Integrated Bulk-Surface Engineering Stabilizes MA-Free Wide-Bandgap Perovskites for Tandem Photovoltaics

Optimal wide-bandgap perovskites are essential for perovskite/silicon tandem solar cells. Conventional wide-bandgap perovskites, typically FA_{1- x - y}Cs_xMA_yPbI_{1- z}Br_z, contain volatile methylammonium (MA) components and mixed halides that compromise device stability and performance. Removing MA to form FA_{1- x}Cs_xPbI_{1- z}Br_z eliminates volatile organic components; however, the absence of MA and high Br content required for bandgap widening inevitably accelerates crystallization, increases defect density, and induces severe voltage losses. Here, we present a coupled bulk-surface regulation strategy that fundamentally overcomes these intrinsic bottlenecks. Incorporation of homopiperidinic acid hydroiodide into the precursor heals bulk lattice defects via ─COOH─Pb²⁺ coordination and suppresses halide migration through N─H…I- hydrogen bonding, while subsequent treatment with trimethylenediamine dihydroiodide salts neutralizes surface unsaturated Pb²⁺ and halide vacancies through amino-Pb²⁺ coordination, hydrogen bonding, and electrostatic interactions. Their coupling effect precisely suppresses defect formation, minimizes non-radiative recombination, and critically stabilizes halide distribution. As a result, the wide-bandgap perovskite solar cells achieve an efficiency of 23.71% and enhanced operational stability with T₉₂ exceeding 1000 h. Integrated into silicon tandem devices, they deliver 32.26% with long-term durability. This work establishes molecular coupling consolidation as a new paradigm for constructing stable, high-efficiency, MA-free wide-bandgap perovskites, advancing the practical realization of reliable tandem photovoltaics.