Compositional engineering has been used to overcome difficulties in fabricating high-quality phase-pure formamidinium perovskite films together with its ambient instability. However, this comes alongside an undesirable increase in bandgap that sacrifices the device photocurrent. Here we report the fabrication of phase-pure formamidinium-lead tri-iodide perovskite films with excellent optoelectronic quality and stability. Incorporation of 1.67 mol% of 2D phenylethylammonium lead iodide into the precursor solution enables the formation of phase-pure formamidinium perovskite with an order of magnitude enhanced photoluminescence lifetime. The 2D perovskite spontaneously forms at grain boundaries to protect the formamidinium perovskite from moisture and suppress ion migration. A stabilized power conversion efficiency (PCE) of 20.64% (certified stabilized PCE of 19.77%) is achieved with a short-circuit current density exceeding 24 mA cm−2 and an open-circuit voltage of 1.130 V, corresponding to a loss-in-potential of 0.35 V, and significantly enhanced operational stability.

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This work shows a bimolecular additive engineering approach to prepare highly efficient wide-band-gap perovskite solar cells. The coupling of PEA + and SCN À provides a synergistic effect that overcomes growth challenges with either additive individually and improves perovskite quality with enhanced crystallinity, reduced defect density, and improved carrier mobility and lifetime. When coupling a semitransparent wide-band-gap perovskite top cell with a low-band-gap CIGS bottom cell, we achieve a 25.9%-efficient polycrystalline perovskite/CIGS 4-terminal thinfilm tandem solar cell.