Coupling Interfacial Defect Regulation With Lattice Relaxation in FACsPbI 3 Perovskite

ABSTRACT FACsPbI 3 perovskites, free of volatile methylammonium and mixed halides, are among promising absorbers for efficient optoelectronics owing to their favourable bandgap and intrinsic stability. However, the structurally soft lattice makes their surfaces prone to distortion and defect generation, creating a heterogeneous interfacial energy landscape that promotes non‐radiative recombination and ion migration. Here, we report an interfacial regulation strategy that stabilizes the surface/interface configuration of FACsPbI 3 through a defect‐lattice coupling mechanism. Introducing morpholinium chloride at the FACsPbI 3 interface/surface enables coordination with undercoordinated lattice sites, thereby relieving local lattice strain, stabilizing surface Pb‐I octahedral network, and reducing the density of defect states. This coupling between interfacial chemistry and lattice energetics increases vacancy formation energies, homogenizes the interfacial electronic environment, and suppresses defect‐assisted recombination and ion migration. The resulting perovskite solar cells yield an efficiency of 26.67% with a 16.1 cm 2 ‐aperture mini‐module reaching an efficiency of 22.79%. Moreover, the devices exhibit enhanced stability, retaining 95.8% of their initial efficiency after 1000 h of continuous operation. This work reveals a defect‐lattice coupling paradigm that links interfacial chemistry, lattice energetics, and ion migration kinetics, offering a general route toward stable and high‐efficiency soft ionic semiconductors.