How Sand Becomes Solar Panels
Solar panels are mostly made of silicon. Here’s how that abundant substance is transformed into something that generates electricity.
Chi Odogwu is a digital consultant, professor, and writer with over a decade of experience in finance and management consulting. He has a strong background in the private equity sector, having worked as a consultant at PwC and a research analyst at Renaissance Capital. Additionally, he has bylines in well-known publications, including Entrepreneur, Forbes, NextAdvisor, and CNET. He has also leveraged his writing talent to create educational email courses for his clients and ghostwritten op-eds published in top-tier publications such as Forbes, CoinDesk, CoinTelegraph, Insider, Decrypt, and Blockworks. In addition to his writing, education, and business pursuits, Chi hosts the top-rated Bulletproof Entrepreneur Podcast. Through this podcast, he engages in insightful conversations with talented individuals from various fields, allowing him to share a wealth of knowledge and inspiration with his listeners.
Sand is coarse, rough, irritating, and it gets everywhere. It’s also the key ingredient in solar panels.
If you’re looking to install a solar system in your home so you can say goodbye to your electric bill.- and the inflation that keeps pushing it up.- you should be grateful for sand.
Now, you can’t just go outside, throw a bunch of sand on your roof and pray for energy to flow through it into your power outlets.- far from it. While sand is an essential raw material for producing solar cells, not every kind of sand will do. The sand used for solar cell production must be rich in silicon dioxide and meet exacting standards to ensure the resulting solar cell most efficiently converts sunlight to electricity.
Can solar panels save you money?
Interested in understanding the impact solar can have on your home? Enter some basic information below, and we’ll instantly provide a free estimate of your energy savings.
It also takes a highly sophisticated manufacturing process to create efficient solar cells, the building blocks of the solar panels you see on rooftops everywhere.
Here’s what goes into a solar panel.
Can solar panels save you money?
Interested in understanding the impact solar can have on your home? Enter some basic information below, and we’ll instantly provide a free estimate of your energy savings.
What are monocrystalline solar panels made of?
The two most common types of household solar panels.- monocrystalline and polycrystalline.- both start with sand that has a high silicon dioxide content being heated and purified to form blocks called ingots, Rohit Kalyanpur, CEO of Silicon Valley-based solar technology company Optivolt, told CNET.
For monocrystalline solar panels, individual wafers are cut from a single ingot. The ingots used for monocrystalline cells have a distinctive black appearance and uniform cell structure. Solar panels made from monocrystalline solar cells are the most efficient, with ratings ranging from 17% to 22%, and offer the best performance. As of 2021, more than half the residential solar panels installed in the US had efficiency ratings above 20%, compared with 0.6% a decade ago, according to the Lawrence Berkeley National Laboratory’s Tracking the Sun report.
The average cost of these panels is between 1 and 1.50 per watt, but may differ depending on location. Monocrystalline panels have a 25-year useful life, a moderate temperature coefficient.- how well they perform on hot days.- and an all-black appearance.
What are polycrystalline solar panels made of?
Polycrystalline solar panels are made from a combination of several silicon crystals. The manufacturing process involves melting multiple silicon crystals and forming them into wafers. These blended wafers have a distinctive bluish hue, due to the random orientation of the silicon crystals. Because the wafer has a nonuniform cell structure, polycrystalline solar cells are less efficient than monocrystalline solar cells. Therefore, solar panels made from polycrystalline solar cells have efficiency rates ranging from 15% to 17%.
The average cost of these panels ranges from 90 cents to 1 per watt. The temperature coefficient of polycrystalline panels is worse than that of monocrystalline panels, meaning they’ll perform worse when it’s hot, but they can still function for up to 25 years. As a result, these panels are great for budget-conscious homeowners.
What are thin-film solar panels made of?
Thin-film solar panels are made by putting a thin layer of photovoltaic material, such as amorphous silicon, cadmium telluride (CdTe), or copper indium gallium selenide (CIGS), on a material like glass or plastic. These panels are known for being both flexible and light, making them suitable for applications where traditional panels may not be feasible, such as on curved surfaces or portable devices. However, thin-film panels are generally less efficient than both monocrystalline and polycrystalline silicon panels, while costing 1 to 1.50 per watt.
