Chemistry Reference
In-Depth Information
The theoretical maximum value is, however, not practically achievable due to many
kinetic processes at the semiconductor/electrolyte interface such as surface recombi-
nation and majority current. As a result, for a given silicon/electrolyte interface, con-
stant photovoltage over a wide range of redox potentials can occur, that is, band bending
is independent of the redox potential value. Thus, photovoltages in practical systems
are often not indicative of the barrier height.
1.5.3.
Efficiency of Energy Conversion
The power efficiency of a photoelectrode can be characterized by the open-circuit
potential (OCP) and short-circuit current:
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where is the photovoltage at the photocurrent At OCP where is the maximum,
but is zero so that is zero. Large is obtained when both and are large.
In the application for photo power conversion devices, the objective is to maximize
The maximum theoretical solar energy conversion efficiency for a silicon elec-
trode is found to be about 25%. Systems with efficiency up to 16% have been realized
experimentally on silicon/liquid junctions.
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The primary source of inefficiency is
the presence of surface states at the silicon/electrolyte interface that act as recombina-
tion centers.
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1.5.4. Surface Recombination
Surface states, like the impurity levels in the bulk, may take part in the recombi-
nation of electrons and holes, which significantly affects the photoelectrochemical
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As described earlier, there are many different kinds of surface states
in terms of their physical nature and energetic distribution in the band gap. Surface
recombination may thus have very different rates depending on the nature of the surface
states. Also, of the total number of surface states, only some of them participate in the
processes of recombination.
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For highly doped material under a large band bending
(deep depletion), surface recombination by tunneling from the surface states to the
bands may also be significant.
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For a simple case where the surface recombination involves monoenergetic
surface states the basic processes are illustrated in Fig. 1.23.
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processes.
represents the flux
due to capture of electrons from the conduction band,
the emission of electrons into
the conduction band,
the capture of holes from the valence band, and
the emis-
sion of holes into the valence band. The recombination rates of electrons,
and of
holes,
can then be expressed as
At equilibrium and
the recombination rates are zero. Equation (1.98)
can be further expressed as
