Tandem Photovoltaics – From the Laboratory into the World

Silicon solar cells can convert a physical maximum of 29.4 percent of sunlight into electricity. Today the silicon photovoltaic industry has come very close to reaching this theoretical limit. In order to continue making increases in solar cell efficiency, solar researchers around the world are now turning to tandem photovoltaics. In this technology two or more sub-cells made of different semiconductor materials are combined so that a larger spectrum of sunlight can be used. While the silicon solar cell primarily converts the red portion of sunlight efficiently into electricity, a top sub-cell made of perovskites, for example, can better utilize the blue portion of light.

In our key research topic "Tandem Photovoltaics", we describe the development, research challenges and global applications of this promising technology.

+ Stability and Service Life of Tandem Solar Cells and Modules

A performance guarantee of over 80% of the original power output after more than 25 years of service is a prerequisite for a marketable product that ensures long-term cost-effective and sustainable PV electricity.  In addition to the material stability of the solar cells, the interconnection and encapsulation processes  in particular are decisive factors in the production of a long-lasting product.  For perovskite-based tandem solar cells, material developments are currently still required in order to prevent premature cell degradation. In addition, the encapsulation process creates challenges in module production, as perovskite materials are particularly sensitive to moisture, high temperatures and mechanical stress.

+ Scalable and Cost-Effective Production Processes

Cost-efficient solutions for production technologies are key for the successful transfer of laboratory cells with record efficiencies into the industrial environment. To this end, researchers are evaluating processes and systems from the established silicon PV production with regard to their suitability for tandem photovoltaics. For III/V-based tandem solar cells, the development of fast epitaxial processes for cell fabrication and the cell's fast integration into concentrating modules and systems are central to market maturity.  Sustainable aspects such as saving energy and materials in the production process also play a role here.

+ Precise Power Measurement and Analysis of Cells and Modules

Tandem solar cells and modules are significantly more complex than single-junction silicon solar cells. This applies not only to their production, but also to the performance determination.  When several sub-cells are combined, new characterization methods must be developed that allow examinations of each of the sub-cells. Imaging methods that enable evaluations of the individual layers and interfaces are necessary for scaling up to industrial wafer sizes so that fast learning cycles in the development phase can be realized.  The power measurement technology must also be precise and fast, as cycle times of less than one second are required in industrial production.  Only with exact power measurements for cells and modules  can precise yield forecasts for integrated PV applications and large power plants be made. 

Each photovoltaic absorber material can optimally convert a limited wavelength range of sunlight into electricity, while the other wavelengths of the solar spectrum are converted poorly or not at all.  The optimal wavelength range for each material, e.g. red, blue, green or infrared light, depends on the material's electron band gap, an inherent property. For this reason, the maximum conversion efficiency of solar cells consisting of only one material is limited to a maximum of around 30 %, depending on the material.

Tandem or multi-junction solar cells use two or more photovoltaic absorber materials with different band gaps. By stacking two or more solar subcells on top of each other, the solar spectrum can be used much more efficiently. The upper solar cells have a large band gap and convert UV and blue light into electricity, while the lower solar cells in the stack have smaller band gaps and efficiently convert red and IR light into electricity. Together they can achieve much higher efficiencies.

Higher Efficiencies Save Resources

Higher solar cell efficiencies, and thus lower costs and resource requirements for solar power, are the aim of our research in tandem photovoltaics. Silicon solar cells have efficiencies of around 25 % today and in a few years will probably reach their achievable limit of around 27 % even in industrial production. However, higher module efficiency also means that the materials used, such as the module glass, aluminum frames, connecting cables and the mounting, are also used much more efficiently. Greater module efficiency also leads to more efficient land use. Certainly, the more efficient use of resources is a necessary prerequisite for the terrawatt photovoltaic world market, which is expected from 2030 on.