Increasing Solar Cell Efficiencies

During last 40 years, Fraunhofer ISE has been conducting cutting-edge research in photovoltaics. It has several significant contributions in reducing the costs of solar electricity…

The new III-V//Si tandem solar cell has 35.9% efficiency. The top subcell glows red, which is a sign of outstanding material quality. The nanostructured back side of the cell shimmers in rainbow colours... © Fraunhofer ISE / Photo: Michael Schachtner

Photovoltaics, along with wind energy, is the central pillar of the energy transition. Aiming to further reduce the area required for PV installations as well as material use, Fraunhofer ISE is focusing on tandem photovoltaics. This cell technology allows the traditional efficiency limits to be exceeded by selectively combining different solar cell materials. The Freiburg researchers have succeeded in setting a new world record with a monolithic tandem cell made of III-V and silicon semiconductors.

The new monolithic triple-junction solar cell – or more specifically, a III-V//Si tandem solar cell – converts 35.9% of sunlight into electrical energy. This value measured under the terrestrial AM1.5g spectrum, sets a new world record and demonstrates the potential of silicon-based tandem photovoltaics. In the new record-breaking cell, the III-V semiconductor layers are connected directly to the silicon sub-cell on the atomic level. From the outside, the record cell resembles a two-terminal conventional solar cell. This two-terminal cell achieves the same efficiency as the best four-terminal solar cells with mechanically stacked structures, as published jointly by NREL, CSEM and EPFL in 2017. “The use of a new compound semiconductor (GaInAsP) for the middle cell was a key step in our success in achieving the improved efficiency value. The new material allowed us to further improve the lifetime of the charge carriers and thus achieve a higher cell voltage. It’s great to see how our material development has successfully contributed to improvements in III-V//Si triple-junction solar cells,” said Patrick Schygulla, doctoral student in the Department of III-V Photovoltaics and Concentrator Technology at Fraunhofer ISE.

I-V characteristic of the new III-V//Si triplejunction solar cell, measured at Fraunhofer ISE
CalLab PV Cells… © Fraunhofer ISE

Initial applications for high efficiency III-V//Si tandem cells are found where the power generation per area plays an important role, for example, in electrically powered aircraft and drones. Today, the production costs of the new cells are still significantly higher than conventional single-junction crystalline silicon solar cells. This is due to the complex epitaxy step of the III-V layers and the many additional semiconductor processes required to manufacture the cells. The researchers at Fraunhofer ISE are working intensively to make the production more cost-effective in the future and thus also address the terrestrial photovoltaic market.

The rainbow colours show the diffraction of sunlight by a mirror with a nanostructured grating, which was applied to the back of the silicon subcell. The sun’s spectrum is thus captured even better in the silicon bottom cell… © Fraunhofer ISE / Photo: Michael Schachtner

“Combining III-V semiconductor materials on silicon is one of the approaches that we are pursuing with tandem structures (i.e., the combination of different high-performance materials) in order to achieve higher solar cell efficiencies. It will be a few years before any PV modules made from the type of solar cell presented here are available on the market; however, this is an important, forward-looking path in the context of the photovoltaic expansion, which is necessary for a sustainable energy supply,” explained Prof. Andreas Bett, Institute Director of Fraunhofer ISE.

Initial applications for high efficiency III-V//Si tandem cells are found where the power generation per area plays an important role, for example, in electrically powered aircraft and drones.

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