Synergistic Efficiency‐Stability Enhancement in Sn–Pb Narrow‐Bandgap Perovskite Solar Cells via Molecular Chain‐Length Engineering

Abstract Tin–lead (Sn–Pb) mixed narrow band gap (NBG) perovskite solar cells (PSCs) hold promise for tandem photovoltaic applications due to their tunable bandgap (≈1.25 eV), but face challenges from rapid crystallization kinetics and operational instability. Conventional ammonium salt passivators improve efficiency but often compromise stability by introducing mobile ions. Herein, alkylammonium halide salts are systematically investigated with varying carbon chain lengths to reconcile this trade‐off. As a result, it is found that the long‐chain oleylammonium iodide (OAmI) achieves unprecedented synergy. It demonstrates superior defect passivation, enhanced crystallographic orientation, and suppressed ion migration. Transient ion current measurements reveal a fourfold reduction in mobile ion density for OAmI‐treated devices. Electrostatic potential calculations and XPS analysis confirm stronger coordination between OAmI and undercoordinated Sn ²⁺ /Pb ²⁺ sites, while GIWAXS reveal improved (001)‐oriented crystallization and reduced surface roughness. These synergies yield a champion power conversion efficiency (PCE) of 22.69% for gas‐quenching produced Sn–Pb PSCs and 14.46% for 6 × 6 cm ² mini‐modules, alongside a T80 lifetime nearly twofold increase compared to the control group.