Semiconductors conduction band

Fig. 11. Schottky diode device used for measurement of chemicurrents. Highly exoergic surface reactions like adsorption of an atom to the surface produce excited electrons and holes. Some of these electrons are able to surmount the Schottky barrier and arrive at the semiconductor conduction band. This results in a detectable chemicurrent. (From Ref. 64.)...

Macroscopic n-type materials in contact with metals normally develop a Schottky barrier (depletion layer) at the junction of the two materials, which reduces the kinetics of electron injection from semiconductor conduction band to the metal. However, when nanoparticles are significantly smaller than the depletion layer, there is no significant barrier layer within the semiconductor nanoparticle to obstruct electron transfer [62]. An accumulation layer may in fact be created, with a consequent increase in the electron transfer from the nanoparticle to the metal island [63], It is not clear if and what type of electronic barrier exists between semiconductor nanoparticles and metal islands, as well as the role played by the properties of the metal. A direct correlation between the work function of the metal and the photocatalytic activity for the generation of NH3 from azide ions has been made for metallized Ti02 systems [64]. 

One of the most important DSSC component is the dye sensitizer that represents the electron pump of the device. It allows an independent electron injection into the semiconductor conduction band and conversion of visible and near-infrared (NIR) photons to electricity. 

In the case of a semiconductor electrode, the existence of the energy gap makes a qualitatively different location of energy levels quite probable (Figs. 23b, 23c). One of them, either the ground or excited, is just in front of the energy gap, so that the direct electron transition with this level involved appears to be impossible. This gives rise to an irreversible photoelectro-chemical reaction and, as a consequence, to photocurrent iph. The photoexcited particle injects an electron into the semiconductor conduction band... 

As mentioned, the photocatalytic activation of Ti02 requires UV irradiation, and hence the semiconductor performance in the solar spectrum is inefficient. A solution to switch the photocatalytic activity to the visible spectral region was described by covalent attachment of an eosin dye monolayer to the semiconductor oxide particles of a Pd-Ti02 catalyst.168 The improved photocatalytic activities, specifically, the efficient formation of formate, are attributed to the effective injection of electrons from the excited dye into the semiconductor conduction band. 

Consider the electronic energy diagram for an n-type semiconductor, as shown in Figure 4. The total potential energy in the semiconductor conduction band at x, relative to the corresponding band energy deep in the bulk may be expressed as ... 

The basic requirement of the redox oxidant in contact with n-type semiconductors is that it has an equilibrium potential more negative than the decomposition potential of the semiconductor and more positive than the lower edge of the semiconductor conduction band. The basic requirement of the reductant electrolyte is that its redox equilibrium potential be negative of the oxidant electrolyte and more positive than the lower edge of the semiconductor conduction band. More work will be necessary at characterizing solid electro-lyte/semiconductor interfaces with those solid electrolytes available before satisfactory solid-state devices capable of photocharge can be realized. 

The sensitization of semiconductors is a special example of electron transfer quenching and may prove to be very important. A photoexcited electron may, for example, be injected with high quantum yield into the semiconductor conduction band, to produce a photovoltaic device. The hole that is left behind may then perform some useful oxidation process. 

The probability of an electron transfer, pet, from an excited donor species to a semiconductor conduction band may be estimated from the following equation ... 

Electron-phonon interaction in a semiconductor is the main factor for relaxation of a transferred electron. There are two different relaxation processes that decrease the efficiency of light conversion in a solar system (1) relaxation of an electron from a semiconductor conduction band to a valence band and (2) a backward electron transfer reaction. The forward and backward electron transfer processes have been already included in the tunneling interaction, HSm-qd, described by Eq. (108). However, the effect of SM e-ph interaction is important for the correct description of electron transfer in the SM-QD solar cell system. In the previous section, we have gradually considered different types of interactions in the quantum dot and obtained the exact expression for the photocurrent (128) where the exact nonequilibrium QD Green s functions determined from Eq. (127) have been used. However, in... 

To maintain eqinlibrinm, there mnst also be a current in the opposite direction that opposes this forward rate, that is, electrons must also be able to leave the metal phase and enter the semiconductor conduction band. Because the electrons enter the empty states of the semiconductor, the concentration of these empty states can be taken as a constant. This leads to the expression... 

Figure 9. Three situations for an n-type semiconductor-electrolyte interface at equilibrium showing overlap of the redox energy levels with the semiconductor conduction band (a) with surface states (b) and with the semiconductor valence band (c). A discrete energy level is assumed for the surface states as a first approximation.

Figure 32. The relative energetic positions of semiconductor conduction band edges, Ecb, for Sn02, ZnO, and Ti02 relative to a sensitizer excited state, S. AE is the apparent energy separation between the conduction band edge and the excited state sensitizers reduction potential.

Cobalt(II) 4,4, 4",4" -tetrasulfophthalocyanine, covalently linked to the surface of titanium dioxide particles, Ti02-CoTSP, was shown [207] to be an effective photocatalyst for the oxidation of sulfur (IV) to sulfur (VI) in aqueous suspensions. Upon bandgap illumination of the semiconductor, conduction-band electrons and valence-band holes are separated the electrons are channeled to the bound CoTSP complex resulting in the reduction of dioxygen, while the holes react with adsorbed S(IV) to produce S(VI) in the form of sulfate. 

The basic theoretical framework for describing electron transfer in bulk solid/Uquid interfaces was developed in the 1960s (Marcus, 1965 Gerischer, 1970 Levich, 1970). Fundamentally, photoinduced electron injection from the molecular excited state to a semiconductor nanoparticle can be described as electron transfer from a discrete and localised molecular state to a continuum of delocalised k states in the semiconductor. As shown in Fig. 11.5, the reactant state corresponds to the electron in the molecular excited state and the product states correspond to the oxidised molecule and the transferred electron in the semiconductor conduction band. There is a continuous manifold of product states, corresponding to the injected electron at different electronic levels in the semiconductor. 

Fig. 2 presents a stack of IPE spectra taken at different Fe coverage, along with the reference spectra corresponding to the substrate (continuous line) and to a clean and well ordered Fe(OOl) surface (top spectra). The features A and D in the spectrum from clean ZnSe can be assigned to transitions between bulk states, as they display a sizable angular dispersion, typical of band-like states. The semiconductor behaviour is clearly evident from the delayed onset of the spectrum with respect to the Fermi level, Ep. The onset corresponds to the semiconductor conduction band minimum (CB), which, as estimated from the graphic extrapolation shown in Fig. 2,... 

The selection of a photocatalyst offers special challenges. Aside from the more conventional Ti02. there are other available photocatalyst choices Ti02/Pt or ZnO. This is pailiculai ly relevant as the overall rale of oxidative degradation appears to be limited by the electron transfer from the valence band to the semiconductor conduction band (Linsebigler et al., 1995 Choi el ah, 1994). 

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