Bifunctional Molecular Regulation Couples Halide Homogenization With Deep‐Level Defect Control in MA‐Free Wide‐Bandgap Perovskites for Tandem Solar Cells

Methylammonium-free (MA-free) wide-bandgap mixed-halide perovskites are highly promising for stable perovskite/silicon tandem solar cells. However, the halide homogenization process readily induces deep-level defects because iodide incorporation into bromide-rich phases is hard to control. Here, we report a bifunctional ammonium salt strategy that achieves sequential defect control during this process. We find that iodide interstitial and antisite incorporation into the perovskite lattice leads to the formation of halide trimer motifs, which constitute a common structural origin of deep-level defects. Our strategy uses 4-(2-aminoethyl)benzenesulfonic acid hydrochloride, where the sulfonic acid group kinetically regulates iodide incorporation within the narrow homogenization window, while the ammonium moiety reconstructs the local atomic configurations of halide trimer motifs, enabling the spontaneous sequential suppression and deactivation of these deep-level defects. This strategy delivers a champion efficiency of 23.81% for MA-free perovskite solar cells and 32.3% for perovskite/silicon tandem devices, which retain over 90% of their initial efficiency after 1000 h of continuous illumination in ambient air. This work advances the understanding and molecular control of deep-level defects arising from halide homogenization in wide-bandgap perovskites.