Anti-Ribbing: Ink Optimization Enables Certified Slot-Die Coated Perovskite Solar Cells with > 22% Certified Power Conversion Efficiency and a Full Year Outdoor Stability

The next step in the exploration of metal-halide perovskite solar cell technology is the demonstration of larger area device prototypes under outdoor operating conditions. When slot-die coating metal-halide perovskite precursor solutions, layer thickness and morphological inhomogeneities may occur due to the so-called ribbing effect. We here demonstrate that ribbing effects can be prevented by adjusting the precursor ink’s rheological properties. For formamidinium lead triiodide (FAPbI3) precursor inks based on 2-methoxyethanol (2-ME) the ink viscosity was adjusted by using acetonitrile (ACN) as a co-solvent leading to smooth FAPbI3 thin-films with high quality and layer homogeneity. The high vapor pressure of ACN ensured its efficient removal during slot-die coating facilitated by gas quenching. In addition, we provide evidence from in-situ X-ray diffraction experiments that the co-solvent accelerates the formation of a crystalline intermediate phase, promoting a more favorable growth of homogeneous perovskite semiconductor thin-films with better morphology. For an optimized content of 46 vol% of the ACN co-solvent, a certified steady state performance of 22.3% was achieved in p-i-n FAPbI3-perovskite solar cells (PSCs). Scaling devices to larger areas by making laser series-interconnected mini-modules of 12.7 cm2, a power conversion efficiency (PCE) of 17.5% is demonstrated. Optimized inks yielding high-quality perovskite thin-films with record FAPI-based PSC performance also exhibited an improved photo-stability. We performed a full year of outdoor stability testing on encapsulated devices and demonstrate these to maintain close to 100% of their initial performance during continuous maximum power point tracking at our testing site in Berlin, Germany. This contribution is hence of significance importance as we demonstrate the potential of perovskite solar cell technology both regarding the scalability and stability of these devices.