While perovskite-on-silicon tandem solar cells are currently the subject of plenty of commercial interest, the academic community is already looking further ahead. All perovskite tandem cells – where two different perovskite cells are stacked on top of each other – promise similar efficiencies well past the 30% mark, with better flexibility, much lighter weight, and a lower environmental impact than technologies relying on silicon wafer.
Mixed lead-tin perovskites are known to have the right narrow bandgap to be used as the top cell in a tandem device. But despite similar theoretical efficiency potential, the development of these materials has lagged behind pure lead perovskites. One reason for this, according to scientists at China’s Nanjing University, is that the tin tends to oxidize during fabrication of the film, leading to high levels of defects and non-uniformity in the film. “Defective grain surfaces are vulnerable to trap generation and Sn2+ oxidation,” state the group led by Nanjing University Professor Hairen Tan. “And this works against the stability, efficiency, and scaling of mixed Pb–Sn perovskite solar cells and all-perovskite tandems.”
The group theorized that by adding a small amount of FSA – a chemical used as a reducing agent by the textile industry, to the precursor solution, they would be able to slow the crystallization process leading to more uniform growth and passivate many of the surface defects. “We chose FSA because it unites functionalities to improve the uniformity, electronic quality and stability of mixed Pb–Sn films,” states the group.
The group went on to fabricate perovskite solar cells with these films, the best of which achieved 21.7% efficiency as a single-junction cell measuring 1.05cm². They went on to incorporate these into tandem cells with an all-lead perovskite as the bottom cell, achieving 24.2% efficiency, certified by JET Laboratories in Japan, and reporting a top in-lab efficiency of 25.6% for cells measuring 1.05cm². The experiments are described in the paper All-perovskite tandem solar cells with 24.2% certified efficiency and area over 1 cm2 using surface-anchoring zwitterionic antioxidant, published in Nature Energy.
To evaluate the potential for larger devices based on this technology, the group also fabricated 12cm² devices that achieved a top efficiency of 21.4%. They note, however, that scalable production methods would need to be developed to investigate this further. These large-area cells retained 88% of their initial performance after 500 hours under constant 1-sun illumination, while control samples produced without the FSA additive lost more than half of their initial performance after just 90 hours in operation.
Thermal stress testing proved more difficult for the tandem cells, however with a pressure-tight encapsulation the cell retained 93% of its initial performance after 228 hours at 85 degrees Celsius. The group noted that, as well as further experimentations with the perovskite material, work on the overall device structure will be key to further improving long-term stability in all-perovskite tandem cells.
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