P-type electrodes

In conclusion it should be mentioned that the same type of effects are possible for p-type electrodes. In this case an anodic dark current occurs whereas the photocurrent corresponds to an electron transfer via the conduction band (cathodic plEiotocurrent). 

Finally cells containing a p-type semiconductor electrode should be mentioned. In principle the application of p-type electrodes would be even more favorable because electrons created by light excitation are transferred from the conduction band to the redox system. Stability problems are less severe because most semiconductors do not show cathodic decomposition (see e.g. earlier review article. However, there is only one system, p-InP/(V " /V ), with which a reasonable efficiency was obtained (Table 1) . There are mainly two reasons why p-electrodes were not widely used (i) not many materials are available from which p-type electrodes can be made (ii)... 

Frequently it has been observed with n-type as well as with p-type electrodes in aqueous solutions that the onset potential of the pure photocurrent differs considerably from the flatband potential. The latter can be determined by capacity measurements in the dark as illustrated by the dashed line in the ij — Ub curve in Fig. 8 a. This effect is usually explained by recombination and trapping of minority carriers created by light excitation at the surface. It is obvious that these effects have a negative effect... 

The photoelectrolysis of H2O can be performed in cells being very similar to those applied for the production of electricity. They differ only insofar as no additional redox couple is used in a photoelectrolysis cell. The energy scheme of corresponding systems, semiconductor/liquid/Pt, is illustrated in Fig. 9, the upper scheme for an n-type, the lower for a p-type electrode. In the case of an n-type electrode the hole created by light excitation must react with H2O resulting in 02-formation whereas at the counter electrode H2 is produced. The electrolyte can be described by two redox potentials, E°(H20/H2) and E (H20/02) which differ by 1.23 eV. At equilibrium (left side of Fig. 9) the electrochemical potential (Fermi level) is constant in the whole system and it occurs in the electrolyte somewhere between the two standard energies E°(H20/H2) and E°(H20/02). The exact position depends on the relative concentrations of H2 and O2. Illuminating the n-type electrode the electrons are driven toward the bulk of the semiconductor and reach the counter electrode via the external circuit at which they are consumed for Hj-evolution whereas the holes are dir tly... 

The OCP etch rate of p-type and highly doped n-type Si electrodes in HF-HNO3 mixtures increases by an order of magnitude under sufficiently anodic bias [Le20]. In the cathodic regime significant dark-currents are observed for p-type electrodes, as shown in Fig. 4.12. This is ascribed to hole injection from the electrolyte [Kol4]. Note that hole injection is not observed in aqueous HF free of oxidants. 

Fig. 3.2 The inset (center left) shows the electrochemical double-cell set-up with the two applied potentials VEB and VBc, which constitute a circuit similar to a solid-state bipolar transistor. The emitter-base current /EB (full line) of a moderately doped p-type electrode illuminated corresponding to a photocurrent of 10 mA cm-2 is shown in the upper part of the figure. Below /EB is shown for an n-type electrode illuminated with an intensity corresponding to 90 mA cmf2. The base-...

Fig. 3.3 The I—V curves, as recorded and compensated for ohmic losses (iRcor.), of Si electrodes in aqueous HF (1M HF, 0.5M NH4CI) are found to shift cathodically with increasing p-type doping density. In a V versus log(i) plot (inset) a p-type electrode (1 2 cm,...

For the p-type substrate a significant number of electrons are collected at the backside, as shown in the top part of Fig. 3.2. This is true not only for the illuminated p-type electrode but also if the electrode is kept in the dark, which indicates that electrons are injected during the tetravalent dissolution reaction. In the regime of oscillations the electron injection current is found to oscillate, too [CalO]. 

For higher VEB (>7V) electron injection increases and fBC oscillates when fEB does. Capacitance measurements indicate a surface charge of holes at the front, this means a p-type electrode is under accumulation. 

Fig. 4.6 Di ssolution valence nv as a function of anodic current density for micropore formation on low doped p-type electrodes anodized in ethanoic HF (1 1, ethanol HF 50%).

Because JPS is limited by reaction kinetics and mass transport a dependency on the HF concentration cHf and the absolute temperature Tcan be expected. An exponential dependence of JPS on cHf has been measured in aqueous HF (1% to 10%) using the peak of the reverse scan of the voltammograms of (100) p-type electrodes. If the results are plotted versus 1/7) a typical Arrhenius-type behavior... 

