Traps occupancy

Several experimental J-V curves taken from the recent literature have been compared with the PFE model [39], We quote a few typical examples here. Extensive measurements of J(V) curves have been made by Crone et al. [59], They used both hole only Au/MEH-PPV/Al and electron only Ca/MEH-PPV/Ca devices. Crone et al. [59] used the mobility model to interpret their results. Comparison of their results for the electron devices with the Field Dependent Trap Occupancy (FDTO) model is shown in Fig. 3.33(a). The thicknesses of the samples are 25 nm (circles), 60 nm (diamonds) and 100 nm (plus symbols). The parameters used in calculations are Tc = 1420 K, //h = 7 x 1018 cm-3, ]Vt = 5x 1019 cm-3, JV, = 1 x 1020 cm-3, p, = 6.5 x 10-6 cm2 V-1 s 1 and e = 3. The agreement of the experimental results is satisfactory. The model has another advantage. The calculations have been made with the same parameters for all the three samples. With the mobility model, Crone et al. [59] had to use different values of the parameters for different samples. 

V. Kumar, S.C. Jain, A.K. Kapoor, W. Geens, T. Aernouts, J. Poortmans, R. Mertens, Carrier transport in conducting polymers with field dependent trap occupancy,./. Appl. Phys. 92 (2002) 7325-7329. 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

This simple model is not exact because it takes no account of the occupancy of the defects, which is different from the equilibrium occupancy. The time-of-flight measurement of the trapping rates is performed with few excess carriers, so that the trap occupancy is the same as in equilibrium, but in a steady state photoconductivity experiment, and the demarcation energies are often far from midgap. The trap occupancy is calculated from the rate equations for band tail electrons and holes... 

Figure 18 Schematics of electro-optical potential including the Casimir-Polder attraction of Eq. (48) for 87Rb atom between leads. The evanescent field, that decreases exponentially on it half wave-length expels atoms away of the surface to the location where attractive forces balance the light pressure. The line in the well cartoons the trap occupation.

Cao et al. and other authors [191] have observed that the photocurrent risetime decreases as the light intensity is increased. The risetime appears to follow a power law of the form t /2 x/o" with n = 0.5-0.6. This effect may arise from intensity dependent occupancy of electron traps. Cao et al. therefore assumed as a first approximation that the diffusion coefficient of electrons is a linear function of electron concentration. Numerical solution of Eq. 88 then yields transients that depend on light intensity. In principle this approach allows the incorporation of any arbitrary dependence of D on but a more satisfactory approach would be to relate the diffusion coefficient directly to the trap occupancy or to separate diffusion and trapping. Cao et al. estimate an upper limit of D = 10 cm s for the diffusion of electrons in the dark, whereas the D value for intensities corresponding to solar illumination levels are two orders of magnitude higher. 

In the case of thermal equilibrium the above two expressions become equal. Thus, we calculate the function of trap occupancy as... 

Optical grating filling factor Trap occupation probability... 

To get an expression for the measured effective diffusion length, L , we need to consider models which consider trapping and detrapping of electrons. Emphasis has been placed on the quasi-static treatment of Bisquert and Vikhrenko since it has the merit of being testable and suitable for incorporation into detailed models of the response of DSSCs to external perturbations. Their treatment shows that D and t depend on trap occupancy and hence on the Fermi level. The quasi-static approximation predicts that effective x is given by... 

The dn ldn term and its inverse reflect the way that the densities of trapped and free electrons, and n, respectively, vary with changes in the quasi-Fermi level. When D and are measured at the same trap occupancy, according to the quasi-static approximation, the effective diffusion length becomes independent of light intensity and one obtains... 

Often, D is obtained under short-circuit conditions and t under open-circuit conditions. Since the trap occupancy in the mesoporous oxide is very different under these two extreme conditions. Equation 3.26 is not valid. A very interesting method has been proposed by O Regan et based on small-amplitude photo voltage rise and... 

Permit space As defined by OSHA, a confined space that contains a hazardous atmosphere, a material that could engulf an occupant, a configuration that could trap an occupant, or any other recognized safety or health hazard. 

This is not true. Lifts/elevators are powered by electricity which may fail or be cut off during a fire. The lift will then automatically stop and any occupants would find themselves trapped. 

Protection from the Humidity Trapped by Weather-Tightness (including threats to occupant health building durability). 

Figure 14 (a) Excitation distribution along the channel axis of a zeolite L crystal consisting of 90 slabs (occupation probability p = 0.3) under the condition of equal excitation probability at f = 0 calculated for front-back trapping. Fluorescence of the donors is taken into account. (1) t = 5 psec, (2) f = 10 psec, (3) t = 50 psec, and (4) t = 100 psec after irradiation, (b) Predicted fluorescence decay of the donors in absence of acceptors (dotted curve), in the presence of acceptors at both ends (solid curve), and fluorescence decay of the acceptors (dashed curve), (c) Measured fluorescence decay of Py -loaded zeolite L (ppy = 0.08) (dotted curve), Py -loaded zeolite L (p y = 0.08) with, on average, one Ox acceptor at both ends of each channel (solid curveX and fluorescence decay of the Ox acceptors (dashed curve), scaled to 1 at the maximum intensity. The experiments were conducted on solid samples of a monolayer of zeolite L crystals with a length of 750 nm on a quartz plate. 

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