Shallow-trap distribution

TABLE I. Intensity Distribution in the Shallow-trap limit. Upright Band, m = 6... 

Another model based on the same experimental results has been proposed [148]. Traps with a distribution of the potential depth are preformed in the matrix. The electrons generated at 4°K are stabilized in the shallower traps. Detrapping and subsequent retrapping in the deeper traps occur during warming from 4 to 77° K. 

The properties of energetically-distributed charge-carrier traps are above all in disordered organic semiconductors of considerable significance for the analysis of real current-voltage characteristics. We consider the following two energy distributions for shallow traps an exponential distribution, with its maximum at the transport level Ee (compare Fig. 8.6) ... 

The photo-generated charge carriers trapped in shallow states turmel from one trap to another and recombine with opposite type of charge carrier. The emission from the recombination in shallow traps appears at a lower wavelength than deep traps. The broad emission band represents the superposition of wide distribution of traps distance (Spanhel and Anderson, 1991). 

The Butler model for the distribution of excitation energy within the photochemical apparatus of photosynthesis assumes that the rate constant for energy transfer to PSII reaction centres exceeds the rate constant for the back-transfer of energy from the reaction centre to the antenna (Butler, 1978). This led to the concept of the PSII unit as an energy funnel. However, more recent fluorescence lifetime measurements indicate that the equilibration of excitation energy between antennae and reaction centres is one order of magnitude faster than charge separation. Thus, PSII appears to be trap limited and the reaction centre appears to act not as a funnel but as a shallow trap (Schatz et al., 1988 Blankenship, 2002). 

The effect of traps on the SCLC has been widely studied, both theoretically and experimentally. In the case of a single shallow trap level of density lyii g at an energy E below the conduction band (or above the valence band), the current is simply multiplied by a factor 9 - nf/( f -f- t)> where and t are the density of free and trapped carriers, respectively. In the case where tif < n, and assuming that the carrier distribution follows a simple Boltzmann statistic, 9 is given by... 

Like excitons, electrons, holes or polarons also carry out a hopping motion between the domains in a conjugated pol3rmer. Each domain provides unoccupied states around the LUMO and HOMO level for electrons and holes, respectively. They are delocalized within the domains and vary, as for the excitons, in their energy with the domain size. The tails of the corresponding density of states distributions act as shallow traps that keep the charges temporarily fixed. 

In a final RTD experiment, a sheet of dye was frozen as before and positioned in the feed channel perpendicular to the flight tip. The sheet positioned the dye evenly across the entire cross section. After the dye thawed, the extruder was operated at five rpm in extrusion mode. The experimental and numerical RTDs for this experiment are shown in Fig. 8.12, and they show the characteristic residence-time distribution for a single-screw extruder. The long peak indicates that most of the dye exits at one time. The shallow decay function indicates wall effects pulling the fluid back up the channel of the extruder, while the extended tail describes dye trapped in the Moffat eddies that greatly impede the down-channel movement of the dye at the flight corners. Moffat eddies will be discussed more next. Due to the physical limitations of the process, sampling was stopped before the tail had completely decreased to zero concentration. 

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