Sn–Pb mixed perovskites with bandgaps in the range of 1.1–1.4 eV are ideal candidates for single‐junction solar cells to approach the Shockley–Queisser limit. However, the efficiency and stability of Sn–Pb mixed‐perovskite solar cells (PSCs) still lag far behind those of Pb‐based counterparts due to the easy oxidation of Sn2+. Here, a reducing agent 4‐hydrazinobenzoic acid is introduced as an additive along with SnF2 to suppress the oxidation of Sn2+. Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn‐ and Pb‐based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn–Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.
2D-3D mixed tin halide perovskites are outstanding candidate materials for lead-free perovskite solar cells (PSCs) due to their improved stability and decreased trap density in comparison with their pure 3D counterparts. However, the mixture of multiple phases may lead to poor charge transfer across the films and limit the device efficiency. Here, a stacked quasi-2D (down)-3D (top) double-layered structure in perovskite films prepared via vacuum treatment is demonstrated, which can result in a planar bilayer heterojunction. In addition, it is found that the introduction of guanidinium thiocyanate (GuaSCN) additive can improve the crystallinity and carrier mobility in the 2D perovskite layer and passivate defects in the whole film, leading to a long carrier lifetime (>140 ns) in photoluminescence measurements. As a result, the PSCs show a high open circuit voltage (V OC ) up to 1.01 V with a voltage loss of only 0.39 V, which represents the record values ever reported for tin-based PSCs. The champion device exhibits a power conversion efficiency (PCE) of 13.79% with decent stability, retaining 90% of the initial PCE for 1200 h storage in N 2 -filled glovebox.
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