Direct versus indirect 2D/3D heterojunction engineering: Ordered interface design for ultastable perovskite solar cells

While 2D/3D heterostructures are widely employed to improve the stability of perovskite optoelectronic devices, their effectiveness is fundamentally governed by the crystallinity of the interfacial structure—a factor often overlooked. Disordered interfaces exhibit thermodynamic metastability, where ion diffusion induces sequential phase transitions from low-n to high-n phases. Here, we construct atomically ordered 2D/3D interfaces using phase-pure 2D perovskite capping layers, which reduce the interfacial phase transition rate by 95 % and effectively suppress ion migration. As a result, devices exhibit outstanding operational stability, retaining over 99 % of their initial power conversion efficiency after 1500 h of continuous operation, along with excellent thermal durability at 85 °C. These findings identify interfacial order as a critical parameter for regulating ion dynamics and phase behavior, providing a robust design principle for achieving high-efficiency, long-lifetime perovskite technologies.