• Ethylhydrazine oxalate (EDO) effectively inhibits Sn2+ oxidation and enhances perovskite crystallinity.
• EDO-modified mixed tin-lead perovskite solar cells achieved a power conversion efficiency of 21.96%.
• All-perovskite tandem solar cells with EDO interfacial passivation reached 27.58% efficiency.
• The study addresses key challenges of high crystallization rate and Sn2+ oxidation in Sn-Pb perovskites.