Photocurrent onset, voltage

One interpretation presumes that the photocurrent onset in the absence of sulfide is determined by electron-hole recombination. The sulfide ions on the surface are then supposed to be bound to these surface recombination levels rendering them unavilable for recombination reactions. The charge transfer reactions could then proceed at lower voltages. In this case the corrosion suppression role of the sulfide ions would be to reduce the oxidized corrosion site before a cadmium ion could go into solution. A variation on this theme is to consider the corrosion site to be the recombination state, i.e., the site on the surface that normally leads to corrosion when oxidized by a photoexcited hole can be... 

Regardless of the nature of the surface state it is clear that it can capture an electron from the conduction band producing cathodic current. This cathodic current balances the anodic current produced when the photoexcited holes produced the oxidized surface state. The net result of these two processes is electron-hole recombination leading to no net current. This recombination process is what controls the voltage of photocurrent onset as can be seen in curve 2 of Figure 5. 

When illuminated with energy equal to or above the band gap hv > Eg) at these operating potentials, minority hole carriers in n-type electrodes drive the OER at the electrode-electrolyte interface while minority electron carriers in p-type electrodes drive the HER at this interface. The potential at which this phenomenon begins to occur is the photocurrent onset potential (Eonset). which is offset relative to the flat-band potential (Efb) by the required kinetic overpotentials for the reaction of interest. The difference between the photocurrent onset potential (Eonset) and the reversible redox potential of interest (E°) is the onset voltage (Eonset)- A band diagram of a n-type photoanode and its hypothetical j-V response is shown in Fig. 6.7. 

This is only possible if the factor k is known. Roy et al. obtained it from the slope of the voltage/pH plot above the inflection point whereas Bard et al. calculated it from the slope of the onset of photocurrent vs. pH (26). However, only poorly reproducible values were obtained by the former and latter method due to high voltage fluctuations and too low photocurrents, respectively. We have determined the k value by a new method through variation of the redox couple (A,2 +/A, + ). In this case a linear relation between the pH0 value and the redox potential of the pH-independent redox couple is expected [Eq. (4)]. 

This brings us to the rear support-film interface. What sort of barrier exists at this junction Are the electron exchange kinetics voltage-dependent at this interface The effect of changing the work function of the substrate on the current-voltage curves (in the dark and under illumination) has been investigated for Ti02 nanocrystalline films [344]. The onset potential for the photocurrent is found to be the... 

At present there is no unambiguous method for determining the energetic position of the conduction band edge of nanocrystalline semiconductor films under experimental conditions relevant to dye sensitization. Clearly, the lack of a depletion layer precludes traditional capacitance measurements with Mott-Schottky analysis described in Section 1.1. The onset of photocurrents in photocurrent-voltage plots can provide a crude working estimate under some conditions, but is problematic for the reasons described previously. This is unfortunate as accurate energetics are necessary to compare experimental results with even the simplest theory [4[. 

It is interesting to note that the onset of photocurrent is substantially shifted to a negative potential in the presence of 0.IM TBAP. The ammonium ion mediated reduction is obvious in the current-voltage curves. 

Figure 9.46 Photocurrent voltage response of suspended TiOj particles at InjOj electrode. The anodic photocurrent indicates the electron transfer from the particles to electrode. From the onset, the potential for the photogenerated electrons was estimated to be —0.46 V vs. NHE. Reprinted with permission from reference (27). Cop5uight 1981, American Chemical Society.

Fig. 9 Current-voltage curves showing the shift of the onset of photocurrent from the flat-band potential (VfbK...

The photocurrent stimulated is limited in all of these processes by die rate of carrier production (light intenaty dependent) or by the carrier mobilities, or both. Systems in which the current is limited entirely by the rate of carrier gmeratim are termed emission limited, whilst if carrier mobility determines the current it is space-charge limited (SCLC). Indicative of the onset of SCLC is supralinearity in the current-voltage dependence theoretically I then varies as V. ... 

---