Slot‐Die Coating Ultra‐Uniform NiO _X Hole Transporting Layer via Eliminating the “Coffee‐Ring” Effect for Flexible Perovskite Solar Modules

Abstract Solution‐processed NiO X nanocrystals can be facilely applied as hole transporting layer (HTL) for flexible perovskite solar cells (F‐PSCs) which have high power‐to‐mass ratio and flexibility. However, emerging evidence demonstrates that the “coffee‐ring” effect, a consequence of intricate fluid dynamics inherent in solution‐processed NiO X thin films, critically impedes the device performance, particularly when at the module scale. Herein, a successful fabrication of large‐area (100 cm 2 ) high‐quality NiO X films on flexible substrates is demonstrated via a slot‐die coating method. By precisely engineering the solvent used for the NiO X ink, an optimized dispersion, balanced capillary and Marangoni flow are achieved, which consequently ensures a high uniformity of the solute and an elimination of the “coffee‐ring” effect. This optimization process yields uniform, dense, and defect‐less NiO X films. Champion efficiencies of 22.3% and 21.4% are achieved for F‐PSCs with active areas of 0.16 and 1.00 cm 2 , respectively. Notably, these efficiencies remained competitive at 18.8% and 17.2% for modules when the size increasing to 13.74 and 56.80 cm 2 , respectively. Furthermore, the F‐PSCs retained 85% of their initial performance after 1100 h of continuous operation. Additionally, both the mechanical durability and reverse bias stability of the PSMs are significantly improved.