Manufacturing

Manufacturing process of Crystalline-Silicon Cells

Information adapted from www.energy.gov/eere/­solar/­solar-photovoltaic-manufacturing-basics

Step 1: Polysilicon preparation Polysilicon is the starting material for the production of crystalline-silicon cells. It is a highly purified form of silicon that is typically produced through a chemical purification process. The steps involved in polysilicon production include:
  1. Silicon Feedstock Preparation: The process begins with the preparation of silicon feedstock, which is typically obtained from silica-rich materials like quartz. The feedstock is then refined and purified to remove impurities.
  2. Chemical Purification: The refined silicon feedstock undergoes a chemical purification process to further remove impurities, such as metals, oxygen, and carbon. This process involves the use of various chemical reactions and refining techniques.
  3. Melting and Solidification: The purified silicon is melted and then solidified to form polysilicon chunks or rods. The solidification process can be carried out using different methods, such as the Czochralski method or the floating-zone method.
  4. Chunk/Wafer Production: The solidified polysilicon rods are then processed into smaller chunks or wafers. These chunks or wafers will serve as the starting material for the subsequent steps in the manufacturing process.
Step 2: Ingot & Wafer Production In this process, polysilicon chunks obtained from the previous step are shaped and refined. Here are the key steps in ingot and wafer production:
  1. Ingot Formation: The polysilicon chunks are loaded into a furnace and heated to a high temperature until they melt. The molten silicon is then poured into a mold or crucible to form a large cylindrical ingot.
  2. Ingot Slicing: Once the ingot has solidified, it is cut into thin slices called wafers using a wire saw or a diamond blade. The wafers are typically sliced to a standardized thickness, such as 180-200 micrometers.
  3. Wafer Surface Treatment: The sliced wafers undergo various surface treatments, including cleaning, etching, and polishing, to remove any impurities and improve their surface quality. These treatments ensure that the wafers are ready for the subsequent fabrication steps.
Step 3: Cell Fabrication Cell fabrication involves the transformation of silicon wafers into functional solar cells. The process typically includes the following steps:
  1. Texturing: The surface of the silicon wafers is texturized to enhance light trapping within the cells. This can be done through a variety of methods, such as chemical etching or plasma etching, to create a textured or pyramid-like surface structure.
  2. Diffusion: The wafers are subjected to a diffusion process, where dopant materials (e.g., phosphorus or boron) are introduced to create regions with different electrical properties. This helps create the necessary p-n junctions within the solar cells.
  3. Anti-Reflective Coating: An anti-reflective coating is applied to the surface of the cells to minimize the reflection of sunlight. This coating improves the cell's light absorption capability.
  4. Metallization: The cells undergo a metallization process, where metal contacts (usually silver or aluminum) are applied to the front and back surfaces. These contacts facilitate the flow of current generated by the absorbed sunlight.
  5. Firing: The metallization contacts are subjected to a firing process, typically using a high-temperature furnace. This ensures proper adhesion of the contacts to the silicon surface.
  6. Electrical Testing: The fabricated solar cells undergo electrical testing to assess their performance characteristics, including current-voltage (I-V) curves and efficiency measurements.
Step 4: Module Assembly Module assembly involves the integration of multiple solar cells into a complete solar module or panel. The steps in module assembly include:
  1. Module Framing: The solar cells are assembled and aligned within a sturdy frame or support structure. This provides mechanical stability and protects the cells from external factors.
  2. Module Interconnection: The solar cells are interconnected using electrical wiring or conductive ribbons to create a series or parallel configuration, depending on the desired electrical characteristics of the module.
  3. Encapsulation: The interconnected cells are encapsulated using a transparent, protective material (typically ethylene-vinyl acetate or EVA) to ensure weather resistance and long-term durability.
  4. Backsheet Application: A backsheet, usually made of a polymer material, is applied to the rear side of the module to provide additional protection against moisture and mechanical stress.
  5. Electrical Testing: The assembled solar module undergoes rigorous electrical testing to verify its performance, electrical output, and compliance with industry standards and regulations.
  6. Packaging: The tested and certified solar modules are packaged and prepared for shipment. This may involve stacking, palletizing, and securing the modules to ensure safe transportation to their intended destinations.
Cycle diagram showing steps required in the manufacture of crystalline-silicon cells