were broadened, the charge transient Q(t) would not follow a single exponential rise as in Eq. (6), resulting in turn in broadened QTS traces with respect to the fit based on Eqs. (5), (6). Such a behaviour was observed for polymer-based diodes [20] and for phthalocyanines [46], but for our bottom-contacted pentacene OFETs, we found no evidence of a broadened DOS of the trap states with a corresponding distribution of de-trapping rates. 

T= 300 K. b T= 40 K. As a result of charge-carrier traps from which the charge carriers can be thermally released with only a small probability at the lower temperature, the displacement current decreases so strongly for times tcharge carriers arrive at the back electrode and therefore, only a small blip (arrow) is discernible in the transient, in spite of the increased field strength. From the transit time, the field strength and the thickness of the crystal, the mobility can be computed directly. From [21]. 

We note that the transient absorption studies also identified a faster (<400 ps) recombination, which is largely temperature independent but lightly intensity dependent. Nelson explained that the early time recombination events are due to charges before trapping (above mobility edge), and described the recombination dynamics covering the nanosecond-millisecond timescale using a Monte Carlo technique [114]. 

Finally, no discussion will be given of the powerful technique of ionic thermoconductivity, also known as thermally stimulated depolarisation ". This procedure is particularly useful in observing the relaxation of the polarisation of dipoles and/or the release of mobile charge from traps in the material. Neither of these processes is included in the present models, but thermally stimulated depolarisation measurements should certainly be made in parallel with ordinary transient and frequency response measurements whenever possible in order to characterize the system investigated more fully. 

The exciton-radical pair equilibrium model predicts that the number of pigments coupled to P680 influences the AG values for the primary charge separation, trapping times and fluorescence lifetimes. In addition, in preparations where the radical pair is transiently formed, the number of pigments coupled to P680 may have an effect on the triplet yield. In view of entropy effects, the AG decreases when the number of coupled antenna pigments increases. ... 

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

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