Laser Manufactured Nano‐MXenes with Tailored Halogen Terminations Enable Interfacial Ionic Stabilization of High Performance Perovskite Solar Cells

Abstract Formamidinium (FA)‐based perovskite promises high power conversion efficiency in photovoltaics while it faces awkward spontaneous yellow phase transition even at ambient conditions. This has spurred intensive efforts which leave a formidable challenge on robust anchoring of the soft perovskite lattice. Present work pioneers the rational design of interfacial ionic‐bonding between halogen‐terminated nano‐MXenes and perovskite for effective retarding of the lattice instability in FA‐based perovskites. The robust heterointerface between perovskite and nano‐MXenes results also in effectively modulating the deep‐energy‐level defects, lowering the interfacial charge transfer barrier, and tuning the work function of perovskite films. Benefiting from these merits, unencapsulated FA‐based perovskite solar cells after the ionic stabilization (champion efficiency up to 24.17%), maintain over 90% of their initial efficiency after operation at maximum power point under continuous illumination for 1000 h, and retain more than 85% of their initial efficiency even after annealing for 1000 h at 85 °C in inert atmosphere.