09 November 2011
Researchers led by Michael Grätzel from École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have developed a new design for dye-sensitized solar cells, bringing the devices closer to the efficiency of standard silicon-based solar cells.
Dye-sensitized solar cells consist of a titanium oxide nanocrystalline film coated with light-absorbing dye molecules and immersed in an electrolyte solution. These solar cells have separate molecules for light absorption and charge transport, which makes them potentially easier and less costly to manufacture than silicon-based solar cells. However, dye-sensitized solar cells can’t yet match the efficiency of silicon solar cells.
Green porphyrin in a dye-sensitized, transparent, high-efficiency solar module. Image courtesy of Michael Grätzel.The EPFL researchers along with colleagues from National Chung Hsing University and National Chiao Tung University, both in China, boosted the efficiency of dye-sensitized solar cells by using a cobalt (II/III) redox electrolyte and a new zinc porphyrin dye based on YD2. YD2 has a donor-bridge-acceptor structure with a diarylamine group as the electron donor, an ethynylbenzoic acid moiety as the acceptor, and the porphyrin chromophore as the bridge. Donor-bridge-acceptor dyes don’t contain expensive rare metal atoms and tend to absorb light more strongly.
The researchers created a variant called YD2-o-C8 that incorporates two octyloxy groups. Its molecular design slows the rate of electron transfer from the titanium dioxide film to the oxidized cobalt mediator and produced photovoltages of close to 1 volt. Combining YD2-o-C8 with another donor-bridge-acceptor dye (Y123) that has an absorption spectrum complementary to that of their new variant further improved the power conversion efficiency of the photovoltaic device to 12.3% under simulated air mass 1.5 global sunlight.
The research shows that the best sensitizing dyes don’t have to contain ruthenium, and the best mediators don’t have to contain iodide. In a Science perspective article Michael D. McGehee notes that as scientists build on this work, dye-sensitized solar cell efficiency could climb toward 15%.
Research Paper: Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency, Science, Vol. 334 no. 6056, pp. 629-634.DOI: 10.1126/science.1209688.
Written by Nancy Lamontagne, Contributing Editor - US, Solar Novus Today