Quantitative investigation of thermally driven interfacial diffusion and front-contact breakdown in organic-inorganic halide perovskites

Elemental migration critically undermines the stability of organic-inorganic halide perovskites, yet the underlying substrate as a source of extrinsic mobile species remains largely overlooked. This study uses Rutherford Backscattering Spectrometry to quantify elemental depth profiles in methylammonium-lead-triiodide films thermally aged at 45, 65 and 85 °C for 96 hours under nitrogen atmosphere. Aging redistributes lead and iodine and produces unexpected oxygen accumulation at the buried interface. Comparison with a non-annealed control confirms that this oxygen originates not from fabrication but from thermally activated out-diffusion of the indium tin oxide substrate, which shows measurable oxygen loss and surface roughening. To mitigate this, a self-assembled monolayer namely [2-(3,6-Dimethoxy-9 H -carbazol-9-yl) ethyl] phosphonic Acid was integrated as an interfacial diffusion barrier, which preserves substrate’s stoichiometry and suppresses oxygen migration into the perovskite stack. Although intrinsic lead and iodine migration persists, suppressing substrate-derived oxygen underscores the necessity of interfacial engineering and validates RBS for quantitative tracking of elemental migration.