Field carrier separation

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... 

Fig. 26. Schematic design of field flow fractionation (FFF) analysis. A sample is transported along the flow channels by a carrier stream after injection and focusing into the injector zone. Depending on the type and strength of the perpendicular field, a separation of molecules or particles takes place the field drives the sample components towards the so-called accumulation wall. Diffusive forces counteract this field resulting in discrete layers of analyte components while the parabolic flow profile in the flow channels elutes the various analyte components according to their mean distance from the accumulation wall. This is called normal mode . Particles larger than approximately 1 pm elute in inverse order hydrodynamic lift forces induce steric effects the larger particles cannot get sufficiently close to the accumulation wall and, therefore, elute quicker than smaller ones this is called steric mode . In asymmetrical-flow FFF, the accumulation wall is a mechanically supported frit or filter which lets the solvent pass the carrier stream separates asymmetrically into the eluting flow and the permeate flow which creates the (asymmetrical) flow field...

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 contrast to the thermodynamical theory, in simulations the process of recombination of free carriers which can result in the creation of CTEs was neglected. Near the threshold where the concentration of free carriers is small the contribution of this process to the number of CTEs will indeed be negligible. However, even at higher concentration, the effect of free carrier recombination can be reduced by applying an electric field along the interface. This field will separate electrons and holes and thus will create the photocurrent which has to be measured (see also subsection 11.3.4 below). 

---