Semiconductor Electrolyte Junctions - Conventional Picture

All phenomena associated with photoelectrochemical systems are-based on the formation of a semiconductor-electrolyte junction when an appropriate semiconductor is immersed in an appropriate electrolyte. The junction is characterized by the presence of a space charge layer in the semiconductor adjacent to the interface with the electrolyte. A space charge layer generally develops in a semiconductor upon contact and equilibration with a second phase whenever the initial chemical potential of electrons is different for the two phases. For semiconducotrs, the chemical potential of electrons is given by the Fermi level in the semiconductor. For liquid electrolytes, the chemical potential is determined by the redox potential of the redox couples present in the electrolyte these redox potentials are also identified with the Fermi level of the electrolyte. 

A charged layer also exists in the electrolyte adjacent to the interface with the solid electrode-the well known Helmholtz layer. This layer consists of charged ions from the electrolyte adsorbed on the solid electrode surface. The width of the Helmholtz layer is generally of the order of a few angstroms. The potential drop across the Helmholtz layer depends upon the specific ionic equilibrium obtaining at the surface. 

A very important consequence of the presence of the Helmholtz layer for semiconductor electrodes is that it markedly affects the band bending that develops in the semiconductor when it equilibrates with the electrolyte. Without the Helmholtz layer, the band bending would simply be expected to equal the difference in initial Fermi levels between the two phases (i.e., the difference between 

All photoelectrochemical cells are based on the semiconductor-electrolyte junction the energy level diagram for this junction is shown below  

Relationships are shown between the electrolyte redox couple (H /H2), the Helmholtz layer potential drop (V ), the semiconductor band gap (Eg), electron affinity Of), work function ( t sc), band bending (V0), and flat-band potential (Uf ). The electrochemical and solid state energy scales are shown for comparison. is the electrolyte work function. 

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