Photogeneration field dependencies

The effective carrier mobilities and their dependence on concentration for benztriazole derivatives embedded in polycarbonate were explained by the percolative aspects in photoconductivity [296]. The observed field dependence of the mobility for polycarbonate films doped with diethynylaminobenzaldehy-de-diphenyl hydrazone cannot be accounted for by any known hopping model [297]. The influence of the nature of the polymer matrix on photogeneration and transport properties of the molecule doped polymers was investigated in some papers [57, 58, 298, 299],... 

The theory predicts a strong dependence of photogeneration efficiencies on the field and it approaches unity at high field. The temperature sensitivity decreases with the increase in field. The theory has found satisfactory explanations in the photogeneration process in many organic disordered systems, such as PVK (Scheme la) [25], and triphenylamine doped in polycarbonate [26], Figure 4 shows an example of the field dependence of c() calculated from Eq. (22) (the solid lines) to fit the quantum efficiency data at room temperature for hole and electron generation in an amorphous material. The material consists of a sexithiophene covalently linked with a methine dye molecule (compound 1) (Scheme 2). 

The tacit assumption here is that the recombination time r between oppositely charged carriers is larger than the transit time Tt of the carriers so that the measured current is equal to the rate at which carriers are being produced (multiplied by e to convert the photon current into electrical current). Any electric field dependence of this current then reflects the electric field dependence of the QE of photogeneration. However, in the presence of substantial carrier recombination (r < Tt) the current with blocking contacts is... 

Amorphous Se has very high sensitivity. Figure 1 shows the field dependencies of the photogeneration efficiency at different wavelengths (Pai and Enck, 1975). For clarity, the very low field data of the original reference has been deleted. The temperature was 296 K. In the near ultraviolet and blue, the efficiency approaches unity at high fields. Field-dependent photogeneration,... 

Figure 1 The field dependencies of the photogeneration efficiency of a-Se at different wavelengths. The thicknesses were 44.0 pm (open and solid circles) and 3.4 pm (crosses).

Figure 8 The field dependencies of the photogeneration efficiency of a-Si at different wavelengths.

Figure 2 shows results calculated from Eq. (28). The calculations assume g = 3.0, L = 20 im. VQ = 500 V. 7J0 = 1.0. and nQ = 10-6 cm2/ys at VQ. The results show the maximum intensities for which emission-limited discharge can be sustained for different values of Pe and Pm. The results show the importance of a field-dependent carrier supply and mobility on the discharge process. Only in the case where Pm = Pe is the discharge independent of the field dependencies of the photogeneration and transport processes. 

While transient photocurrent and photoinduced discharge techniques are the conventional methods for measuring photogeneration efficiencies, these cannot be readily employed in the presence of trapping. A further limitation is that it is difficult to separate the field dependence of the photogeneration process from field dependencies associated with an injection or interfacial process (Seki, 1970, 1972a). As such, it is difficult to apply the method to dispersions or two-phase materials. Experimental methods that may avoid these limitations are... 

Figures 1 through 3 show the field and temperature dependencies of r]/7]0 calculated from Eq. (20) for different values of rQ. The dielectric constant was assumed to be 3.0. The Onsager formalism leads to strongly field-dependent photogeneration efficiencies that approach a limiting value at high fields. At...

The applicability of the Onsager theory can be determined from the field dependence of the photogeneration efficiency at low fields. From Eq. (20), the efficiency should have a linear field dependence. From plots of the efficiency versus field, the slope-to-intercept ratio is... 

For = 3.0, the ratio is 3.5 x 10-5 cni/V at 296 K. Although based on the assumption that g(r,0) is spherically symmetric, the ratio is independent of the function selected to represent the distribution of thermalized pair separations and contains no adjustable parameters. It thus provides a very critical test of the theory. Batt et al. (1968,1969) were the first to demonstrate the applicability of the Onsager formalism by use of the low-field slope-to-intercept ratio. The primary quantum yield and the thermalization distance can be determined by comparing experimental and theoretical values of the field dependence of the photogeneration efficiency at high fields, or by the temperature dependence of the zero-field quantum efficiency. The latter technique is based on the assumption that the primary quantum yield is independent of temperature. In most cases, thermalization distances and primary quantum yields have been determined from the field dependencies of photogeneration efficiencies at high fields. 

The effect of the TTA concentration of the transport layer on the field dependencies is shown in Fig. 4. The solid lines were calculated from the Onsager theory with r Q = 0.60 and different values of rQ. The results show that rQ increases with increasing TTA concentration while r Q remains constant. The most likely explanation of this is that the photogeneration efficiency is determined by the probability of a donor molecule being in contact with the photoexcited aggregate phase, as proposed by Umeda and Hashimoto (1992). 

Photogeneration efficiencies of 4,4 -(9-oxo-9H-fluorene-2,7-diyl)bis(azo)-bis[N-(2-chlorophenyl)-3-hydroxy-2-naphthalenecaiboxamide] (AZO-FO) in single- and dual-layer configurations were measured by Umeda et al. (1990). The dual-layer materials contained 4-N,N-bis(4-methylphenyl)amino-a-phenyl-stilbene (MAPS) in the transport layer. Figure 5 shows the field dependencies. 

Figure 5 The field dependencies of the photogeneration efficiencies of single- and dual-layer photoreceptors prepared with AZO-FO. For the dual-layer structures, the transport layer contained a stilbene derivative (MAPS).

Figure 7 The field dependencies of the photogeneration efficiencies of dual-layer photoreceptors containing AZO-TPA. The transport layer contained the triarylamine derivative MAPS at different concentrations.

Figure 13 The field dependence of the photogeneration efficiency of a dual-layer photoreceptor, a single-layer photoreceptor, and the photoluminescence quenching of the dual-layer photoreceptor. The generation layer contained AZO-FO. The transport layer contained a stilbene derivative (MAPS).

Figure 19 The field dependencies of photogeneration and fluorescence quenching efficiencies for PPECI.

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