Solar power is the fastest growing renewable energy segment, with a rapidly expanding range of products and innovations. One of the technologies in the spotlight for photovoltaic (PV) power generation is monocrystalline PV cells. Monocrystalline cell-based solar panels are emerging as a strong choice of solar PV technology as compared to polycrystalline cell-based panels. While the latter, being cheaper, are still widely used, the trend is slowly moving towards monocrystalline cells. This change can primarily be attributed to the increased efficiency of monocrystalline cells over polycrystalline ones.
Monocrystalline solar panels are usually made with high-grade silicon, and have higher efficiencies than polycrystalline panels and are sleeker in appearance. The key difference between the two technologies lies in the types of silicon solar cell they use. While monocrystalline cells are made from single crystals of silicon, polycrystalline cells are made from several silicon fragments melted together. Since a monocrystalline cell is composed of a single crystal, the electrons generating the flow of electricity have more room to move, resulting in higher efficiency – usually in the range of 18-24 per cent.
These higher efficiency solar panels are particularly useful when space is limited. Thus, monocrystalline solar panels are a good choice for solar rooftop systems with small project sizes. In such smaller spaces, the higher cost of setting up monocrystalline panels can be offset by the efficiency gains that will be realised. The panels can help maximise electricity production by harnessing more solar energy. Some of the major manufacturers of monocrystalline modules are Canadian Solar, Sunpower, LG, Hyundai, Solar World, Longi, JinkoSolar and JA Solar.
Increasing wafer size
As the segment evolves, the innovations within the segment are also progressing with new and improved versions of monocrystalline cells. Since 2018, larger PV modules and wafers have been entering the market. Although this was initially considered to be an advantage on the customer side due to the resultant increase in power, it was soon realised that such an increase could also help save on the balance-of-system costs of a power plant. Further, the increase in size could reduce the cost of cell and module manufacturing. In June 2020, seven companies, including LONGi, JinkoSolar, JA Solar and Canadian Solar, jointly released the M10 monocrystalline silicon wafer, which is 182 mm in size.
Modules account for a major part of the cost of a solar power system, and are thus an integral target for reducing costs. Some manufacturers, such as Trina Solar, have been focusing on increasing the efficiency of solar panels. In July, Trina Solar unveiled the Vertex series PV module, which can generate as much as 600 W. Due to a low-voltage and high-module string power output, the Vertex series helps to reduce balance-of-system costs further. The power output is estimated to be about 35 per cent higher than the previous models. The Vertex series can achieve efficiencies of up to 21.3 per cent. They also follow the trend of larger wafers by having monocrystalline cells of 210 mm format.
In the last few years, solar cell and module efficiency degradation phenomena have come to the industry’s attention. These include light-induced degradation (LID) and light- and elevated temperature-induced degradation (LeTID). Through research and testing, LONGi’s researchers concluded in a white paper that LID and LeTID problems could be effectively solved without the need for regeneration treatment by using gallium-doped monocrystalline silicon wafers in combination with cell process control. In June 2020, LONGi launched the Hi-MO 5, a module designed for application in utility-scale power plants, with an efficiency of over 21 per cent. The module has a size of 2,256 x 1,133 mm and uses the gallium-doped M10 standard silicon wafers (182 mm) to produce a p-type mono-PERC module. This reportedly shows lower LID with stable, long-term power generation when compared to boron-doped wafers.
Race for highest efficiency
In another instance of efficiency improvement, JinkoSolar achieved 24.79 per cent conversion efficiency for an n-type TOPCon monocrystalline silicon cell in July 2020. According to the company, the result has been certified by Germany’s Institute for Solar Energy Research in Hamelin, and has set a world record for large-size, contact-passivated cells, breaking Jinko’s own January record of an efficiency rate of 24.2 per cent. According to JinkoSolar, it used several technologies to get the result, including passivating contacts, an advanced diffusion system, surface passivation, and an advanced anti-reflection technique. Material upgrades were also integrated into the cell process.
In September 2020, Sharp launched its 440 W monocrystalline silicon PV panel. The NU-JD440 module has an efficiency rate of 19.9 per cent and uses 144 M6 wafer-size half-cells. The increased efficiency reduces balance-of-system costs and lowers the levellised cost of energy (LCoE). This, in turn, results in a higher return on investment. The temperature coefficient for these modules is also lower, at -0.347 per cent per degree Celsius for the power output, ensuring higher performance at high environmental temperatures. This is an important advantage, given the rising temperatures in some parts of the globe due to climate change. The module employs a nine-busbar technology using round ribbons, which increases the power gain from each cell, making it less sensitive to microcracks and offering higher module reliability. Earlier, in January 2020, Sharp had launched a PERC monocrystalline PV module series based on half-cut cell technology. The three five-busbar modules in the series were NU-JC330, NU-BA385 and NU-JB395. While the smallest module, featuring a 330 W, 120 half-cut cell panel, claimed an efficiency of 19.5 per cent, the other two, featuring 385 W and 395 W 144-cell panels respectively, claimed an efficiency of 19.6 per cent.
The higher efficiencies and falling costs of solar panels have been made possible by certain key technological developments. These include PERC, passivated emitter rear totally diffused (PERT), and bifacial modules, which in combination with monocrystalline technology push the efficiency of the modules further. Advanced PERC technology is an innovative step beyond traditional mono facial solar modules, involving the coming together of back surface passivation, front surface advanced passivation and anti-light-induced degradation technologies. A PERC solar cell is similar to a typical monocrystalline PV solar cell. Both technologies use silicon wafers to generate a flow of electrons by absorbing solar radiation, and their overall construction is very similar. The main difference between PERC cells and typical monocrystalline PV cells is the integration of a back-surface passivation layer in the former – a layer of material on the back of each cell that provides key benefits and boosts cell efficiency. Bifacial solar modules are also gaining popularity since they can produce power by absorbing sunlight on both sides of the solar panel. These panels often use monocrystalline cells to have greater overall efficiency and power output.
Some unique solar module structures are also being researched. For instance, a group of scientists from Saudi Arabia’s King Abdullah University of Science and Technology, the University of Jeddah, and the University of California, Berkeley, have come up with an interdigitated back contacts spherical solar cell with stronger heat dissipation and reduced dust accumulation compared to conventional flat solar cells. The monocrystalline, three-dimensional device is reported to have an efficiency of 18.93 per cent and a power yield that is 101 per cent higher than flat cells. Through a corrugation technique, the research group was able to develop a cell with 138 micrometre-wide grooves. This made it possible to achieve a flexible structure, with a total area loss of only 5.6 per cent. The measurements have also shown that spherical cells accumulate less dust than flat cells.
The way forward
New innovations are paving the way for the solar segment. Different technologies are being implemented in tandem, leading to increasing efficiencies and lower LCoEs for solar power. Monocrystalline PV cells form a key part of these developments, as they are currently some of the best performing solar cells. Monocrystalline cells have higher efficiencies, better performance in low-sunlight areas, and long lives of over 25 years, compared to polycrystalline cells. As more manufacturers opt to produce monocrystalline cells, their costs may be expected to fall owing to economies of scale. Further, as the rate of efficiency keeps rising with new developments in research, the net benefit from installing monocrystalline modules will increase. This will make these cells more accessible for larger projects as well in the long term.