Photocurrent current onset

The result of this analysis is a plot of photocurrent density as a function of the potential measured versus a reference electrode. An example j—V curve for a WO3 film is shown in Fig. 6.10. In this case, the photocurrent onset occurs at approximately 0.27 V versus SCE (0.54 V vs. RHE) which corresponds to a VonsecoER of 0.69 V (1.23-0.54 V). Since the onset of photocurrent does not occur cathodic of the reversible potential for HER ( er = —0.27 V versus SCE in this electrolyte), this electrode is unable to split water without an additional bias. At potentials more anodic than 1.2 V versus SCE, the photocurrent density saturates at 3.5 mA/cm. In addition to photocurrent, reverse bias dark current onsets at 1.65 V versus SCE due to shunting or breakdown as mentioned previously. 

As shown in Fig. 10-9, the photoexcited reaction current occurs only when an appreciable electric field exists in the space chai ge layer. No photocurrent occurs at the flat band potential because no electric field that is required to separate the photoexcited electron-hole pairs is present. The photocurrent occurs at any potentials different from the flat band potential hence, the flat band potential may be regarded as the potential for the onset of the photocurrent. It follows, then, that photoexcited electrode reactions may occur at potentials at which the same electrode reactions are thermodynamically impossible in the dark. 

Figure 28.5 Current-potential curves for p-GaP under low- to moderate-intensity illumination a 1 M NaCl (pH = 1) electrolyte is employed. Illumination is from a 200-W high-pressure mercury lamp filtered with neutral density filter. Intensity is relative to the full lamp output. The H2/H+ redox potential is -0.3 V vs. SCE in this cell. Thus, this cell yields approximately 400 mV of open-circuit photovoltage. Note that increased illumination increases both the saturation photocurrent and the onset potential. Although the photocurrent is increased at higher light intensities, a calculation of the quantum yield for electron flow indicates that this parameter decreases with increased light intensity.

The photocurrent onset potential is often taken as the flatband potential, since the measurement of the flatband potential is typically only good to 100 mV and the onset of photocurrent is often observed with less than 100 mV of band bending. This practice is dangerous, however, since the onset potential is actually the potential at which the dark cathodic current and the photoanodic current are equal. Even though in the case of the p-GaP illustration, the observation of an anodic current and a photocathodic current are separated by several hundred millivolts, in many systems these two currents overlap. In those cases, the relationship between the flatband potential and the onset potential becomes unclear. 

When an n-CdS electrode is suddenly illuminated with light capable of producing holes in the CdS, j, would almost immediately reach some large value (equal to or less than the saturation current of curve 1) and then decay to the steady state value of curve 1 as the steady state value of N is approached according to equation 3. If such a transient does not occurthe oxidized corrosion site acting as a recombination state is not the controlling factor in the photocurrent onset. 

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. 

The Effect of Illumination. In an alkaline solution, an n-GaP electrode, (111) surface, under illumination shows an anodic photocurrent, accompanied by quantitative dissolution of the electrode. The current-potential curve shows considerable hysterisis as seen in Fig. 2 the anodic current, scanned backward, (toward less positive potential) begins to decrease at a potential much more positive than the onset potential of the anodic current for the forward scanning, the latter being slightly more positive than the Ug value in the dark, Us(dark). 

A typical time response for a short-circuited photocurrent in the presence of hydroquinone ( Q) as an added solution redox species is shown in Figure 9. These photocurrents were stable for several hours. In the absence of in the electrolyte, the photocurrent also increased rapidly upon the onset of illumination, but subsequently decayed exponentially to 70% of its initial value in a half-decay time of ca. 25 s. This behavior is similar to that observed for chlorophyll monolayers deposited on SnC (12). Photocurrents under potentially-controlled conditions were also stable upon illumination, but exhibited slower decay characteristics when the light was turned off. This effect is unusual and is currently under further investigation. 

