Tandem photovoltaics is a relatively new field in solar technology in which high-performance PV cell materials are used together in various combinations in order to capture the solar spectrum even more efficiently. The Fraunhofer Institute for Solar Energy Systems (ISE) has now achieved record-breaking solar efficiency of 25.9 percent for a III-V/Si tandem solar cell grown directly onto silicon.

Pure III-V semiconductor solar cells are already used in space and in concentrator photovoltaics. Silicon absorbs the infrared part of the spectrum and layers of III-V materials are deposited on top, which convert the ultraviolet, visible and near infrared light into electricity.

Fraunhofer researchers have been developing the technology since 2007 with TU Ilmenau, Philipps University Marburg and the company Aixtron, in projects funded by the German Federal Ministry of Education and Research (BMBF). Fraunhofer ISE had previously achieved 34.5 percent efficiency for a tandem solar cell in which the III-V semiconductor layers were transferred from a gallium arsenide substrate to silicon connected by special wafers, but this technique proved too expensive for general photovoltaics.

The new tandem solar cells, developed by student researcher Markus Feifel, can be made inexpensively. The III-V wafer layers are grown directly onto a special silicon wafer (produced by the Danish company Topsil) and treated in a simple process using inexpensive grinding and etching.

Commenting on Feifel’s discovery in a press release, head of the Photovoltaics Research Division Prof. Stefan Glunz says: "Not only are we working to make silicon solar cell production more sustainable and cost-effective, but we are also breaking new ground. We are aiming to reach even higher efficiencies using the tried and tested silicon in combination with other semiconductor materials and have achieved this in the field of tandem photovoltaics.” Tandem photovoltaics is also ideally suited for electrolysis. Thus, this technology also contributes to the production of hydrogen as an energy storage medium and an important building block for the energy turnaround.