Fermi levels splitting

Fig. 2.20 Band diagram for a PEC cell based on an n-type semiconducting photoanode that is electrically connected to a metal counter electrode in equilibrium in the dark (left) and under illumination (right). Illumination raises the Fermi level and decreases the band bending. Near the semiconductor/eiectroiyte interface, the Fermi level splits into quasi-Fermi levels for the electrons and holes...

The concept of the quasi-Fermi level split is extremely important to how much useable photopotential a semiconductor device can generate, specifically in relation... 

Fermi Level Splitting in the Semiconductor-Electrolyte Junction... 

The maximum quasi-Fermi-level splitting in a semiconductor under illumination is also affected by nanostructuring. This potential difference (V c) will be smaller for the nanowire photoelectrode for several reasons. For example, in a traditional planar photoelectrode geometry under low-level injection, the highest achievable np product in Si is ultimately limited by SRH recombination in the bulk. For a macroscale, planar n-type semiconductor photoelectrode operating under this limitation, the maximum possible is given by ... 

Suspended semiconductor nanoparticles also differ from high-aspect-ratio semiconductor photoelectrodes in another aspect. High-aspect-ratio nanowire/macroporous photoelectrodes in low-level injection are connected to current collectors and are thus intended to operate at a specific power point (i.e., at a particular combination of current-potential values that maximizes the product of the quasi-Fermi-level splitting and the net photocurrent). In contrast, a suspended semiconductor nanoparticle functions without any external contacts at precisely open-circuit conditions. That is, at the operational conditions, photoexcited semiconductor nanoparticles suspended in a solution pass no net current, that is, 0=0, and their quasi-Fermi levels are offset by the maximum value possible under the operative illumination and recombination conditions. 

As long as A and B are chemically distinct species, a net chemical conversion in solution will be effected under illumination since the values of O for each respective electron and hole current are themselves not zero. Further, the quasi-Fermi-level splitting induced by illumination is not necessarily applied evenly to the electron and hole currents, that is, n and p, can be different in order... 

If the interaction between the atomic orbital and the d-band is weak, the splitting between the bonding and the antibonding orbital of the chemisorption bond is small. The antibonding orbital falls below the Fermi level and is occupied. This represents a repulsive interaction and does not lead to bonding. 

Fig, 10-1. Splitting of Fermi level, cnsci, into both quasi-Fermi level of electrons, bCp, and quasi-Fermi level of holes, pSp, in photoezcited semiconductors (a) in the dark, (b) in photon irradiation. SC = semiconductor hv = photon energy. 

In the dark, thermal equilibrium is established between electrons in the conduction band and holes in the valence band so that both the quasi-Fermi level of electrons and the quasi-Fermi level of holes equal the oiiginal Fermi level of the semiconductor (nCr = pC, = ep). Under the condition of photoexdtation, however, the quasi-Fermi level of electrons is higher and the quasi-Fermi level of holes is lower than the original Fermi level of the semiconductor (nSp > cp > pCp). Photoexdtation consequently splits the Fermi level of semiconductors into two quasi-Fermi levels the quasi-Fermi level of electrons for the conduction band and the quasi-Fermi level of holes for the valence band as shown in Fig. 10-1. 

Fig. 10-2. Splitting of Fenni level of electrode, cnsci. into quasi-Fermi levels of electrons, ep, and of holes, pCp, respectively, in a surface layer of photoexcited n-type and p-type semiconductors a shift of quasi-Fermi levels from original Fermi level is greater for minmity charge carriers than for majority charge carriers.

Since photoexcited electron-hole pairs are formed only within a limited depth from the semiconductor surface to which the irradiating photons can penetrate, the photon-induced split of the Fermi level into the quasi-Fermi levels of electrons and holes occurs only in a surface layer of limited depth as shown in Fig. 10-2. 

Third, the light intensity must be high enough to split sufficiently the quasi-Fermi levels for electrons and holes, i.e. by more than 1.23 eV, such that the level for holes at the interface is below that for the O2/H2O redox system, as shown in Figure 6. 

Fig. 11. A plot of the Au5

Our XPS results on AU55 can also be examined to decide whether metallic shielding is present. The presence of a finite density of states at the Fermi level in AU55 was clearly detected in our XPS valence band spectrum, as indicated by the arrow in Fig. 10. This presence can be considered as an indication of metallic character in a cluster, even though this view has been questioned [74, 152,157]. In addition, the near full bulk value of the valence band splitting of AU55 is also... 

---