Polycrystalline silicon, also known as poly-Si or polysilicon, is produced by exposing mono- or multicrystalline silicon to high temperatures which causes it to melt and then recrystallize into multiple small silicon crystals or "grains". While it may not be a household name, polysilicon plays a crucial role in the production of solar photovoltaic cells and is responsible for enabling the growth of the global solar energy industry.

What is Polysilicon?
Polysilicon is a type of silicon used in making solar panels and integrated circuits. It consists of multiple small silicon crystals or grains joined together. While a single crystal silicon wafer has a consistent crystalline structure throughout, polysilicon contains randomly oriented grain boundaries where the crystals meet. This gives polysilicon somewhat inferior electrical properties compared to single-crystal silicon, however it can be produced more cheaply at scale. Due to its lower cost, polysilicon has become the dominant silicon material used in photovoltaics and helped drive down the price of solar panels over time, improving their affordability and adoption worldwide.

Production of Polysilicon
There are two main industrial processes for producing polysilicon - the Siemens process and fluidized bed reactors. In the Siemens process, which currently dominates commercial production, trichlorosilane gas is decomposed and deposited on heated filaments in vacuum chambers, slowly building up polysilicon blocks. This high temperature chemical deposition process requires large amounts of electricity. Fluidized bed reactors offer a lower energy alternative by depositing silicon particles in a fluidized bed, but have not been widely commercialized yet. Producing high purity polysilicon suitable for semiconductors and solar cells is an involved process that requires removing contaminants. Overall polysilicon manufacturing is energy intensive and a large source of carbon emissions in the electronics supply chain.

Role in Solar Energy Industry
Solar photovoltaic (PV) cells convert sunlight directly into electricity using semiconducting materials, most commonly polysilicon. In a solar cell, polysilicon serves as the light absorbing material where electrons gain energy from photons and begin flowing. A PV module is made from dozens of electrically connected solar cells sandwiched between a front sheet and backsheet. The declining cost of polysilicon production over the decades has allowed for cheaper solar panels, driving down costs per watt of installed solar power capacity. This has enabled solar energy to become competitive with fossil fuel power sources in an increasing number of markets worldwide each year. Without inexpensive polysilicon as the core material, the solar energy revolution simply would not have been possible.

Outlook for Polysilicon Demand

With global solar PV installation growing at a strong double-digit pace year-over-year and more countries committing to renewable energy targets, demand for polysilicon is projected to continue rising significantly. Solar industry analysts estimate polysilicon demand will increase by over 25% between 2021 to 2023 alone to meet new solar cell and module manufacturing requirements. This continued polysilicon consumption growth is necessary to sufficiently scale up global solar capacity and help reduce greenhouse gas emissions from the energy sector. While China currently dominates polysilicon production, new plants are coming online in other Asian nations and Europe to diversify supply. With long-term solar adoption expected worldwide, polysilicon will remain a essential material underpinning the expansion of clean energy worldwide.

In summary, low-cost polysilicon has been the linchpin enabling the enormous growth of the global solar industry. Though an unsung material in the shadows, polysilicon's significant price reductions over time have lowered solar electricity costs and allowed solar energy to increasingly compete with fossil fuels in numerous markets. Looking ahead, continued consumption growth of polysilicon will be needed to achieve ambitious climate change mitigation goals through more widespread solar deployment internationally. Polysilicon's role in a sustainable energy future remains crucial.

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