Lead-halide perovskite photovoltaics(PVs)have achieved impressive progress in power conversion efficiencies(PCEs)over the past ten years,accelerating their step forward to next-generation commercial PV technologies[1].As a leading candidate for perovskite PVs,monolithic all-per-ovskite tandem solar cells(PTSCs)have achieved the best 24.8％for small-area devices(0.049 cm2)in 2019,and have the potential to deliver higher PCE in comparison to their single-junction counterparts that are already close to the PCE limit according to the Shockley-Queisser theory[2].PTSCs generally consist of two stacked perovskite sub-cells:a back narrow-bandgap cell(～1.2 eV)combined with a front wide-bandgap cell(～1.8 eV).Fabricating high-quality mixed lead-tin(Pb-Sn)narrow-bandgap sub-cells remains the key challenge for efficient and stable PTSCs because of the poor electronic quality and instability of the mixed Pb-Sn perovskite polycrystalline film[3].In general,even a trace amount of oxygen or effects of solvent could induce the Sn2+oxidation that results in a high density of Sn vacancies and distorted lattices during film crystallization.Moreover,non-uniform nucleation and fast crystallization of the Sn-containing perovskites are likely to cause abundant in-terfacial defects and serious film heterogeneities,which re-tard the development of efficient,stable,and scalable PTSCs[4].