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  1. Reduced open-voltage deficit and enhanced light absorption enable bifacial tandem device with equivalent efficiency of 29.3% 
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  2. null (Ed.)
  3. Abstract

    High‐performance tin‐lead perovskite solar cells (PSCs) are needed for all‐perovskite‐tandem solar cells. However, iodide related fast photodegradation severely limits the operational stability of Sn‐Pb perovskites despite the demonstrated high efficiency and thermal stability. Herein, this work employs an alkylammonium pseudo‐halogen additive to enhance the power conversion efficiency (PCE) and photostability of methylammonium (MA)‐free, Sn‐Pb PSCs. Density functional theory (DFT) calculations reveal that the pseudo‐halogen tetrafluoroborate (BF4) has strong binding capacity with metal ions (Sn2+/Pb2+) in the Sn‐Pb perovskite lattice, which lowers iodine vacancy formation. Upon combining BF4with an octylammonium (OA+) cation, the PCE of the device with a built‐in light‐scattering layer is boosted to 23.7%, which represents a new record for Sn‐Pb PSCs. The improved efficiency benefits from the suppressed defect density. Under continuous 1 sun illumination, the OABF4embodied PSCs show slower generation of interstitial iodides and iodine, which greatly improves the device photostability under open‐circuit condition. Moreover, the device based on OABF4retains 88% of the initial PCE for 1000 h under the maximum‐power‐point tracking (MPPT) without cooling.

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