Solar cell technology is marching ahead, though it still struggles with
the two problems: efficiency and high production costs. In collaboration
with Satoshi Uchida at the University of Tokyo, Michael Gr�tzel and his
research group at the Swiss Federal Institute of Technology in Lausanne
have now developed new sensitizers that should help an inexpensive type
of solar cell to be more efficient. As they report in the journal
Angewandte Chemie, the sensitizers are based on the dye indoline.
Some years ago, Gr�tzel developed
photoelectrochemical solar cells that are inexpensive, easy to produce,
and able to withstand long exposure to light and heat. These "Gr�tzel
cells" contain a mesoscopic layer of titanium oxide (TiO2)
particles coated with a sensitizing dye. Upon irradiation with light,
electrons are injected from the dye adsorbed on the TiO2,
which are then transferred to the conducting band of the TiO2
and collected at the back contact, and carried away by an external
circuit. In order for the cell to work, the electrons that are
injected into the TiO2 must not recombine with the oxidized
dye. To prevent this, the cell contains an electrolyte solution with
negatively charged iodide and triiodide ions as a redox couple
dissolved in a solvent, which immediately reduce the holes created in
the dye. The main disadvantage of using volatile organic solvent in
the electrolyte is the need for encapsulation of the electrolyte.
Ionic liquids are an alternative to the use of these volatile solvent.
These salts exist as liquids at low temperatures and do not evaporate.
However, the high viscosity of these electrolytes is detrimental to
the mass transport and consequently a problem for obtaining high
efficiency.
Gr�tzel and his team compensated for this loss of efficiency by
optimizing the sensitizer. In place of the usual ruthenium dyes, they
used tailor-made organic dyes based on indoline, which have a higher
molar extinction coefficient. This allows the TiO2 films to
be thinner, in turn reducing the electron path length. The combination
thus attained an energy conversion yield of 7.2 %. This is a record
for this type of cell (organic dye, ionic liquid, titanium oxide).
In this case the efficiency of the dye as a sensitizer is not only
dependent on its chromophore, but also on its interfacial properties.
So using a dye with an additional hydrocarbon chain has improved the
performance by retarding the back electron reaction.
Quellen und Artikel:
-
Daibin Kuang, Dr., Satoshi Uchida, Prof., Robin Humphry-Baker, Dr.,
Shaik M. Zakeeruddin, Dr., Michael Gr�tzel, Prof.: Organic Dye-Sensitized Ionic Liquid Based Solar Cells: Remarkable
Enhancement in Performance through Molecular Design of Indoline
Sensitizers.
In: Angewandte Chemie International Edition 2008, 47,
1923�1927;
doi: 10.1002/anie.200705225.
