Roll-to-Roll Preparation of Perovskite Coaters for Flexible Perovskite Solar Cells

Coating process principle

The preparation of flexible perovskite solar cells requires uniform coating of perovskite precursor solution on the flexible substrate. Roll-to-roll coating technology is the key to achieving large-scale continuous production, including slit extrusion, inkjet printing and other methods. Slit extrusion controls film thickness by adjusting gap, flow rate and substrate speed, which is suitable for high-speed production. Inkjet printing is suitable for patterned deposition. The viscosity, surface tension and solvent volatilization rate of the precursor solution directly determine the uniformity of film formation, and the process parameters need to be optimized according to the Reynolds number and capillary number.

During the coating process, the crystallization kinetics of perovskite are interfered with by the substrate temperature and airflow. The anti-solvent assisted method can induce rapid nucleation and reduce the defect density. Wet film thicknesshand coating speeduThe relationship can be approximated by the following formula:

h = (Q / (W * u)) * η

Among themQfor the flow rate,Wfor the coating width,ηTo correct the coefficient, it needs to be calibrated by experiments.

Key material requirements

Flexible substrates such as polyethylene terephthalate need to have high light transmittance and thermal stability. The perovskite precursor adopts a formamidine-lead-iodine system, and additives can be introduced to control crystallization. Solvent selection needs to balance evaporation rate and toxicity, and mixed solvents of γ-butyrolactone and dimethyl sulfoxide are more common. The following table lists the characteristics of different solvent combinations:

Roll-to-roll system composition

The roll-to-roll coating machine includes a unwinding unit, a coating head, a drying channel and a winding unit. The position accuracy of the coating head needs to be at the micron level to avoid streak defects. The drying section adopts segmented temperature control, the temperature of the preheating zone is low to prevent solvent boiling, and the subsequent zone heats up to promote crystallization. The tension control needs to be constant to prevent wrinkling or tensile deformation of the substrate. The online monitoring system feeds back the uniformity of film thickness through optical or electrical means.

The key parameters of the system are listed in the table below:

Process control essentials

The coating environment needs to be low humidity, and the water and oxygen concentrations are less than 10 ppm and 100 ppm, respectively. The precursor solution is filtered with an accuracy of 0.2 microns to prevent particles from causing pinholes. In-situ annealing after coating is done by infrared or hot air, and the time needs to match the roll-to-roll speed. For perovskite films, crystallization needs to be completed under inert atmosphere or nitrogen protection. Defect detection can be controlled in time with the help of dark-field imaging.

Performance evaluation methodology

The film uniformity was evaluated by scanning electron microscopy and X-ray diffraction, and the crystal phase purity was measured by the half-height and width of the main peak. The photoelectric conversion efficiency was tested using a standard solar simulator, and the outer quantum efficiency reflected carrier collection. The flexible bending test was performed with repeated bending with a curvature radius of 5 mm, and the attenuation of performance was recorded. The batch stability was monitored by sampling, and the mean and standard deviation were counted.

Cited Literature

1. Wang et al. Research on roll-to-roll coating process of flexible perovskite solar cells. Materials Review, 2022: 35(6), 100-108.
2. Li, J. et al. Slot-die coating of perovskite films. Advanced Energy Materials, 2021: 11(15), 200-215.
3. Zhang et al. Effect of perovskite precursor solvent on thin film crystallization. Functional Materials, 2023: 54(3), 50-56.