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Silicon Crystal Growing or Casting

Crystal growing and casting are relatively energy-intensive metallurgical processes which transform molten silicon at around 1600°C. A large number of units of process equipment operate in parallel. This modular nature makes for relatively easy expansion of plant throughput.

The starting material is chemically pure polycrystalline silicon with a quality close to semiconductor-grade. The solar industry has historically utilized off-specification material that has been rejected by the semiconductor industry. However, as growth of the PV industry overtakes the semiconductor industry, this scrap material is in short supply. An increasing proportion of more expensive prime-grade silicon (Si) is now being used as melt-stock.

A small number of companies, either integrated PV companies or independent wafer production operations, use one of two main methods of manufacture. The traditional method for altering monocrystalline wafers is the Czochralski process, in which a single crystal, about 150 mm in diameter, is pulled from molten Si held in a large heated quartz crucible. In the more recently developed method, Si is cast in a re-useable graphite mold to produce blocks of multicrystalline silicon (cubes of over 0.5 m dimensions). When sawed into bars, and then wafers (just bigger than a compact disc) using a wire saw, the cleaned product is ready for cell manufacturing.

Crystal growing and casting plants are best sited where there is an abundant source of reliable, cheap energy to power the high temperature operations. Many crystal growing and casting plants are located near solar cell plants that were built by PV manufacturers to secure a wafer supply to their cell plants.

Thin film plants do not utilize crystalline Si wafers, so this whole piece of the manufacturing chain is avoided. Instead, as a starting point for manufacture, they generally use large-area glass sheets coated with a transparent conducting oxide layer. This is manufactured either on-line in a float glass factory, or off-line in a large scale chemical vapor deposition plant. Some thin film plants operate using roll-to-roll stainless steel sheets, rather than glass, as the substrate for the cell.

Solar Cell Manufacturing

Solar cell plants take the wafer through a high technology semiconductor processing sequence to create working solar cells. In c-Si, wafers typically undergo a process sequence of etching, diffusion, and screen-printing steps before they are tested and graded for incorporation into modules. For thin films, glass or stainless steel substrates are processed through steps of transparent conducting oxide deposition, semiconductor layer growth, laser scribing, and metallization. The sequence is dependent on the substrate being used. Today, thin film plants are designed to handle large substrates in sheet or roll form. Therefore, the process equipment is much larger than for the wafer-based c-Si plants.

Module Assembly

The assembly of crystalline Si solar modules is most commonly carried out in the cell plant, but can be done in smaller plants closer to the end market. Smaller plants can be preferable because, while solar cells are relatively inexpensive to transport, modules with a glass front sheet and an aluminum frame are heavy and bulky. In general, thin film modules must be assembled in the cell plant because the cells are too susceptible to mechanical damage during transportation, unless they are packaged within a module.

Solar module assembly usually involves soldering cells together to produce a string of cells, and then laminating it between toughened glass on top and a polymeric backing sheet on bottom. Frames are usually created to allow for mounting in the field. The laminates may also be separately integrated into a mounting system for a specific application, such as building integration.

Systems Assembly

The final part of the overall manufacturing process is the solar system assembly and installation. First, an array structure is chosen for the mechanical integration of the solar module. This array structure will depend on the final location of the system, which could involve retrofitting onto a roof, integrating into building materials for roofs or vertical walls, or pole-mounting, ground-mounting, or attaching to an industrial structure.

Second, the electrical components are integrated with other parts of the solar energy system. This will include the connection of elements such as inverters, batteries, wiring, disconnects, and regulators (charge controllers). This process also requires matching the equipment to the electrical load as required by the customer. A sales company will use computer software, known as a sizing program, to make this calculation.



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