A Transferable Aqueous KCl–OPLT Co‐Treatment for Buried‐Interface Regulation in SnO ₂ ‐Based Perovskite Solar Cells

Regulating the SnO 2 /perovskite buried interface is critical for suppressing interfacial recombination and J–V hysteresis in n–i–p perovskite solar cells, yet many treatments remain formulation‐dependent or fail to establish a confined and reproducible interface after deposition. Here, we report a one‐step aqueous ion–molecule co‐treatment that forms a KCl–O‐phospho‐L‐tyrosine (OPLT) interlayer on solution‐processed SnO 2 without altering the perovskite precursor or fabrication process. In a representative rigid hybrid device, the champion power conversion efficiency (PCE) increases from 24.44% to 26.10%, with an improved open‐circuit ( V OC ) from 1.156 to 1.196 V and reduced hysteresis ( HI ) from 8.4% to 2.7%. Across four additional SnO 2 ‐based platforms, including rigid/flexible and hybrid/all‐inorganic devices, this strategy consistently delivers PCE gains of 1.44–1.75%, V OC increases of 20–50 mV, and suppressed hysteresis. Depth‐resolved characterization reveals preferential SnO 2 ‐side localization of the interlayer. Combined spectroscopic, electrostatic, and theoretical analyses indicate a cooperative interfacial reconfiguration driven by the coupled K + –phosphonate environment, leading to reduced defects, more uniform electrostatics, and improved energetics. These effects enable more efficient electron extraction, faster V OC build‐up, and reduced photovoltage decay. This work provides a modular and transferable aqueous strategy for regulating buried interfaces in high‐performance perovskite solar cells.