Polysilicon
Silicon purified to nine or eleven nines — the traded commodity the entire solar industry rests on, and the most energy-intensive step in making a panel.
Polysilicon is silicon made pure enough to be a semiconductor, and it is the form the material is actually traded in. Metallurgical-grade silicon out of an arc furnace is about 98 or 99 per cent pure, which is fine for aluminium alloying and silicones and useless for anything electronic. Polysilicon is 99.9999 per cent or better for solar and 99.999999999 per cent for semiconductors — six nines and eleven nines respectively.
It is polycrystalline, which is what the name says: a mass of randomly oriented crystals rather than a single one. That is the input to the Czochralski puller or the casting furnace, which turns it into the single crystal or the directionally solidified ingot the wafers are cut from.
The reason it is worth holding separately from silicon the element is that the purification is the industry. Polysilicon is where most of the energy in a solar panel goes, its price has swung by a factor of ten inside a decade and taken the whole solar supply chain with it, and the argument about where panels come from is an argument about where polysilicon is made.
Processing
Almost all of it by the Siemens process, which is a vapour deposition route and is elegant and expensive in equal measure.
Metallurgical silicon is reacted with hydrogen chloride to make trichlorosilane, a liquid that boils at 32 °C. Distilling a liquid is a far better purification than anything that can be done to a solid, and repeated fractional distillation is what actually removes the impurities — this is the step that does the work.
The purified trichlorosilane is then decomposed onto thin heated silicon rods inside a bell jar at around 1,100 °C, and silicon deposits on them until they have grown into thick polysilicon rods, which are broken into chunks.
Heating those rods is where the energy goes. The Siemens process is one of the most electricity-intensive chemical processes in commercial use, and it is the largest single contributor to a solar panel's embodied energy. The fluidised bed reactor route uses a fraction of it by depositing onto circulating seed particles instead, gives granular rather than chunk polysilicon, and has taken a modest share against a long-established process that everyone knows how to run.
Economic significance
Polysilicon is the pinch point of the solar supply chain, and it has behaved like one twice.
Between 2004 and 2008 the price rose roughly tenfold as solar demand outran a capacity base built for semiconductors, capacity was built in response, and the price collapsed by around ninety per cent — bankrupting a good deal of the industry that had built it. It rose sharply again in 2021 on a fire, an outage and a demand surge, and fell again as capacity caught up.
Production is now overwhelmingly Chinese, and within China heavily concentrated in Xinjiang, where cheap coal-fired electricity makes the Siemens process economic. That concentration is the basis of the forced-labour restrictions several countries have placed on solar imports, and it means a panel's carbon footprint depends substantially on which grid smelted its silicon — solar manufactured on coal power has an energy payback of a year or two rather than months.
The honest summary is that solar's material bottleneck was never silicon, which is the second most abundant element in the crust. It is the purification, the electricity to run it, and where both happen to be located.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is sourced from
- Silicon — element · metallurgical-grade silicon at 98 or 99 per cent, which is fine for alloying and useless for anything electronic — the purification from there is the entire product
is produced by
- Vapour deposition — process · the Siemens process: trichlorosilane distilled to purity, then decomposed onto heated silicon rods at 1,100 °C until they grow thick enough to break up
is an input to
- Czochralski process — process · melted and pulled into the single crystal the wafers are sliced from — polysilicon is the feedstock, and the puller changes its arrangement rather than its purity
contains
- Silicon — element · to nine or eleven nines, which is a purity nothing else is produced at in bulk
is used as
- Photovoltaics — application · the material the whole industry rests on, and the largest single contributor to a solar panel's embodied energy
is used in
- Semiconductor manufacturing — industry · at eleven nines for electronics against six to nine for solar, which are different products from the same process run harder
- Energy generation — industry · and production is heavily concentrated in Xinjiang because the Siemens process needs cheap electricity, which is the basis of both the forced-labour import restrictions and the panel's carbon footprint
- Solar panel — object · by way of the wafer — around 150 micrometres of it, thinner than a business card, and the expensive ingredient the whole design works to use less of
is an alternative to
- Cadmium telluride — compound · the thin-film alternative in photovoltaics: far less material and far less energy per panel, at a lower efficiency and with cadmium and tellurium to account for — it holds a steady minority share rather than losing
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)