Interfacial Bidirectional Anchoring for CsPbI₃ Phase Stabilization in Inverted Perovskite Solar Cells

Despite the inherent chemical stability of CsPbI₃, 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 [PbI₆]^4- octahedras due to its small size, thus inducing [PbI₆]^4- tilting and CsPbI₃ 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 [PbI₆]^4- to inhibit its tilting, thus stabilizing black CsPbI₃. Besides, the simultaneous anchoring with Cs⁺ and [PbI₆]^4- can also introduce a compressive strain in CsPbI₃ 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.