Tandem Photovoltaics –
The Road to Higher Conversion Efficiencies

©Fraunhofer ISE

Our research aim is to increase the energy conversion efficiency of photovoltaics and bring down the costs. Tandem photovoltaics holds great promise for achieving these goals and is among the fastest-developing solar technologies today. The high efficiency of tandem solar cells allows for more energy output per surface area, thus creating potential savings in solar cell and module materials – an important aspect in regard to the sustainability of photovoltaics.

The successful research on tandem photovoltaics began at Fraunhofer ISE over thirty years ago with III-V multi-junction photovoltaic cells, with which several record efficiency values have been reached. Recently new materials like the perovskites also lead to very high performance devices and the focus is now on optimizing the most promising photovoltaic absorbers to reach high efficiency and to bring tandem photovoltaics to large scale industrial production.

Combining this expertise with the long-standing successes in silicon cell research and development, Fraunhofer ISE has been moving forward to address the challenges facing this fast-paced technology. Our newest state-of-the-art facilities with over 1000 m² new laboratory space help us to maintain our international standing in this competitive field and bring on the energy transition.

Every photovoltaic absorber material has the ability to convert only a limited wavelength range of sunlight into electricity optimally while the other wavelengths of the sun’s spectrum are used poorly or not at all. This is mainly due to the electronic bandgap, an attribute inherent to the material. Therefore, the maximum conversion efficiency of solar cells made from only one material is limited to around 30 % at most, depending on the material.

Tandem or multi-junction solar cells utilize two or more photovoltaic absorbers each with different properties. By combining solar cells with different bandgaps into a so-called “tandem stack”, the sun’s spectrum can be used much more efficiently. As a rule, the solar cells at the top of the monolithic stack have a large bandgap and convert UV and blue light into electricity, while the solar cells at the bottom of the stack have smaller bandgaps and convert the red and IR light efficiently into electricity. This team of solar cells working in tandem makes it possible to reach much higher conversion efficiencies. At the same time the cells look like conventional solar cells as we are working solely on monolithically integrated devices – only one contact on the front and one on the back. Everything else is integrated into the device structure. This keeps things simple on the module integration level.