Guiding Vertical Growth and Improving Buried Interface of Pb-Sn Perovskite Film by 2D Perovskite Seeds for Efficient Narrow Bandgap Perovskite Solar Cells and Tandems

Developing narrow-bandgap Pb-Sn perovskite solar cells (PSCs) for all-perovskite tandem device has been the hotspot during the past few years. To maximumly absorb infrared light, sufficient thickness of Pb-Sn perovskite film is required, yet it introduces problems of unbalanced crystallization and poor buried interface. Therefore, effective strategies are desired to precisely control the vertical growth of Pb-Sn crystal and improve the buried interface for efficient charge transportation and extraction to construct efficient Pb-Sn PSCs. Herein, F-PEA2PbI3SCN 2D perovskite seeds layer was developed to guide the crystal growth and improve the buried interface of FA0.7MA0.3Pb0.5Sn0.5I3 perovskite film. 2D perovskite seeds was found to eliminate the formation of SnI2 phase and promote the energy level alignment that improve the buried interface, while the uniform distribution of 2D seeds could facilitate the crystallization and guide the vertical growth of Pb-Sn crystals to produce film with reduced defect density and released residual strain. Therefore, the optimized PSCs yielded champion PCE of 22.71% with a broadened antisolvent-processing window and robust stability. Notably, the four-terminal all-perovskite tandem device exhibited a PCE of 27.68% with stable power output of 27.2%. This work opens up new avenue for fabricating efficient Pb-Sn PSCs by rationally controlling their crystallization behavior.