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In the world of photovoltaic solar energy we find two large families of cell technologies: crystalline and thin film. Crystalline technology reigns supreme over thin films, which have a very small share of the market. Thin films are produced with different types of materials: amorphous silicon, microcrystalline silicon, cadmium 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. In 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 first is manufactured from the growth of crystalline ingots, the second is manufactured from a silicon casting and molding process. The result of the different manufacturing processes is a slight reduction in efficiency in poly 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 passivation – 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. You may also be interested in these other articles previously published on Solar Channel, which address in more detail some of the subjects mentioned in the previous paragraphs:
- What are N-type solar cells
- CIGS thin films, an alternative to crystalline silicon
- Monocrystalline modules will dominate the solar energy market
- Photovoltaic cell technologies with passivation
- PERC technology: the new generation of photovoltaic cells
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).
In the midst of the rise of monocrystalline silicon and the fall of polycrystalline silicon, a third way emerged: the so-called cast-mono silicon, which brought survival to polycrystalline silicon factories.
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 polycrystalline silicon. The result of this process are photovoltaic cells that are not perfectly monocrystalline, but also not completely polycrystalline.
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 silicon (technically known as multicrystalline) is produced by a foundry process. Silicon is melted in a container 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. The multicrystalline silicon produced in this way is an agglomeration of crystalline grains. In wafers produced with this process, the grain orientation is random. The random orientation of the grains makes it difficult to texture the wafer surface, which is the manufacturing basis of the photovoltaic cell. Texturing is used to improve photovoltaic cell efficiency by reducing light reflection and improving the absorption of light energy across the cell surface. Furthermore, defects existing 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 electrical current), which leads to a decrease in the cell's efficiency. For this reason, polycrystalline silicon photovoltaic cells are less efficient than monocrystalline ones. However, due to relative simplicity and lower production cost, poly- or multi-crystalline silicon was originally the most widely used form of silicon for manufacturing 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 deposited in 2008 by the US Department of Energy. The invention of the technique was motivated by the need to find a method for faster and cheaper manufacturing 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 a description found in the patent text, the term “almost monocrystalline” refers to a crystalline silicon body having a crystal orientation consistent with greater than 50% in body volume, 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, the 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 or cast-mono silicon is a type of silicon that has monocrystalline parts and polycrystalline parts. In practical terms, in the wafers (and cells) produced using this technique, it is possible to find parts of polycrystalline silicon embedded in monocrystalline regions. The idea of the cast-mono process is to produce monocrystalline silicon in the same furnace that was previously used to produce standard polycrystalline silicon. The secret of 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.
Commercially available cast-mono
One of the global manufacturers that 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 that reached 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 almost single-crystalline cells. When speaking with Canadian Solar, the vice president of the photovoltaic modules division, Thomas Koerner, added: “This P5 technology is based on our own process, used to achieve a semi-monocrystalline structure from cooled polysilicon in a cubicle. The general behavior is close to that of monocrystalline, but with a cost level close to that of poly – combining the best of both worlds. Except for efficiency, the module behavior is almost identical to our modules based on P4 [conventional poly-PERC silicon] technology, so the only noticeable difference is the higher powers than regular poly-PERC modules. All modules have been subjected to the same long-term degradation and reliability tests and have the same certification standards (IEC and UL tests). We saw the P5 as a great balance between cost and efficiency in a time when mono-PERC modules are still significantly more expensive than their poly-based brethren.”
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 other words, we can say that they are almost monocrystalline polycrystalline cells. Whatever the point of view, the result of the cast-mono manufacturing process are mostly monocrystalline cells, with some traces of polycrystalline grains. Cast-mono cells are produced through a simpler and cheaper process than that used in conventional monocrystalline cells, but they are not inferior. 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 suffering less from 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 yet another reason why monocast 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 have some spots, such as those 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
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