Swiss and Chinese Researchers Hit New Efficiency Benchmark for Solvent-Free Dye-Sensitized Solar Cells

30 June 2008

EPFL (École Polytechnique Fédérale de Lausanne) professor Michael Grätzel, Shaik Zakeeruddin and colleagues from the Changchun Institute of Applied Chemistry at the Chinese Academy of Sciences (CAS) have achieved a record light conversion efficiency of 8.2% in solvent-free dye-sensitized solar cells. A report on their work was published online 29 June in the journal Nature Materials.

Dye-sensitized solar cell technology, developed by Grätzel at EPFL in the 1990s, shows promise as an inexpensive alternative to silicon solar cells. Currently, state-of-the-art dye-sensitized cells have achieved a highest validated efficiency of 11.1%—still about two times lower than silicon cell technology—and remarkable stability.

However, the cells with the best performance use volatile solvents in their electrolytes and must be carefully sealed. This, along with the fact that the solvents permeate plastics, has precluded large-scale outdoor application and integration into flexible structures.

Solvent-free room-temperature ionic liquids have been pursued as an attractive solution to this dilemma, and device efficiencies of more than 7% were achieved by using some low-viscosity formulations. Unfortunately, the authors note in their paper, apart from tetracyanoborate, all of these low-viscosity melts proved to be unstable under prolonged thermal stress and light soaking.

To overcome these limitations, Grätzel and his colleagues developed a new concept—a mixture of three solid salts as an alternative to using organic solvents as an electrolyte solution. When the three solid components are mixed together in the right proportion they turn into a melt showing excellent stability and efficiency.

This breakthrough in efficiency without the use of volatile organic solvents will make it possible to pursue large scale, outdoor practical application of lightweight, inexpensive, flexible dye-sensitized solar films that are stable over long periods of light and heat exposure.

Resources

• Yu Bai, Yiming Cao, Jing Zhang, Mingkui Wang, Renzhi Li, Peng Wang, Shaik M. Zakeeruddin & Michael Grätzel, “High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts”, Nature Materials  29 June 2008 doi:10.1038/nmat2224