Cathodic Behavior and Redox Couples - Electrochemistry of Silicon and Its Oxide - page 274

A continuous metal deposit layer may behave as an ohmic contact or a Schottky
barrier. For a relatively thick metal film the silicon can still behave like a semicon-
ductor before the onset of current. For example, for n -Si deposited electrochemically
with 150nm Au, the electrode behavior is similar to that of bare silicon electrode: At
positive potentials the anodic current is small whereas at cathodic potentials current
from hydrogen evolution increases with increasing polarization. 848 In the potential
region before the onset potential for the cathodic current a linear Mott-Schottky plot
is obtained giving a flatband potential similar to that of bare silicon sample. In the
potential region where hydrogen evolution occurs, it behaves like a metal with poten-
tial drops mostly in the Helmholtz layer.
Silicon electrodes can also be stabilized by depositing a layer of compounds of
oxides and silicides such as molybdenum dinitrogen complexes, 882 manganese
oxide, 191 indium-tin oxide, 814 aluminum oxide and aluminosilicate. 596 In contrast to the

noble metals, deposition of islands of Pb and Cd tends to inhibit hydrogen evolution

solution. 688 in

6.7.2. Polymer Coatings

Coating the surface of silicon electrodes with a polymer coating can also be an
effective method of stabilizing the electrodes and improving the photovoltage and
kinetics of electrode reactions 78,546 The polymer film effectively insulates the semi-
conductor from the superoxide ion and prevents chemical reaction and deteriora-
tion. At the same time, the polymer behaves like a surface-bond redox couple to
mediate the charge transfer between the semiconductor and the redox species in the
solution. Various types of polymers can be used to coat silicon electrodes as shown in
Table 6.6.
For n -Si the surface-confined material can serve two functions: (1) photogener-
ated holes are transferred to the attached redox reagent to preclude silicon oxidation
and (2) the photooxidized polymer oxidizes the solution species. 553 Because whether
the species in solution can be oxidized depends on the nature of the surface-confined
polymer, the electrode can be designed to oxidize certain species selectively.
An example is shown in Fig. 6.31 for a polymer with a redox couple
that is attached to the silicon surface through chemical bonding. 78 On illumination

is converted to M. The resulting oxidized surface is then capable of oxidizing
to The redox species B is not efficient in capturing holes
directly from silicon due to its distance from the valence band. When the Si surface
is covered with a polymer, the location of electron transfer is within the polymeric
film rather than at the solid interface. 546,815 The photocurrent response of derivatized
electrode is similar to that of bare silicon surface but the stability is significantly
improved. 827
The presence of a derivatized surface layer can affect the energetics as well as
the kinetics of electrode reactions. 712 It has been found that the flatband potentials of
n -Si and p -Si electrodes coated with conducting polypyrrole films are shifted by 300
and 500 mV in solution. 402 The reaction kinetics on polymer derivatized surface
can further be enhanced by impregnation of noble metals such as Pt particles into the

a second species B