Multisite‐Functionalized Thiazole Spacer Enables Synergistic Grain‐Boundary Healing and Energy‐Level Alignment for High‐Performance 2D/3D Perovskite Solar Cells

ABSTRACT Although 2D/3D perovskite heterostructures enhance performance in perovskite solar cells (PSCs), conventional mono‐functional spacers display inadequate defect passivation and energy‐level misalignment, accelerating grain‐boundary degradation and nonradiative losses, thus limiting power conversion efficiency (PCE) and stability. Herein, we designed a multisite‐functionalized thiazole‐based spacer, 2‐(2‐amino‐4‐thiazolyl)acetic acid (ATAA), capable of synergistically healing grain boundaries (GBs) and realigning energy levels in 2D/3D perovskites. ATAA features polyfunctional groups that coordinate undercoordinated Pb 2+ simultaneously stabilizing FA + /MA + and I − /Br − through hydrogen‐bond networks. This multi‐site interaction mediates interfacial structural reorganization and elevates defect formation energy, thereby effectively suppressing degradation at high‐risk GBs while reducing non‐radiative recombination. Furthermore, ATAA upshifts the valence band maximum (VBM) by 0.12 eV, optimizing interfacial energy alignment and facilitating charge extraction. The optimized device achieves a champion PCE of 25.82%, while large‐area device (1 cm 2 ) maintains a high PCE of 24.68%. Enabled by triple synergistic effects of ATAA in establishing 2D perovskite shielding, reinforcing hydrogen‐bond networks, and healing GBs, the devices exhibit outstanding humidity, thermal, and light stability, retaining 91% of the initial PCE after 1 200 h of maximum power point tracking (ISOS‐L‐2 protocol) under continuous illumination. This work provides both a novel material solution and fundamental design principles toward high‐performance manufacturable PSCs.