This review discusses the challenges and solutions for improving the performance of all-perovskite tandem solar cells, focusing on mixed Sn–Pb perovskite films. The main issues include low-quality films due to Sn(II) oxidation, difficult-to-control crystallization, and surface defects that reduce device efficiency and stability. Surface modifications are critical for enhancing the performance of these films. The review covers the origins of efficiency losses, including crystallization, oxidation, mass loss, ion movement, and charge carrier transport. It also discusses interface engineering strategies, such as surface protection, defect passivation, and the use of interlayers and 2D capping layers. The review outlines the important aspects of integrating narrow bandgap (NBG) subcells into all-perovskite tandem devices and highlights the need for further research on surface characterization and modification. The review also discusses the challenges of Sn-containing perovskites, including their faster crystallization kinetics, oxidation issues, and the impact of ion movement on device performance. The review concludes with a discussion on the future potential of surface engineering and the need for further research to achieve stable and efficient all-perovskite tandem solar cells.This review discusses the challenges and solutions for improving the performance of all-perovskite tandem solar cells, focusing on mixed Sn–Pb perovskite films. The main issues include low-quality films due to Sn(II) oxidation, difficult-to-control crystallization, and surface defects that reduce device efficiency and stability. Surface modifications are critical for enhancing the performance of these films. The review covers the origins of efficiency losses, including crystallization, oxidation, mass loss, ion movement, and charge carrier transport. It also discusses interface engineering strategies, such as surface protection, defect passivation, and the use of interlayers and 2D capping layers. The review outlines the important aspects of integrating narrow bandgap (NBG) subcells into all-perovskite tandem devices and highlights the need for further research on surface characterization and modification. The review also discusses the challenges of Sn-containing perovskites, including their faster crystallization kinetics, oxidation issues, and the impact of ion movement on device performance. The review concludes with a discussion on the future potential of surface engineering and the need for further research to achieve stable and efficient all-perovskite tandem solar cells.