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How Crystalline Silicon Becomes a PV Cell

Solar photovoltaic (PV) cells are semiconductor devices that convert sunlight directly into electricity. The photovoltaic effect was first observed in 1839 by French physicist Edmond Becquerel. The first practical photovoltaic cell wasn’t developed until 1954 by scientists at Bell Labs. Today, solar PV provides a clean and renewable source of energy that helps combat climate change. The global solar PV market has experienced massive growth over the last decade, with total installed capacity reaching over 600 gigawatts in 2019.

Solar cells have become a mainstream technology today, providing emissions-free electricity around the world. They are used in a wide range of applications, from small consumer devices to large utility-scale solar farms. The modular nature of solar PV makes it highly versatile and scalable. Solar energy has many benefits, including reduced dependence on fossil fuels, lower electricity bills, and democratized power generation that supports energy independence. With continued advancements, solar PV will play a major role in the global transition to sustainable energy.

Raw Materials

Solar PV cells are primarily manufactured from silicon, one of the most abundant materials on Earth. Silicon is found in sand and quartz. To make solar cells, high purity silicon is needed. The silicon is refined through multiple steps to reach 99.9999% purity. This hyper-purified silicon is known as solar grade silicon.

The silicon acts as the semiconductor, allowing the PV cell to convert sunlight into electricity. The silicon is treated with other elements like boron and phosphorus, which act as dopants. Adding controlled amounts of these dopants alters the silicon’s electrical properties. This process is known as doping.

Beside the silicon, other raw materials are needed in PV cell manufacturing. The cells are encased in glass to provide protection. Plastic polymers like ethylene-vinyl acetate (EVA) are used as sealing and adhesive materials. Aluminum is used for the framing and structural support. Various other metals are used for electrical contacts and connections.

Ingot Creation

The silicon used in solar panels starts as quartzite rock. The quartzite is crushed into a gravel-like consistency and placed into a furnace along with carbon in the form of coal, wood chips, or sawdust. The carbon and quartzite are heated to temperatures exceeding 2,000°C, which separates the oxygen from the silicon and creates metallurgical grade silicon. This process results in a material that is 98% pure silicon.

To further purify the silicon, it goes through a process called the Siemens process. The metallurgical grade silicon is ground into a powder and reacted with gaseous hydrogen chloride. This reaction produces trichlorosilane gas, which is distilled and condensed into a liquid. When heated, the trichlorosilane decomposes into silicon, forming polysilicon rods that are 99.999999% pure.

The polysilicon rods then go through the Czochralski process to create monocrystalline silicon ingots. In this process, a polysilicon rod is suspended in a quartz crucible along with a seed crystal. The crucible is heated until the polysilicon melts. As the crucible is slowly rotated, the seed crystal is slowly pulled upwards. This pulls a single silicon crystal ingot from the molten polysilicon. The diameter of the ingot depends on the rate at which it is pulled from the melt.

After the monocrystalline ingot is formed, it is sliced into thin wafers using a wire saw. The wire saw uses a steel wire with slurry to cut the ingot into wafers between 180-240 microns thick. These wafers are then polished to create a smooth surface. The monocrystalline silicon wafers serve as the substrate for solar cells.

Cell Fabrication

The cell fabrication process turns the silicon wafers into interconnected solar cells ready for module assembly. There are several key steps:

Texturing

Texturing creates tiny pyramids on the surface of the silicon wafer. This increases the amount of light absorbed as light is reflected multiple times between the pyramids, increasing the chance of absorption. Texturing allows the cell to trap more light energy.

Diffusion

Diffusion introduces dopant atoms into the silicon to form the p-n junction that generates electricity. Phosphorus provides extra electrons (n-type) while boron creates electron deficiencies (p-type). The junction between p-type and n-type silicon allows electrons to flow when exposed to light.

Anti-Reflective Coating

An anti-reflective coating is added to reduce light reflection off the surface. This allows more light to enter the cell. Silicon nitride is commonly used as it has excellent anti-reflective properties.

Screen Printing

Screen printing deposits the metal contacts onto the cell. Silver paste collects electrons and busbars transport them. The front gets thin wires while the back is entirely covered in metal. Firing burns the contacts into the silicon.

The cell fabrication transforms the raw silicon into a working solar cell ready to be connected and encapsulated into a complete module. The specialized manufacturing steps enable high efficiency electricity generation.

Stringing

Once the solar cells have been fabricated, they need to be connected into strings to build a module with the desired voltage and current. This process is known as stringing.

The individual solar cells are quite small, typically 6 inches across. To produce useful amounts of electricity, they need to be connected together in series into long strings. The number of cells in a string determines the voltage, while the number of parallel strings determines the current.

The cells are carefully laid out and interconnected using thin tabs that have been pre-soldered onto the front and back of each cell during fabrication. The cells are soldered together in a straight line to form a string with the required number of cells.

This soldering process requires precision and care to ensure proper electrical connections between each cell. The solder joints must be strong and durable to withstand decades exposed to weather. Imperfect connections can impede performance and reliability.

Once the cell strings are assembled, they undergo testing to verify the string’s performance matches specifications. Parameters like open-circuit voltage, short-circuit current, and peak power are measured and compared to expected values. This ensures each string will function properly when connected in parallel in the final module assembly.

The stringing process is critical for constructing a properly working solar panel from individual cells. Careful interconnection and testing helps maximize module efficiency and lifetime in the field. Proper stringing sets the stage for a high-quality finished solar panel.

Laminating

Laminating solar cells into a solar panel is a multi-step process that protects the cells and wires from the environment.

