Facile fabrication of large-grain CH₃ NH₃ PbI_3-x Br_x films for high-efficiency solar cells via CH₃NH₃ Br-selective Ostwald ripening

Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technology. Structural and electro-optical properties of the perovskite absorber layer are most critical to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH₃NH₃PbI_3-x Br_x (MAPbI_3-xBr_x) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concentration converts MAPbI₃ thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI_3-xBr_x thin films following an Ostwald ripening process, which is strongly affected by MABr concentration and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concentration enhances I-Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.