Toward Green Processing of Perovskite Solar Cells: Protic Ionic Liquids Enable Water‐ and Alcohol‐Based MAPbI3 Precursors Inks for Slot‐Die Coating

Abstract Halide perovskite solar cells are approaching commercialization, with solution processing emerging as a key method for large‐scale production. This study introduces a significant advancement: using non‐toxic solvents like water and alcohol in perovskite precursor inks facilitated by the protic ionic liquid methylammonium propionate (MAP). MAP effectively dissolves perovskite precursors such as lead acetate and methylammonium iodide, enabling the first stable water‐based perovskite precursor ink suitable for one‐step slot‐die coating. This new ink formulation contrasts with conventional dimethylformamide (DMF) and dimethylsulfoxide (DMSO)‐based inks, as evidenced by in‐situ grazing incidence wide‐angle X‐ray scattering (GIWAXS), which revealed an intermediate‐free liquid‐to‐solid transition. In‐situ mass spectrometry also showed that organic molecules evaporate during annealing, resulting in a crystalline perovskite phase. Optimization of the solvent mixture to H 2 O/IPA/MAP enabled successful slot‐die coating, yielding perovskite solar cells with an efficiency of up to 10%. This eco‐friendly ink reduces toxicity and environmental impact compared to DMF‐based inks, offering a longer shelf life and the possibility of using the ink in ambient conditions. This pioneering work represents the first report of a water‐based green ink formulation for one‐step thin film coating at room‐temperature conditions by slot‐die coating, highlighting its potential for sustainable commercial applications.