How are solar panels made?
Now that we’ve covered how different types of solar cells are made, you’re probably wondering how solar panels themselves are assembled. Here’s a step-by-step outline of the process.
- Step 1: Silicon purification and ingot formation: Sand with a high silicon content is refined to remove impurities and produce high-purity silicon. The silicon is melted and formed into cylindrical ingots.
- Step 2: Wafer slicing: The silicon ingots are sliced into a precisely measured and prespecified thickness with a diamond-edged saw. The thickness of the wafer is carefully controlled to optimize the balance between light absorption and electrical conductivity. These wafers are the foundation of each solar cell, Kalyanpur said.
- Step 3: Cell processing: The wafers are then put through a series of treatments to enhance their photovoltaic properties. The treatments include a texturing process to increase light absorption. Metal contacts are added to the front and back of each cell to allow the cells to conduct electricity.
- Step 4: Solar cell assembly and encapsulation: The solar cells are assembled into a solar panel by connecting them in series or parallel configurations. The configuration will determine the desired voltage and current output. The cells are then encased in durable glass or another transparent material. The encased cells are then attached to a frame.
- Step 5: Integration of electrical components: The junction box and other electrical components are added to the panels. These components enable the solar panel to transmit the electricity it generates to the inverter and charge controller.
Solar panels undergo a rigorous quality assurance process at every stage of the manufacturing process, including after assembly. This quality assurance process helps to ensure that each component meets the required standards for performance and reliability.
Are solar panels safe?
You should always have a healthy dose of skepticism and concern regarding any equipment, but you’ll probably have nothing to worry about as long as they are used as originally intended. But if they’re misused or damaged during installation, there could be some cause for concern.
Solar panels can include some materials that might be classified as hazardous, according to the Environmental Protection Agency. Different types and manufacturers of panels have different levels of potentially hazardous chemicals, such as lead and cadmium. These are typically part of semiconductors or solder.
Solar panels are safe and do not pose a significant risk to human health or the environment, said Kalyanpur. While some toxic materials are used in the manufacturing process, similar to the presence of … cobalt in phone batteries, these materials are safe as long as they are not ingested and are properly disposed of or recycled at the end of their life cycle. Most solar panels are made from silicon, a nontoxic and abundant element that poses no risk to human health. As long as solar panels are properly handled, recycled, or disposed of at the end of their life, people should have no concerns regarding health or safety.
Advances in crystalline silicon solar cell technology for industrial mass production
Crystalline silicon photovoltaic (PV) cells are used in the largest quantity of all types of solar cells on the market, representing about 90% of the world total PV cell production in 2008. Crystalline silicon solar cells are also expected to have a primary role in the future PV market. This article reviews the current technologies used for the production and application of crystalline silicon PV cells. The highest energy conversion efficiency reported so far for research crystalline silicon PV cells is 25%. Standard industrial cells, however, remain limited to 15–18% with the exception of certain high-efficiency cells capable of efficiencies greater than 20%. High-efficiency research PV cells have advantages in performance but are often unsuitable for low-cost production due to their complex structures and the lengthy manufacturing processes required for fabrication. Various technologies for mono- and polycrystalline PV cells are compared and discussed with respect to the corresponding material technologies, such as silicon ingot and wafer production. High energy conversion efficiency and low processing cost can only be achieved simultaneously through the development of advanced production technologies and equipment, and some of the latest technologies that could lead to efficiencies of greater than 25% and commercially viable production costs are reviewed.
Main
In 2008, the world annual production of photovoltaic (PV) cells reached more than 7.9 GWp (Wp, peak power under standard test conditions) 1. and the average annual growth rate in PV cell production over the last decade has been more than 40%. Yet the electrical power generated by all PV systems around the world has been estimated to be less than 0.1% of the total world electricity generation 1. Nevertheless, the strong growth in PV cell production is expected to continue for many years. Crystalline silicon PV cells, with over 60 years of development, have the longest production history and now account for the largest share of production, comprising up to 90% of all the solar cells produced in 2008 1. Silicon is safe for the environment and one of the most abundant resources on Earth, representing 26% of crustal material. The abundance and safety of silicon as a resource grants the silicon solar cell a prominent position among all the various kinds of solar cells in the PV industry. World annual PV cell production of 100 GWp is expected to be achieved by around 2020, and the silicon PV cell is the most viable candidate to meet this demand from the point of view of suitability for large-volume production.
