An In Situ Multihalide Blocking Layer for Minimizing Energy Loss in High‐Performance TOPCon/Perovskite Tandems

The efficiency of perovskite/silicon tandem solar cells is inherently constrained by energy losses, which originate from surface defect‐mediated recombination in wide‐bandgap (WBG) mixed‐halide perovskites. While ammonium‐based passivation layers can mitigate this to some extent, their weak hydrogen bonding often degrades under operational stress. In this article, we propose a novel in situ reconstructed cohesive multihalide blocking layer, fabricated via a simple n‐butylammonium chloride (BACl) post‐treatment process. The incorporation of Cl − ions induces the formation of a trihalide‐based surface phase, which exhibits substantially higher lattice cohesive energy compared to conventional I/Br‐mixed perovskites. Meanwhile, BA + cations are stably immobilized through strong ionic interactions, effectively passivating cationic vacancies and reducing nonradiative recombination losses. The resulting semi‐transparent WBG (1.67 eV) perovskite solar cell achieves a power conversion efficiency (PCE) of 20.53% and a high bifaciality factor of 91.60%, retaining 92.2% of its initial PCE after 1000 h of continuous illumination. When integrated with a planar TOPCon silicon bottom cell, the two‐terminal tandem device achieves an remarkable open‐circuit voltage of 1.925 V and a PCE of 30.41% (active area = 1 cm 2 ), ranking among the highest efficiencies reported for planar silicon‐based tandem photovoltaic devices.