Process background
By stacking absorbing layers with different bandgaps, the upper limit of single-junction conversion efficiency can be broken. As a key functional layer connecting the front and rear subcells, the interlayer needs to take into account the optical transmittance, electrical contact quality, and interface defect passivation effect. The coating method is gradually becoming the mainstream solution for the preparation of interlayer due to its large-area film formation, high material utilization, and process compatibility. By precisely controlling the spacing between the coating head and the substrate, the solution supply rate, and the drying atmosphere, the perovskite coating machine can control the uniformity of the film thickness at the nanoscale, providing a guarantee for the interlayer to achieve the required photoelectric characteristics.
The core control parameters of the coating machine
The quality of the interlayer is highly dependent on the stability and repeatability of the coating process. Key parameters include:
- Coating gap: typically set between 50 μm and 200 μm, affecting the wet film thickness and shear stress distribution.
- Coating speed: range 0.5 mm/s to 5 mm/s, too slow can lead to streaking, too fast can cause bubbles.
- Solution concentration and solvent boiling point: match the evaporation rate to avoid pinholes or uneven crystallization in the film layer.
- Ambient temperature and humidity: Temperature fluctuations should be controlled at ±0.5 °C, and the dew point below -20 °C can inhibit water vapor intrusion.
Intermediate material and coating adaptability
Commonly used interlayer materials include metal oxides, organic small molecules, and two-dimensional perovskite layers. Metal oxides (e.g., ZnO, SnO₂) usually require hydrolysis and condensation of precursor solutions, and need to be annealed and converted after coating. Organic layers rely on π-π stacking and self-assembly, and the coating speed should be lower than the critical value to prevent molecular orientation confusion. Two-dimensional layered perovskites contain long-chain organic cations and have high solution viscosity, so a narrow-gap coating head is required to maintain uniform spreading. Table 1 summarizes the relationship between typical materials and coating parameters.
| Material type | Recommended coating parameters |
| Metal oxide precursors | Gap 80 μm, speed 1.5 mm/s, annealing temperature 150 °C |
| Organic small molecules | The gap is 120 μm, the speed is 0.8 mm/s, and the annealing temperature is 80 °C |
| 2D perovskite | The gap is 60 μm, the speed is 0.5 mm/s, and the annealing temperature is 100 °C |
Film thickness and defect control
The thickness of the interlayer directly affects the series current matching. The target thickness range is generally from 10 nm to 50 nm, and the corresponding optical interference peak should be aligned with the absorption edge of the subcell. The mathematical expression of film thickness uniformity can be simplified to:
h = (C × Q) / (W × V)
Among themhis the thickness of the wet film,Cis the solid content of the solution,Qis the flow rate of the liquid supply,Wfor the coating width,Vis the coating speed. The thickness of the dry film after actual drying is multiplied by the solid phase shrinkage coefficient. In terms of defect control, the edge thickening caused by Marangoni convection can be suppressed by adjusting the exhaust rate at the edge of the coating head, and the typical exhaust flow rate is set between 5 L/min and 15 L/min.
Drying and annealing work together
After coating, the wet film must go through the pre-drying, main drying and annealing stages. The pre-drying is done using an infrared lamp plate, and the temperature is set at 40 °C to 60 °C for 20 s to 60 s, and the low boiling point solvent is removed. The main drying is carried out on the hot plate at 80 °C to 120 °C for 2 minutes to 5 minutes. The annealing temperature should be lower than the decomposition temperature of the underlying perovskite (usually below 150 °C). For the organic transport layer, annealing time of more than 10 min may trigger molecular aggregation, so rapid thermal annealing (heating within 60 s) is advisable to maintain the nanoscale morphology.
Online monitoring and feedback
Advanced coaters integrate a film thickness reflectance spectrometer and a vision inspection system. The reflectance spectrum measures the interference curve immediately after coating, inverting a thickness error of less than 2 nm. The vision system captures macroscopic defects with a width greater than 30 μm, such as streaks, broken films, or particle scratches. When the real-time detection deviation exceeds a threshold (e.g., thickness error of more than ±5%), the system automatically adjusts the coating speed or supply flow rate for closed-loop control. This in-line regulation significantly reduces batch-to-batch variability and improves the repeatability of stacked devices.
Use cases and effects
The SnO₂ interlayer (about 20 nm) was prepared in the stacked structure using a perovskite coating machine, which increased the current density matching between the front and rear subcells from 88% to 96%, and the filling factor increased by 3% to 5%. In another case, the organic interlayer (PEDOT: PSS derivative) was used with the coating annealing process to reduce the open circuit voltage loss of the stacked cell from 120 mV to 80 mV. The above data comes from work published in the journal Materials Science, which verifies the feasibility of the coating process in the preparation of the interlayer.
Development trend
In the future, coating machines will evolve towards higher precision slit adjustment (resolution up to 0.1 μm) and multi-component gradient coating to meet the needs of gradient interlayers or composite structures. At the same time, multimodal fusion of non-contact in-line inspection (such as elliptic spectroscopy and photoluminescence imaging) is expected to achieve simultaneous monitoring of electronic and optical properties, thereby achieving tighter yield control in industrial production.
References
1. A Review of Interface Engineering of Stacked Solar Cell Interlayers, Advanced Energy Materials, 2023.
2. Parameter Optimization of Oxide Film Deposition by Coating Method, Solar Energy Materials & Solar Cells, 2022.
3. Self-assembly and coating process of two-dimensional perovskite interlayer, Journal of Materials Chemistry A, 2024.