The crystalline silicon PV cell is one of many silicon-based semiconductor devices. The PV cell is essentially a diode with a semiconductor structure (Figure 1), and in the early years of solar cell production, many technologies for crystalline silicon cells were proposed on the basis of silicon semiconductor devices. The synergy of technologies and equipment developed for other silicon-based semiconductor devices, such as large-scale integrated circuits and the many different kinds of silicon semiconductor applications, with those developed for PV cells supported progress in both fields. Process technologies such as photolithography helped to increase energy conversion efficiency in solar cells, and mass-production technologies such as wire-saw slicing of silicon ingots developed for the PV industry were also readily applicable toother silicon-based semiconductor devices. However, the value of a PV cell per unit area is much lower than that for other silicon-based semiconductor devices. Production technologies such as silver-paste screen printing and firing for contact formation are therefore needed to lower the cost and increase the volume of production for crystalline silicon solar cells. To achieve parity with existing mains grid electricity prices, known as ‘grid parity’, lower material and process costs are as important as higher solar cell efficiencies. The realization of high-efficiency solar cells with low process cost is currently the most important technical issue for solar cell manufacturers. Cutting the cost of producing expensive high-purity crystalline silicon substrates is one aspect of reducing the cost of silicon solar cell modules. This review covers the historical and recent technological advances in crystalline silicon solar cells from the perspective of industrial application.
Features of standard crystalline-silicon PV cells and modules
Crystalline silicon PV cells are the most popular solar cells on the market and also provide the highest energy conversion efficiencies of all commercial solar cells and modules. The structure of typical commercial crystalline-silicon PV cells is shown in Figure 1. Standard cells are produced using one of two different boron-doped p-type silicon substrates; monocrystalline and polycrystalline. The cells of each type are typically 125 mm (5 inches) or 156 mm (6 inches) square, respectively. Monocrystalline solar cells are produced from pseudo-square silicon wafer substrates cut from column ingots grown by the Czochralski (CZ) process (see Figure 2). Polycrystalline cells, on the other hand, are made from square silicon substrates cut from polycrystalline ingots grown in quartz crucibles. The front surface of the cell is covered with micrometer-sized pyramid structures (textured surface) to reduce reflection loss of incident light. An anti-reflection coating (ARC) of silicon nitride (SiNx) or titanium oxide (TiOx) is overlayed on the textured silicon surface to further reduce reflection loss. Crystalline silicon solar cells have highly phosphorous-doped n (electron-producing) regions on the front surface of boron-doped p-type (electron-accepting) substrates to form p–n junctions. Back-surface p field (BSF) regions are formed on the back surface of the silicon substrate to suppress recombination of minority carriers (photogenerated electrons). The BSF regions are usually formed by firing screen-printed aluminum paste in a belt furnace. The carriers (electrons) generated in the silicon bulk and diffusion layers are collected by silver contacts (electrodes) formed on the front and back silicon surfaces. The front contact consists of gridlines connected by a busbar to form a comb-shaped structure. The back contact is usually a series of silver stripes connected to the front bus bar of the adjacent cell via soldered copper interconnects. The contacts are usually formed by firing of screen-printed silver paste at the same time as firing for formation of the BSF regions. The front contact is similarly formed using screen-printed silver paste applied on top of the ARC layer. Contact between the front electrode and the n region of the silicon substrate is achieved by firing such that the silver penetrates through the ARC layer. The screen-printed front silver contact prepared by firing to penetrate the ARC is one of the most important techniques for large-volume fabrication of modern standard crystalline silicon cells. Other techniques, such as using boron-doped BSF and nickel–copper plating contacts, are used by a small number of cell manufacturers. The efficiencies of typical commercial crystalline silicon solar cells with standard cell structures are in the range of 16–18% for monocrystalline substrates and 15–17% for polycrystalline substrates. The substrate thickness used in most standard crystalline cells is 160–240 μm. The solar cells are assembled into modules by soldering and laminating to a front glass panel using ethylene vinyl acetate as an encapsulant. The energy conversion efficiency of modules of standard solar cells is roughly 2% lower than the individual cell efficiency, falling in the range of 12–15%.
