The study focuses on the so-called multivalent amidinium ligands, which can bind the perovskite surface more strongly and more stably than common monovalent ammonium ligands.[1] The team developed a strategy for the controlled transition between 1D and 2D structures by systematically varying the shape and properties of the ligands, thereby affecting hydrogen bonding, π–π interactions, and basicity.[1] The one-dimensional (1D) amidinium perovskite layer had significant anisotropy and could not uniformly cover the surface, so defect passivation was limited.[1] A two-dimensional (2D) amidinium perovskite layer, on the other hand, formed a continuous, homogeneous interlayer that effectively passivated defects and improved energy matching at the interface.[1] The electron-rich pyridine rings in these ligands coordinated the surface defects better than the phenyl rings and replaced the weaker π–π stacking with stronger hydrogen bonds.[1] Inverted 3D/2D amidinium perovskite cells achieved a certified energy conversion efficiency of 25.4% on an area of 1.1 cm².[1] In continuous operation at an intensity of 1 sun and a temperature of 85 °C, these cells retained more than 95% of their original efficiency even after 1100 hours.[1]