Charge generation efficiency

Fig. 7. The field-dependence of the charge-generation efficiency of a 2.0- lm thick (0) a l.l-).tm thick ( ), and 1.8-).tm thick (A) fuUerene/PMPS film obtained with positive charging and 340-nm irradiation (A). The soHd lines are calculated from the Onsager model. The best-fit curve is obtained with Tq = 2.7 nm and = 0.85. Also plotted is the charge-generation efficiency of a fuUerene/PVK film (+) obtained with positive charging and 340-nm irradiation (B). The soHd lines are calculated from the Onsager model. The best-fit curve is obtained with = 1.9 nm and = 0.9 (13).

Table 3. Charge-Generation Efficiency of Selected Polymeric Photoconductors ...

Experimental Values of Charge-Generation Efficiencies. In this section the charge-generation efficiencies of many polymeric photoconductors are compared (Table 3). When the experimental data has been fitted to the Onsager model, the initial electron—hole separation distance,... 

The intrinsic charge-generation efficiency of polymers is often low and needs to be enhanced by the addition of sensitizers. The sensitizer can be dissolved in the polymer to enhance the bulk charge-generation efficiency of the polymer. Effective sensitizers include 2,4,7-trinitro-9- uorenone [129-79-3] (TNF), hiUerene, thiapyryhum dye, CdS nanoclusters, etc (Table 3). Molecular stmctures of selected sensitizers are shown in Figure 8. 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

Several classes of organic compounds have been developed as carrier generation materials. The most important characteristics of a charge generation material are the spectral response, which is determined by the absorption spectrum, and the sensitivity, which depends on the charge generation efficiency. 

For light of sufficiently (ow intensity, absorbed within a small fraction of the film thickness, the charge generation efficiency, discharge rate of the surface potential, (dV/d/), o (refs 6, 7)... 

FIG. 2 The wavelength dependence of the charge generation efficiency for a 28 (i,m fullerene/PVK film, obtained for both positive ( ) and negative (O) chargif. The dashed line shows the t tical absorption spectrum of the film. The applied voltage is 3xlO Vcm. ... 

FIG. 3 The field dependence of the charge generation efficiency for 4.5 (i.m (O) and 9.3 p.m +) fullerene/PVK films, with positive charging and 340 nm irradiation. The solid lines are calculated from the Onsager model. The best-fit curve is obtained with ro=19 A and 4>o=09. 

Recently, several photoactive polymers have been examined that fit these criteria. They include JV-polyvinylcarbazole (PVK) [101,102], (phenyl-methyljpolysilane (PMPS) [103], and amine-doped polycarbonate [104], All are known hole-transporting polymers. A large number of semiconductor nanoclusters can be doped into these polymers and interact with the polymer and facilitate carrier injection into the polymer [101-104]. Dramatic enhancement in charge generation efficiency has been observed. The availability of these semiconductor nanocluster/polymer composites opens the doorway for exploring transport-related applications. In the following sections, I review their photoconductive properties and discuss possibilities in other related areas. 

The charge generation and charge transport efficiency of a polymer can also be measured separately by other techniques. For example, laser flash photolysis can measure the charge generation efficiency directly, if the generated electron or hole (anion or cation) has a distinct absorp-... 

The two parameters, r0 and [Pg.220]

With the addition of 2.7 wt% of fullerene, the photoinduced discharge rate of PVK shows a dramatic enhancement [101]. Wavelength dependence of the charge generation efficiency shows that fullerene acts as the sensitizer for the enhanced photoconductivity [101]. Figure 21 shows the field... 

The Onsager charge recombination model [113-114] is commonly used for analyzing the field dependence of charge-generation efficiency. Limitations and inadequacies do exist for this model. In this section, I discuss the... 

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