Recombination efficiency

Some battery designs have a one-way valve for pressure rehef and operate on an oxygen cycle. In these systems the oxygen gas formed at the positive electrode is transported to the negative electrode where it reacts to reform water. Hydrogen evolution at the negative electrode is normally suppressed by this reaction. The extent to which this process occurs in these valve regulated lead —acid batteries is called the recombination-efficiency. These processes are reviewed in the Hterature (50—52). 

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

Recent work with multi-layer polymer LEDs has achieved impressive results and highlights the importance of multi-layer structures [46]. Single-layer, two-layer and three-layer devices were fabricated using a soluble PPV-based polymer as the luminescent layer. The external quantum efficiencies of the single-layer, two-layer, and three-layer devices were 0.08%, 0.55%, and 1%, respectively, with luminous efficiencies of about 0.5 hn/W, 3 lm/W, and 6 lm/W. These results clearly demonstrate improvement in the recombination current because of the increase in quantum efficiency. The corresponding increase in luminous efficiency demonstrates that the improvement in recombination efficiency was achieved without a significant increase in the operating bias. 

The processes of charge injection, transport, and recombination dictate the recombination efficiency h(/), which is the fraction of injected electrons that recombine to give an exciton. The recombination efficiency, which is a function of the device current, plays a primaty role in determining the amount of emitted light, therefore determining the OLED figurcs-of-meril. For example, the quantum efficiency /y(/) (fraction of injected electrons that results in the emission of a photon from the device) is, to a first approximation, given by ... 

Figure 13-11. (a) A diagram showing ihc spatial distribution of lire relative hole and electron currents in an OLED. The recombination efficiency h is equal to the fraction of the electron (hole) current that docs not make it to the anode (cathode) (b) cll icicncy-currcni balance diagram for OLEDs. Sec text for details. 

Recombination reactions between two different macroradicals are readily observable in the condensed state where molecular mobility is restricted and the concentration of radicals is high. Its role in flow-induced degradation is probably negligible at the polymer concentration normally used in these experiments (< 100 ppm), the rate of radical formation is extremely small and the radicals are immediately separated by the velocity gradient at the very moment of their formation. Thus there is no cage effect, which otherwise could enhance the recombination efficiency. 

A detailed analysis of Koenig, Hay, and Finke s model is outside the scope of this book. Only one of its main conclusions is quoted here. If, for a given solvent, F is the recombination efficiency of D and E in the cage (0 < F < 1), then... 

The photoconductivity and absorption spectra of PVC with pynacyanol as a sensitizer are given in Fig. 11 [59]. It can be seen that a photo response appears in the range of the absorption maximum of the monomolecular form of the dye. The results are typical for all sensitization data [60-62]. The addition of the dopant molecules leads to the change of photogeneration, transfer and recombination efficiency. The effectiveness of the sensitization increases with lowering of the first excited state of the dye molecule. 

If we compare the rate constants for the recombination of alkyl radicals ( 1010,5 liter/mole-sec.) with collision frequencies of these same radicals (1011-3 liter/mole-sec.) we are struck by the very high efficiency (1 in 6 collisions) of these recombination processes. For the younger generation of kineticists these values are by now well established and occasion no surprise. However, one has only to turn back in the literature some 15 or more years to discover that the older generation was quite prepared for recombination efficiencies of the order of 10 a to 10 4 while a number of respected workers anticipated activation energies of the order of 3 to 15 kcal. What was the origin of such speculations and why was the range so broad ... 

Marcus and Rice6 made a more detailed analysis of the recombination from the point of view of the reverse reaction, the unimolecular decomposition of ethane, C2Ha - 2CH3. By the principle of microscopic reversibility the transition states must be the same for forward and reverse paths. Although they reached no definite conclusion they pointed out that a very efficient recombination of CH3 radicals would imply a very high Arrhenius A factor for the unimolecular rate constant of the C2H6 decomposition which in turn would be compatible only with a very "loose transition state. Conversely, a very low recombination efficiency would imply a very tight structure for the transition state and a low A factor for the unimolecular decomposition. 

There are, however, a number of facts which make such a proposal untenable. In the first place, the fact that disproportionation is much faster than recombination by a factor of 3.2 for fer -butyl radicals indicates that such structures cannot be general since this would imply that for recombination structure which is less favored, is arrived at through the tighter 4-centered structure (reaction H) of the disproportionation state.f It is much more reasonable to assume for er -butyl that steric repulsion of CH3 groups lowers the recombination efficiency but does not affect the disproportionation rate which goes through a different transition state. 

Interrelationships of Excited-State Decay Routes. The iLV curves conveniently display the competitive nature of photocurrent and luminescence intensity as excited-state deactivation pathways. Our analysis is limited in the sense that we have obtained absolute numbers for X but have had to content ourselves with relative < > r measurements. We lack measures of nonradiative recombination efficiency (4>nr) although they now appear to be... 

As already said in the introduction, simultaneous doping with n- and p-type impurities represents a way to overcome the low radiative recombination efficiency in our systems, so, starting from the already described hydrogenated Si-NCs, and following the work of Fujii et al. [32-34], we have... 

Additional notes on the AdEasy system are available at www.coloncancer.org/ adeasy.htm. The most challenging step in the procedure is the generation of successful recombinants. Several factors affected recombination efficiency, including the quality of the preparation of the electrocompetent BJ5183 cells. We found that increasing the time of incubation of the cells on ice from 30 min to 1 to 2 h improved recombination efficiency. We typically isolated at least 10 colonies to ensure identification of a recombinant vector. 

FIGURE 7. The proposed effect of stress in the several stages of the decreased radical recombination efficiency hypothesis. 

At any start, the recombination efficiency levels off with the diffusion approaching plateau Z = Zq. This is actually the kinetic limit of recombination. It is reached when the fraction of ions that recombine in the reaction layer during the residence time there, xe = RL/D, is small ... 

According to Eqs. (3.320) and (3.195), the averaged charge separation quantum yield is expressed through the mean recombination efficiency Z(AG,D,D) ... 

Figure 3.39. The viscosity dependence of the recombination efficiency Z at fixed starts from the interior of the remote rectangular recombination layer (thin solid line), from inside of this layer (long dashed and dashed lines) and outside of it (dotted line). The true Z(D) dependence, calculated from cp by means of IET, is indicated by the thick solid line. The horizontal line represent Z = 0.0585 A2/ns. The parameters, defining the rectangular recombination layer are the following / = 1.1 a, re — 1.4a, Wo — 0.156ns-1, a — 10A. (From Ref. 151.)...

Figure 3.41. The diffusional dependence of the recombination efficiency Z in the contact approximation (dotted line) at starting distance ro — 1.124 a and the same for the remote recombination in a normal (solid line) and inverted (dashed line) Marcus region, in highly polar solvents. The horizontal dashed-dotted line represents the exponential model result, Z — z — const. (From Ref. 152.)...

To our knowledge, there are only two reactions of electron transfer, (3.329) and (3.330), studied in solvents of different viscosity. They were investigated separately and presented differently in Figures 3.23 and 3.38. These data, brought together in Figure 3.43, indicate an essential difference between the results related to reactions I and II. The recombination efficiency in reaction II... 

Figure 3.43. The diffusional dependence of the recombination efficiency Z in reactions (I) and (II) found experimentally in Refs. 148 and 151, respectively.

Now we are ready to explain the qualitative difference between reaction I, where the recombination efficiency monotonously increases with diffusion, and reaction II, where it passes through the maximum. This difference could not be attributed to either the different charge or spin states of the excited reactants nor... 

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