Trap depths, distribution

Rothe C, Monkman A (2002) Dynamics and trap-depth distribution of triplet excited states in thin films of the light-emitting polymer poly(9,9-di(ethylhexyl)fluorene). Phys Rev B 65(7) 073201... 

Figure 9-19. Bund diagram of LPPP with hole traps and gold electrodes with Va<- vacuum level. Ec conduction band, Eva valence band. E, Fermi level. . baudgup energy. and , " trap depths. ,( ) trap distribution, X electron affmity, and All work function of the gold electrodes.

In the inverted band a quite different pattern of intensity distribution is to be expected. In the pure crystal the topmost level alone is active it remains the strongest under all conditions. As the trap is deepened, some intensity moves from the topmost level downward through the band into the bottom level, which breaks out of the band and eventually becomes practically a localized state of the trapping molecule. Thus the presence of guest molecules awakens spectral activity in normally inactive levels, and should enable the extent and character of the pure crystal band structure to be studied experimentally. The point is illustrated in the diagrammatic spectra in Fig. 6, illustrating the transitions in one-dimensional mixed crystals for trap depths from zero (pure crystal) to d = 3.6. In each case the intensities are adjusted to make the lowest transition have unit intensity this... 

Similar to 234 Th, downcore profiles of 7Be can also be used to determine seasonal changes in sedimentation and sediment mixing rates in estuaries (Canuel et al., 1990). The basic assumption here, as described earlier, is that the nuclide (e.g., 7Be) traces movements of particles during sediment accumulation and that the delivery and trapping of the nuclide to surface sediments is uniform across habitats within an estuary. The three basic processes controlling the depth distribution are (1) supply rate from sedimentation (2) radioactive decay and (3) postdepositional particle mixing processes. Finally, it should be noted that using 7Be for the aforementioned purposes also requires concurrent measurement of 7Be in atmospheric fallout (Canuel et al., 1990). 

In the case of exponentially distributed traps, the effect of high fields on the trap depths (Poole-Frenkel effect) can be taken into account by the same procedure. The trapped hole density is modified and Eq. (3.40) changes to [38],... 

The triplet-triplet interaction, postulated to explain increased quantum yield of thin film organic LEDs, has been well known in EF of organic single crystals [2,21,41], One of the most spectacular manifestation of this type excitonic interactions is spatial distribution of EL emission (see Sec. 3.3). Interestingly, the EL light output resulting from the free-trapped carrier recombination (Oel) with respect to that underlain by free carriers recombination (Oel) does not depend on the trap depth [2]... 

Depth-distribution of fluorescent dopants in cast polymer film (4) Fluorescence spectra of poly(N-vinylcarbazole) (PVCz) film doped with perylene are shown in Fig. 6. They consist of two broad structureless excimer bands of the polymer with a shoulder at 375 nm and a peak at 420 nra, and perylene band with a vibrational structure above 450 nm. It is worth noting that the perylene fluorescence intensity under the TIR condition is relatively weaker than that under the normal one. Since the boundary surface is selectively excited under the former condition, the structure near the surface should be different from the bulk. It is well known that the excitation energy migrates over carbazolyl chromophores and is trapped in the doped perylene efficiently. Therefore, the present result means that energy migration efficiency in the host polymer and/or the dopant concentration are a function of the depth from the interface. 

Here, N is the overall density of the trapping states, is the width of the distribution, and ft is the average trap depth. 

There are many electrons in the sample, each in its own trap or in the conduction band (delocalized state), and there is a distribution of trap depths. The trap depths are not all the same because orientational disorder of the molecules provides a variety of polarization potential wells. Furthermore, an electron in a trap seems to couple with vibrational and librational modes of the trapping molecules. The optical absorption band of solvated electrons is very broad (Fig. 8). Part of the broadening might be caused by the distribution of trap depths, and part by the coupling with molecular modes. These parts are sometimes... 

In liquids where the trap depths are density dependences of electron mobilities are dominated by changes in the conduction band, not by the distribution of trap depths. In further development of the model we would ignore the possibility that localized states might lie above the delocalized level, and integrate Eq. (20) only over energies from zero to infinity. We would then need a formal couple between the values of E and a, which the present model does not have. 

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