Phase Separation Suppression by Guanidine Phosphate Enables 21.54% Wide‐Bandgap Perovskite Cells for All‐Perovskite Tandem Solar Cells

Wide‐bandgap perovskite materials with high bromine content (> 1.7 eV) suffer from severe light‑induced phase separation, leading to irreversible device degradation—yet effective suppression strategies remain scarce. Here, we introduce guanidine phosphate (GP) as a bifunctional additive that simultaneously passivates defects and regulates crystal orientation. The Gua + and PO 4 3− ions act cooperatively: Gua + passivates undercoordinated Pb 2+ and reacts with excess surface Pb 2+ , while PO 4 3− stabilizes iodide vacancies through strong Pb–O–P bonds. This dual passivation suppresses ion migration and promotes (100)‐oriented crystallization, thereby improving film quality. Consequently, GP‑modified 1.78 eV perovskite solar cells achieve a champion power conversion efficiency (PCE) of 21.54% (control: 19.72%)—the highest reported for devices in the 1.74–1.79 eV range. Unencapsulated devices retain 90.64% of their initial efficiency after 1400 h in N 2 atmosphere. The strategy also yields 19.64% efficiency for 1.85 eV perovskites, demonstrating its generality. Furthermore, all‑perovskite tandem cells incorporating this approach achieve a PCE of 28.72%.