ZSW boosts efficiency of cadmium-free thin-film cells to world record level

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Germany’s Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) has managed to boost the efficiency of cadmium-free CIGS thin-film solar cells to 21%.

Scientists at the Stuttgart-based research center replaced the system of intermediate films consisting of cadmium sulfide and zinc oxide with a combination of zinc oxysulfide and zinc magnesium oxide to achieve the score.

"This combination promises to harvest even more light than the material used in earlier CIGS cells," ZSW said in a statement, adding that with the improvement, its researchers "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 copper indium gallium diselenide (CIGS) is 21.7%. ZSW scientists set this record and have now come very close to matching it with the new cell type. Prof. 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 cadmium sulfide. In theory, we could use it to achieve even higher efficiency than with previous CIGS cells. 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. This means more sunlight penetrates to the underlying CIGS absorber layer, which then converts more light energy into electricity, according to ZSW.

Another innovation in the cell is its improved front contact. The researchers made it 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 0.5 square centimeters, 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 said. German engineering group Manz AG, ZSW’s industry partner, has already made the first test modules.

ZSW researchers expect the German-made modules to hit the market in a few years.

ZSW, which comprises sites in Stuttgart, Ulm and Widderstall, 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.

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