Guiding vertical growth and improving the buried interface of Pb-Sn perovskite films with 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 a hotspot during the past few years. To maximize infrared light absorption, a sufficient thickness of the Pb-Sn perovskite film is required. However, this introduces problems with unbalanced crystallization and poor buried interfaces. Therefore, effective strategies are desired to precisely control the vertical growth of Pb-Sn crystals and improve the buried interface for efficient charge transport and extraction, in order to construct efficient Pb-Sn PSCs. Herein, an F-PEA2PbI3SCN 2D perovskite seed layer was developed to guide crystal growth and improve the buried interface of FA0.7MA0.3Pb0.5Sn0.5I3 perovskite film. The 2D perovskite seeds were found to eliminate the formation of the SnI2 phase and promote energy level alignment, which improved the buried interface. Additionally, the uniform distribution of 2D seeds facilitated crystallization and guided the vertical growth of Pb-Sn crystals, resulting in a film with reduced defect density and released residual strain. Therefore, the optimized PSCs yielded a 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 a stable power output of 27.2%. This work presents a new avenue for fabricating efficient Pb-Sn PSCs by rationally controlling their crystallization behavior.