Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Fig. 2: structure of spirobifluorene hole-conducting p-type organic semiconductor. The fluorene structures
are perpendicular to each other, but conjoined through a carbon site common to both.
Attention has also been given to the use of dye-sensitised solar cells as a component of a tandem
device. In the report of 2006 use of a DSC component in association with a second photoactive surface
with a suitable oxygen-evolution photoanode for direct hydrogen production by photoelectrolysis
(4) was presented. The oxide photoanode itself, of course, does not provide the potential difference at
least 1.23V, necessary for electrolysis of water so the complementary DSC is added in electrical series
to achieve the required voltage. It is also convenient that the cells be in optical series and have
complementary optical absorption spectra in order to maximise the energy drawn from sunlight. The
cell with the larger bandgap photoabsorber is effectively transparent for longer wavelengths, which can
be harvested by a second cell whose sensitivity extends towards the red. For the oxide semiconductor
photoanode, iron or tungsten is favoured. A schematic of the energetics of this system is shown in
Fig.3. A further development, associating a dye-sensitised cell with a chalcogenide-based thin film
solid state cell with spectral complementarity and in optical and electrical series will be presented later
in this report.
In the development of cell components, the introduction of the K-series of dyes featured in the report
for the year 2006 (5). These compounds carry long-chain aromatic or aliphatic substituents, as well as
the standard carboxylate linking groups. As a result the monomolecular film represents effectively an
integrated hydrophobic encapsulant on the semiiconductor surface, so that trace aqueous contaminants
in an organic electrolyte cannot desorb the dye or otherwise degrade cell performance. Electrolyte
specifications also receive attention and continuous improvement. In consequence, proven stability
and reliability have been recognised in industry.
Fig. 3: schematic of energy levels and electrical series connection of a tandem arrangement for solar
photoelectrolysis. As stated in the text, the two photosystems are also in optical series.
Dye-Sensitised Nanocrystalline Solar Cells, M. Grätzel, EPFL Seite 109 von 288