Carrier Control in Sn-Pb Perovskites via 2D Cation Engineering for All-Perovskite Tandem Solar Cells with Improved Efficiency and Stability

All-perovskite tandem solar cells are promising for achieving photovoltaics with power conversion efficiencies above the detailed balance limit of single-junction cells, while retaining the low cost, light weight and other advantages associated with metal halide perovskite photovoltaics. However, the efficiency and stability of all-perovskite tandem cells are limited by the Sn–Pb-based narrow-bandgap perovskite cells. Here we show that the formation of quasi-two-dimensional (quasi-2D) structure (PEA)₂GAPb₂I₇ from additives based on mixed bulky organic cations phenethylammonium (PEA⁺) and guanidinium (GA⁺) provides critical defect control to substantially improve the structural and optoelectronic properties of the narrow-bandgap (1.25 eV) Sn–Pb perovskite thin films. This 2D additive engineering results in Sn–Pb-based absorbers with low dark carrier density (~1.3 × 10¹⁴ cm⁻³), long bulk carrier lifetime (~9.2 μs) and low surface recombination velocity (~1.4 cm s⁻¹), leading to 22.1%-efficient single-junction Sn–Pb perovskite cells and 25.5%-efficient all-perovskite two-terminal tandems with high photovoltage and long operational stability.