Modifying Surface Termination of CsPbI3 Grain Boundaries by 2D Perovskite Layer for Efficient and Stable Photovoltaics

It is highly desirable for all‐inorganic perovskite solar cells (PVSCs) to have reduced nonideal interfacial charge recombination in order to improve the performance. Although the construction of a 2D capping layer on 3D perovskite is an effective way to suppress interfacial nonradiative recombination, it is difficult to apply it to all‐inorganic perovskites because of the resistance of Cs+ cesium ions in cation exchange reactions. To alleviate this problem, a simple approach using an ultra‐thin 2D perovskite to terminate CsPbI3 grain boundaries (GBs) without damaging the original 3D perovskite is developed. The 2D perovskite at the GBs not only enhances the charge‐carrier extraction and transport but also effectively suppresses nonradiative recombination. In addition, because the 2D perovskite can prevent the moisture and oxygen from penetrating into the GBs and at the same time suppress the ion migration, the 2D terminated CsPbI3 films exhibit significantly improved stability against humidity. Moreover, the devices without encapsulation can retain ≈81% of its initial power conversion efficiency (PCE) after being stored at 40 ± 5% relative humidity for 84 h. The 2D‐based champion device exhibits a high PCE of 18.82% with a high open‐circuit voltage of 1.16 V.