While formamidinium lead iodide (FAPbI3) displays favorable optoelectronic properties, its photoactive black phase is thermodynamically unstable at room temperature and prone to transformation into non-functional phases under light, heat, or moisture. Existing stabilization strategies, including compositional mixing and surface passivation, often introduce new degradation pathways, including interfacial strain and accelerated ion migration.
To address this issue, researchers from Huazhong University of Science and Technology, Hainan University, Shantou University, Chinese Academy of Sciences and Wuhan University of Technology have developed a stabilization strategy based on co-evaporated cesium lead iodide (CsPbI3) capping layers. The team developed a 3D/3D bilayer perovskite structure, in which an ultrathin (~5 nm) cesium lead iodide (CsPbI3) layer is deposited on top of a formamidinium-based perovskite absorber using a vapor-phase co-evaporation process. Unlike conventional low-dimensional surface treatments, both layers retain a three-dimensional perovskite framework, enabling robust structural coupling across the interface.
Structural analyses show that strong lattice matching at the interface favors the formation of a photoactive cubic phase in both layers, despite each material being individually metastable under ambient conditions. This interfacial coupling reduces strain, suppresses phase transitions during aging, and maintains structural integrity under prolonged thermal stress. Importantly, the inorganic capping layer also acts as an effective barrier against ion migration, blocking both intrinsic ions and ligand-derived species from diffusing across the interface, one of the primary causes of long-term degradation in perovskite devices.
When integrated into inverted perovskite solar cell devices, the bilayer design delivered a certified power conversion efficiency exceeding 27% for small-area cells, while large-area devices achieved efficiencies close to 25%. Operational stability tests demonstrated that the cells retained over 94% of their initial performance after more than 1,100 hours of continuous operation at elevated temperatures. These results place the devices among the most stable and efficient perovskite solar cells reported to date.
According to the researchers, the key advance is not a single material improvement but the cooperative behavior between two perovskite layers. By allowing the hybrid and inorganic components to stabilize each other through lattice matching, the design avoids many of the degradation pathways that typically limit perovskite devices. The team emphasizes that this approach shifts the focus from compositional complexity to interface control, offering a more universal and scalable route for improving both efficiency and durability in perovskite photovoltaics.