In I/E curves the onset of photocurrent is expected from classical theories to occur near the Hatband potential as measured in the dark (Efb (d)), i.e. where the majority carrier current starts too. However, a large shift of the onset potential is seen especially if no additional redox couple is present in the aqueous electrolyte, in cathodic direction for p-, in anodic direction for n-type materials (Fig. 1). This shift depends on the light intensity but saturates already at relatively low intensities (Memming, 1987). If minority carrier acceptors (oxidants for p- and reductants for n-type semiconductors) are added to the solution, the onset can be shifted back to Efb (d) if they have the appropiate redox potential. In principal two types of redox couples can be found those which lead to a shift of the photocurrent onset potential and those which don t. The transition between the two classes occurs at a specific redox potential. 

It should be mentioned further, that the shift of energy bands upon creation of minority carriers does not only occur upon light excitation. Holes can also be produced in the dark via hole transfer from a hole donor into valence band. This kind of process occurs for instance by using the oxidized species of the redox couple [Ru(bipy)3]2+/3+. A corresponding cathodic dark current starts at the same potential at which the photocurrent onset was found in the presence of the reduced form of the same couple. This shift of Efb by hole injection from the electrolyte has been found also with Ce4+ at n-WSe2 (McEvoy et al., 1985) and n-GaAs (Schroder et al., 1985). 

The experimental observations217 of an apparent light intensity threshold for the photocurrent onset have been rationalized218 on the basis that a critical photon flux must be exceeded to counteract the dark current of opposite polarity flowing through the cell. Thus, there appears to be confusion between alternate definitions of a light intensity threshold a threshold for incipient product (say H2) formation and a threshold for product formation in a specific (e.g., standard) state,218... 

They checked this mechanism by using TiOj-electrodes instead of particles and found that the 2-phenylindazole was only formed in a small potential range around the photocurrent onset, i.e. at potentials at which also a comparable cathodic current occurs. At more anodic potentials, the hydroxymethyl radical is further oxidized [175]. It should be mentioned here that the oxidation of CH3OH (Eq. (83)) or other organic compounds must not necessarily occur via direct hole transfer. There are strong indications that at first an OH -radical is formed at the surface of Ti02-particles by hole transfer, and that this radical oxidizes the organic molecule in a second step, as proved in the case of acetate oxidation [176]. 

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... 

The current-potential curve for the p-InP photocathode under illumination in CO2 (40 atm)-methanol exhibited a relatively large photocurrent (solid line), while the dark current was negligibly small (dotted line, < 1 mA cm 2) at potentials down to -2.0 V vs. Ag-QRE (Fig. 1). The onset photopotential was approximately -0.6 V. When CO2 was replaced with Ar, the onset of the cathodic photocurrent shifted toward the negative direction by 0.4 V (dashed line). This indicates that, in the highly concentrated CO2 solution, CO2 reduction on the p-InP surface occurs in preference to the reaction occurring under Ar atmosphere, which is predominantly hydrogen evolution. The cathodic photocurrent reached 20 mA (approximately 100 mA cm 2) at a potential of -2.4 V vs. Ag-QRE. 

Reaction intermediates are a special group of surface states that can cause band edge shift (or Fermi level pinning). When this occurs, the flatband potential tends to change with current. Figure 2.32 shows that the flatband potential of -type silicon in O.IM K4Fe(CN)6 + 0.5 M KCl changes with photocurrent which induces surface states. The shift of 0.55V shown in Fig. 2.32 corresponds to a density of 2 x 10 cm. This result can be used to explain the difference between the flatband potentials determined by Mott-Schottky plot and by measurement of the onset potential for photocurrent... 

The photocurrent in nonfluoride solutions is affected by the amount of preanodic current passed through the sample as shown in Fig. 5.10. It is also seen that the photocurrent onset potential is shifted to more anodic values with formation of an oxide film and the amount of shift is related to the thickness of the film. This shift is due to the potential drop across a growing oxide layer and is one of the reasons for the difference between the photocurrent onset potential and the flatband potential. ... 

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