Structure of the Dye-Sensitized
Solar Cell |
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The dye-sensitized solar cell is often
compared to photosynthesis in plants. This
is because both convert the energy in light
absorbed by dyes or pigments into other
forms of energy.
In plants, light is converted to chemical
energy, and glucose is produced. In contrast,
the dye-sensitized solar cell converts light
to electrical energy. First, let's look at
the structure of the dye-sensitized solar
cell. (See figure 1.) This device is like the
LCD display, in which liquid materials
and circuits are sandwiched between two
glass plates, but in the dye-sensitized solar
cell, the following four main materials are
sandwiched between two conductive glass
plates.
(1) TiO2 (semiconductor electrode)
The white areas in figure 1 on the previous page
In figure 1, there is a reason these areas
are drawn to look like snowmen consisting
of stacked spheres. When TiO2 paste is
applied to the conductive glass and sintered
at around 500°C, the solvent included in
the paste is removed, and tiny particles
(with diameters of 10 to 30 nm) of TiO2 are
connected to each other. This forms a porous
film with many tiny holes.
(2) Dye
The red areas in figure 1 on the previous page
The dye is applied to the porous TiO2 layer. The energy conversion efficiency (the
efficiency with which light is converted
to electricity) is increased by the dye
being attached in a three-dimensional
manner to the TiO2, whose surface area
has increased radically. This is thought to
make stable generation possible without
being significantly affected by the angle of
incidence of the light. Actually, it turns out
that it is possible to generate electricity with
just TiO2. However, as one might expect due
to its use as a sunscreen, TiO2 only absorbs
UV, and does not absorb other wavelengths.
Therefore, the energy conversion efficiency
is increased by adding a dye that absorbs
light with wavelengths in the visible light
range. That is, these devices are, as the name
indicates, sensitized using dyes.
(3) Electrolyte
The yellow area in figure 1 on the previous page
Electrolyte refers to materials in which a
material (the solute) is disassociates into
positive and negative ions when dissolved
in a solvent. Electrolytic solutions conduct
electricity by the action of ions accepting
and releasing electrons.
Through the development and manufacture
of lithium-ion batteries, Sony has
accumulated a rich store of know-how
about these electrolytes. Sony has verified
that there is almost no change in the energy
conversion efficiency when the electrolyte
solvent is made into a gel (is semisolidified).
Sony has created whole new
classes of applications for dye-sensitized
solar cells by increasing their reliability in a
variety of ways, including using gels.
(4) Catalytic electrode (counter electrode)
The black area in figure 1 on the previous page
The counter electrode returns the electrons
taken from the cell back to the ions.
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Electricity Generation
Principles and Issues |
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Figure 2 shows the mechanisms and flows
that occur during electricity generation.
When the dye absorbs light and ejects
electrons (oxidation), those electrons pass
through the TiO2, the transparent electrode,
the external circuit, the catalytic electrode,
and the electrolyte, and are returned once
again to the dye (reduction). Electricity flows
as a result of this cycle.
Sony has succeeded in increasing the
conversion efficiency to 8.2% in a prototype
module. Sony is currently working on
several issues, including the following three,
to improve the performance even further:
(1) increasing the current density by using
an even wider range of wavelengths, (2)
reducing the dark current to increase the
cell voltage, and (3) suppressing the internal
resistance by increasing the conductivity of
the electrolyte.
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