Hybrid perovskite absorbers are poised to become a crucial part of next-generation photovoltaics (PVs) for mitigating the impact of energy production on the climate. In one projected application, two-terminal silicon-perovskite tandem PVs, a high-bandgap perovskite absorber is coated on top of a silicon absorber (with intermediate layers) - achieving power conversion efficiencies superior to the Shockly-Queisser limit of single-junction silicon PVs. One of the major advantages of the two-terminal tandem architecture is that the perovskite deposition step can be integrated conveniently into the high-throughout production of mono-crystalline silicon modules that currently dominate the global market. In practice, these modules are assembled from individual silicon wafers in batch-to-batch operation. Consequently, for seamless integration, the perovskite absorber must be deposited onto a surface of silicon-wafer size, which are typically from about 20 cm to 30 cm wide. There are two principal routes of perovskite absorber deposition: Solvent-free vapor deposition and coating or printing from solution. While the first route offers advantages in homogeneity and compatibility with textured silicon substrates, the second route is more cost-effective and less operationally complex. However, solution processing of perovskite thin films is challenging to control on the targeted substrate scales for high-throughput coating techniques such as slot-die coating and spray coating (spin coating is most likely not an option for commercial PVs due to excessive material waste and throughput limitations). Therefore, there is a great merit in detailed investigation of how to deposit perovskite thin films homogeneously on substrates of typical silicon-wafer sizes.

For perovskite deposition from solution, not only the homogeneity of the coated wet films, but also the homogeneity of the drying process comes into play. This is because perovskite crystallization is highly drying-rate dependent. Typical perovskite precursor solutions require very high drying rates at moderate temperatures, which is why slot-nozzles purged by pressurized air or nitrogen, so-called “air knives” are often employed for drying. It is common knowledge that the mass transport under these air knives is highly inhomogeneous. That is to say that a liquid film under an air knife will dry much faster directly under the nozzle opening than on the edges of the film farther away from the nozzle. To counter-balance this inhomogeneity, technologists typically move the wet solution film under the air knife linearly. Still, the part of the substrate that experiences the air stream first, will dry differently than the part of the film situated at the center of the substrate. In turn, the part of the film that passes under the air knife last will have different drying dynamics from the other two parts mentioned before. Conclusively, it is challenging to homogenize drying dynamics of thin films moving under an air knife. In this talk, we investigate how this problem can be addressed by a systematic parameter variation during drying - the available paramters being the air flow velocity, the air knife distance, slot-width and mounting angle as well as the movement speed of the air knife. The investigation is implemented by a) developing a suitable and computationally efficient drying model of a high number of positions on the substrate b) defining how a homogeneous drying process is characterized and c) optimizing the available parameter space as a function of time for obtaining homogeneous drying. The presented results showcase the high potential and prediction power of employing drying models to homogenize perovskite drying. Further, the dependence of the controllability of drying on the homogeneity of the coated wet film as well as the time and film composition at crystallization onset is demonstrated.