In the production process of perovskite solar cells, slit coating is one of the key processes; however, edge effects seriously affect coating uniformity, thereby impacting cell performance and efficiency. How to overcome the marginal effect and achieve a breakthrough in uniformity has become a focal point of attention in the industry.

Surface tension difference: At the outlet of the slit die head, there is a difference in surface tension between the edge and center of the coating solution. At the edges, the contact area between liquid and air is large, resulting in relatively high surface tension, which tends to cause the liquid to accumulate toward the edges, resulting in more coating at the edge than at the center, resulting in edge bulge.

Evaporation rates differ: During coating, the liquid at the edges is in better contact with air, so the evaporation rate is usually faster than in the center. This difference is especially pronounced when coating over large areas, increasing the liquid viscosity at the edges and increasing flow resistance, further affecting coating uniformity, and possibly causing the "coffee ring effect," where the crystallization rate at the edges is faster than in the central area, causing a thick film to form at the edges.

Hydrodynamic factors: When the coating liquid flows out of the slit die, the fluid dynamics at the edge differ from the center. The liquid flow at the edge of the die head is easily disturbed by external factors such as airflow, and the flow velocity distribution at the edge is uneven, which may cause vortices or sudden changes in flow velocity, affecting coating uniformity.

Special design of the die lip: By optimizing the internal geometry of the die lip, the outflow profile of the coating solution can be adjusted. For example, designing special runner structures allows the flow velocity at the edges to be properly adjusted when liquid flows inside the die head, compensating for liquid accumulation caused by surface tension and other factors. Research shows that by reasonably designing the die tip lip, the raised edges of the coating edge can be almost eliminated, significantly improving coating uniformity.

Suction system application: Installing a suction system behind the coating die head can improve edge effects to some extent. By creating a low-pressure and rapid gas flow environment on the wet membrane surface, solvent evaporation is accelerated while the liquid flow state is adjusted. However, if the traditional suction device aligns the coating direction with the liquid outlet position of the coating die blade lip, eddy currents form between the die head blade lip and the substrate, affecting coating uniformity. The improvement is to adjust the suction angle so that it is angled toward the substrate to reduce eddy currents. Additionally, a reasonable storage area can prevent condensing solvent from dripping back onto the substrate from the suction runner, further improving film quality and uniformity.

Other auxiliary technologies: For example, using an air-floating coating platform is also an effective way to address uniformity issues. The floating platform uses high-pressure gas to form a suspended air cushion, keeping the coating head in "zero contact" with the substrate. Its dual Y, dual Z-axis structure addresses edge collapse challenges in coating large substrates by enabling bidirectional synchronous drive to eliminate inertial bias and Z-axis adaptive leveling to suppress substrate deformation, controlling film thickness fluctuations within a minimal range and greatly improving the uniformity of perovskite film coating.

Nano-scale bore diameter air floating bearings

To achieve uniformity breakthroughs in perovskite slit coating equipment, it is necessary to deeply understand the mechanism of edge effects and comprehensively apply various advanced equipment technologies and solutions. With continuous technological advancements, it is believed that more efficient methods will emerge in the future, helping the perovskite solar cell industry develop toward higher efficiency and lower costs.