Flat-Band Potential Techniques

It is important to determine the conductivity and flat-band potential ( ft) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. Photoanodes (n-type conductivity) drive the oxygen evolution reaction (OER) at the electrode-electrolyte interface, while photocathodes (p-type conductivity) drive the hydrogen evolution reaction (HER). The conductivity type is determined from the direction of the shift in the open circuit potential upon illumination. Illuminating the electrode surface will shift the Fermi level of the bulk (measured potential) towards more anodic potentials for a p-type material and towards more cathodic potentials for a n-type material. The conductivity type is also used to determine the potential ranges for three-electrode j-V measurements (see section Three-Electrode J-V and Photocurrent Onset ) and type of suitable electrolyte solutions (see section Cell Setup and Connections for Three- and Two-Electrode Configurations ) used for the electrochemical analyses. 

---

Now we briefly touch upon certain practical applications concerning the measurement of photopotential. Its measurement is a convenient method to determine the flat band potential

limiting case of very intensive illumination the bands unbend completely, i.e., the flat band potential is attained. Tyagai and Kolbasov (1975) used this technique to measure (plb for several AnBVI semiconductors the values obtained are in good agreement with those measured by the differential capacity method. 

The above considerations are illustrated by Fig. 38, which shows the dependences of electroreflection of a silicon electrode on its potential. In fact, the signal changes the sign at the flat band potential (the latter was measured independently by the differential capacity technique). 

The magnitude of the errors in determining the flat-band potential by capacitance-voltage techniques can be sizable because (a) trace amounts of corrosion products may be adsorbed on the surface, (b) ideal polarizability may not be achieved with regard to electrolyte decomposition processes, (c) surface states arising from chemical interactions between the electrolyte and semiconductor can distort the C-V data, and (d) crystalline inhomogeneity, defects, or bulk substrate effects may be manifested at the solid electrode causing frequency dispersion effects. In the next section, it will be shown that the equivalent parallel conductance technique enables more discriminatory and precise analyses of the interphasial electrical properties. 

For semiconductor colloidal particles pulsed radiolysis techniques [103] have also been used to derive the flat-band potential. 

Tab.l Typical data for passive films taken from Ref. [1], density p, dielectric permittivity e, band gap energy g, flat band potential Ufb, equilibrium potential of oxide electrode Uqx (Reaction 2 [16]), donor concentration N, difference of electronegativity Ax, transference number of cations f+, formation factor dd/dU, and initial oxide thickness do. Because of the strong dependence of properties on the preparation technique, the microstructure and the sensitivity of thin films, the reliability of these data is less than for bulk, crystalline solids... 

---