Mar. 10, 2008 - Japan's National Institute of Advanced Industrial Science and Technology (AIST) recently made a pair of announcements about its research into solar cell materials, saying it has achieved 11.0% energy conversion efficiency with a tandem type dye-sensitized solar cell, and also is making a new lightwave conversion material using europium.

The first news involves a tandem type solar cell, in which two types of dye-sensitized solar cells are attached to each other, explains Japan's TechOn!. The upper cell in a tandem type solar cell uses visible light to generate a high voltage, while the lower cell uses light in the near-infrared and infrared regions, generating a higher current though lower voltage. The upper cell needs to transmit near-infrared light without loss while also absorbing visible light.

In its work, AIST used a finer powder of highly transparent TiO₂ to form the film holding the dye, which reduced light scattering and allowed infrared light to pass more easily through the first layer down to the second, noted the Nikkei Business Daily. The lower cell incorporates semiconductor films made from different-sized stacked titanium oxide particles. The resulting structure confines photons inside the cell, increasing current; to achieve higher voltage AIST also developed a method to control leakage current.

To get the technology ready for commercialization, AIST needs to simplify the production process and the shape, as well as reduce costs, without losing the high energy conversion efficiency. The most recent version of the cell uses dyes suitable for single-cell type products, so optimizing a dye for tandem type cells, particularly one that can better utilize light in the longer wavelength region, should further improve efficiencies, according to AIST.

Meanwhile, AIST also is touting its joint work with Sanvic to make a light wavelength conversion material using a europium (Eu) metal complex for crystalline silicon solar cells, that helps the cells utilize a wider portion of the sunlight spectrum, notes TechOn!.

The work involves making a solar cell sealing sheet "kneaded" with the Eu complex, containing organic molecules coordinated around metal atoms. Researchers found that the Eu complex can absorb the UV part of sunlight and emit fluorescence -- and this wavelength conversion can help enhance efficiency of crystalline silicon solar cells. Applying the Eu metal complex (supplied by an unidentified chemical manufacturer) involves "kneading" it as a powder into beads of an ethylene vinyl acetate, which is then formed into sheets of sealing material, measuring 0.6mm x 115cm. "We had difficulty in preventing the Eu complex from reacting with other materials mixed in EVA," noted Masatoshi Kanesato, AIST team leader on the project.

Prototypes of solar cells with the new EVA sealing sheet have been installed on the roof of Sanvic's plant in Hamamatsu City, Shizuoka Prefecture, where durability tests are now underway. Early results indicate generation efficiency of the cells has been improved by 1.75%, they say.