From the production of silicon to the end use in solar power plants or in solar panels used for distributed power generation, the solar PV value chain consists of a number of steps, and brief inputs about the business entities in the various stages of the value chain are provided.
I. Silicon suppliers
Silicon is the main component of a solar cell and is melted from sand – the element with the second largest quantity in the earth’s crust. Mono-crystalline technology uses thin wafers sliced from a single, pure crystal silicon ingot. Poly-crystalline silicon has a structure in which many crystals grow next to each other during solidification. A polycrystalline cell is cut from multifaceted silicon crystal.
Globally there are only a handful of silicon suppliers that supply to the total fast growing PV market, where demand significantly outstrips supply. Significant silicon production capacity increases have been planned by current suppliers, and new parties are planning to enter the market.
II. Wafer manufacturers
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The high-purity silicon, melted into blocks, is prepared in stacks, from which, in turn, very thin discs (wafers) are cut with the help of modern wire-cutting technology. After cleaning and intensive final checks, the mono-crystalline and poly-crystalline wafers form the basis for the production of solar cells. Worldwide, about 50 companies supply wafers suitable for solar cell manufacturing.
III. Solar cell producers
Processes in Solar PV Value Chain
The value chain for a solar PV industry starts with the production of raw materials. The raw material of most solar cells is crystalline silicon. Before silicon can be cut into thin wafers, however, it has to be purified.
There are 3 main steps to produce high-purity polycrystalline silicon.
1. Coke reduction: Metallurgical-grade silicon with 98.5% purity is produced from quartz sand in an arc furnace at very high temperatures.
2. Distillation: In the second step, the metallurgical grade silicon powder is dissolved in hydrogen chloride and subsequently distilled to form a silane gas. In most instances, this is the trichlorosilane, but could be others.
3. Siemens Process: In the so-called Siemens Process the polycrystalline silicon is developed at very high temperatures.
It requires hydrogen and produces more hydrogen-chloride as a by-product.
Manufacturing Wafers
In wafer manufacturing, three different wafer types can be produced.
• Monocrystalline wafer: Silicon with a single, continuous crystal structure is grown from a small seed crystal that is slowly pulled out of a polysilicon melt into a cylindrical shaped ingot (Czochralski process). The ingot is cut into wafers using a diamond saw. Silicon waste from the sawing process can be re-cycled into polysilicon.
• Polycrystalline wafer: Polycrystalline silicon consists of small grains of monocrystalline silicon. Cube-shaped ingots can be made directly by casting molten polysilicon, which are then cut into wafers similar to monocrystalline wafers.
• Silicon ribbons: This is a continuous process whereby thin ribbons or sheets of multicrystalline silicon are drawn from a polysilicon melt. The subsequent cutting into wafers does not produce waste, as the drawn sheets are already wafer-thin. Silicon ribbons require around 5g of silicon per Watt rather than 8g/W using crystalline wafers.
• Wafers are doped, cleaned and casted to produce cells. Then the cells are connected together and wired to form modules.
III. Manufacturing Cells
Three different types of PV cells and modules are produced, namely, PV crystalline, thin-film PV and concentrating PV. Similar steps are followed in manufacturing different types of PV cells and modules
Crystalline Solar Cells
Crystalline cells are made from silicon wafers by cleaning and doping the wafer. In a separate manufacturing process, a number of cells are wired up to form a module. As such the manufacturing process of crystalline modules consists of four distinct processes: polysilicon production, ingot & wafer manufacturing, cell manufacturing and module manufacturing.
Thin-Film Solar Cells
In thin-film, the semiconductor layer is about 100 times “thinner” than in crystalline cells. The manufacturing process starts by depositing the thin photoactive film on the substrate, which could be either glass or a transparent film. Afterwards, the film is structured into cells similar to the crystalline module. Unlike crystalline modules, the manufacturing process of thin-film modules is a single process that cannot be split up.
IV. Solar module producers
A solar module is a term for a unit that can be electrically connected, consisting of many solar cells, with weather protection (glass), embedding and framing. The direct current produced by the module is converted to alternating current with the help of an inverter for feeding into a public power network.
The module production branch is largely based on assembly allowing for a larger number of players, especially from low-cost labor regions and countries.
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