Dicyandiamide-Driven Tailoring of the n-Value Distribution and Interface Dynamics for High-Performance ACI 2D Perovskite Solar Cells

• Dicyandiamide (DCD) dual-functional modification simultaneously passivates buried interface defects and regulates phase distribution in quasi-2D ACI perovskites, reducing interfacial trap density by 73%. • The optimized perovskite solar cells achieve a record efficiency of 21.54% (vs. 19.05% control) and retain 94% of initial efficiency after 1200 h, outperforming unmodified counterparts (84% retention). • DCD's guanidine group passivates undercoordinated Pb2+ and MA+ vacancies, while cyano groups eliminate oxygen vacancies in TiO2 via Ti4+–CN coordination, enhancing charge transport and stabilizing energy-level alignment. • This molecular bridge strategy decouples stability-efficiency trade-offs in low-dimensional perovskites, providing a universal framework for interface engineering in high-performance optoelectronics.