Better Dye Yields Better Solar Cell

By judicious modification of a dye molecule to better absorb light, and use of a more efficient electron-transfer reagent, a multi-institution research team has created a new class of dye-sensitized solar cell (DSC) that sets a record for power conversion efficiency. The achievement pushes DSC efficiency closer to—but still only about half of—that of standard silicon-based solar cells.

Unlike silicon solar cells, DSCs use separate molecules for absorbing light and electron transport, which makes them more versatile, easier to make, and less costly. Until now, ruthenium complexes have typically been used as a light-absorbing dye to transfer electrons to a titanium dioxide nanocrystal semiconductor layer on an electrode. An iodide/triiodide (I^-/I₃^-) redox shuttle completes the circuit by mediating electron transfer from a counterelectrode back to the dye.

In the new DSC, the researchers turned to a zinc porphyrin and used a cobalt(II/III) bipyridine redox shuttle (Science, DOI: 10.1126/science.1209688). The research team was led by Michael Grätzel and Shaik M. Zakeeruddin of the Swiss Federal Institute of Technology, Lausanne; Chen-Yu Yeh of Taiwan’s National Chung Hsing University; and Eric Wei-Guang Diau of Taiwan’s National Chiao Tung University.

Tailoring the porphyrin ring with octyloxy groups turned out to be an important part of the new DSC’s efficiency. The alkoxy chains inhibit the cobalt mediator’s access to the titanium dioxide layer, thereby reducing incidental back electron transfer to boost power conversion efficiency at higher voltage and current. The researchers achieved a record 12.3% efficiency, breaking the DSC record of 11.4% for ruthenium-iodide systems.

The development represents “a significant breakthrough in the field of dye-sensitized solar cells that will have a strong impact on research and perhaps also on industrial development of this technology,” says Gerrit Boschloo of Uppsala University, in Sweden. Last year, Boschloo and coworkers first showed that an alternative redox mediator—the cobalt bipyridine system—could beat the iodide/triiodide system in DSC ­efficiency (J. Am. Chem. Soc., DOI: 10.1021/ja1088869).