External bias potential

Fig. 4. An interfacial energetic situation in a photoelectrolysis cell where the flat-band potential of the n-type semiconductor photoanode lies positive of the HER potential. 1W is the external bias potential needed in this case to drive the photoelectrolysis process.

In Eq. (14), ket is the rate constant for electron transfer, Cqx is the concentration of empty (acceptor) state in the redox electrolyte, Hs and Hjo are the surface concentrations of electrons, the subscript o in the latter case denoting the equilibrium situation. Thus, as long as the semiconductor-electrolyte interface is not perturbed by an external (bias) potential, Ug = n o and the net current is zero. The voltage... 

The operation of a PEC cell can be influenced by applying an external bias potential to the semiconductor. When the bias is applied with respect to a reference electrode, the potential difference will be distributed over the space charge layer and the Helmholtz layer. These layers act as two capacitances in series [41] ... 

It has been reported long time ago that some of perovskite-type oxides, e.g. SrTiOs, can decompose H2O into H2 and O2 with no external bias potential. [36] The principle for this process is that the conduction band (CB) edges of some of the perovskite oxides are more negative than the H /H2 energy level. [1] However, only a few work on using perovskite oxides as photocatalyst is reported up to date and study on this field is still in its infancy. 

Fig. 2. Representation of the band edges in a semiconductor p—n junction where shallow donor, acceptor energies, and the Fermi level are labeled Ejy E, and E respectively, (a) Without external bias is the built-in potential of the p—n junction (b) under an appHed forward voltage F. ...

Another way to measure the Vhi is by means of photovoltaic measurements [97, 113. The technique is based on the fact that, at near zero applied bias, the OLED acts as a photovoltaic cell, where photogencraled carriers drift under the influence of Vhi to produce a current in an external cireuit. In a way similar to electroabsorption, an external bias is applied in order to compensate the built-in potential and null the net pholocurrent (Fig. 13-6). However, it has been shown that the measurement produces accurate results only at low temperatures, where diffusive transport of charges that are phoiogcneraled at the interlaces is negligible [97]. 

A constant bias potential is applied across the sensor in order to form a depletion layer at the insulator-semiconductor interface. The depth and capacitance of the depletion layer changes with the surface potential, which is a function of the ion concentration in the electrolytic solution. The variation of the capacitance is read out when the semiconductor substrate is illuminated with a modulated light and the generated photocurrent is measured by means of an external circuit. 

The dissolution of PS during PS formation may occur in the dark or under illumination. Both are essentially corrosion processes, by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The material in the PS, which is distant from the growing front is little affected by the external bias due to the high resistivity of PS and is essentially at the open circuit potential (OCP). Such corrosion process is responsible for the formation of micro PS of certain thickness (stain film) in HF solutions containing oxidants under an unbiased condition. 

Semiconductor structures that develop space charge layers and contact potentials, like films of proper thickness, films with applied external bias, homo- and hetero-(nano)junctions, permit significant suppression of bulk recombination processes and, potentially, allow high quantum yields. Spatial separation of electron and holes also allows the separation of cathodic and anodic processes in a photoelec-trochemical cell (eventually at the micro and nano level), minimizing surface re-... 

The overall reaction of the photoelectrochemical cell (PEC), H2O + hv H2 -I- I/2O2, takes place when the energy of the photon absorbed by the photoanode is equal to or larger than the threshold energy of 1.23 eV. At standard conditions water can be reversibly electrolyzed at a potential of 1.23 V, but sustained electrolysis generally requires -1.5 V to overcome the impedance of the PEC. Ideally, a photoelectrochemical cell should operate with no external bias so as to maximize efficiency and ease of construction. When an n-type photoanode is placed in the electrolyte charge distribution occurs, in both the semiconductor and at the semiconductor-... 

As has been previously discussed,(5, 6) the amount of band bending in the depletion region determines the amount of external bias (Vb, ) needed for chemically producing PECs (photoelectrochemical cells) or the open-circuit potential (V ) for wet... 

Figure 1. Energy level diagram for a photoelectrochemical cell illustrating the relationship between the electron affinity (EA) and the flatband potential (V/t). Energy levels are shown for zero external bias (Ec = 4.75 eV).

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