Interstitial doping has been considered as an effective strategy to passivate and immobilize the ionic defects of metal halide perovskites to enhance performance and stability of perovskite solar cells. However, high dopant dosage causes lattice distortion which results in micro-strain and subsequent phase destabilization. This highlight discusses the latest report regarding optimal interstitial doping with a multivalent alkali metal cation for perovskites and awaiting issues associated with it.