Lifetime of carriers

Polymer-electrolyte junctions, lifetimes of carriers with, 496... 

This classification based on water splitting is important to understanding the redox potential of a given semiconductor. Although this classification is simple, it is convenient in selecting a semiconductor that is appropriate for a desired reaction. For a more detailed reactor design, factors such as the lifetimes of carriers energy levels of surface states adsorption and desorption of molecules on the surface kinetic nature of the surface and electron kinetics must be considered (Serpone and Pelizzetti, 1989). 

Wang X, Liu G, Chen Z-G, Li F, Wang L, Lu GQ, Cheng H-M (2009) Enhanced photocatalytic hydrogen evolution by prolonging the lifetime of carriers in ZnO/CdS heterostructures. Chem Commun 45 3452-3454... 

Especially with LTG GaAs, materials became available that were nearly ideal for time-resolved THz spectroscopy. Due to the low growth temperature and the slight As excess incorporated, clusters are fonned which act as recombination sites for the excited carriers, leading to lifetimes of <250 fs [45], With such recombination lifetunes, THz radiators such as dipole anteimae or log-periodic spirals placed onto optoelectronic substrates and pumped with ultrafast lasers can be used to generate sub-picosecond pulses with optical bandwidths of 2-4 THz. Moreover, coherent sub-picosecond detection is possible, which enables both... 

The equihbtium lever relation, np = can be regarded from a chemical kinetics perspective as the result of a balance between the generation and recombination of electrons and holes (21). In extrinsic semiconductors recombination is assisted by chemical defects, such as transition metals, which introduce new energy levels in the energy gap. The recombination rate in extrinsic semiconductors is limited by the lifetime of minority carriers which, according to the equihbtium lever relation, have much lower concentrations than majority carriers. Thus, for a -type semiconductor where electrons are the minority carrier, the recombination rate is /S n/z. An = n — is the increase of the electron concentration over its value in thermal equihbtium, and... 

The increased lifetime of photogenerated minority carriers can be measured experimentally. This is shown for a single-crystal ZnO-electrode (Fig. 22). Both the stationary PMC peak and the potential-dependent lifetime in the depletion region, measured with transient microwave conductivity techniques are plotted.25 It is seen that the stationary PMC peak coincides with a peak in the lifetime of minority carriers. This... 

Therefore, no experimental knowledge is available on interfacial reaction mechanisms under such conditions. These now become accessible via PMC measurements. As theory shows [Fig. 13(b)], the PMC signals in the accumulation region are controlled by potential-dependent surface recombination and charge-transferrates, as well as by the bulk lifetime of charge carriers. 

This relation shows that the lifetime of PMC transients indeed follows the potential dependence of the stationary PMC signal as found in the experiment shown in Fig. 22. However, the lifetime decreases with increasingly positive electrode potential. This decrease with increasing positive potentials may be understood intuitively the higher the minority carrier extraction (via the photocurrent), the shorter the effective lifetime... 

Self-doped polymers, 334 Semicircles, Albery and Mount interpretation of, 584 Semiconductor electrodes with polymer layers, 499 diffusion length in, 492 Semiconductors, lifetime for carriers and, 495... 

In semiconductors, which have a bandgap, recombination of the excited carriers— return of the electrons from the conduction band to vacancies in the valence band—is greatly delayed, and the lifetime of the excited state is much longer than in metals. Moreover, in n-type semiconductors with band edges bent upward, excess electrons in the conduction band will be driven away from the surface into the semiconductor by the electrostatic held, while positive holes in the valence band will be pushed against the solution boundary (Fig. 29.3). The electrons and holes in the pairs produced are thus separated in space. This leads to an additional stabihzation of the excited state, to the creation of some steady concentration of excess electrons in the conduction band inside the semiconductor, and to the creation of excess holes in the valence band at the semiconductor-solution interface. 

The incorporation of elements as carbon, nitrogen, or germanium, however, leads to material with a low mobility and lifetime of charge carriers. This would limit the application of these alloys, and a large research effort has been undertaken to find ways around this problem. 

The mechanism of facilitated transport involves using the metal ion only in its reduced state in the oxidized state the oxygen-carrying capacity is virtually nil. It is thus natural that electrochemical processes should be attempted to improve both the flux and selectivity obtained with the membranes described above by exploiting this 02 capacity difference. For example, the best of the ultra-thin membranes developed by Johnson et al. [24] delivered oxygen at a rate equivalent to a current density of only 3 mA/cm2, at least an order lower than that achievable electrochemically. Further, the purity was but 85% and the lifetime of the carrier less than a year. 

The main problem to be overcome with these organic-ligand oxygen-transfer processes is the lifetime of the carrier, which is measured in weeks and not months or years, although improvements are continuing. 

W., Lifetime of neutral-carrier-based liquid membranes in aqueous samples and blood and the lipophilicity of membrane components, Anal. Chem. 1991 63 596-603. 

E. Bakker and E. Pretsch, Lipophilicity of tetraphenylborate derivatives as anionic sites in neutral carrier-based solvent polymeric membranes and lifetime of corresponding ion-selective electrochemical and optical sensors. Anal. Chim. Acta 309, 7-17 (1995). 

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