Copper(I) polypyridine complexes. the sensitizers of the future for dye-sensitized solar cells (DSSCs)

The thesis concerns the development of novel dyes for the photosensitization of titanium
dioxide for incorporation into dyes-sensitized solar cells (DSSCs). The majority of dyes
utilized to date are based upon heavy transition metals such as ruthenium. Although these
are efficient (>10%), they have disadvantages in the cost of the materials and also in the
availability of the very rare platinum group metals. The thesis investigates the use of
copper(I) complexes of oligopyridines, which are known to have similar photophysical
properties to ruthenium(II) tris(oligopyridine) species but which have not been widely
used in solar cells. The labile nature of the copper(I) centre precluded previous systematic
investigation of the complexes for this application.
Synthetic methods for a series of carboxylate and phosphonate functionalized 2,2'-
bipyridine ligands with substituents at the 6- and 6'-positions have been developed. The
carboxylate or phosphonate functionality is required for the binding of the ligands and
complexes to the titanium dioxide surface and the substituents adjacent to the nitrogen
stabilise the photoexcited state with respect to quenching and oxidation.
DSSCs were prepared using the basic protocols established for ruthenium dye-sensitizers,
involving the doctor-blading of a TiO2 paste onto an ITO or FTO electrode followed by
annealing, absorption of the dye and cell construction with conventional iodide-triiodide
electrolyte. The cells were tested in Basel using a home-built cell tester or a modified
scanning electrochemical microscope and in Lausanne at the EPFL in the laboratory of
Prof. Michael Graetzel using an industry standard protocol. The surprising results were
that the copper-functionalised DSSCs had efficiencies approaching 2.5% for the
prototype compounds.
The lability of the copper(I) complexes has allowed us to develop an entirely novel
strategy for the design of solar cells in which the carboxylated or phosphonated ligand L
is first attached to the TiO2 surface and subsequently metallated by reaction with any
[CuL'2] complex to give a surface bound [CuLL'] species and the method is likely to lead
to the future use of libraries of complexes to achieve full spectrum coverage rather than
the design of "black" dyes.