Recombination surface/interface

The resulting output characteristic for Pt and Rh nanoisland catalysts (Figure 2.102) shows a rather sluggish increase of the cathodic photocurrent, indicative of surface recombination, and, at the potential of the hydrogen redox couple, current saturation has not yet been reached. Even at 0.2V cathodic from Er,o (H2/H ), where band bending is increased, carrier collection does not yet reach a saturation value. The low observed efficiencies are attributed to pronounced surface/interface recombination and to the reduced contact potential difference between p-Si and... 

Figure 12. Energy diagram of a semiconductor/electrolyte interface showing photogeneration and loss mechanisms (via surface recombination and interfacial charge transfer for minority charge carriers). The surface concentration of minority...

Figure 14. PMC potential dependence, calculated from analytical formula (18) for different interfacial rate constants for minority carriers S = 1 cm, minority carrier flux toward interface I,- 1 cm-2s 1, a= 780enr1, L = 0.01 cm, 0=11.65 cmV, Ld = 2x 0"3cm), (a) sr = 0 and different charge-transfer rates (inserted in the figures in cm s 1), (b) Constant charge-transfer rate and different surface recombination rates (indicated in the figure).

Experimental evidence with very different semiconductors has shown that at semiconductor interfaces where limited surface recombination and a modest interfacial charge-transfer rate for charge carriers generate a peak... 

Surface recombination processes of charge carriers are mechanisms that cannot easily be separated from real semiconductor interfaces. Only a few semiconductor surfaces can be passivated to such an extent as to permit suppression of surface recombination (e.g., Si with optimized oxide or nitride layers). A pronounced dip is typically seen between the potential-dependent PMC curve in the accumulation region and the photocurrent potential curve (e.g., Fig. 29). This dip may be partially caused by a surface... 

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. 

Between 0.20 and 0.30 V, a decay of the initial photocurrent and a negative overshoot after interrupting the illumination are developed. This behavior resembles the responses observed at semiconductor-electrolyte interfaces in the presence of surface recombination of photoinduced charges [133-135] but at a longer time scale. These features are in fact related to the back-electron-transfer processes within the interfacial ion pair schematically depicted in Fig. 11. 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

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