Structure-Property Relationships for Linear, Extended, and Branched Alkane Ammonium Iodide Derivatives as Interface...

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 (V_OC). However, this gain is often associated with increased instability. Here, we investigate the interface passivation of a Cs_0.1FA_0.6MA_0.3Pb_0.5Sn_0.5I₃ 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 V_OC 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 V_OC. 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 (J_SC). 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, V_OC, FF, and operational stability.