Chemistry Reference
In-Depth Information
a conduction band electron in the first step, which is not possible without an electron-
injecting species in the solution. The quenching effect of hydrogen evolution on lumi-
nescence is ascribed to the surface adsorption of hydrogen atoms leading to the formation
of Si-H bonds which prevent the injection of an electron into the conduction band. On
the other hand, for
-Si in containing photoemission only occurs at the
cathodic potentials at which hydrogen is evolved, which may be attributed to the inter-
action of the reduction intermediates of with the hydrogen evolution reaction.
In certain situations, photoemission occurs only when both cathodic potential and
light are present.
87,646
For example, blue emission is observed by illuminating
p-
Si in
acetonitrile containing
at cathodic potentials where is reduced. The emission does not occur with
illumination or cathodic potential alone because neither the energy of the light, less
than 1.7eV, nor the electrical energy reaches the threshold energy for the production
of the emitted photons of 3.0 eV.
87
n
6.6. OPEN-CIRCUIT PHOTOVOLTAGE
The open-circuit photovoltage of silicon depends on the minority carrier diffu-
sion length, majority carrier dopant density, photocurrent density, and oxide growth.
161
It has been calculated that for an
n-
Si material without any recombination
processes other than bulk recombination, i.e., the maximum photovoltage at
is about In order to obtain the largest photovoltage it is necessary
to choose the redox couple with a potential located in the most favorable position rel-
ative to the energies of the band edges. For example, for an
n
-type semiconductor it is
desirable to have a redox couple located as close to as possible in order to maxi-
mize the output voltage.
553
Photovoltages close to the maximum value have been
obtained in nonaqueous solutions.
276,569,583
A crucial factor for achieving open-
circuit voltage is the initial photoelectrochemical oxide formation which passivates
surface recombination sites.
The open-circuit photovoltage of silicon electrodes may or may not change with
redox potentials depending on the system. When follows the variation with redox
potential, the band bending is related to the redox potential:
for
n
-Si and for
p-
Si. Figure 6.27 shows on
n
-Si and
p-
Si as a func-
for redox couples
with very negative potentials on
p-
Si or very positive potentials on
n-
Si vary very little
with redox potential. The lack of variation in with redox potentials may be due to
the effect of carrier inversion or Fermi level pinning.
Silicon electrodes in nonaqueous solutions with appropriate redox couples are
more stable than in aqueous solutions. The in solvents such as acetonitrile is gen-
erally not very large (max In methanol it is found to be considerably
larger (up to 0.67 V), which is attributed to the formation of bonds which
result in an improved stability and low density of surface states. Figure 6.28 shows the
dependence of
275
tion of redox potentials in a methanol solution.
However, the
on temperature; it decreases as temperature increases.
699
The effect
of temperature on
is mainly by changing the magnitude of the minority dark satu-
ration current.
