Nowadays, state of the art solar cells are based on monocrystalline silicon wafers. The manufacturing of silicon wafers for photovoltaic (PV) applications involves a series of precise and carefully controlled processing steps. This blog post delves into the critical stages of production between sawing and texturing of the substrates, while highlighting key parameters and quality characteristics of the final product.
The actual wafers are produced through sawing of mono-Si ingots which are glued to a support beam. Multiple wire saws cut thin silicon slices of down to 100 µm thickness and an edge length of up to 230 mm.
Nowadays, diamond wires are commonly used to cut through the silicon ingot to ensure higher quality wafer surfaces compared to traditional slurry-based wire sawing. Set to the right speed and tension, the diamond wire cuts easily
through the Si-block, ensuring high yield and a faster process compared to the slurry process. A perfect cut can make a big difference in subsequent processes and can even influence the efficiency of the solar cell. Those improvements additionally lead to less material loss and reduced cost per wafer due to thinner wafers as well as thinner wires. However, the surface of the wafers are smooth but still have saw damages, that need to be improved.
Automation in wafer handling has greatly improved manufacturing efficiency in the last decade. These systems include AGVs (Automated Guided Vehicles), conveyor belts, automated inspection, and sorting systems.
Especially in between processing steps, the wafers have to be moved from one machine to another. Modern Solar-Cell factories utilize AGVs, rail systems or conveyor belts to do this job. The transport carriers are loaded onto the platform which in turn navigates the lot to the next required processing step. The benefits are obvious — increased throughput and yield, reduced human error and labor costs as well as enhanced cleanliness and reduced contamination.
After the sawing process, the wafers are still dirty and glued to a support structure made of steel or similar material (often called “beam”). A cleaning step is necessary to wash off particles, coolant/cutting fluid and de-glue the wafers. This is done in a wet process fashion using tools like RENAs PreWaClean — through several cleaning and rinsing steps the whole array of wafers is treated by liquid chemical through spray nozzles and immersion tanks to wash it free from contaminants and residues.
De-gluing from the support beam is also performed during the pre-cleaning step. The key to good cleaning results is the right choice of chemical solutions and combination of spraying and immersion processing. Additionally, process times and bath temperatures can be used to influence the results. Experience as well as technical knowhow does make the difference in this Pre-Cleaning step.
Some cleaning processes can also be combined with ultrasonic high-frequency treatment and/or higher temperatures to strip and remove contaminants from the wafer surfaces.
After cleaning the de-glued wafers can stick together due to surface tension and need to be separated (also called singulation). Through an highly automated process the wafers are carefully separated. Modern separation units can handle the thin and fragile wafers with an astonishing throughput of more than 5,000 wafers per hour. Microcracks and other defects are detected and the damaged wafers are sorted out before they are loaded into transport or process carriers for further treatment.
To maintain highest quality and to prepare the wafers for texturing in the subsequent cell process, a final cleaning step is performed. This process step removes the remaining contaminants from the wafers in a batch type fashion without etching the surface e.g. by RENAs BatchWaClean. Typically alkaline treatment with subsequent organic cleaning combined with rinsing is used for this final process step. Benefits of this process are high throughput and high cleaning efficiency for different texturing methods.
After Cleaning a drying step is carried out to ensure that no watermarks and particles remain on the wafer, which may cause efficiency loss or inhomogeneity in the latter cell processing.
After this final step the solar wafers are ready for texturing and for the following process steps of becoming a highly efficient solar cell.
Advanced metrology and inspection tools check the solar wafers in every stage of the manufacturing process. High-precision metrology enables manufacturers to monitor wafer quality and process parameters in real-time after every stage, ensuring high yields and consistent performance. Early detection of defects and deviations in wafer quality as well as enhanced overall yield and efficiency are obvious benefits of utilizing these systems.
The wafer manufacturing process in photovoltaics is extremely throughput driven and highly automated. It involves several critical steps between sawing and texturing, each requiring meticulous control over various parameters. From the precision in sawing to the detailed chemical treatments in cleaning, etching, and texturing, every stage plays a vital role in
The first process step in the cell process (conversion from wafer to solar cell) is the so-called alkaline texturing: This etching step is the first, and also one of the most critical process steps in enhancing the light absorption of the wafers. By creating a rough surface with specifically shaped and homogeneously distributed pyramids reflection is reduced and the effective surface area is increased. Previous treatment of the wafer surface has a significant influence on the texturing.
State-of-the-art texturing techniques, such as alkaline texturing for mono-crystalline silicon wafers, have significantly improved light absorption by creating surface patterns that reduce reflectivity. Depending on the desired technology like TOPCon or Heterojunction the surface of the solar cells have to be shaped in different ways. However, the cleanliness of the processed wafers is crucial and even more important for the efficiency of the final solar cells.
