[JSAP] Optical Fiber Turns Into Solar Cell

A Japanese research group developed the "fiber-type TCO-less dye sensitized solar cell" and unveiled it at the 56th Spring Meeting, 2009 of the Japan Society of Applied Physics, which took place from March 30 to April 2, 2009, at the University of Tsukuba and other events.

The group is led by Shuji Hayase, professor at the Graduate School of Life Science and Systems Engineering of the Kyushu Institute of Technology (KIT).

The solar cell is composed of dye sensitized solar cell layers concentrically formed around a glass rod with a length of 3.5cm and a diameter of 9mm. Specifically, a layer of titanium oxide (TiO2) and a sensitizing pigment, a porous titanium (Ti) layer serving as an electrode (anode), a porous layer containing an iodine solution and other electrolytes, and a layer of platinum (Pt) and Ti serving as another electrode (cathode) are formed in this order around the rod. The solar cell thus obtained is covered with Ti except for both ends of the glass rod.

Light entering from one end of the glass rod is absorbed by the pigment in the solar cell and converted into electricity if it is at an angle that does not result in total reflection on the glass inner surface before exiting the other end of the rod.

At present, the solar cell has a conversion efficiency of slightly higher than 1% when using one type of pigment, Hayase said. The value may seem to be low, but the device made this time has a large diameter of 9mm, and the length of the portion serving as a solar cell is only about 1.5cm. As a result, about 90% of light incident from one end goes out from the other end without being converted.

In consideration of this fact, the net conversion efficiency of the solar cell itself is likely to reach about 10%. The problem of wasted light can be overcome by increasing the fiber length or by reducing the fiber diameter, he said.

Aimed at using near-infrared light

The main difference between the new solar cell and standard dye sensitized solar cells lies not in their shapes but rather in the fact that the former does not use a transparent electrode (TCO). KIT intended to "utilize near-infrared energy, which has not been used for existing dye sensitized solar cells, to generate electricity," Hayase said.

However, both ITO and FTO transparent electrodes have low transmittances to near-infrared light, even though they have high transmittances to visible light. The members of the research group came up with the idea for this new solar cell when they were searching for ways to make a solar cell without a TCO.

KIT has already prototyped a fiber-type solar cell with a so-called "tandem structure" by selectively applying two kinds of pigments having absorption peaks in different wavelength ranges. If a pigment with a high absorption rate can be found for each of the near-infrared and other ranges, "the number of contacts may be increased without limitation" by using the pigments in different parts of the fiber, Hayase said.