Interfaces between the perovskite active layer and the charge-transport layers (CTLs) play a critical role in both 
efficiency and stability of halide-perovskite photovoltaics. One of the major concerns is that surface defects of 
perovskite could cause detrimental nonradiative recombination and material degradation. In this work, we 
addressed this challenging problem by inserting ultrathin alkali-fluoride (AF) films between the tri-cation lead- 
iodide perovskite layer and both CTLs. This bilateral inorganic “walls” strategy makes use of both physical- 
blocking and chemical-anchoring functionalities of the continuous, uniform and compact AF framework: on 
the one hand, the uniformly distributed alkali-iodine coordination at the perovskite-AF interfaces effectively 
suppresses the formation of iodine-vacancy defects at the surfaces, thus reducing the trap-assisted recombination 
at the perovskite-CTL interfaces and therewith the open-voltage loss; on the other hand, the impermeable AF 
buffer layers effectively prevent the bidirectional ion migration at the perovskite-CTLs interfaces even under 
harsh working conditions. As a result, a power-conversion efficiency (PCE) of 22.02% (certified efficiency 20.4%) 
with low open-voltage deficit (<0.4 V) was achieved for the low-temperature processed inverted planar 
perovskite solar cells. Exceptional operational stability (500 h, ISOS-L-2) and thermal stability (1000 h, ISOS-D- 
2) were obtained. Meanwhile, a 35.7% PCE was obtained under dim-light source (1000 lux white LED light) with 
the optimized device, which is among the best records in perovskite indoor photovoltaics.