An Inert Fluoride Interlayer Enabling Efficient and Stable Inverted Perovskite Solar Cells

The commercialization of inverted perovskite solar cells (PSCs) is critically hindered by interfacial instability, especially the chemical degradation of top organic passivation layers that accelerates device performance decay. Here, we present a physical-barrier stabilization strategy by introducing an ultrathin interlayer of inert alkaline-earth metal fluorides between the passivation layer and the electron-transport layer. The interlayer simultaneously homogenizes the interfacial contact-potential distribution without sacrificing transport properties. Owing to its pronounced chemical inertness, the interlayer exhibits negligible interaction with the perovskite substrate, thereby preserving interfacial integrity and preventing detrimental reactions. Moreover, the dense and conformal morphology of the fluoride layer reinforces interfacial contact and functions as a robust barrier against iodide-ion migration and metal-electrode diffusion. This strategy shows good universality among typical compositions, delivering power-conversion efficiencies above 26%. As a result, the devices incorporating the CaF₂ interlayer exhibit significantly improved operational stability, retaining over 91% of the initial power conversion efficiency after 1000 h of continuous operation. The ultrathin inert-interlayer approach provides a general and scalable pathway toward highly efficient and durable inverted PSCs.