Laying Strings into Panel

The first step is to lay the connected strings of solar cells into the panel frame. The strings are arranged according to the panel’s voltage and wattage design. Care must be taken to ensure the cell strings do not overlap or touch, as this can cause shorts and power losses. The strings are laid out in a grid pattern that matches the dimensions of the panel frame.

EVA Encapsulation

Once the strings are in place, the next step is EVA encapsulation. EVA stands for ethylene-vinyl acetate, which is a clear elastic polymer used to seal and protect the solar cells. The EVA is sandwiched between the cells and a top sheet of glass or plastic. EVA has high transparency, electrical resistivity, and weather resistance, making it ideal for solar panel encapsulation. It helps adhere the cells to the top sheet and provides shock absorption.

The EVA is laid over the cells, heated to a liquid state, and then compressed and vacuum sealed to remove air bubbles. This protects the cells from moisture, UV rays, and electrical shorts. The EVA fills in gaps between the cells and wires, providing a water-tight seal.

Vacuum Sealing

The last step is to use a vacuum laminator to seal the EVA encapsulant and remove any remaining air pockets. The laminator uses pressure and heat to melt the EVA into a solid, optically transparent layer. Removing air prevents clouding and moisture penetration over time. Vacuum sealing compresses the EVA tightly around the cells, wires, and edges. This bonds everything into a solid laminate and provides long-term weather protection. The result is a securely sealed panel that can withstand harsh environmental conditions.

Framing

Once the solar cells have been laminated into panels, they need to be framed and prepared for installation. The most common framing material for solar panels is aluminum. Aluminum offers high strength and durability while remaining lightweight. It also conducts heat well, helping to keep the solar cells cool.

The solar cells are encapsulated between layers of ethylene vinyl acetate (EVA) plastic. An aluminum frame is then fastened around the edges to hold everything in place. The frame has small lips that overlap the edges of the laminate material to prevent moisture ingress. Silicone sealant is also applied for additional waterproofing.

Junction boxes are mounted onto the back of the frame. These house the electrical connections for linking panels together and feeding power into the inverter. The cables from the solar cells are fed into the junction box and terminated at terminal blocks or MC4 connectors.

For rooftop solar, tempered glass is commonly used to cover the front of the panel. This provides protection against weather and impacts while remaining transparent to sunlight. The glass is sealed to the frame to prevent water ingress. Anti-reflective coatings are often applied to maximize light transmission.

Rigorous testing is performed on finished panels including insulation testing, wet leakage current tests, mechanical load testing and more. Safety certifications are also required, such as IEC 61730 for photovoltaic module safety qualification. Once the appropriate tests and certifications are passed, the solar panels are ready for shipment and installation.

Testing

Solar PV cells undergo rigorous testing and quality checks before being certified for installation. Some key tests include:

Flash Testing

Flash testing involves exposing the PV module to pulses of light at set intervals and measuring the current response. This tests the module’s performance parameters like open-circuit voltage, short-circuit current, and maximum power point. Any underperforming cells or issues like shunts and bad electrical connections can be identified.

Electroluminescence (EL) Imaging

EL imaging uses a bias voltage to induce luminescence in the PV cell. An EL camera captures an image that shows any microcracks, breaks, or dead areas in the cell. It is useful for quickly inspecting large numbers of cells.

Wet Leakage Current Test

In this test, the PV module is immersed in a conductive liquid and a voltage bias is applied. By measuring leakage current, any sites of potential corrosion or flaws in encapsulation can be identified. A good quality module will show very low leakage.

Certification

Safety, performance, and reliability certifications are critical for solar PV modules and systems before they can be sold and installed. There are several major certification standards that manufacturers must meet:

Safety Certification

  • IEC 61730 – provides requirements for construction and testing to ensure electrical, mechanical, and fire safety. Certified by bodies like TUV Rheinland.
  • UL 1703 – similar safety standard as IEC 61730, but specific to the North American market. Certified by Underwriters Laboratories.

Performance Certification

  • IEC 61215 – tests performance and reliability over time, including UV exposure, temperature cycling, and damp heat tests. Certified by bodies like TUV Rheinland.
  • IEC 61646 – similar to IEC 61215 but specifically for thin-film PV modules.

Other Certifications

  • IEC 62446 – provides minimum requirements for PV system documentation, commissioning tests, and inspection.
  • ISO 9001 – quality management system certification for manufacturing facilities and processes.

Earning these internationally-recognized certifications demonstrates that PV modules and systems meet stringent safety and reliability standards, assuring customers of their quality. The certification process can take months and often requires manufacturers to modify designs and production methods to achieve compliance. Keeping up with evolving certification requirements is an ongoing necessity in the solar industry.

Conclusion

Solar photovoltaic cell manufacturing has come a long way in recent decades. The raw silicon materials are converted into ingots, sliced into wafers, fabricated into cells, assembled into panels, and tested for safety and performance. While the basic manufacturing process is well established, there are always opportunities for improvement.

Manufacturers continue to find ways to increase cell efficiency and lower costs through improved production techniques and new materials. For example, thin-film and multi-junction cells can achieve higher efficiencies than traditional crystalline silicon cells. Automation and quality control procedures also help minimize waste and defects.

The future is bright for solar PV manufacturing as costs continue to decrease and more homes and businesses adopt this renewable energy source. With expanded production and further innovations, solar PV promises to play an increasingly important role in clean electricity generation worldwide. The manufacturing process will evolve to support mainstream adoption of this clean and sustainable technology.