Investigating the Effects of a High Boiling Point Solvent in Slot Die-Coated Halide Perovskite Solar Cells

To commercialize perovskite solar cells and advance beyond lab-scale comparisons, understanding large-area film formation using slot-die coating is essential to improve film homogeneity. Adding high-boiling-point solvents like N-methyl-2-Pyrrolidone (NMP) to the perovskite ink extends film′s processing window, but the effects of varying NMP levels on gas-quenched slot-die coatings remain unclear. This article examines how different NMP ratios impact film quality, showing that a moderate amount of NMP as a co-solvent reduces defects, as observed through photoluminescence, hyperspectral absorbance, and back-illuminated optical absorptions. However, the decreased vapor pressure with the addition of NMP impairs crystallization and film coverage, highlighting the need for balanced amounts. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis indicate that the most volatile option tested at Dimethylformamide (DMF) : NMP ratio of 8 : 1 yields the most homogeneous and compact films. Slot-die-coated devices fabricated with this optimized ratio were subsequently compared with using NMP as an additive to increase the volatility of the perovskite inks further. The additive method demonstrates improved performance and uniformity, suggesting that minimizing high-boiling-point solvents to maintain ink volatility supports effective large-area coatings and fabrication of perovskite solar cells. Furthermore, this article provides insights on important metrics to narrow down suitable perovskite inks for large-area coatings.