Tailoring the SnO2 Ink for Roll-to-Roll Manufacturing of Flexible Perovskite Solar Cells

Roll-to-roll (R2R) fabrication of high-performance perovskite solar cells (PSCs) remains challenging for PSC commercialization. Herein, the R2R-suitable electron transfer layer (ETL) SnO2 ink system for flexible PSCs is investigated. Water-dispersed SnO2 inks, commonly used for flexible PSCs, form dewetting pinholes during R2R slot-die coating owing to slower evaporation compared to spin coating. To prevent the dewetting of the pinhole, a low surface tension solvent (isopropyl alcohol, IPA) is added to the SnO2 ink. The IPA addition reduced surface tension, enhancing wetting behavior, but increased SnO2 nanoparticles (NPs) aggregation. Interestingly, the fabricated ETL films with highly aggregated SnO2 NPs are smooth, in contrast to the rough layers obtained via the spin-coating process, owing to the high shear rate. Herein, NP aggregation and surface morphology are explored using Brownian dynamics simulations under shear conditions. Consequently, when the particles are agglomerated, the PSCs with slot-die coating exhibit higher power conversion efficiency compared to ETL spin-coated PSCs. Therefore, IPA addition to SnO2 ink is preferred for the R2R slot-die process by improving the wetting properties and forming smooth films. In this study, a framework is provided for obtaining high-quality films using the slot-die coating of ETL and promotes the commercialization of PSCs.