Dual-Functional Guanidinium Iodide Interfacial Engineering for Efficient and Stable Dion–Jacobson Perovskite Solar Cells

Perovskite solar cells (PSCs) have achieved over 27% efficiency in just a decade, yet their long-term stability remains a commercialization bottleneck. Quasi-two-dimensional (quasi-2D) perovskites, particularly Dion–Jacobson (DJ) phases, have garnered significant attention due to their superior environmental stability compared with three-dimensional (3D) counterparts. Despite this, quasi-2D PSCs still lag behind in efficiency owing to intrinsic defects at grain boundaries and interfaces; these defects also facilitate moisture ingress and accelerate degradation. To address these limitations, this work introduces a stable DJ-phase perovskite based on 1,4-cyclohexanedimethanamine (CDMA) and employs guanidinium iodide (GUAI) as an interfacial modifier. The multifunctional GUAI layer synergistically passivates cation vacancies, heals deep-level defects, and smooths grain boundaries through interactions between its electron-rich guanidinium groups and iodide ions. This dual-action strategy enhances charge extraction, suppresses nonradiative recombination, and strengthens moisture resistance. Consequently, GUAI-modified 2D PSCs achieve a champion PCE of 15.07% (vs 13.25% for control devices) with an open-circuit voltage of 1.09 V and fill factor of 70.68%. Remarkably, the unencapsulated device retains 92% of its initial PCE after 2500 h in ambient air (35–75% RH), demonstrating exceptional operational stability. This work provides a scalable approach to enhance the efficiency and stability in perovskite photovoltaics.