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Achieving high‐performance perovskite solar cells hinges on the precise management of interfacial properties. However, the current research predominantly concentrates on addressing individual interfaces, often overlooking the potential synergies between the upper and lower interfaces crucial for efficient carrier transfer. Herein, an effective interfacial strategy is introduced, which combines 2‐aminoterephthalic acid (2‐AA) pre‐processing with post‐processing techniques to prepare superior mixed tin‐lead perovskite films suitable for both single‐junction and all‐perovskite tandem solar cells. Incorporating 2‐AA at both the upper and lower interfaces of the perovskite films serves to regulate interfacial defects, alleviate interface residual stress, and improve film crystallinity. In addition, the 2‐AA molecule acts as an interface protective layer, anchoring tin and lead atoms, facilitating band energy alignment, and improving film and device stability. As a result, mixed tin‐lead perovskite solar cells achieved an improved open‐circuit voltage of 0.87 V and PCE of 21.57%. Furthermore, the integration of these optimized tin‐lead perovskite subcells into four‐terminal all‐perovskite tandem solar cells yielded an impressive efficiency of 27.46%. This work thus offers an effective strategy for comprehensive interfacial engineering and stress relief, paving the way for the production of high‐performance perovskite solar cells (PSCs). [ABSTRACT FROM AUTHOR]