Japan's National Institute of Advanced Industrial Science and Technology (AIST) developed a tandem type dye-sensitized solar cell with an energy conversion efficiency of 11.0%. The new solar cell outperforms any existing solar cells, according to AIST.

A tandem type solar cell has a structure in which two types of dye-sensitized solar cells are attached to each other. To enhance the conversion efficiency, AIST formed a highly transparent TiO2 electrode and used it in the upper cell.

A tandem type dye-sensitized solar cell can utilize a wider range of sunlight spectrum compared with the standard single cell type products. The new solar cell uses red (N719) and black (N749) sensitizing dyes in the upper and lower cells, respectively.

The upper cell utilizes visible light to generate a high voltage while the lower cell uses light in the near-infrared and infrared regions with wavelength longer than that of visible light. The lower cell generates a higher current although the voltage is lower.

The upper cell in a tandem type solar cell needs to transmit near-infrared light without loss while absorbing visible light. AIST succeeded in the formation of highly transparent TiO2 electrode with the adoption of new production process. The lower cell has a structure in which semiconductor films made of titanium oxide particles with different sizes are stacked on top of one another.

This structure enhances the "light trapping effect" to confine photons inside the cell, thereby increasing the current. To achieve a higher voltage, AIST also developed a method to control the leakage current.

In order to commercialize the new solar cell, AIST plans to promote the simplification of production process and the shape as well as the cost reduction while maintaining the high energy conversion efficiency. The latest cell uses dyes suitable for single cell type products, so the conversion efficiency is expected to improve by developing a dye optimized for tandem type cells, AIST said.

In particular, AIST intends to develop a new sensitizing dye that can efficiently utilize light in the longer wavelength region.

AIST will present the new technology at the 75th Meeting of the Electrochemical Society of Japan, which runs from March 29 to 31, 2008, in Kofu City, Yamanashi Prefecture, Japan.