Carrier separation

Space charge layers and contact potential for efficient charge carrier separation can be achieved with proper semiconductor structure in several ways. When possible semiconductor structures are considered, the charge separation can be attained in an active mode, i.e., by the use of a potential bias in a photoelectrochem-ical cell, or in a passive mode, i.e., with the use of proper contact between different phases. 

Powders give statistically mixed phases and, possibly, spatially unseparated reduction and oxidation sites, as well as poor space charge layers for carrier separation. This leads to high rates of bulk and surface recombination, as well as solution species back reactions. Light scattering losses add a further decrease in... 

In chromatography the components of a mixture are separated as they pass through a column. The column contains a stationary phase which may be a packed bed of solid particles or a liquid with which the packing is impregnated. The mixture is carried through the column dissolved in a gas or liquid stream known as the mobile phase, eluent or carrier. Separation occurs because the differing distribution coefficients of the components of the mixture between the stationary and mobile phases result in differing velocities of travel. 

Nanocrystalline semiconductor thin film photoanodes, commonly comprised of a three dimensional network of inter-connected nanoparticles, are an active area of photoelectrochemistiy research [78-82] demonstrating novel optical and electrical properties compared with that of a bulk, thick or thin film semiconductor [79,80]. In a thin film semiconductor electrode a space charge layer (depletion layer) forms at the semiconductor-electrolyte interface charge carrier separation occurs as a result of the internal electric... 

This has implications for the design of high-surface-area solar cells in general If the bulk of the device is essentially field-free at equilibrium, then mobile electrolyte and nanoporosity are required to eliminate the photoinduced electric fields that would otherwise inhibit charge-carrier separation. On the other hand, if the particle size is substantially larger than in the conventional dye cell or if there is no mobile electrolyte, then an interfacial or bulk built-in electric field... 

Taking a general view of the above studies, we note that Chl-coated metal (platinum) electrodes commonly function as photocathodes in acidic solutions, although the photocurrent effcien-cies tend to be lower compared to systems employing semiconductors. This cathodic photoresponse may arise from a p-type photoconduc-tive nature of a solid Chi layer and/or formation of a contact barrier at the metal-Chl interface which contributes to light-induced carrier separation and leads to photocurrent generation. 

Zhivkova, S., Dimitrov, K., Kyuchoukov, G. and Boyadzhiev, L. (2004) Separation of zinc and iron by pertraction in rotating film contactor with Kelex 100 as a carrier. Separation and Purification Technology,... 

Li, S.-J., Chen, H.-L. and Zhang, L. (2009) Recovery of fumaric acid by hollow-fiber supported liquid membrane with strip dispersion using N7301 as carrier. Separation and Purification Technology, doi 10.1016/j. seppur.2008.12.004. 

Fig. 10.28. Model of charge carrier separation and charge transport in a nanocrystalline film. The electrolyte has contact with the individual nanocrystallites. Illumination produces an electron-hole pair in one crystallite. The hole transfers to the electrolyte and the electron traverses several crystallites before reaching the substrate. Note that the photogenerated hole always has a short distance (about the radius of the particle) to pass before reaching the semiconductor/electrolyte interface wherever the electron-hole pair is created in the nanoporous film. The probability for the electron to recombine will, however, depend on the distance between the photoexcited particle and the tin-coated oxide back-contact. (Reprinted with permission from A. Hagfeldt and Michael Gratzel, Light-Induced Redox Reactions in Nanocrystalline Systems Chem. Rev. 95 49-68, copyright 1995, American Chemical Society.)...

The authors included that the excited states created by the optical transition at peak absorption ( 620 nm) are not involved in carrier generation. Very interesting is the observation that photocurrents increased with decreasing temperature to the peak value at 175—225 °K. They proposed that carrier mobilities increase with decreasing temperature and that the increase is high enough to offset the exponential decrease in carrier separation and lifetime. 

Fig. 16.10 Coupling two different semiconductor particles and charge-carrier separation...

In organic cells, however, the steps involved in the generation of photo-current are (1) light absorption, (2) exciton creation, (3) exciton diffusion, (4) exciton dissociation in the bulk or at the surface, (5) field-assisted carrier separation, (6) carrier transport, and (7) carrier delivery to external circuit. Assuming that only the excitons which reach the junction interface produce free carriers, if the blocking contact is illuminated [65],... 

By the HTOF technique, an interference pattern from two ps or ns laser pulses creates a sinusoidal distribution of carriers. Under the influence of an applied field, the carriers separate. As the charge separation proceeds, a space-charge field is created that can be probed with a cw laser through the electrooptic effect. The space-charge field reaches a maximum when the carriers have drifted to a position of anticoincidence with the immobile distribution of carriers of opposite polarity. Further drift causes a decrease of the space-charge field until coincidence is reached again. The diffraction efficiency versus time shows oscillatory behavior. From the time tmax that corresponds to the first maximum, the mobility can be derived from the relationship... 

Figure 2.5 Schematic diagram showing approximate time sequence for the space-charge separation of the carriers, (a) Initial distribution. The photoelectrons and photoholes coincide spatially in each diagram, (b) The space-charge field lines prior to transport effects, (c) After 200 fs, the carrier distribution for a moderately doped p-type semiconductor has the minority electron carrier localised at the surface, (d) The field lines after carrier separation. There is a transient field line that creates a coulombic barrier to carrier diffusion into the space-charge region from the bulk. The blue area highlights the space-charge region.

In order to achieve higher photovoltages, the rates of photogenerated carrier separation, transport and interfacial transfer across the semiconductor interface must all be fast compared with the rate of carrier cooling (Boudreaux et al, 1980 Nozik, 1980 Williams and Nozik, 1984). The achievement of higher photocurrent requires that the rate of II (rn) or electron-hole pair multiplication is greater than the rates of... 

Fignre 5.6 illnstrates the temporal evolution of the transient spectra of one of the sols examined, the 23 A TiOi specimen, at varions delay times after excitation with the 30 ps, 2.5 mJ laser pulse (Serpone et al, 1995b). Subsequent to electron-hole pair formation and charge-carrier separation, in competition with recombination, a fraction of these carriers are trapped at lattice sites and some migrate to the surface in about 0.05-10 ps, depending on particle size, where they also get trapped to give Ti " species for the electron and Ti" -0 -Ti" - OH for the hole (Lawless et al., 1991). 

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