Cytidylic acid improves crystal growth, defect passivation and flexibility of inorganic CsPbI2Br film for inverted photovoltaics towards versatile applications

Cesium-based all-inorganic perovskites have been of great interest due to their excellent thermal and light stability. However, these inorganic perovskites more often suffer from poor crystallization and unfavorable mechanical flexibility, hence delivering inferior photovoltaic performance and limited device lifetime. In this work, cytidine 5ʹ-monophosphate (5ʹ-CMP), a natural organics with multiple functional groups, is used as an additive to facilitate the crystallization of CsPbI2Br film, and meanwhile to mitigate the interfacial defects and residual strain, which could be attributed to the strong interactions between 5ʹ-CMP and PbI2 precursor as well as perovskite lattice. As a result, the optimal devices with an inverted structure exhibit an enhanced efficiency of 15.94% or 33.22% along with excellent operational stability under one-sun or white light emitting diode (WLED) illumination conditions. In addition, the perovskite film with 5ʹ-CMP exhibits a reduced modulus, hence rendering the flexible devices with improved mechanical stability. This work thus demonstrates a promising strategy to optimize inverted all-inorganic perovskite devices for versatile applications.