The high-power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) on lab-scale devices trigger the need to develop scalable manufacturing processes to accelerate their commercialization transition. A roll-to-roll (R2R) vacuum-free printing on flexible substrates allows for high-volume and low-cost manufacturing which is especially well-suited for PSCs due to its solution processibility and low-temperature annealing requirements. Herein, a facile hot deposition technique is reported to fabricate triple-cation (Cs 0.07 FA 0.79 MA 0.14 -Pb(I 0.83 Br 0.17 ) 3 ) perovskite films in an ambient environment using a R2R slotdie coating method. This perovskite composition, whilst being most studied in lab devices due to its high efficiency and stability, has not been applied in R2R fabrication thus far. The demonstrated R2R slot-die coated flexible PSCs achieve stabilized PCE reaching 12% at maximum power point in inverted "p-i-n" architectures, the highest efficiency reported to date for R2R inverted PSCs. To achieve this, the underlying hole transport layer (poly(3,4-ethylenedio xythiophene):polystyrene sulfonate) is modified with guanidinium iodide additive which leads to the formation of large millimeter-sized perovskite clusters, improved perovskite crystallinity, and enhanced charge-transfer efficiency. This study highlights the potential of the facile hot-deposition method while providing critical insights into the role of interfacial engineering in eliminating performance losses and fabricating efficient printed flexible PSCs.

Use of the guanidinium iodide (GAI) cation is widely recognized as an interface engineering technique for perovskite solar cells that deliver stability improvements via defect passivation on surfaces and grain boundaries. However, a comprehensive understanding of the relationship between the structural and photophysical properties is lacking. Herein, GAI‐induced perovskite structural modifications, including derivative phases and underlying transitions, are detected in GAI surface‐treated Cs0.07MA0.14FA0.79Pb(I0.83Br0.17)3 through an analysis of X‐ray and electron diffraction and microscopy data. An optimum GAI solution concentration at 10 mg mL−1 can eliminate excess PbI2, improve crystallinity, and increase grain size of the as‐prepared perovskite films. However, a further increase to 20–40 mg mL−1 induces new (FAPbI3) x (GA2PbI4) x phases and a reduction in crystallinity and grain size. In addition, from confocal photoluminescence imaging, it is observed that 10 mg mL−1 GAI also helps to remove the microscale spatial heterogeneities, demonstrating optimum device performance. These results show that understanding the impact on structure and microstructure of the selection and concentration of surface treatment agents is critical for the homogenization of perovskite optoelectronic properties and achieving efficient device.