Synergistic Modulation of Buried Interface and 2D–3D Assisted Growth of Bulk via Surface‐Functionalized Layer Enables >85% Fill Factor in Inverted Perovskite Solar Cells

Abstract The NiOx/perovskite buried interface critically influences the power conversion efficiency (PCE) and long‐term stability of perovskite solar cells (PSCs) due to high defect density, suboptimal charge transport and associated interfacial redox reactions. Herein, this study introduces interfacial molecular layers of 1‐(4‐cyanophenyl) guanidine hydrochloride (CPGH) and 4‐guanidinobenzoic acid hydrochloride (GBAH) onto NiOx. Theoretical calculations and experimental validation reveal the synergistic effect of functional groups in CPGH (─CN) and GBAH (─COOH) that interact with Ni^>3⁺ species and surface hydroxyls (─OH), effectively passivating redox‐active sites and improving the Ni³⁺/Ni²⁺ ratio. This passivation leads to a valence band downshift, enhancing hole extraction. Concurrently, the guanidine (─NH₂) groups interact with residual PbI₂ at the buried perovskite interface, promoting 2D perovskite formation, attained via cautious mechanical exfoliation which passivates Pb²⁺ related defects and iodine vacancies, thereby reducing ion migration. The 2D PVSK further facilitates the growth of high‐quality 3D perovskite films with larger grains, lower defect density, and reduced residual stress, improving film morphology and optoelectronic properties. Consequently, the CPGH and GBAH treated devices achieve high fill factors (FF) of 85.5% and 82.9%, PCEs of 24.72% and 23.46%, respectively, compared to 80% FF and 21.79% PCE (control). Long‐term stability tests show PCE retention of >82% after 500 h of thermal aging and >87% after 2000 h of ambient exposure. These results underscore the critical role of buried interface engineering in device performance and stability.