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 cm²), ranking among the highest efficiencies reported for planar silicon-based tandem photovoltaic devices.