A team of researchers, led by China’s Nanjing University, have found that a chemical most commonly used in the textile industry can also serve as a performance-enhancing additive for mixed lead/tin perovskite thin films. They have used this additive to create a two perovskite tandem cell measuring 1.05cm² that achieved 24.2% efficiency.
Mixed lead-tin perovskites are known to have the right narrow bandgap for use as the top cell in a tandem device. However, despite similar theoretical efficiency potential, the development of these materials has stayed behind that of pure lead perovskites. One reason for this, according to scientists, 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 hypothesized 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 fabricated 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.
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 said, however, that scalable production methods would have to be developed in order 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, but with a pressure-tight encapsulation the cell retained 93% of its initial performance after 228 hours at 85 degrees Celsius. The group stated that, as well as further experimentations with the perovskite material, work on the overall device structure will be vital to further improving long-term stability in all-perovskite tandem cells.