  We demonstrate four and two-terminal perovskite-perovskite tandem solar cells
with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap
perovskite, $FA_{0.75}Cs_{0.25}Sn_{0.5}Pb_{0.5}I_3$, that can deliver 14.8 %
efficiency. By combining this material with a wider bandgap
$FA_{0.83}Cs_{0.17}Pb(I_{0.5}Br_{0.5})_3$ material, we reach monolithic two
terminal tandem efficiencies of 17.0 % with over 1.65 volts open-circuit
voltage. We also make mechanically stacked four terminal tandem cells and
obtain 20.3 % efficiency. Crucially, we find that our infrared absorbing
perovskite cells exhibit excellent thermal and atmospheric stability,
unprecedented for Sn based perovskites. This device architecture and materials
set will enable 'all perovskite' thin film solar cells to reach the highest
efficiencies in the long term at the lowest costs.
