Chemistry Reference
In-Depth Information
caused by the large density of surface states, which results in the potential drop in the
Helmholtz layer instead of in the space charge layer.
In aqueous solutions, where the dissolution of silicon and formation of oxide film
is significant, the photopotential cannot be predicted according to the energetic posi-
tions of the bands and the redox couples in the solution. For example, in solution
without HF, photovoltage has been found to be relatively large on
n
-Si but very small
on
p
-Si.
392
In HF solutions the photovoltage on
p-
Si is manyfold larger. In another study,
open-circuit voltage of
p-
Si of about 0.4V is measured in 0.5% HF solution but only
about 0.02V on
n
-Si.
968
The complex photoresponses in these solutions can be attrib-
uted to the presence of surface states associated with surface defects, dissolution inter-
mediates, adsorbed species, and oxide film. Also, surface states may affect the reduction
and oxidation of a redox couple differently; they can facilitate the reduction of a redox
couple
from
the electrons in the surface states.
553
to A but not the oxidation from A to
without light to detrap
Numerous studies have been conducted to optimize the product of
to make
161,275,573,812
silicon an efficient photocathodic material.
Depending on the system, the
energy conversion efficiencies of silicon/electrolyte electrodes have been found to
greatly vary, from a fraction of 1% to more than 10%. Efficiency can vary from 0.2 to
2% by simply changing the pH of the solution.
812
An efficiency exceeding 10% for
n
-
Si in and for
p
-Si in has been realized.
622,623,847
A surface textured
sample shows a higher efficiency than a polished surface due to lower reflectivity and
increased surface area.
552
6.7. SURFACE MODIFICATION
Silicon is highly unstable in aqueous electrolytes due to the formation of an insu-
lating oxide film which prevents the use of
n
-Si as photoanode. On the other hand, the
silicon electrode has poor kinetics for hydrogen evolution which is not desirable for its
use as a photocathode. Many methods have been explored to stabilize Si electrodes in
aqueous solutions for possible applications as photochemical cells. They include
coating the surface with noble metals, metal oxides, metal silicides, or organic materi-
als as shown in Table 6.6. Also, some redox species, the reduction of which can favor-
ably compete with the oxidation of silicon, can be used to stabilize silicon anodes