The sequence of crystalline silicon solar cell production, from raw materials to modules, is shown in Figure 2. The value chain for crystalline silicon solar cells and modules is longer than that for thin-film solar cells. There are generally three industries related to crystalline silicon solar cell and module production: metallurgical and chemical plants for raw material silicon production, monocrystalline and polycrystalline ingot fabrication and wafer fabrication by multi-wire saw, and solar cell and module production. The cost of PV production is roughly divided in half between solar cell module production and balance-of-system fabrication, which includes the inverter, cables and installation. The fabrication cost for solar cell modules includes the cost of the silicon substrate (50%), cell processing (20%) and module processing (30%). The cost share is therefore strongly affected by the market price for poly-silicon feedstock, and reducing the cost of the silicon substrate remains one of the most important issues in the PV industry.
The industrial goal for PV power is to reduce the electricity generation cost to the equivalent of that for commercial grid electricity. The energy conversion efficiency of solar cells is another important issue because the efficiency influences the entire value-chain cost of the PV system, from material production to system installation. The solar cell efficiency is limited by the three loss mechanisms: photon losses due to surface reflection, silicon bulk transmission and back contact absorption; minority carrier (electrons in the p region and holes in the n region) loss due to recombination in the silicon bulk and at the surface; and heating joule loss due to series resistance in the gridlines and busbars, at the interface between the contact and silicon, and in the silicon bulk and diffusion region. In the design of solar cells and processes, these losses are minimized without lowering the productivity of the solar cells.
The electrical performance of a solar cell is determined by the short-circuit current (Isc), open-circuit voltage (Voc), current at the maximum power point (Imp), voltage at the maximum power point (Vmp), maximum power (Pmax), fill factor (FF) and energy conversion efficiency (η). In research and development, short-circuit current density (Jsc) is also used. An air mass 1.5 (AM1.5) spectrum condition (1,000 W m–2) is the standard test condition for terrestrial solar cells. The AM1.5 condition is defined as 1.5 times the spectral absorbance of Earth’s atmosphere; in contrast, the spectral absorbance for space is zero (air mass zero, AM0). The solar energy under the AM1.5 condition is used as the input energy for calculation of solar cell efficiency. The solar cell fill factor and efficiency are calculated using the following equations.
Efficiency improvements
Historical development
Bell Laboratory fabricated the first crystalline silicon solar cells in 1953, achieving 4.5% efficiency, followed in 1954 with devices with 6% efficiency [2,3]. In the ten years since the first demonstration, the efficiency of crystalline silicon cells was improved to around 15%, and were sufficiently efficient to be used as electrical power sources for spacecraft, special terrestrial applications such as lighthouses, and consumer products such as electronic calculators. The improvements in research-cell efficiencies achieved for various kinds of solar cells over the past 30 years are shown in Figure 3. Although crystalline silicon solar cell technologies are not yet as efficient as cells based on single-junction GaAs and multi-junction concentrators, they currently provide a good compromise between efficiency and cost.
The basic cell structure used in current industrial crystalline solar cells, which includes features such as a lightly doped n layer (0.2–0.3 μm) for better blue-wavelength response, a BSF formed by a p/p low/high junction on the rear side of the cell, a random pyramid-structured light-trapping surface, and an ARC optimized with respect to the refractive index of the glue used to adhere it, were developed for space and terrestrial use in the 1970s. The efficiency of monocrystalline cells for space use is in the range of 14–16% under ‘1 sun’ AM0 test conditions, equivalent to 15–17% at AM1.5. These standard structures for crystalline silicon cells are still used in standard industrial crystalline cells, which offer efficiencies in the range of 14–17%.
The key technologies needed to realize efficiencies of higher than 20% were developed in the 1980s and ’90s, and the latest high-efficiency crystalline silicon cells possess most of these features (Table 1).
