Selective Ion-Blocking Strategy for Stabilizing Perovskite Tandem Solar Cells

Wide-bandgap (WBG) perovskite solar cells employing iodide-bromide mixed halide compositions are essential for tandem integration but suffer from open-circuit voltage (V_OC) losses and photo-instability. Lithium fluoride (LiF) interlayers are widely adopted to enhance V_OC via interfacial defect passivation and energy-level alignment, yet their adverse impact on operational stability and the associated degradation mechanism under realistic conditions remains poorly understood. Here, we systematically investigate the origin of operational instability in LiF-based WBG devices and demonstrate a strategy for selectively blocking fluoride ion (F⁻) migration into perovskite layer to improve device stability. We found that LiF does not remain confined to the interface but diffuses into the perovskite layer, where F⁻ accelerate halide segregation and compromise device photostability. To address this issue, we introduce lithium bis(trifluoromethanesulfonyl)imide (Li─T) as a selective ion-blocking interlayer (i.e. ion-fence), which suppresses F⁻ diffusion while preserving the benefits of LiF, yielding a high V_OC of 1.284 V and efficiency of 19.55%. This strategy markedly enhances operational stability without sacrificing efficiency and is successfully extended to monolithic all-perovskite tandem cells, where Li─T maintains high efficiency of 28.41% while significantly improving device stability. These findings provide critical insights into interfacial engineering of high-performance WBG perovskites for next-generation tandem solar cells.