TNO, Eindhoven University of Technology, imec and TU Delft – partners in Solliance – have joined forces to further increase the conversion efficiency of tandem solar cells beyond the limits of current commercial photovoltaic (PV) modules. Four-terminal perovskite/silicon tandems, with the top cell certified, break the 30% barrier for the first time. Such a high efficiency makes it possible to realize more power per m2 and less costs per kWh. The result was presented at the 8th World Conference on Photovoltaic Energy Conversion (WCPEC-8) in Milan and was achieved by combining the newly emerging perovskite solar cell technology with conventional silicon solar cell technology. The perovskite cell which has transparent contacts and forms the stacked tandem has been independently certified.
Achieving high power density will create more opportunities to integrate these solar cells into construction and building elements, so that more existing surfaces can be covered with PV modules. Breaking the 30% barrier is therefore a major step in accelerating the energy transition and improving energy security, as it reduces our dependence on fossil fuels.
The best of both worlds
Higher efficiencies can be achieved with tandems than with single solar cells; after all, the solar spectrum is better utilized. The currently emerging tandems combine commercial silicon technology for the bottom module with a perovskite technology on top: the latter offers a very efficient conversion of ultraviolet and visible light and excellent transparency for near infrared light. In tandems with four terminals (4T: four electrical contacts), the upper and lower cells operate independently of each other, making it possible to use different lower cells in this type of module. Commercial PERC technology as well as high-end technologies such as heterojunction, TOPCon or thin film technology such as CIGS can be applied in a 4T tandem without significant changes to the solar cells. In addition, the four-terminal architecture makes it easy to implement bifacial (two-sided light-capturing) tandems, which further increase energy yield.
Researchers from the Netherlands and Belgium managed to improve the efficiency of the semi-transparent perovskite cells to 19.7% with a surface area of 3×3 mm2 and obtained certification of this value from ISPRA (Italy). ‘This type of solar cell has a highly transparent back contact that allows more than 93% of the near-infrared light to reach the bottom module. This result was achieved by first optimizing all layers of the semi-transparent perovskite solar cells using advanced optical and electrical simulations, as a guideline for the experimental work in the laboratory,’ says Mehrdad Najafi of TNO. “The silicon module is a 20 x 20 mm2 heterojunction solar cell with optimized surface passivation, transparent conductive oxides and copper-plated front contacts for state-of-the-art conduction of electric charge,” says Yifeng Zhao, PhD student at TU Delft, whose results recently assessed by colleagues. The silicon module optically under the perovskite contributes 10.4% efficiency points to the total conversion of solar energy. Combined, the conversion efficiency of these “non-area matched” 4T tandems, operating independently, is 30.1%. This best return in the world has been measured according to generally accepted procedures.
Future of tandem PV modules with four terminals
But that’s not all. Combining this highly transparent perovskite cell with other silicon-based technologies such as back contact cells and TOPCon solar cells has also yielded conversion efficiency values approaching 30%. This proves the potential of these flexible, highly transparent perovskite solar cells to be combined with a variety of already commercialized technologies.
These best efficiency values in the world, achieved by combining a large number of established technologies, are another milestone towards industrial application. ‘We now know the ingredients and can control the layering required to achieve an efficiency of more than 30%. Combined with the scalability expertise and knowledge we have gained in recent years to apply materials and processes on a large scale, we can focus together with our industrial partners on the mass production of this technology with an efficiency of more than 30%, ‘ said Gianluca Coletti, Tandem Technology & Application program manager at TNO.
More information
Would you like to know more about Delft’s contribution to this new record? Please contact:
- Olindo Isabellahead of the Photovoltaic Materials & Devices group – O.Isabella@tudelft.nl / +31 15 27 81947