In Situ Formation of Guanidinium Derivative and 2D/3D Heterostructure for 26.49% Efficient and Stable Inverted Perovskite Solar Cells

Inverted perovskite solar cells (IPSCs) face challenges in achieving both high efficiency and stability due to defects that induce non‐radiative recombination. Here, we introduce 1H‐1,2,4‐triazole‐1‐ammonium chloride (HTA) as a multifunctional additive that reacts with formamidinium iodide (FAI) to generate a multi‐amino guanidinium derivative. This in situ formed compound enables dual passivation by coordinating with Pb 2+ ions and forming hydrogen bonds with I − ions, while simultaneously templating a 2D/3D heterostructure. HTA effectively regulates nucleation kinetics, reduces defect density, and optimizes energy‐level alignment. Consequently, the HTA‐treated device achieves a champion power conversion efficiency (PCE) of 26.49% (certified 26.20%) with significantly enhanced stability. The unencapsulated device retains 89.15% of its initial efficiency after 2000 h in ambient air, and the encapsulated device maintains 95% of its initial PCE after 1000 h of maximum power point tracking. This strategy provides a promising route for developing high‐performance and stable IPSCs.