Structure–Property Relationships for Linear, Extended, and Branched Alkane Ammonium Iodide Derivatives as Interface Passivators for Narrow-Bandgap Lead–Tin Perovskite Solar Cells

Interface passivation is crucial to reduce nonradiative recombination losses at the perovskite-electron transport layer interface and enhance the power conversion efficiency (PCE) of perovskite solar cells. Various molecules are known to result in a gain in open-circuit voltage (VOC). However, this gain is often associated with increased instability. Here, we investigate the interface passivation of a Cs0.1FA0.6MA0.3Pb0.5Sn0.5I3 narrow-bandgap (1.26 eV) perovskite by ammonium iodide derivatives with multiple, primary, secondary, or tertiary ammonium iodide groups, connected via alkane linkers of different lengths to establish structure–property relationships. The impact of these passivators on interfacial recombination, charge extraction, and device stability is elucidated by tracking the quasi-Fermi level splitting (QFLS) of perovskite layers and perovskite/C60 bilayers, together with the VOC of complete solar cell devices over one month. All tested passivators reduce the nonradiative recombination losses at the perovskite/C60 interface, but the extent to which this translates into an improved photovoltaic performance strongly depends on the molecular structure. Short-chain primary diammonium passivators provide the most favorable balance between effective passivation and charge extraction, yielding QFLS values that closely match the device VOC. In contrast, extended and branched multiammonium passivators improve photovoltage stability but impede charge carrier extraction, leading to reduced fill factors (FF) and short-circuit current densities (JSC). Secondary ammonium terminal groups promote crystallite formation on the perovskite surface, which reduces the efficacy of passivation. Importantly, combining small with larger-sized passivators in a single layer enables simultaneous enhancement of QFLS, VOC, FF, and operational stability.