Interfacial Bidirectional Anchoring for CsPbI ₃ Phase Stabilization in Inverted Perovskite Solar Cells

ABSTRACT Despite the inherent chemical stability of CsPbI ₃ , its metastable black phase tends to transform into the non‐photovoltaic yellow phase, particularly under thermal stress. One key reason lies in the Cs ⁺ displacement among [PbI ₆ ] ⁴⁻ octahedras due to its small size, thus inducing [PbI ₆ ] ⁴⁻ tilting and CsPbI ₃ phase transition. Here, we propose a bidirectional anchoring strategy with 1‐Methyl‐3‐(3‐sulfopropyl)‐1H‐imidazol‐3‐ium chloride (SMCl) introduction. The sulfonate group in SMCl can strongly interact with Cs ⁺ to inhibit its displacement, and the imidazole group can coordinate with [PbI ₆ ] ⁴⁻ to inhibit its tilting, thus stabilizing black CsPbI ₃ . Besides, the simultaneous anchoring with Cs ⁺ and [PbI ₆ ] ⁴⁻ can also introduce a compressive strain in CsPbI ₃ film with SMCl, further increasing its phase stability. Resulting inverted PSCs exhibit high efficiency of 21.17% with good operational stability even under ISOS‐L‐2 protocol, retaining 85% of their initial efficiency after continuous maximum power point tracking for 1000 h with continuous illumination at 65°C.