Semiconductors flat-band potential

FIGURE 22.5 Electrode potential consisting of interfacial potential A(f>H and space charge potential Ac()sc for an intrinsic semiconductor = flat band potential, v = valence band edge potential, and... 

Figure 3.23 Semiconductor flat-band potentials and the open circuit photovoltage (Toe) of dye-sensitised semiconductor cells measured under 520 nm monochromatic light. Reprinted from Sayama et al., (1998) . Copyright (1998) American Chemical Society...

Fig. V-14. Energy level diagram and energy scales for an n-type semiconductor pho-toelectrochemical cell Eg, band gap E, electron affinity work function Vb, band bending Vh, Helmholtz layer potential drop 0ei. electrolyte work function U/b, flat-band potential. (See Section V-9 for discussion of some of these quantities. (From Ref. 181.)...

Primarily connected to corrosion concepts, Pourbaix diagrams may be used within the scope of prediction and understanding of the thermodynamic stability of materials under various conditions. Park and Barber [25] have shown this relevance in examining the thermodynamic stabilities of semiconductor binary compounds such as CdS, CdSe, CdTe, and GaP, in relation to their flat band potentials and under conditions related to photoelectrochemical cell performance with different redox couples in solution. 

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

Fig. 5.8 The energy levels of n-type M0S2 at the flat band potential relative to the positions of various redox couples in CH3CN/[n-Bu4N]C104 solution. The valence band edge of the semiconductor as revealed by accurate flat band potential measurement is at ca. +1.9 V vs. SCE implying that photooxrdations workable at Ti02 are thermodynamically possible at illuminated M0S2 as well. (Reproduced with permission from [137], Copyright 2010, American Chemical Society)...

Lemasson P, Dalbera JP, Gautron J (1981) Flat band potential determination of an elec-trolyte/semiconductor junction by an electro-optical method. J Appl Phys 52 6296-6300... 

Fig. 4.12 Dependence of concentrations of negative charge carriers (ne) and positive charge carriers (np) on distance from the interface between the semiconductor (sc) and the electrolyte solution (1) in an w-type semiconductor. These concentration distributions markedly differ if the semiconductor/electrolyte potential difference A cp is (A) smaller than the flat-band potential AF

Because of the adsorption equilibrium for H+ and OFT ions between the surface of semiconductors and an aqueous (aq) solution, the semiconductor surface attains the point of zero charge (PZC). The flat-band potential U[h of most semiconductors including all oxides and also other compounds such as n- and p-type GaAs, p-type GaP, and n- and p-type InP in an aqueous solution is determined solely by pH and shifts proportionately with pH with a slope of -59 mV/decade, that is, pH, for example,... 

Flat-Band Potentials and Positions of the Valence Band Maximum Evs and Conduction Band Minimum Ecs of Oxide Semiconductors, Group IV and III/V Semiconductors, and Mixed Oxide Semiconductors with Respect to the H+/H2 Scale, Where Minus Represents above Zero and Plus Represents below Zero... 

Energy level diagram for an n-type semiconductor-metal photoelectrolysis cell in which the flat-band potential lf(b lies above the H+/H2 potential, whereas the 02/H20 potential lies above the valence band of the n-type semiconductor. 

Direct splitting of water can be accomplished by illuminating two interconnected photoelectrodes, a photoanode, and a photocathode as shown in Figure 7.6. Here, Eg(n) and Eg(p) are, respectively, the bandgaps of the n- and p-type semiconductors and AEp(n) and AEF(p) are, respectively, the differences between the Fermi energies and the conduction band-minimum of the n-type semiconductor bulk and valence band-maximum of the p-type semiconductor bulk. lifb(p) and Utb(n) are, respectively, the flat-band potentials of the p- and n-type semiconductors with the electrolyte. In this case, the sum of the potentials of the electron-hole pairs generated in the two photoelectrodes can be approximated by the following expression ... 

For Eredox more negative than the flat band potential (b) there is little band bending (no barrier) and the electrode is reversible with respect to the redox couple and is in ohmic contact (a). For Ere between the flat band potential and some positive potential represented in (c) the drop in potential occurs across the semiconductor and the behavior is ideal because the band bending varies following Equation 1. For more positive Ereadditional potential drop across the semiconductor does not occur because the semiconductor is inverted at the surface, and the band edges effectively shift more positive as the potential drop occurs across the Helmholtz... 

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