Solar Cells Can Take the Heat

Jan 09, 2008

Solar cells have attracted global attention as one of the cornerstones of alternative energy. In theory, it seems to make abundant sense to tap into the energy of the sun to convert light to electricity with little or no emission of noxious pollutants.

However, in practical terms, progress has been slow because of technological impediments and the many different factors that need to be optimized to obtain stable and high-efficiency devices.

Dye-sensitized solar cells based on dye molecules adsorbed onto titanium dioxide electrodes have emerged as one of the most attractive solar-cell constructs, combining low cost and relative ease of fabrication with high-efficiency performance. Indeed, state-of-the-art solar cells with this architecture show greater than 11% light-conversion efficiency.

However, the catch is that these high-efficiency solar cells typically use volatile organic solvents as electrolytes, which essentially precludes their use in outdoor applications because of the high vapor pressure of the solvents. Solvent-free solar cells fabricated so far show poor performance owing to the high viscosity of the alternative electrolytes.

Now, a team of researchers at the Ecole Polytechnique Fédérale de Lausanne in Switzerland have fabricated a solvent-free dye-sensitized solar cell based on a binary ionic liquid electrolyte. These devices show a light-conversion efficiency of 7.6% under simulated sunlight conditions, which sets a new record for a solvent-free device.

Shaik Zakeeruddin, Michael Grätzel, and their colleagues have used a mixture of two ionic liquids as the redox electrolyte in conjunction with a novel highly conjugated ruthenium-based dye to prepare a solvent-free solar cell.

Ionic liquids essentially have a vapor pressure of zero, which makes them preferable even to robust and low-volatile organic solvents. However, the typically high viscosities of ionic liquids and the fact that most of the promising ionic liquids are composed of iodide ions have precluded their application in viable cells since iodide ions tend to have a deleterious impact upon the photoconversion efficiency by quenching the dye sensitizer.

Grätzel and co-workers have tackled these challenges by adding in a low-viscosity ionic liquid comprising an inert anion. The low viscosity helps to overcome mass-transfer challenges, whereas the inert anion counteracts the deleterious influence of the iodide anions.

The obtained solar cells show a record 7.6% conversion efficiency. More importantly, these cells are stable at 80 °C in the dark and under visible-light soaking at 60 °C for over a thousand hours, which points to the possibility of their use in outdoor applications in warm climates. The solvent-free nature of the electrolyte also suggests that it may be possible to construct flexible and lightweight devices based on these electrolytes.

“This is a big step in the search for nonvolatile electrolytes”, said Zakeeruddin, adding that the performance of devices based on ionic liquids had lagged so far behind that it was widely thought that these systems would be limited to indoor applications at low lighting levels. Grätzel added that the results indicate that it should be possible to further optimize the performance of these solvent-free systems.

Citation: Michael Grätzel, Stable, High-Efficiency Ionic-Liquid-Based Mesoscopic Dye-Sensitized Solar Cells, Small 2007, 3, No. 12, 2094–2102, doi: 10.1002/smll.200700211

Source: Wiley-VCH

Explore further: Ice cream goes Southern, okra extracts may increase shelf-life

add to favorites email to friend print save as pdf

Related Stories

Nano-supercapacitors for electric cars

Jul 01, 2014

Innovative nano-material based supercapacitors are set to bring mass market appeal a good step closer to the lukewarm public interest in Germany. This movement is currently being motivated by the advancements ...

Surprising nanotubes: Some slippery, some sticky

Jun 01, 2014

Nanotubes—microscopic cylinders the shape of drinking straws, but just one-thousandth the diameter of a human hair—have been the subject of intensive research, with potential uses ranging from solar cells ...

The Future in Two Words: Ionic Liquids

Nov 13, 2009

(PhysOrg.com) -- Ionic liquids are molecular solutions that have a wide range of potential applications, including next-generation solar cells, hydrogen fuel cells and lithium batteries.

Recommended for you

The fluorescent fingerprint of plastics

8 hours ago

LMU researchers have developed a new process which will greatly simplify the process of sorting plastics in recycling plants. The method enables automated identification of polymers, facilitating rapid separation ...

Water and sunlight the formula for sustainable fuel

12 hours ago

An Australian National University (ANU) team has successfully replicated one of the crucial steps in photosynthesis, opening the way for biological systems powered by sunlight which could manufacture hydrogen ...

Rice chemist wins 'Nobel Prize of Cyprus'

12 hours ago

Rice University organic chemist K.C. Nicolaou has earned three prestigious international honors, including the Nemitsas Prize, the highest honor a Cypriot scientist can receive and one of the most prestigious ...

Researchers create engineered energy absorbing material

14 hours ago

(Phys.org) —Materials like solid gels and porous foams are used for padding and cushioning, but each has its own advantages and limitations. Gels are effective as padding but are relatively heavy; gel performance ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Doug_Huffman
not rated yet Jan 09, 2008
The Solar Constant is less than 1.35 kW m^-2 at the collector. Deciding how much power to accumulate decides how many acres to pave over.
Ashibayai
not rated yet Jan 09, 2008
It's less than 1.35kW/m^2 at the edge of the atmosphere. At the surface it's much less, and you still have the variable efficiency of the energy conversion and storage. If you can define all those AND how much power you want, then you can how many acres to pave.