Beyond the Surface: Three-Dimensional Distribution and Defect Passivation of Self-Assembled Monolayers in Perovskite Solar Cells

Self-assembled monolayers (SAMs) are widely employed to tune interfacial properties in perovskite solar cells (PSCs), yet they have so far been regarded as strictly two-dimensional surface modifiers. Here, we uncover the three-dimensional (3D) nature of perfluorinated silanol-based SAMs applied onto perovskite films and their impact on both the photovoltaic performance and stability of PSCs. Using high-resolution 3D nanoscale secondary ion mass spectrometry, which directly maps their lateral and vertical distribution, we reveal that these SAMs not only decorate the perovskite surface but also selectively penetrate and anchor at grain-boundary iodine-deficient regions within ∼100 nm of the film. This dual surface-bulk passivation reshapes the defect landscape in perovskite thin films, yielding reduced nonradiative recombination, more favorable interfacial energetics, enhanced photovoltaic performance, and increased environmental stability. Our findings reposition SAMs as volumetric chemical modifiers rather than merely interface dipole layers, highlighting their potential for molecular-level engineering in perovskite optoelectronics.