Nanoscale Facet Engineering Stabilizes α-FAPbI₃ and Suppresses Ion Migration to Realize Ambient Stable Perovskite...

Formamidinium (FA) based perovskites have evolved as a promising candidate for high performance optoelectronic devices due to their near-ideal band structure. However, demonstration of high performance field effect transistors with FA based perovskites remains largely unexplored due to the inherent phase instability of the photoactive FA phase and field induced ionic defect migration. Here, we developed an α-phase-assisted anti-solvent method which utilizes facet-rich CsPbBr₃ nanocrystals (NCs) to demonstrate hysteresis-free field effect transistors with n-type transport at least 30 times higher than pristine FAPbI₃ reaching values > 1 cm²/Vs. These devices demonstrate exceptional operational bias stress stability with a marginal threshold voltage shift (∆V_th) ∼ 0.7 V for 10 h of continuous operation which self-heals, > 1500 h of ambient stability and > 8500 h of performance retention under nitrogen atmosphere making them one of the champion n-type perovskite compositions till date in terms of stable devices for practical applications. Interestingly, these composite perovskites do not exhibit significant ion migration indicated by an activated temperature dependence in the mobility which is generally not observed in 3D Pb-based perovskite compositions. By integrating electronic/ionic transport studies along with terahertz time-domain spectroscopy, single-particle blinking measurements, and structure-property correlations, we establish the microscopic mechanism through which facet polarity stabilizes the thermodynamically elusive α-phase of FAPbI₃ without affecting the band-structure, ultimately yielding desirable long-term operational stability of the field effect devices. Our findings establish facet engineering of NCs as a transformative route toward developing reliable and stable perovskite based optoelectronic devices.