Sn-Pb mixed perovskite solar cells (PSCs) are developing rapidly and making great progress due to their environmentally friendly advantages. High-crystalline quality perovskite films are essential for obtaining high-efficiency and -stability PSCs. Here, the DJ-phase two-dimensional (2D) perovskite (4-AMP)PbI₄ (4-AMP is 4-(aminomethyl) piperidine) was used as a substrate to regulate the crystallization growth of the Sn-Pb mixed perovskite for preparing high-quality perovskite films, and the regulation mechanism was analyzed in detail. The results indicate that the suitable amount of the 2D perovskite substrate is favorable for the formation of PbI₂/SnI₂ films with wide intergranular gaps and vertical distribution grain boundaries. Moreover, the suitable hydrophobicity of the PbI₂/SnI₂ film made on the 2D perovskite substrate also provides a better template for regulating the formation and dissolution of prophase perovskite capping. In addition, the 4-AMP cations from the collapsed 2D perovskite substrate can diffuse into PbI₂/SnI₂ films and interact with PbI₂ to form the intermediate (4-AMP)-PbI₂-(4-AMP) and with SnI₂ to form the 2D perovskite (4-AMP)SnI₄. All of these promote the diffusion of FAI/MAI molecules and decrease the crystallization growth rate of the Sn-Pb perovskite and thus increase the conversion levels of the perovskite phase and improve the crystallization orientation and quality of the perovskite, which helps mitigate the erosion of water and oxygen. In addition, the 2D perovskite used as a substrate can passivate the buried interface defects and improve the interfacial contact. Moreover, the diffusion behavior of 4-AMP cations regulates the perovskite energy levels, which match more with those of the electron transport layer. As a result, the champion device made on the (4-AMP)PbI₄ substrate acquires a power conversion efficiency (PCE) of 17.7% with an open-circuit voltage (V_oc) of 0.806 V, a short-circuit current density (J_sc) of 28.97 mA cm^-2, and a fill factor (FF) of 75.86%, far exceeding those of the control device. Meanwhile, the unencapsulated PSCs modified with 4-AH retain above 70% of the initial efficiency value after storage for 1200 h in N₂ at room temperature and about 25% of its initial efficiency after exposure to air for nearly 300 h with RH = 30 ± 10% at room temperature, while the control device has only 30% of the initial efficiency and near-zero efficiency at the same conditions.

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