Beyond Strain: Controlling the Surface Chemistry of CsPbI3 Nanocrystal Films for Improved Stability Against Ambient Reactive Oxygen Species

Colloidal halide perovskite nanocrystals (NCs) have the possibility of easy scale-up due to their batch synthesis and have demonstrated excellent optoelectronic properties. In particular, perovskite NCs have remarkably high photoluminescence quantum yields in solution and as thin films and impressive open circuit voltages in photovoltaic devices. Despite these promising results, little work has been done to understand the stability of CsPbI3 NCs for optoelectronic device applications. It has been previously shown that the ligands impart tensile surface strain, which stabilizes the black three-dimensional (3D) perovskite phase against phase degradation, making CsPbI3 NCs some of the most structurally robust inorganic halide perovskites to date. However, understanding exactly how CsPbI3 NCs degrade under ambient conditions is critical. We demonstrate that the degradation mechanism of NCs is unique from, and 2 orders of magnitude slower than, their polycrystalline thin-film counterparts. Under specific conditions, CsPbI3 NC films show a compositional instability instead of the phase instability seen in large grain CsPbI3. This is mediated through reactions with superoxide and other reactive oxygen species, which are initiated from surface defect states, O2 and light. We then use this mechanistic insight to identify multiple strategies to prolong the lifetimes of CsPbI3 NC films, by going beyond surface strain to mitigate key surface chemistries. We demonstrate that (1) minimizing the number of surface defects (2) using an alkylammonium bromide ligand surface treatment and (3) encapsulation with an oxygen scavenging layer all increase NC film lifetimes by inhibiting various steps in the photo-oxidation degradation reaction.