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

Organic-inorganic hybrid perovskite solar cells achieve remarkable efficiencies (> 26%) yet face stability challenges. Quasi-2D alternating-cation-interlayer perovskites offer enhanced stability through hydrophobic spacer cations but suffer from vertical phase segregation and buried interface defects. Herein, we introduce dicyanodiamide (DCD) to simultaneously address these dual limitations in GA(MA)PbI perovskites. The guanidine group in DCD passivates undercoordinated Pb and MA vacancies at the perovskite/TiO interface, while cyano groups eliminate oxygen vacancies in TiO via Ti-CN coordination, reducing interfacial trap density by 73% with respect to the control sample. In addition, DCD regulates crystallization kinetics, suppressing low-n-phase aggregation and promoting vertical alignment of high-n phases, which benefit for carrier transport. This dual-functional modification enhances charge transport and stabilizes energy-level alignment. The optimized devices achieve a record power conversion efficiency of 21.54% (vs. 19.05% control) and retain 94% initial efficiency after 1200 h, outperforming unmodified counterparts (84% retention). Combining defect passivation with phase homogenization, this work establishes a molecular bridge strategy to decouple stability-efficiency trade-offs in low-dimensional perovskites, providing a universal framework for interface engineering in high-performance optoelectronics.