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

Ongoing Work and Results 2007
Dye development is particularly important, with attention to molecular engineering for improvement of
stability, particularly at higher temperatures. The substitution of hydrophobic side chains on the dye
molecule, so that traces of water in the ionic liquid electrolytes are not deleterous for cell performance
as mentioned above, has been further developed and is patent-protected (5). Similar protection has
been extended for coadsorbents such as decylphosphonate to be used in association with dyes in the
formation of the structured monomolecular sensitising layer on the mesoporous substrate (6). The
molecular structure of a dye in the “K-series” dyes with these side chains is shown in fig. 4.
Fig. 4: Molecular structure of the dye "K77". Note the tetrabutylamine counterion.
Electrolyte performance has also been enhanced recently. The particular objective is the long-duration
stability of cells in full sunlight operation, since the temperature on the photoactive surface may reach
80�C. Ionic liquid electrolytes, which are normally less volatile, were investigated for their applicability
for nanocrystalline dye-sensitized solar cells with an acceptably high current density of over 15
mA/cm 2 . The mass transport of tri-iodide had previously been considered a limiting factor because of
its lower diffusion coefficient and lower concentration in electrolytes compared with that for iodide.
Initial results obtained by simply replacing the organic solvent with some ionic liquids were disappointing.
In a recent publication (7) however, the EPFL team reports efficient and stable mesoscopic dyesensitized
solar cells by introducing a low-viscosity binary ionic liquid (1-propyl-3-methyl-imidazolium
iodide (PMII) and 1-ethyl-3-methyl-imidazolium tetracyanoborate (EMIB(CN)4) electrolyte in combination
with a new high-molar-extinction-coefficient dye, Ru(2,2'-bipyridine-4,4'-dicarboxylic acid)(4,4'bis(2-(4-tert-butyloxy-phenyl)ethenyl)
-2,2'-bipyridine) (NCS)2. The dependence of photovoltaic performance,
charge transport and electron lifetime on the composition of the binary ionic-liquid electrolyte
with different ratios of PMII/EMIB(CN)4 were investigated by electrochemical impedance and
photovoltage transient techniques. A photovoltaic conversion efficiency of 7.6 % was obtained under
simulated full sunlight illumination, which is a record for solvent-free DSCs. These devices exhibit excellent
stability at 80ºC in darkness or under visible-light soaking at 60 degrees C during 1000 hours of
accelerated testing.
Fig. 5: Stability test: photovoltaic parameter variations (Jsc, Voc, FF and �) with aging time for the
device based on dye K77 and the non-volatile electrolyte (Z646) subjected to aging under light
soaking at 60 °C. (8)
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Dye-Sensitised Nanocrystalline Solar Cells, M. Grätzel, EPFL
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