From sand to solar panels: Unveiling the journey of solar panel manufacturing

Jun 09, 2023

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2023. The world is striving to transition to more sustainable energy sources and reduce its dependence on fossil fuels. As a result, renewable energy is becoming increasingly popular. In fact, international accounting firm BDO Global predicts that, by 2024, almost 33 percent of the world's electricity will be produced from renewable sources.

Solar power, in particular, is one of the most promising clean energy options, and its use is growing rapidly worldwide. Some sources report that solar power now accounts for more than half of the new generating capacity in the US.

In this context, becoming more familiar with solar panels is relevant. One of the most surprising facts about them is that they are actually made of sand. But how does sand transform into solar panels?

Here's all you need to know about the engineering behind silicon photovoltaic technology.

Sand is one of the primary raw materials in solar panel production.

Unlike other raw materials, sand is pretty ordinary and widely available in most parts of the world. It is not infinite, though. According to CNBC, sand is the most consumed natural resource after water, and there could be a shortage of sand anytime soon.

This is not due to solar panel manufacturing but because the construction sector has a high demand for sand. After all, sand is used as a fine aggregate in concrete production.

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Sand is also one of the primary raw materials of the glass in our windows and the screens of our smartphones and one of the raw materials of silicon chips in phones, computers, and other electronic devices.

Sand has several properties that make it suitable for all these applications:

Sand consists of grains, and these grains can fill in gaps in cement particles, which is why it is mixed with gravel to produce concrete. Sand provides structural strength to concrete.

The high silica content in the sand allows it to melt at high temperatures and form a molten glass material. When cooled, the glass retains its transparency, allowing the transmission of light, protection, and insulation.

The crystalline structure of silicon, derived from sand, has unique semiconductor properties that allow a high control of the flow of electrical currents -which is why silicon is present in electronic devices. It is a key component of diodes, transistors, and circuits.

However, in solar panel manufacturing in particular, not just any sand will do. The sand used must have a high silicon dioxide content. This is important because silicon dioxide is the primary silicon source and is essential in wafer production, as we will explain below.

To build solar panels, silica-rich sand must be extracted from natural deposits, such as sand mines or quarries, where the sand is often composed of quartz, a form of crystalline silica.

The sand is washed to remove impurities like clay, organic matter, and other minerals. It is then refined with chemical processing methods. One common method is acid leaching, where the sand is mixed with an acid solution - such as sulfuric acid - to dissolve impurities and separate the silica.

After that, silica is heated at high temperatures, typically in a furnace, to eliminate any residual organic material and turn it into high-purity silica.

High-purity silica is key for producing polysilicon, also known as polycrystalline silicon. This high-purity form of silicon is used as the raw material for solar cells.

To obtain it, purified quartz sand is mixed with carbon-rich materials, such as coal or petroleum coke. After that, the mixture is exposed to a stream of chlorine gas at high temperatures, forming trichlorosilane (SiHCl3). This process is called chlorination.

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Trichlorosilane, the result of chlorination, is further processed with distillation and purification techniques. In the distillation process, the trichlorosilane is heated to separate into its components. The purified trichlorosilane is converted back into very high-purity silicon by reacting with hydrogen gas (H2). The result of this process is polysilicon.

The production of polysilicon requires strict quality assurance measures to ensure the high purity levels that are needed to achieve optimal performance of solar cells.

The polysilicon is melted in a crucible or furnace under controlled conditions. The molten polysilicon is carefully maintained at high temperatures to ensure uniformity and consistency.

The molten polysilicon is then solidified by a crystal growth process known as the Czochralski (CZ) method. A seed crystal, usually made of a single crystal of high-purity silicon, is dipped into the molten polysilicon and slowly pulled out while rotating. As the seed crystal is raised, it forms a cylindrical shape and draws the molten polysilicon. This liquid mass is cooled in the directional solidification process until it forms a large-grained multi-crystalline-silicon ingot.

(A less common process is sometimes used, involving using gaseous silicon compounds to deposit a thin layer of silicon atoms onto a crystalline template in the shape of a wafer.)

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The silicon ingots are then mechanically sliced into thin, circular wafers using precision sawing techniques. These wafers are typically around 200-300 micrometers thick and have a 150-200 millimeters diameter. Larger wafers - with diameters of 300 millimeters or more - are even more efficient.

The sliced wafers must undergo several surface treatment processes to eliminate any impurities, roughness, or flaws. This includes chemical etching to remove mechanical damage to the wafer surface, polishing with alumina abrasive in a lapping machine to improve the surface parallelism, and cleaning to ensure the wafer's surface is smooth, clean, and optimized for subsequent processing.

The cleaned and inspected wafers are then doped with specific materials, such as phosphorus or boron, to create different regions with different electrical properties.

After doping, the wafers go through a passivation process to improve their efficiency and reduce surface recombination. Passivation involves depositing a thin layer of insulating material, such as silicon nitride or silicon dioxide, onto the wafer surface to minimize electron and hole recombination, thus enhancing the overall performance of the solar cells.

The wafers produced from the silicon ingots serve as the building blocks for individual solar cells. These wafers undergo further fabrication, including applying contacts, anti-reflection coatings, and other essential layers, encapsulating with glass and polymer encapsulants, and lamination to transform them into fully functional solar cells.

The finished panel has a frame, edge sealant, and a junction box. Electrical cables, which carry the current from one panel to the next, are also run.

These interconnected, encapsulated, and assembled solar cells form complete solar modules or panels, which are then installed at homes and other buildings.