In the world of solar photovoltaic energy, we find two major families of cell technologies: crystalline and thin-film. Crystalline technology reigns supreme over thin-film, which has a very small market share.
Thin films are produced with several types of materials: amorphous silicon, microcrystalline silicon, cium telluride, CIGS (copper-indium-gallium-selenium), polymeric materials and others. Within the crystalline family, silicon is the material chosen by the photovoltaic industry.
When we talk about crystalline cells, we are always referring to silicon. Within the crystalline family, we know two traditional divisions: polycrystalline cells (also known as multicrystalline) and monocrystalline cells.
The difference between monocrystalline and polycrystalline silicon lies in the manufacturing process. While the former is manufactured from the growth of crystalline ingots, the latter is manufactured from a silicon casting and molding process.
The result of the different manufacturing processes is a slight reduction in efficiency in polycrystalline silicon compared to monocrystalline silicon. However, the production of polycrystalline silicon has always been much cheaper – which has enabled the mass production of photovoltaic cells and the dissemination of the technology on a global scale.
Currently, some advances in the manufacture of monocrystalline cells (which include the use of N-type wafers and cells with ivation – PERC) have increased market preference for this technology, to the detriment of polycrystalline.
There are projections in the market that indicate that the manufacture of polycrystalline silicon could become extinct, giving way to the absolute dominance of monocrystalline silicon.
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In short, polycrystalline silicon is cheaper to manufacture, while monocrystalline silicon has always been more expensive and more efficient.
The production of monocrystalline silicon has become more economically attractive with the introduction of new technologies (N and PERC wafers, as mentioned above) and industrial processes such as sawing wafers with diamond tape (which allows the production of thinner wafers and increases the productivity of the manufacturing process, reducing the final cost of the cells).
After all, is cast-mono poly or monocrystalline?
The best answer is that this type of silicon is a hybrid between poly and mono. The term cast-mono is an allusion to cast or molded monocrystalline silicon.
In other words, it is a type of monocrystalline silicon produced using a manufacturing process similar to that of polycrystalline silicon. The result of this process is photovoltaic cells that are not perfectly monocrystalline, but are not completely polycrystalline either.
monocrystalline silicon
Monocrystalline silicon is generally produced by methods known as Czochralski (CZ) or Floating Zone (FZ), both processes that result in cylindrical-shaped ingots.
In the CZ process the ingot is slowly extracted (or grown, as they say in the industry) from the molten silicon contained in a container. In the FZ process, a raw silicon ingot is heated and crystallized gradually along its length by means of a high-frequency inductive heating ring.
Monocrystalline silicon produced by any of these techniques is more expensive than its multicrystalline silicon, but it allows the production of higher efficiency cells.
Polycrystalline or multicrystalline silicon
Conventional polycrystalline (technically known as multicrystalline) silicon is produced by a casting process. Silicon is melted in a vessel and then cooled in a controlled manner to allow the silicon to crystallize.
The resulting block of multicrystalline silicon is cut into bricks with a cross section equal to the size of the wafer to be used to manufacture the photovoltaic cells.
Multicrystalline silicon produced in this way is an agglomeration of crystalline grains. In wafers produced using this process, the orientation of the grains is random.
The random orientation of the grains makes it difficult to texture the wafer surface, which is the basis for manufacturing the photovoltaic cell. Texturing is used to improve the efficiency of the photovoltaic cell by reducing light reflection and improving the absorption of light energy through the cell surface.
Furthermore, defects at the boundaries between multiple silicon grains reduce cell performance. These defects and the impurities they tend to attract form carrier recombination centers (mobile charges that form electric current), which leads to a decrease in cell efficiency.
For this reason, polycrystalline silicon photovoltaic cells are less efficient than monocrystalline ones. However, due to their relative simplicity and lower production costs, polycrystalline or multicrystalline silicon was originally the most widely used form of silicon for the manufacture of photovoltaic cells.
Cast-mono silicon
The idea of manufacturing cast-mono silicon is not new. A patent for this technique, called “Methods for manufacturing monocrystalline or near-monocrystalline cast materials” was filed in 2008 by the U.S. Department of Energy.
The invention of the technique was motivated by the need to find a method for the faster and cheaper manufacture of monocrystalline silicon. The result of the technique is an intermediate product between monocrystalline and polycrystalline, which can be called quasi-monocrystalline silicon.
According to the description found in the patent text, the term “quasi-monocrystalline” refers to a crystalline silicon body having a consistent crystal orientation over more than 50% by volume of the body, where, for example, such nearly single-crystalline silicon may comprise a single-crystalline silicon body proximate to a multicrystalline region, or may comprise a large consistent contiguous silicon crystal that partially or wholly contains smaller silicon crystals of other crystal orientations, where the smaller crystals make up no more than 50% of the total volume. Preferably, nearly single-crystalline silicon may contain smaller crystals that constitute no more than 25% of the total volume. More preferably, the nearly single-crystalline silicon may contain smaller crystals that constitute no more than 10% of the total volume. Even more preferably, the nearly single-crystalline silicon may contain smaller crystals that constitute no more than 5% of the total volume.
