Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Experimental Validation of New Classes of Dyes
for Dye-Sensitised Solar Cells
Michal G. Zubel, Jacqueline M. Cole, Paul G. Waddell
Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, United Kingdom
Solar power is numbered, along with wind, water (waves, tides,
dams, run-on-the-river systems), biomass growth (traditional
and advanced) and geothermal heat among renewable energy
sources. Although the accuracy of any long-term forecast can be
considered questionable, especially in the times when the spectacular
failure of the predictions of the Club of Rome, expressed
in their 1972 book “Limits to Growth”, has become evident, it is recognised
that the photovoltaic energy harvesting seems to be the
only branch able to provide cheap and clean energy in the future
[1]. This is because the other above-mentioned “green” technologies
have not enough growth potential and area of availability to
be regarded major sources of cheap energy for the future.
Energy production from renewable sources encompass still
emerging technologies. They constitute only 6% of the global final
energy consumption (2009), excluding traditional biomass [2].
Their share of the global electricity consumption (2010) is higher
but it is still smaller than 20%, whereas about 16% is attributed
to hydroelectricity [2], the most established renewable energy
source. All the other sources contribute around 3.3% of the global
share. Obviously, fossil fuels dominate both rankings.
The main parameter to characterise energy sources is the
average cost of production per kilowatt‐hour or megawatt-hour.
Looking at the comparison of the cost [2] it is no wonder that photovoltaics
are not in widespread use. The cost of electric energy
from this source is 5–10 times higher than from the cheapest renewable
one, i.e. hydroelectricity. It is also estimated to be around
3 times more expensive than electric energy from coal [3]. Various
reports give different figures on the average costs of energy, but
in each of them the tendency for photovoltaic energy to be much
more expensive is clear.
Solar cells will not be competitive with other energy sources as
long as they have not reached grid parity. This has been achieved
in some locations (e.g. mountain areas), in which the cost of energy
transport is substantial. Use of solar cells can appear to be
the only solution in some areas without access to the grid. They
can also be used to power off-grid, self-standing devices (road
and water signs, meteorological stations etc.). Despite these niche
applications a real breakthrough can only be achieved by
reaching gird parity for regardless of location.
Nowadays, silicon dominates the photovoltaics market as it
profits from the microelectronic technology that has been optimised
over many years. However, the high price of silicon solar
cells, which is unlikely to be lowered further (due to the substantial
cost of the material itself, the high quality and purity required
and the high energy processes involved in the production) limits
their application and drives research towards alternative methods
that would be more cost-effective [1]. The prospective solutions
encompass organic, nanocrystalline and dye-sensitised solar
cells (DSSC). The latter branch is the focus of this work.
In silicon solar cells silicon itself serves to absorb photons as
well as to separate and transport charge carriers. This means that
one type of material has to be optimised with regards to three different
processes. As the electron-hole pair can only be separated
in the p-n junction region, a high value of diffusion length of electrons
and holes is necessary for the charge carriers to reach the
junction. This can only be achieved by providing crystalline material
of very high quality, which is expensive to produce. Silicon has
a relatively small value of absorption coefficient compared to dye
molecules which means that the Si wafers used to produce photovoltaic
cells cannot be thinner than 300–500 µm. This makes
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substantial reduction of the material amount per wafer impossible
and thus maintains the high price of the device. Moreover, charge
carriers generated far from the junction region can recombine at
defects of crystalline lattice and at the surface. These remarks
show contradicting requirements to be met by silicon that are limiting
the performance of Si-based PV cells.
Overview of Dye-Sensitised Solar Cells
Structure and Principles of Operation
Dye-sensitised solar cell (DSSC) are based on the concept of
separating the elements responsible for absorption of photons,
separation and transport of charge carriers and thus let them be
optimised separately. For this various types of material must be
used. The principles of operation are similar to the process of
photosynthesis taking place in green plants. The main parts of a
dye-sensitised solar cell in the configuration proposed by Grätzel
[4] (Fig.1) include a substrate plate (transparent to visible light)
covered with a thin conductive oxide (TCO) layer (usually indium
tin oxide), a wide-band-gap semiconductor film (usually titanium
dioxide), to which dyes attach, a liquid electrolyte layer (typically
an iodide/triiodide redox couple) and a counter electrode. A photon
of energy corresponding to the visible range passes through
both the upper substrate and the semiconductor to be absorbed
by the dye (Fig. 1, (1)). The electron of the chromophore that has
been excited (S S* transition) is subsequently injected into the
conduction band of the semiconductor (e cb
) (2) and further to the
outer circuit. This process is only possible if the LUMO level of the
dye lies above or at the same energy as the conduction band of
the semiconductor. The ionised dye (S + state) is then regenerated
from the electrolyte via a redox reaction between the dye molecule
and the counter electrode. Iodide ions (I - ) transmit electrons to the
sensitiser transforming into triiodide (I 3-
) (3). By means of diffusion
in the electrolyte triiodide eventually reaches the counter electrode
where it is reduced back into iodide (4). The electrons reducing
the triiodide (e Pt
) come from the external circuit after losing energy
through resistance. As in the case of (2), the processes (3)
and (4) occur only when the energy level of electrolyte is equal or
higher than the ground state of the dye. The maximum energy per
electron that can be obtained from the cell (or open circuit voltage
V oc
in voltage scale) is the difference between Fermi level of the
semiconductor and the redox potential of the electrolyte.
In the Grätzel design separate elements associated with the
fundamental processes in the cell operation can be differentiated.
The dye molecule is responsible for the light absorption. Charge
separation takes place at the dye-semiconductor and dye-electrolyte
interfaces for electrons and holes, respectively. Electrons are
transported in the conduction band of the semiconductor, whereas
holes are carried by the electrolyte by means of redox reaction.
The basic Grätzel setup can be further investigated and improved.
The materials utilised can be changed as long as they
are able to play their given roles. Other nanocrystalline semiconducting
compounds (e.g. ZnO and SnO 2
) have already been tried
as electron conductors but they have turned out to offer reduced
performance [5]. There has recently been much investigation
into alternative liquid electrolytes (e.g. because of sealing issues)
such as ionic liquids [6] as well as gel electrolytes [7] and solid
polymeric hole conductors [7]. The latter in particular seems to
be promising as polymeric hole conductors have superior performance
to their electron‐conducting counterparts [8]. Nonetheless,
the liquid iodide electrolyte remains the most efficient for the time
Elektronika 6/2012