ZSW boosts efficiency of cadmium-free CIGS solar cells
The Centre for Solar Energy and Hydrogen Research Baden-Wuerttemberg (ZSW) in Germany has managed to boost the efficiency of cadmium-free CIGS thin-film solar cells to 21.0 percent.
The Stuttgart scientists replaced the system of intermediate films consisting of CdS and ZnO with a combination of ZnOS and ZnMgO to achieve this performance. This combination promises to harvest even more light than the material used in earlier CIGS cells. With this improvement, the researchers at Stuttgart relegated their Japanese colleagues, the former record holders, to second place and took the lead in the global efficiency stakes.
The record for conventional solar cells made of CIGS is 21.7 percent. ZSW scientists set this record and have now come very close to matching it with the new cell type. Michael Powalla, ZSW board member and head of the Photovoltaics division, believes 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 CdS. In theory, we could use it to achieve even higher efficiency than with previous CIGS cells. The alternative buffer layer and the CdS buffer are both deposited in a chemical bath, so a transition to manufacturing is possible without requiring additional processes," he said
As a buffer layer, ZnOS is more transparent to light in the blue wavelength range. This means 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. The researchers made it with ZnMgO in place of the high resistance, thin ZnO film. Made in a ZSW manufacturing lab, the solar cell has a surface area of 0.5 square centimetres, a standard size for experimental cells. The Fraunhofer Institute for Solar Energy Systems ISE confirmed the results.
Alternative CIGS technology is still in the initial stages of development, so a further significant increase in efficiency is possible. ZSW's industry partner Manz AG has already made the first test modules. ZSW researchers expect that the modules made in southwest Germany can be brought to market within a few years.
The Centre for Solar Energy and Hydrogen Research Baden-Wuerttemberg (ZSW) is one of the leading institutes for applied research in the fields of photovoltaic energy, renewable fuels, battery technology, fuel cells and energy systems analysis. The three ZSW sites at Stuttgart, Ulm and Widderstall are currently staffed with around 230 scientists, engineers and technicians supported by 120 research and student assistants.