In other words, quasi-monocrystalline silicon or cast-mono is a type of silicon that has monocrystalline parts and polycrystalline parts. In practical , in the wafers (and cells) produced using this technique it is possible to find polycrystalline silicon parts embedded in monocrystalline regions.
The idea behind the cast-mono process is to produce monocrystalline silicon in the same furnace that was previously used to produce standard polycrystalline silicon. The secret to the process lies in placing monocrystalline ingots at the base of the casting vessel.
During solidification, the silicon crystallizes and takes the monocrystalline form under the influence of the monocrystalline ingots, which act as seeds for crystal formation.
In the end, the ingot produced will not be perfectly monocrystalline – on the other hand, it will not be a polycrystalline ingot either. Figure 4 illustrates the difference between the manufacturing processes for polycrystalline silicon and cast-mono silicon.
Like many other ideas and technologies in the photovoltaic market (such as PERC technology, developed more than 20 years ago and only recently brought to the market), the cast-mono technique was already known but did not become important until the industry became interested in it, in a recent effort to keep polycrystalline silicon manufacturing plants operational, allowing the production of more efficient cells with little investment in improving and adapting manufacturing plants.
Cast-mono commercially available
One of the global manufacturers that has adopted cast-mono technology is Canadian Solar. In September 2020, the company announced the production of cast-mono cells, which it called P5 technology, with efficiencies reaching 23,81% according to press announcements.
For example, the CS3W-420-435P family, from the HiKu line, is sold in Brazil under the name “poly PERC”, which means that the manufacturer classifies this product in the polycrystalline category. However, the cells used are high-efficiency, manufactured using the cast-mono process, which means that they are quasi-monocrystalline cells.
Speaking to Canadian Solar, Thomas Koerner, vice president of PV modules, added: “This P5 technology is based on our proprietary process, which is used to achieve a semi-monocrystalline structure from polysilicon cooled in a cubicle. The overall behaviour is close to monocrystalline, but at a cost level close to poly – combining the best of both worlds. Except for efficiency, the module behaviour is almost identical to our modules based on P4 technology [conventional poly-PERC silicon], so the only noticeable difference is the higher power ratings than regular poly-PERC modules. All modules have undergone the same degradation and long-term reliability tests and have the same certification standards (IEC and UL tests). We saw P5 as a great balance between cost and efficiency at a time when mono-PERC modules are still significantly more expensive than their poly-based siblings.”
Conclusion
Cast-mono cells are a hybrid version of mono and polycrystalline technologies. In a very simplified way, we can say that cast-mono are monocrystalline cells produced through the polycrystalline process. Or, in another way, we can say that they are almost monocrystalline polycrystalline cells.
Whatever the point of view, the result of the cast-mono manufacturing process is cells that are mostly monocrystalline, with some traces of polycrystalline grains. Cast-mono cells are produced using a simpler and cheaper process than that used for conventional monocrystalline cells, but they are not inferior for that reason.
On the contrary, with cast-mono technology it is possible to produce high-efficiency photovoltaic modules (polycrystalline, strictly speaking). In addition to allowing the use of existing polycrystalline production lines, without requiring much investment in machinery, cast-mono silicon also has the advantage of being less affected by the well-known boron-oxygen effect, which reduces the efficiency of silicon.
In addition to the quasi-monocrystalline nature, the reduction of the boron-oxygen effect is another reason why cast-mono cells achieve high efficiencies. In practice, the consumer who purchases a photovoltaic module with cast-mono cells does not even realize that the product carries this technology. Commercially, the product is classified as a high-efficiency polycrystalline module.
The cells of this type of module have a monocrystalline appearance and present some stains, as shown in Figure 5, which can be observed with a closer look and are perfectly normal.
References
- About the origin of low wafer performance and crystal defect generation on seed‐cast growth of industrial mono‐like silicon ingots, Progress in Photovoltaics, Wiley, 2012
- Advantage in solar cell efficiency of high-quality seed cast-mono Si ingot, Applied Physics, 2015
- Seed-Assisted Growth of Cast-Mono Silicon for Photovoltaic Application: Challenges and Strategies, Solar RRL, Wiley, 2020
- Methods and apparatuses for manufacturing monocrystalline cast silicon and monocrystalline cast silicon bodies for photovoltaics, US Patent US8048221B2
- Methods for manufacturing monocrystalline or near-monocrystalline cast materials, US Patent US8709154B2
