Unpinned Band Edges

The possibility of hot carrier effects is discussed in a later section. 

Te) indicate that the band edges can become unpinned, and that applied potentials can shift the semiconductor band positions with respect to the electrolyte redox levels. The effect here is to be able to oxidize or reduce redox couples that lie outside the band gap as determined from values of the flat-band potential obtained in the dark. Several explanations for the unpinning of the bands have been proposed. 

Although the p-Si sample is etched in HF solution before each o run, it is also exposed to air. Hence, a thin oxide layer (10-20 A) exists on the surface. Therefore, the similar behavior of the capacitance of the p-Si electrode in non-aqueous electrolyte to that of 

Current voltage curves for p-Si v/ith methanol and acetonitrile containing redox couples above the conduction band edge as determined from flat-band potential measurements. 

Fermi level pinning would produce junction effects that are completely independent of the redox species in the electrolyte since the junction would be controlled strictly by semiconductor surface and bulk properties. However, experimental results (28) show that the junction properties are only independent of the electrolyte redox species for certain critical values of the redox potential. 

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The unpinned band edges can move with applied potential, making the semiconductor electrode behave similar to a metal electrode in that changes in applied potential occur across the Helmholtz layer rather than across the semiconductor space charge layer. 

In the former case, the positions of the conduction and valence band edges at the interface (E and E ) are fixed with respect to the electrolyte redox levels, while in the latter case the positions of E and E with respect to the electrolyte redox levels vary with electrode potential. Thus, with unpinned band edges, redox couples lying outside the band gap under flat band conditions can lie within the band gap under band bending conditions or under illumination. 

Figure 2. Pinned vs. unpinned band edges. In the former case the position of Ec and E reman fixed with respect to the electrolyte redox couples as the electrode potential is varied. In the latter case, Ecs and E s change with applied potential.

Band-Edge Unpinning Produced by the Effects of Inversion... 

Experiments using p-Si in non-aqueous electrolytes (5.6) indicate that the reduction of redox species with redox potentials more negative than the conduction band edge could apparently beachieved. However, further analysis showed that these results are not caused by hot electron injection, but by the unpinning of the semiconductor band edges at the semiconductor-electrolyte interface this unpinning effect is caused by the creation of an inversion layer at the p-Si surface. This is an important effect, especially for small band gap semiconductors, that has received little attention 0n Sabbatical leave from the Weizmann Institute of Science, Rehovot, Israel. 

Moderately doped diamond demonstrates almost ideal semiconductor behavior in inert background electrolytes (linear Mott -Schottky plots, photoelectrochemical properties (see below), etc.), which provides evidence for band edge pinning at the semiconductor surface. By comparison in redox electrolytes, a metal-like behavior is observed with the band edges unpinned at the surface. This phenomenon, although not yet fully understood, has been observed with numerous semiconductor electrodes (e.g. silicon, gallium arsenide, and others) [113], It must be associated with chemical interaction between semiconductor material and redox system, which results in a large and variable Helmholtz potential drop. 

Unpinning of band edges at the semiconductor/electrolyte interface is understood as a common phenomenon for n- and p-type materials. Thus, the band edge positions as obtained from Hatband potential measurements in the dark, cannot be taken as a fixed value for the interpretation of charge transfer processes. More investigations in this direction are necessary. 

Unlike the case illustrated in Fig. 10, changes in the solution redox potential have been observed to cause no change in the magnitude of Vsc- This situation is termed Fermi level pinning in other words, the band edge positions are unpinned in these cases so that the movement of iiredox accommodated by Vh rather than by Vsc- As mentioned earlier, it appears [37] that surface state densities as low as 10 cm ( 1% of a mono-layer) suffice to induce complete Fermi level pinning in certain cases. Of course, intermediate situations are possible. Thus, the ideal case manifests a slope of 1 in a plot of Vsc (or an equivalent parameter)... 

Another important role for surface states has been proposed to explain how semiconductor band edges can become unpinned at the semiconductor-electrolyte interface. This important effect is discussed in detail below in the next section. 

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