Over the last few years, scientists have been working hard to develop dye-sensitized solar cells (DSSCs) that are both cheaper and easier to manufacture than conventional photovoltaic cells. To date, the best results in this field were based on organic compounds of Ru, unfortunately a rather expensive element. Now, researchers from the University of Basel and the École Polytechnique Fédérale de Lausanne in Switzerland have synthesized complexes using oligopyridine ligands and the much cheaper element, Cu, to produce a material that can be used as a dye-stuff in DSSCs [Bessho et al., Chem. Commun., doi: 10.1039/b808491b].
Whereas in conventional photovoltaic systems both light absorption and charge carrier transport occur in the same material, these two tasks are accomplished by separate substances in DSSCs. The sensitizer – a metal-organic dye-stuff – absorbs photons and releases electrons. The semiconductor only provides the potential barrier to separate the charges and conduct the electrons in order to create a current. Since both systems need to interact closely, layers of sensitizer molecules are coated on nanocrystalline particles of the semiconductor, TiO2. This arrangement makes production much easier, and is suited especially for thin-film solar cells.
Presently, state-of-the-art Ru-based DSSCs possess conversion efficiencies of about 11% (the values for common low-cost Si cells range from 12 and 15%, flexible thin-film cells operate at around 8%). Their newly-developed Cu-counterparts do not reach that value, but further development and refinement may lead to Cu-DSSCs with at least comparable efficiencies. “A number of factors are in favour of the copper systems,” says Edwin Constable. “The ideal is a lower price and higher efficiency,” he explains.