Hierarchical Microfluidic Epitaxy of Sn4+‐Stabilized Core–Shell Perovskites with Lead‐Free Double Perovskite Shells for Highly Efficient and Ultra‐Stable Emission

Abstract Colloidal lead halide perovskite (LHP) nanocrystals (NCs) exhibit promising potential in photovoltaics and optoelectronics owing to their unique photophysical properties. Their practical applications remain limited by challenges, including uncontrolled instantaneous nucleation in conventional synthesis, intrinsic structural instability, and lead toxicity risks. Herein, a multistage microfluidic‐based epitaxial growth strategy is presented to achieve precise multilevel control over the structural parameters and core–shell configurations of CsPbBr 3 NCs. By employing a quasi‐1D thermal flow field within microchannels, the nucleation and growth processes of CsPbBr 3 are successfully decoupled. A lead‐free Cs 2 SnBr 6 double perovskite shell is further epitaxially grown on CsPbBr 3 , forming a type‐I band‐aligned core–shell heterostructure. The resulting core–shell nanocrystals demonstrate significantly enhanced optical performance and stability, maintaining intact lattice structures after 75 days of ambient exposure. First‐principles calculations reveal that interfacial Sn 4+ migration reinforces the Pb─Br bonding network and passivates halogen vacancies, thereby improving structural integrity. This microfluidic system achieves 1.48 g h −1 per channel production, offering scalable, eco‐friendly perovskite heterostructures.