A p-type electrode is in depletion if a cathodic bias is applied. Illumination generates one electron per absorbed photon, which is collected by the SCR and transferred to the electrolyte. It requires two electrons to form one hydrogen molecule. If the photocurrent at this electrode is compared to that obtained by a silicon photodiode of the same size the quantum efficiencies are observed to be the same for the solid-state contact and the electrolyte contact, as shown in Fig. 4.13. If losses by reflection or recombination in the bulk are neglected the quantum efficiency of the electrode is 1. 

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

Fig. 4.13 Number of observed charge carriers per absorbed photon as a function of the current density. The photoinduced current at n-type electrodes in HF (squares) is increased compared to a photodiode or p-type electrode...

The photocurrent doubling discussed above can be understood as a consequence of the divalent dissolution reaction as shown in Fig. 4.3. Dissolution for current densities below JPS is initiated by a hole in step 1 and proceeds under injection of an electron in step 2. For the case of an n-type electrode, one photon is required to generate one hole, but the electron injected in the dissolution process doubles the current without consumption of another photon. Hence the resulting current density is twice as large as observed at a reference photodiode. Because step 2 of the reaction depicted in Fig. 4.3 is independent of type of doping it can be concluded that electron injection also takes place at p-type electrodes. There is, however, no simple way to detect these injected electrons because the electrode is under depletion in this regime, as discussed in Section 3.2. 

For p-type electrodes with doping densities below 1018 cm-3 diffusion and thermionic emission of charge carriers across the SCR is dominant. For p-type doping densities below 1016 cm4 this charge transfer is associated with the formation of macropores, as discussed in Chapter 9. 

Fig. 7.4 XRD signal for powder samples prepared from bulk Si (top) and micro PS films produced on p-type electrodes using different anodization current densities, as indicated in...

In contrast to p-type electrodes, an n-type electrode is under reverse conditions in the anodic regime. This has several consequences for pore formation. Significant currents in a reverse biased Schottky diode are expected under breakdown conditions or if injected or photogenerated minority carriers can be collected. Breakdown at the pore tip due to tunneling generates mainly mesopores, while avalanche breakdown forms larger etch pits. Both cases are discussed in Chapter 8. Macropore formation by collection of minority carriers is understood in detail and a quantitative description is possible [Le9], which is in contrast to the pore formation mechanisms discussed so far. 

M NH4C1 are determined to be 0.14 V (SCE) and -0.54 V (SCE), respectively [Otl], A similar value of -0.35 V (SCE) is observed for n-type Si in 1 M HF by microwave reflectivity measurements [Na7]. Figure 10.3 summarizes values of Vh, obtained by different methods. Note that the scatter in these data is much larger for p-type silicon electrodes than for n-type. A similar scatter has been observed in the determination of the OPC potential of p-type electrodes, which is found to be more sensitive to parameters such as, for example, illumination intensity than that of n-type electrodes, as discussed in Section 3.2. 

Figure 5-47 shows the Mott-Schottky plot of n-type and p-type semiconductor electrodes of gallium phosphide in an acidic solution. The Mott-Schottl plot can be used to estimate the flat band potential and the effective Debye length I D. . The flat band potential of p-type electrode is more anodic (positive) than that of n-type electrode this difference in the flat band potential between the two types of the same semiconductor electrode is nearly equivalent to the band gap (2.3 eV) of the semiconductor (gallium phosphide). 

As the polarization (the overvoltage t) ) increases of a redox reaction that requires the transport of minority charge carriers towards the electrode interface (anodic hole transfer at n-type and cathodic electron transfer at p-type electrodes), the transport overvoltage, t)t, increases from zero at low reaction currents to infinity at high reaction current at this condition the reaction current is controlled by the limiting diffusion current (iu.)tm or ip.um) of minority charge carriers as shown in Fig. 8-25. 

For p-type electrodes, the cathodic current is carried at low overvoltages by the minority carriers (electrons) in the conduction band and is controlled at high overvoltages by the limiting current of electron diffusion the anodic current is carried by the mtqority carriers (holes) in the valence band and the concentration of interfacial holes increases with increasing anodic overvoltage until the Fermi level is pinned in the valence band at the electrode interface, where the anodic current finally becomes an electron injection current into the electrode. 

Figure 10-17 shows the polarization ciirves for the cathodic hydrogen reaction (cathodic electron transfer) on a p-type semiconductor electrode of galliiun phosphide. The onset potential of cathodic photoexcited hydrogen reaction shifts significantly from the equilibrium electrode potential of the same hydrogen reaction toward the flat band potential of the p-type electrode (See Fig. 10-15.). 

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