Tandem Photovoltaics –
The Road to Higher Conversion Efficiencies

Photos: ©Fraunhofer ISE, ©Fraunhofer ISE/Dirk Mahler

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.

Why Tandem?

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.

Our Research and Development

Fraunhofer ISE is the largest solar energy research institute in Europe and nearly half of all our research activities are in the field of photovoltaics. Our wide-ranging in-house expertise in this area enables us to offer research services on all aspects of the technology and provide our customers with comprehensive expert solutions. Our research in tandem photovoltaics includes:

Perovskite Tandem Solar Cells (Pero-on-Si, Pero/Pero)

Perovskite-based tandem solar cells are promising candidates for providing high efficiencies at low costs and thus wide-spread solar energy installation. To meet this challenge, our research at the Center for High-Efficiency Solar Cells and our PeroLab facility focuses on perovskite-silicon and perovskite-perovskite tandem solar cells as well as the development of silicon bottom cells tailored for the specific tandem technology.

III-V Tandem Solar Cells (III-V-on-Si, III-V/III-V)

Multi-junction solar cells made of III-V semiconductors have the potential to take efficiencies to new, previously unimaginable levels approaching conversion efficiencies of 50 % under concentrated solar irradiation. Inspired by this, we have been successfully working on this technology for decades. Our focus at the Center for High-Efficiency Solar Cells is on the development of III-V on Si and III-V/III-V tandem solar cells with the goal of high efficiency and lower costs.

Production Technology

Fraunhofer ISE has long-term experience in scaling up PV production processes. Our pilot line facility PV-TEC features a high throughput baseline industrial solar cell production process along the entire process chain with the possibility to carry out new processes using innovative functions. The service center has been created to support the German and European photovoltaic industry with large scale research opportunities and features high-quality characterization tools for intensive process characterization.

Characterization

We apply optical, electrical and microstructural methods for quantitative quality assessment and process optimization along the entire process chain of the tandem technology – from the materials (e.g. perovskites or III-V compounds) and solar cell development up to the production technology of solar cells and modules.

Module Technology

Module technology is a critical step towards the widespread and reliable use of photovoltaics. Besides carrying out product development and material qualification, we are also looking to develop new module concepts for perovskite-based solar cells, including in-situ concepts, using our expertise to address the special requirements of the different cell technologies as well as the specified PV application.

Calibration and Durability Tests

Complementing its research and development activities, Fraunhofer ISE has special accredited laboratories as CalLab PV Cells and CalLab PV Modules which perform testing and certification procedures on solar cells and modules for its own research purposes as well as for commercial enterprises and research institutes. 

Applications

Integrated photovoltaics opens up many new opportunities which are advantageous for the energy transition and facilitate widespread dissemination of photovoltaics. For example, new types of technology and design options allow free choice of formats and colors for integrated modules, such that their connection circuits and cell architecture can be completely covered if necessary. Tandem photovoltaics demonstrates the potential to open up new opportunities for the wide-spread use of photovoltaics. 

Contact

Stefan Glunz

Contact Press / Media

Prof. Dr. Stefan Glunz

Director, Division Photovoltaics­ Research

Fraunhofer ISE
Heidenhofstr. 2
79110 Freiburg

Phone +49 761 4588-5191