Mechanistic insight into α-to-δ phase transition and stabilizing α-phase FAPbI₃ via regulating δ-phase orientation

The structural instability of formamidinium-based perovskite primarily originates from spontaneous α-to-δ phase transition. Although various suppression strategies have been proposed, developing a definitive solution remains challenging due to incomplete understanding of the physical mechanisms governing this phase transition. This work reveals that the coherent interface with tensile strain between perovskite α-phase (011) and δ-phase (110) planes acts as an active site for α-to-δ phase transition. Building on this insight, two-dimensional perovskitoid materials are introduced into the perovskite matrix, which strongly interact with the δ-phase (110) plane to form the 2D/δ interface. The large interfacial strain disrupts δ-phase orientation, suppresses the formation of α/δ transition interfaces, and thus enhances α-phase stability. This strategy enables FA-based perovskite solar cells to achieve an efficiency of 25.61%, with unencapsulated devices maintaining 90% initial efficiency after 1,000 h at 70% relative humidity. This work provides a mechanistic understanding and a universally applicable strategy to stabilize FA-based perovskites.