Quantum efficiency of sensitization

Fig. 27. Quantum efficiency of sensitized hole injection in organic crystals (injected holes per incident photon) versus applied field strength (ratio of voltage to crystal thickness) Pe = perylene. Ar = anthracene, Ch = chrysene, Phen = phenanthrene...

In the previous subsection the case of Cu fluorescence quenching was discussed. One of the most important features of these results was the very high second-order quenching constant. The results for anthracene were described in terms of Xt the quantum efficiency of sensitization. This is related to a rate constant as follows ... 

Some of these devices have a respectable quantum efficiency of charge generation and collection, approaching 0.4 (20). The nature of the polymeric binder has a large effect on the device performance (21), and so does the quaUty and source of the dye (22). Sensitivity to the environment and fabrication methods results in some irreproducibiUties and batch-to-batch variances. However, the main advantage of the ZnO-based photoreceptor paper is its very low cost. 

In sensitization studies, the 7 state of anthrabarrelene (45) proved to be totally unreactive. However, direct photolysis of 2,3-anthrabarrelene afforded 2,3-anthrasemibullvalene via vinyl-vinyl bridging with a quantum efficiency of 0.25 ... 

EXAMPLE 1.5 The sensitivity of luminescence. Consider a photoluminescence experiment in which the excitation source provides a power of 100 ptW at a wavelength of400 nm. The phosphor sample can absorb light at this wavelength and emit light with a quantum efficiency of r] = O.I. Assuming that kg = 10 fii.e., only one-thousandth of the emitted light reaches the detector) and a minimum detectable intensity of l(f photons per second, determine the minimum optical density that can be detected by luminescence. 

Experimental methods for determining 0 are well documented (2). These experiments are conveniently carried out and require only a method of producing reasonably narrow-bandwidth radiation, a method of measuring the flux of that radiation per unit area, and a UV-visible spectrophotometer. The quantum efficiency of typical diazonaphthoquinone sensitizers of the type that are used in the formulation of positive photoresists ranges from 0.2 to 0.3, whereas the quantum efficiency of the bis-arylazide sensitizers used in the formulation of two-component negative photoresists, ranges from 0.5 to 1.0. 

Table 7 Quantum Efficiencies of Ring Opening by Direct Excitation and by Sensitization...

The quantum efficiency of a given fluorochrome ultimately determines the sensitivity attainable. Thus protein fluorochromes derived from marine algae, such as phycoerythrin, have very high quantum efficiency in comparison to small chemical fluorochromes, such as fluorescein. For analysis of low antigen densities, phycoerythrin is to be preferred. 

Chlorophyll-Coated Semiconductor Electrodes. Chi has first been employed by Tributsch and Calvin (55,56) in dye sensitization studies of semiconductor electrodes. Solvent-evaporated films of Chi a, Chi b, and bacteriochlorophyll on n-type semiconductor ZnO electrodes (single crystal) gave anodic sensitized photocurrents under potentiostatic conditions in aqueous electrolytes. The photocurrent action spectrum obtained for Chi a showed the red band peak at 673 nm corresponding closely to the amorphous and monomeric state of Chi a. The addition of supersensitizers (reducing agents) increased the anodic photocurrents, and a maximum quantum efficiency of 12.5% was obtained for the photocurrent in the presence of phenylhydrazine. 

Muenter and Cooper (30) measured the room-temperature fluorescence of two J-aggregated dyes, l,l -diethyl-2,2 -quino-cyanine and 1,1, 3,3 -tetraethyl-5,5, 6,6 -tetrachlorobenzimid-azolocarbocyanine, adsorbed on cubic AgBr grains. The quantum efficiency of spectral sensitization was inversely related to the relative fluorescence. The fluorescence by the dyes in the molecular state was low compared to that for the aggregated dyes. Addition of a styryl or thiohydantoin dye as a supersensitizer for the quinocyanine quenched its fluorescence and increased the relative efficiency from 0.06 to nearly 1.0. 

It has been assumed that desensitization by spectral sensitizers occurs to an equal degree in the regions of inherent absorption by the silver halide and absorption by the dye, but data on the effects of oxygen and moisture in these two absorption regions show that the degree of desensitization can be different under some conditions. The effect of the environment on the relative quantum efficiency of some dyes can depend on whether the dye is in the monomeric or aggregated state (266). This is illustrated in Table A. 

TABLE 4. Relative Quantum Efficiencies of Dyes for Spectral Sensitization of Reduction-Sensitized Emulsion (100-s Exposures)... 

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