Abstract The interfacial contact between NiOx and self-assembled monolayers (SAMs) in wide-bandgap (WBG) subcells limits the efficiency and stability of all-perovskite tandem solar cells (TSCs). The strongly acidic phosphoric acid (PA) anchors in common PA-SAMs corrode reactive NiOx, undermining device stability. Moreover, SAM aggregation leads to interfacial losses and significant open-circuit voltage (VOC) deficits. Here, we introduce boric acid (BA) as a milder anchoring group that chemisorbs onto NiOx via strong – $${{\rm{BO}}}_2{\mbox{-}}$$ BO 2 - –Ni coordination. A benzothiophene-fused head group enhances interfacial bonding through S–Ni orbital interactions, yielding higher binding energy than PA-SAMs. This design also promotes homogeneous SAM formation without aggregation. Resultantly, the WBG cell exhibits an improved PCE to 20.1%. When integrated with narrow bandgap (NBG) subcell, the two-terminal (2T) TSCs achieve an ameliorative PCE of 28.5% and maintain 90% of the initial PCE after maximum power point tracking (MPP) under 1 sun illumination for 500 h.