Contacts semiconductor-redox electrolyte

If we leave the equilibrium situation and apply a voltage to such semiconductor electrodes in contact with redox electrolytes, the position of the band edges at the interface will normally not vary to a considerable extent but the space charge layer varies and with it the surface concentration of electrons or holes. Fig. 11.12... 

Figure III.4 Energy correlations in n type semiconductor electrode in contact with redox electrolyte at open circut.

Semiconductor - Electrolyte Interlace The electric field in the space charge region that may develop at the semiconductor electrolyte interface can help to separate photogenerated e /h 1 couples, effectively suppressing recombination. When a semiconductor is brought into contact with an electrolyte, the electrochemical potential of the semiconductor (corresponding to the Fermi level, Ey of the solid [50]) and of the redox couple (A/A ) in solution equilibrate. When an n-type semiconductor is considered, before contact the Ey of the solid is in the band gap, near the conduction band edge. After contact and equilibration the Ey will... 

Fig. 3.8 Electron energy distribution at the contact between a semiconductor and a redox electrolyte for two different redox systems at equilibrium, (a) n-type semiconductor, and (b) p-type semiconductor.

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. 

Fig. 16.9 CB and VB energy levels of several semiconductors. (The semiconductors are in contact with aqueous electrolyte at pH 1. The energy scale is indicated in electron volts using either the normal hydrogen electrode (NHE) or vacuum level as reference. On the right the standard potentials of several redox couples are presented against the standard hydrogen electrode potential.) [Reprinted by permission from Macmillan Publishers Ltd [Nature] (Gratzel 2001), copyright (2001)]...

Figure 5. (a) Energy levels in a semiconductor (left-hand side) and a redox electrolyte (right-hand side) shown on a common vacuum reference scale. / and are the semiconductor electron affinity and work function, respectively, (b) The semiconductor-electrolyte interface before (left) and after (right) equilibration (i.e., contact of the two phases) shown for an n-type semiconductor, (c) As in (b) but for a p-type semiconductor. 

When a semiconductor is immersed in this redox electrolyte, the electrochemical potential (Fermi level) is disparate across the interface. Equilibration of this interface thus necessitates the flow of charge from one phase to the other and a bandbending ensues within the semiconductor phase. The situation before and after contact of the two phases is illustrated in Figure 5b and c for an n-type and p-type... 

Let us return to the equilibrium situation of an n-type semiconductor in contact with a redox electrolyte and reconsider the situation in Figure 9a. This is shown again in Figure 12a to underline the fact that the interface is in a state of dynamic equilibrium. That is, the forward and reverse (partial) currents exactly balance each other and there is no net current flow across the interface. In fact, the situation here is much like that at a metal redox electrolyte interface at the rest potential. We can... 

Analogous expressions may be developed for majority-carrier flow for a p-type semiconductor in contact with a redox electrolyte with the important caveat that the valence band is involved in this process instead. 

The current-voltage characteristics of an illuminated semiconductor electrode in contact with a redox electrolyte can be obtained by simply adding together the majority and minority current components. The majority carrier component is given by the diode equation (Eq. 17) while the minority carrier current (iph) is directly proportional to the photon flux (see, e.g., Eq. 24). Thus, the net current is given by... 

If a semiconductor is brought into contact with an electrolyte containing one or more redox couples, charge transfer between the two phases occurs until electrostatic equilibrium (equality of the free energies of the electron in both phases) is attained. 

FIGURE 11.8 Schematic representation of (a) n- and (b) p-type semiconductor in contact with an electrolyte solution containing an Ox/Red redox couple under irradiation of photons with energy larger than the band gap energy. 

Another important result is that the flatband potentials and therefore the position of the energy bands at the semiconductor surface contacting an aqueous electrolyte, are usually independent of any redox system added to the solution. Hence, the interaction between semiconductor and H2O determines the Helmholtz layer and the position... 

The simplest photoelectrochemical cells consist of a semiconductor working electrode and a metal counter electrode, both of which are in contact with a redox electrolyte. In the dark, the potential difference between the two electrodes is zero. The open circuit potential difference between the two electrodes that arises from illumination of the semiconductor electrode is referred to as the photovoltage. When the semiconductor and counter electrode are short circuited, a light induced photocurrent can be measured in the external circuit. These phenomena originate from the effective separation of photogenerated electron-hole pairs in the semiconductor. In conventional photoelectrochemical studies, the interface between the flat surface of a bulk single crystalline semiconductor and the electrolyte is two dimensional, and the electrode is illuminated from the electrolyte side. However, in the last decade, research into the properties of nanoporous semiconductor electrodes interpenetrated by an electrolyte solution has expanded substantially. If a nanocrystalline electrode is prepared as a film on a transparent conducting substrate, it can be illuminated from either side. The obvious differences between a flat (two dimensional) semiconductor/ electrolyte junction and the (three dimensional) interface in a nanoporous electrode justify a separate treatment of the two cases. 

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