Thermally-induced drift of A-site cations at solid-solid interface in physically paired lead halide perovskites

The promise of hybrid organic-inorganic halide perovskite solar cells rests on their exceptional power conversion efficiency routinely exceeding 25% in laboratory scale devices. While the migration of halide ions in perovskite thin films has been extensively investigated, the understanding of cation diffusion remains elusive. In this study, a thermal migration of A‑site cations at the solid-solid interface, formed by two physically paired MAPbI₃ and FAPbI₃ perovskite thin films casted on FTO, is demonstrated through continuous annealing at comparably low temperature (100 °C). Diffusion of methylammonium (CH₃NH₃⁺, MA⁺) cations into the low‑symmetry yellow δ‑FAPbI₃ phase triggers a transition from the yellow (δ) to black (α) phase evident in the distinctive color change and verified by shifts in absorption bands and X‑ray diffraction patterns. Intermixing of the A‑site cations MA⁺ and FA⁺ (CH(NH₂)₂⁺) occurred for both systems, α‑MAPbI₃/δ‑FAPbI₃ and α‑MAPbI₃/α‑FAPbI₃. The structural and compositional changes in both cases support a thermally activated ion drift unambiguously demonstrated through changes in the absorption and X-ray photoelectron spectra. Moreover, the physical contact annealing (PCA) leads to healing of defects and pinholes in α‑MAPbI₃ thin films, which was correlated to longer recombination lifetimes in mixed MA_xFA_1-xPbI₃ thin films obtained after PCA and probed by ultrafast transient absorption spectroscopy.