Flat band potential interface states

The point at which the straight line of (tph) versus Eintersects the coordinate of electrode potential represents the flat band potential. Equation 10-15 holds when the reaction rate at the electrode interface is much greater than the rate of the formation of photoexcited electron-liole pairs here, the interfadal reaction is in the state of quasi-equilibrium and the interfadal overvoltage t)j, is dose to zero. 

The flat band potential of semiconductor electrodes is determined by the potential across the compact las r at the electrode interface and is characteristic of individual semiconductor electrodes. For semiconductor electrodes in the state of band edge level pinning, the potential across the compact layer remains constant and independent of the electrode potential. For some semiconductor electrodes, however, photon irradiation changes the potential across the compact layer and, hence, shifts the flat band potential of the electrode. 

Thus, a potential-dependent smooth background in IR spectra can be due to modulation of concentration/population of free carries and surface states. In contrast to common capacitance measurements, IR spectra of free carrier can provide information on the space charge capacitance of the semiconductor electrode under accumulation as well as under depletion conditions, without interference from the surface state capacitance. The DA-potential plots under depletion conditions allow measurements of the flat-band potential, while absorption of charge carriers in conducting polymer films can be used for estimating the film conductivity. It is also possible to follow the filling of surface states and relate these energy levels to the chemical composition of the interface. 

Several key features of this study should be emphasized. IS clearly can be used to successfully model a semiconductor-electrolyte interface in a PESC. The ability to probe the physics of this interface using IS while controlling the applied potential can allow significant insight into the important parameters of the device. In particular, the surface states at the semiconductor-electrolyte interface may be determined, as can their relative importance after several different pretreatments or in different cell configurations. The electrical characteristics of the interface, for example the flat-band potential and the space charge capacitance, can also be determined. 

Uosaki K, Shigcanatsu Y, Kaneko S, Kita H (1989) Photoluminescence and impedance study of p-GaAs/ electrolyte interfaces under cathodic bias Evidence for flat-band potential shift during illumination and introduction of high-density surface states by Pt treatments. J Phys Chem 93 6521... 

For purposes of discussion of Figure 5, the difference between the O2/H2O redox potential and the valence band edge at the interface is defined as the intrinsic overpotential of the semiconductor anode, na- This overpotential is not the usual overpotential or overvoltage of conventional electrochemistry since it is current independent and is determined only by the band gap, the flat-band potential, and the redox potential of the electrolyte donor state. 

Here, Ws is the work function of electrons in the semiconductor, q is the elementary charge (1.6 X 1CT19 C), Qt and Qss are charges located in the oxide and the surface and interface states, respectively, Ere is the potential of the reference electrode, and Xso is the surface-dipole potential of the solution. Because in expression (2) for the flat-band voltage of the EIS system all terms can be considered as constant except for tp (which is analyte concentration dependent), the response of the EIS structure with respect to the electrolyte composition depends on its flat-band voltage shift, which can be accurately determined from the C-V curves. 

Fig. S-41. Band edge levels and Fermi level of semiconductor electrode (A) band edge level pinning, (a) flat band electrode, (b) under cathodic polarization, (c) under anodic polarization (B) Fermi level pinning, (d) initial electrode, (e) under cathodic polarization, (f) imder anodic polarization, ep = Fermi level = conduction band edge level at an interface Ev = valence band edge level at an interface e = surface state level = potential across a compact layer.

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