The work aims to improve the uniformity of large-scale solvent evaporation in perovskite precursors assisted by air knife (AK). A computational fluid dynamics (CFD) model was developed based on the finite volume method, coupled with the Hertz-Knudsen-Schrage evaporation equation, to investigate the evaporation kinetics of dimethylformamide (DMF) in perovskite precursors. Firstly, the thermal-assisted evaporation flux of DMF at different substrate temperatures was experimentally measured, and a two-dimensional model was established based on these experimental conditions. At a substrate temperature of 333.15 K, the evaporation-condensation coefficient in the evaporation equation was adjusted to align the simulated values with the experimental data. The accuracy of this coefficient was verified by comparing the flux values of simulated DMF evaporation at various temperatures with the corresponding experimental values. The results demonstrated excellent agreement among the experimental, simulated, and literature values. Furthermore, it was confirmed that the evaporation-condensation coefficient remained constant across variations in temperature and pressure, with a determined value of 1.12×10-4. Next, an AK was incorporated into the two-dimensional evaporation model to systematically investigate the effects of key parameters, including the blowing angle, outlet speed, and distance from the liquid film, on the uniformity of DMF evaporation flux. By analyzing the velocity distribution on the liquid film surface and the DMF partial pressure distribution, it was observed that the AK effectively removed solvent molecules from the liquid film surface, reduced the DMF partial pressure, and ultimately enhanced the evaporation flux. These findings validated the mechanism of AK-assisted solvent evaporation. Selecting appropriate operating conditions for the AK was critical to ensuring uniform laminar flow on the liquid film surface, thereby improving the uniformity of solvent evaporation. The optimized two-dimensional AK conditions were applied to three-dimensional simulations, including a blowing angle of 40°, an outlet velocity of 10 m/s, and a distance of 10 mm from the liquid film height. Additional conditions, such as a blowing angle of 50°, an outlet velocity of 10 m/s, and a distance of 5 mm, as well as a blowing angle of 60°, an outlet velocity of 5 m/s, and a distance of 10 mm, were also tested. The results revealed that the evaporation flux directly below the AK was high and uniform, while the surface velocity of the liquid film downstream of the AK decreased, leading to a reduction in evaporation flux on both sides. In practical production, maintaining a uniform outlet speed at the AK outlet is challenging. Therefore, the effect of outlet speed amplitude at the AK outlet on the uniformity of solvent evaporation across large-area substrates is investigated. The horizontal evaporation flux distribution shows a gradual decrease from directly below the AK to the sides, with larger outlet speed amplitudes resulting in greater flux non-uniformity. Specifically, at outlet speed amplitudes of 0, 1, 3, and 5, the widths of the regions where the longitudinal evaporation flux varies within 10% are 22 mm, 18 mm, 11 mm, and 7 mm, respectively. These results highlight the importance of controlling outlet velocity uniformity, providing methodological guidance from a numerical simulation perspective for achieving stable solvent evaporation in large-scale perovskite precursor fabrication.
Key words
perovskite solar cells /
evaporation dynamics /
evaporation and condensation coefficient /
uniformity /
air knife /
computational fluid dynamics
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Funding
National Natural Science Foundation of China (22078091); Shanghai Pujiang Program (2022PJD016)
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