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 FA_0.7MA_0.3Pb_0.5Sn_0.5I₃ 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.