A Japanese institute announced that that it confirmed a cell conversion efficiency of 17.7% by using a CIGS solar cell made with a flexible substrate.
The efficiency of the solar cell, which was developed by Japan's National Institute of Advanced Industrial Science and Technology (AIST), is one of the highest in the world for a flexible CIGS solar cell.
The production of a flexible CIGS solar cell has thus far faced an issue in the formation of a p-type semiconductor, according AIST. The carrier density of a p-type semiconductor in a CIGS solar cell is controlled by adding an alkali metal such as sodium.
Specifically, sodium selenide or sodium fluoride is added to the semiconductor. But Na2Se and NaF are unstable and only have a poor reproducibility, AIST said.
This time, AIST developed a technology called "alkali-silicate glass thin layer (ASTL) method." According to this method, a silicate glass layer is first formed on the substrate, and the amount of alkali metal that passes through the backside electrode layer and diffuses into the light absorbing layer is controlled by adjusting the film formation conditions of the silicate glass layer.
The method facilitates the addition of alkali metal and increases the reproducibility, thus significantly improving the conversion efficiency of the solar cell, AIST said.
AIST used three kinds of substrates, namely, a ceramic substrate, a transparent plastic film provided by Teijin Ltd and a titanium foil with a coarse surface. The cell conversion efficiency of 17.7% was achieved when using the ceramic substrate. With the use of the plastic film and the Ti foil, the efficiencies were 14.7% and 17.4%, respectively.
Details of the technology will be unveiled at the 4th Annual Symposium of Research Center for Photovoltaics, which runs from July 28-29, 2008, at the National Museum of Emerging Science and Innovation.