ZSW (Zentrum für Sonnenenergie- und Wasserstoff-Forschung - or Center for Solar Energy and Hydrogen Research - Baden-Württemberg) of Stuttgart, Germany has boosted the efficiency of cadmium-free copper indium gallium diselenide (CIGS) thin-film solar cells to a record 21%.
The researchers achieved this performance by replacing the system of intermediate films consisting of cadmium sulfide (CdS) and zinc oxide (ZnO) with a combination of zinc oxysulfide and zinc magnesium oxide. This combination promises to harvest even more light than the material used in earlier CIGS cells, it is reckoned.
The record for conventional CIGS solar cells made of 21.7% was set last September by ZSW, which has now come very close to matching that with the new type of cell. Professor Michael Powalla, ZSW board member and head of the Photovoltaics division, believes that the lack of heavy metals in the new cell's buffer layer is an advantage, but not the key benefit. The metal in conventional CIGS modules is chemically bound. "First and foremost, the buffer layer transmits more light without the cadmium sulfide," he says. "In theory, we could use it to achieve even higher efficiency than with previous CIGS cells," Powalla adds. "The alternative buffer layer and the cadmium sulfide buffer are both deposited in a chemical bath, so a transition to manufacturing is possible without requiring additional processes."
As a buffer layer, zinc oxide-sulfide is more transparent to light in the blue wavelength range. So, more sunlight penetrates to the underlying CIGS absorber layer, which then converts more light energy into electricity. Another innovation in the cell is its improved front contact, made with zinc magnesium oxide in place of the high-resistance thin zinc oxide film. Made in a ZSW manufacturing lab, the solar cell has a surface area of 0.5cm2 (a standard size for experimental cells). The results was confirmed by the Fraunhofer Institute for Solar Energy Systems ISE.
Alternative CIGS technology is still in the initial stages of development, so a further significant increase in efficiency is possible, reckon the researchers. ZSW's industry partner Manz AG of Reutlingen, Germany has already made the first test modules. ZSW expects that the modules made in southwest Germany can be brought to market within a few years.
