Researchers have developed perovskite-perovskite tandem solar cells with optimally matched bandgaps, achieving high efficiency and stability. The study introduces a new infrared-absorbing perovskite with a 1.2 eV bandgap, FA0.75Cs0.25Pb0.5Sn0.5I3, which achieves 14.8% efficiency. When combined with a wider bandgap perovskite, FA0.83Cs0.17Pb(I0.5Br0.5)3, the tandem cells achieve 17.0% efficiency with over 1.65 V open-circuit voltage. Mechanically stacked four-terminal tandem cells reach 20.3% efficiency. The infrared-absorbing perovskite shows excellent thermal and atmospheric stability, a significant improvement for tin-based perovskites. Metal halide perovskites, with the formula ABX3, have shown great promise in photovoltaics due to their high power conversion efficiencies and low processing costs. Single-junction perovskite devices have reached 22% efficiency, but tandem configurations are expected to enhance silicon cell efficiency. An all-perovskite tandem cell could offer lower fabrication costs but requires specific bandgaps that have not yet been realized. The study demonstrates a stable 14.8% efficient perovskite solar cell using a 1.2 eV bandgap material. Combining this with a 1.8 eV perovskite cell results in 17.0% efficient monolithic all-perovskite 2-terminal tandem solar cells. The study also fabricates 20.3% efficient four-terminal tandems using a semitransparent 1.6 eV perovskite front cell. The research team developed a technique called precursor-phase antisolvent immersion (PAI) to deposit uniform layers of tin-containing perovskites. This method allows for the creation of smooth, pinhole-free layers with high crystallinity. The study also investigates the optical and electronic properties of the perovskite materials, finding that the band gap narrows with increasing tin content. The team performed optical pump-probe terahertz spectroscopy to determine the diffusion length, mobility, and recombination lifetimes of the materials, finding that the charge-carrier diffusion lengths are comparable to those of lead-based perovskites. The study also evaluates the stability of the perovskite materials under various conditions, finding that the infrared-absorbing perovskite shows excellent thermal and atmospheric stability. The team fabricated planar heterojunction devices with an inverted p-i-n architecture, achieving high efficiency and stability. The study demonstrates that the perovskite materials can be used in both 2-terminal and 4-terminal tandem configurations, with the 4-terminal configuration showing higher efficiency dueResearchers have developed perovskite-perovskite tandem solar cells with optimally matched bandgaps, achieving high efficiency and stability. The study introduces a new infrared-absorbing perovskite with a 1.2 eV bandgap, FA0.75Cs0.25Pb0.5Sn0.5I3, which achieves 14.8% efficiency. When combined with a wider bandgap perovskite, FA0.83Cs0.17Pb(I0.5Br0.5)3, the tandem cells achieve 17.0% efficiency with over 1.65 V open-circuit voltage. Mechanically stacked four-terminal tandem cells reach 20.3% efficiency. The infrared-absorbing perovskite shows excellent thermal and atmospheric stability, a significant improvement for tin-based perovskites. Metal halide perovskites, with the formula ABX3, have shown great promise in photovoltaics due to their high power conversion efficiencies and low processing costs. Single-junction perovskite devices have reached 22% efficiency, but tandem configurations are expected to enhance silicon cell efficiency. An all-perovskite tandem cell could offer lower fabrication costs but requires specific bandgaps that have not yet been realized. The study demonstrates a stable 14.8% efficient perovskite solar cell using a 1.2 eV bandgap material. Combining this with a 1.8 eV perovskite cell results in 17.0% efficient monolithic all-perovskite 2-terminal tandem solar cells. The study also fabricates 20.3% efficient four-terminal tandems using a semitransparent 1.6 eV perovskite front cell. The research team developed a technique called precursor-phase antisolvent immersion (PAI) to deposit uniform layers of tin-containing perovskites. This method allows for the creation of smooth, pinhole-free layers with high crystallinity. The study also investigates the optical and electronic properties of the perovskite materials, finding that the band gap narrows with increasing tin content. The team performed optical pump-probe terahertz spectroscopy to determine the diffusion length, mobility, and recombination lifetimes of the materials, finding that the charge-carrier diffusion lengths are comparable to those of lead-based perovskites. The study also evaluates the stability of the perovskite materials under various conditions, finding that the infrared-absorbing perovskite shows excellent thermal and atmospheric stability. The team fabricated planar heterojunction devices with an inverted p-i-n architecture, achieving high efficiency and stability. The study demonstrates that the perovskite materials can be used in both 2-terminal and 4-terminal tandem configurations, with the 4-terminal configuration showing higher efficiency due