CsPbBr Nanocrystals as Bottom Interface Nucleation Seeds for Printing Oriented FAPbI Thin Films: An In Situ Study.

The exceptional optoelectronic properties of lead halide perovskites are highly sensitive to processing conditions, as uncontrolled crystallization driven by random nucleation often results in defect-rich active layers that impair device performance. Achieving controlled and oriented crystallization in printed films remains a major challenge. To address this, we introduce a pre-deposited CsPbBr nanocrystal seed layer at the bottom interface to guide crystallization and suppress defect formation. This strategy is evaluated via an in situ study on FAPbI, offering mechanistic insights into the influence of seeding on film growth and optoelectronic quality. Using in situ grazing-incidence wide-angle X-ray scattering, transmission-mode UV-vis absorption, and photoluminescence spectroscopy, phase evolution and seed-mediated growth kinetics are tracked. Seeding accelerates the transition from the photoinactive δ-phase to the photoactive α-phase, yielding a crystallization rate constant over six times higher than in unseeded films. Moreover, the seed layer governs the crystallographic orientation of the resulting perovskite film, leading to improved optical absorption and reduced defect density.