The paper presents the development and performance of perovskite-perovskite tandem solar cells with optimized bandgaps. The authors demonstrate two-terminal and four-terminal tandem solar cells, achieving efficiencies of 17.0% and 20.3%, respectively. They develop a 1.2 eV bandgap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, which delivers 14.8% efficiency. By combining this with a wider bandgap FA0.85Cs0.17Pb(0.5Br:0.5)I3 material, they achieve high open-circuit voltages and stable performance. The study also highlights the thermal and atmospheric stability of the infrared-absorbing perovskite cells, which is unprecedented for Sn-based perovskites. The results suggest that "all perovskite" thin-film solar cells could reach high efficiencies and low costs in the long term.The paper presents the development and performance of perovskite-perovskite tandem solar cells with optimized bandgaps. The authors demonstrate two-terminal and four-terminal tandem solar cells, achieving efficiencies of 17.0% and 20.3%, respectively. They develop a 1.2 eV bandgap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, which delivers 14.8% efficiency. By combining this with a wider bandgap FA0.85Cs0.17Pb(0.5Br:0.5)I3 material, they achieve high open-circuit voltages and stable performance. The study also highlights the thermal and atmospheric stability of the infrared-absorbing perovskite cells, which is unprecedented for Sn-based perovskites. The results suggest that "all perovskite" thin-film solar cells could reach high efficiencies and low costs in the long term.