Polymer assisted deposition of high-quality CsPbI₂Br film with enhanced film thickness and stability

Inorganic halide perovskites such as cesium lead iodide (CsPbI₃) have drawn tremendous attention, as their tunable band gaps are desirable for solar cells as well as light emitting diodes. However, due to their low Goldschmidt tolerance factor, the cubic phase of bulk CsPbX₃—the variant with desirable band gap—is not stable in ambient, especially in humid air. Besides, the low solubility of CsX in precursor makes it difficult to control the film thickness and morphology of CsPbX₃, which becomes another obstacle for the practical application of inorganic perovskite. Here, we report a polymer assisted deposition of high-quality CsPbI₂Br film by spin-coating a polymer-blended CsPbI₂Br precursor. The long-chained polymer increases the viscosity of the solution, which enables us to achieve a ca. 700-nm thick film with a low solution concentration of CsPbI₂Br. Moreover, the polymer network helps to regulate the crystallization process and provides more crystallization sites for perovskite film, reducing grain size and thus improving the film coverage. Perovskite solar cells with the polymer network exhibit improved efficiency and reproducibility (0.72% standard deviation). Moreover, the device demonstrates excellent robustness against moisture and oxygen, and maintains 90% of its initial power conversion efficiency (PCEs) after aging 4 months in ambient conditions. The conception of polymer incorporation into inorganic perovskite films paves a way to further increase the performance, stability and reproducibility of inorganic perovskite devices.