Energy Diagram of Semiconductor/Electrolyte Junctions

In electrochemistry and photoelectrochemistry, it is most common to refer the voltage to a reference electrode (RE) or in a two-electrode cell with respect to a counterelectrode (CE).(1, 2) However, in a semiconductor film that is composed of several junctions, as in semiconductor nanoheterostructures including catalytic layers, it is convenient to use the energy scale with reference to the vacuum level (VL).(3) This type of reference in the energy diagram allows one to track the local variations of energy, electrostatic potential, and Fermi energy inside the solid, and it is the standard representation in solid-state electronics as well as in materials characterization involving ultraviolet photoelectron spectroscopy (UPS) measurements.(4, 5) In contrast, the measurement with respect to RE only takes the voltage at a point on the surface of the solid. Therefore, the relationship between the two pictures can lead to some confusion. In this Guest Commentary, we provide a consistent set of definitions and a diagram that allow one to combine both types of conventions. It is far from the scope of this paper to define the physical meaning of all quantities involved, but we do provide a set of mathematical relationships that allow us to relate the different expressions of voltage and energies such as the flatband potential and the energy (or potential) of the semiconductor conduction band at the surface.