Surface recombination and charge transfer

If recombination in the neutral region is negligible, the steady-state concentration of minority carriers in the surface region (cm ) is determined by the incident photon flux Jo (cm s ) and the rate constants for interfacial charge transfer and surface recombination (both cm s ). 

The microwave conductivity is proportional to the number of free electronic charge carriers multiplied by their respective mobilities (the contribution of ions and dipoles can be neglected in a first approximation.). The microwave sensitivity constant S must be obtained by calibration. The potential-dependent photoinduced microwave conductivity (PMC) of a semiconductor resulting from illumination has been calculated analytically as a function of the interfacial charge-transfer and surface recombination rates (Schlichthorl and Tributsch, 1992). The starting point is the general set of equations of the form... 

Separation of Charge Transfer and Surface Recombination Rate... 

Thus, the recorded signal corresponds to the integral of the excess minority carrier profile throughout the sample modulated by the relative carrier mobilities. This finding was the basis for the analysis of the charge transfer and surface recombination velocity profiles above (cf Figures 2.16 and 2.17). 

N.S. Lewis, Chemical control of charge transfer and recombination at semiconductor photoelectrode surfaces,... 

Recombination in the depletion layer can become important when the concentration of minority carriers at the interface exceeds the majority carrier concentration. Under illumination minority carrier buildup at the semiconductor-electrolyte interface can occur due to slow charge transfer. Thus surface inversion may occur and recombination in the depletion region can become the dominant mechanism accounting for loss in photocurrent. 

The accumulated surface charge Qs t) oscillates periodically, and the time dependent charging, transfer and recombination terms are replaced by the sums of their steady state and periodic equivalents. 

Time-resolved photoinduced microwave conductivity measurements can be made as a function of applied potential. It has been shown that the measured minority-carrier lifetime r for moderately fast or slow interfacial charge transfer depends not only on the interfacial rate constant and surface recombination Ukc, but also on the energy band bending (AE) and the Debye length Ld (Tributsch, 1999). 

The competition between charge transfer and recombination via surface states can be treated exactly by Hall-Shockley-Read statistics, taking proper account of... 

Schoenmakers et al. [77] have used IMPS, PEIS and potential dependent photoluminescence to study the competition between charge transfer and recombination at ZnO electrodes. Their results indicate that recombination in the bulk solid as well as via surface states is responsible for the reduction in photocurrent conversion efficiency in the photocurrent onset region. 

Guibaly et al.154-156 took into account not only charge transfer kinetics and surface recombination but also the recombination in the space charge region and the effect of separation between the quasi-Fermi level of minority carrier and that of majority carrier. 

Less vigorous approaches to describe surface recombination were taken by Kelly and Memming,158 Rajeshwar,134 and Peter et al.159,160 These authors considered the possibility of charge transfer via surface states. Thus, as shown in Fig. 20, the minority carriers which are trapped by surface states would either recombine with majority carriers (surface recombination via surface states) or transfer to the electrolyte (charge transfer via surface states or surface state mediated charge transfer). There are many experimental results which support the surface state mediated charge transfer under illumination134,158-161 and in the dark.162-164... 

Khan and Bockris167 presented a model which accounts for charge transfer kinetics and surface recombination as well as potential drop in the Helmholtz layer. In their paper, they estimated the transit time across the space charge region and the lifetime of the minority carrier and showed that it is reasonable to neglect the... 

In a model that considers the situation at the semiconductor-electrolyte contact, the charge transfer rate and the surface recombination velocity have to be included [59]. In such a model, the currents related to charge transfer and to surface recombination are expressed by the excess carrier concentration at the surface (x = 0) according to... 

Fig. 11.7 Important processes in the surface photocatalysis processes photoexcitation of electron-hole pair, charge transfer processes, bulk and surface recombination processes, and electron- and charge-induced chemistry at surfaces (Reproduced with permission from Ref [27]. Copyright 1995 American Chemical Society)...

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