Steric hindrance driven passivating cations for stable perovskite solar cells with an efficiency over 24%

In this work, a series of passivating salts for perovskite solar cells (PSCs) based on ortho-methylammonium iodide functional units are molecularly engineered to study the steric hindrance driven passivation effect. The incorporation of fluorine atoms into passivating agents has been found to be beneficial not only for maximized defect passivation effect, ensuring improved charge transport, but also for significantly enhanced hydrophobicity of the perovskite film, leading to enhanced device stability. The highest power conversion efficiency (PCE) of over 24% has been achieved for surficial passivated PSCs based on fluorinated cation PFPDMAI2. Importantly, long-term operational stability over 1500 h is demonstrated showing the great prospect of a simple passivation strategy forming a thin organic halide salt layer instead of a 2D perovskite layer on the surface.