The best output parameters reported for the HIT cell, which was developed for industrial use, are 729 mV, 39.5 mA cm–2, 0.800 and 23.0% (Voc, Jsc, FF and η) for a large 100.4 cm 2 cell 9. This cell has a unique heterojunction structure consisting of very thin, amorphous p- and n-doped layers and intrinsic amorphous layers on the front and rear surfaces of a CZ n-type monocrystalline-silicon substrate. This heterojunction structure improves Voc considerably by the effects of the large energy bandgap of the front amorphous silicon layer and the excellent quality of the interface between the amorphous layer and the crystalline substrate. This cell has the additional advantage of a low temperature coefficient of about 0.30 % K –1 at Pmax compare to about 0.45 % K –1 for standard industrial crystalline silicon PV cells. This cell has a transparent conductive oxide (TCO) ARC, which reduces the sheet resistivity of the front amorphous layers. The distinctly lower Jsc compared to other high-efficiency PV cells appears to be due to suppressed photocurrent collection by the front amorphous silicon layers and the bulk silicon by the effects of the lower transparency of the TCO layer compared to other ARCs and/or the lower internal quantum efficiency of the amorphous layers. The result is a weaker blue response and lower Jsc.
The BC-BJ cell has interdigitated n- and p-doped regions and n and p contacts on the back surface. The original BC-BJ cell, called the front surface field (FSF) cell or interdigitated back contact (IBC) cell, was fabricated and studied for space applications in the late 1970s [10,11]. The BC-BJ-structured point contact (PC) cell developed by Stanford University in the 1980s gave efficiencies of more than 20% from the outset 12. BC-BJ cells were first fabricated for unmanned aircraft and solar race cars by SunPower in the 1990s. The cells were then extended to large-scale production for PV generation systems in the 2000s. The best conversion efficiency reported so far for a large-area industrial BC-BJ cell is 23.4% 13. The BC-BJ cell has front and rear surface passivation layers, a random-pyramid light-trapping surface, FSF, interdigitated n- and p-doped regions on the back surface, n and p contact gridlines on n- and p-doped regions, a single-layer ARC and CZ n-type single-crystalline silicon substrate with a minority carrier lifetime of longer than 1 ms. Of all the crystalline silicon PV cell modules on the market at this time, only those based on BC-BJ cells provide the possibility of module efficiencies exceeding 20%. Several laboratories and manufactures are studying methods for improving the design and processing of BC-BJ cells [14,15]. BC-BJ cells have several advantages compared to the conventional front-contact cell structure: no gridline (sub-electrode) or busbar (main electrode) shading, a front surface with good passivation properties due to the absence of front electrodes, freedom in the design of back contacts (electrodes), and improved appearance with no front electrodes. They also provide advantages in module assembly, allowing the simultaneous interconnection of all cells on a flexible printed circuit (see Figure 4(d)). The low series resistance of interconnection formed by this type of surface-mount technology results in a high FF of 0.800, compared with around 0.75 for standard silicon PV cell modules [16,17].
Industrial cells
Monocrystalline solar cells
p-Type monocrystalline substrates sliced from boron-doped CZ ingots have been used for standard industrial PV cells for many years. In the early era of terrestrial PV cell production, small 2–5-inch-diameter CZ ingots were used, the small size and high cost of which obstructed cost reduction for monocrystalline cells. Much research and development has been devoted to reducing the production costs for CZ ingots and wafer processing over the past 20 years. CZ wafers with side lengths of 125 and 156 mm, sliced from 6- and 8-inch-diameter ingots, respectively, are now widely used for monocrystalline silicon PV cell fabrication. The fabrication of monocrystalline cells and modules using wafers of the same size as those used for polycrystalline cell production has improved the competitiveness of monocrystalline cells against their polycrystalline counterparts in terms of manufacturing cost per output watt. Monocrystalline cells represented 38% of all solar cells manufactured in 2008 1.
There are large differences between the efficiencies of the best research crystalline silicon PV cells and the corresponding industrial cells. The efficiencies of standard industrial monocrystalline PV cells remain in the range of 16–18%, considerably lower than the 25% efficiency levels of the best research cells. Industrial cells are restricted by economic factors to simple cells that are suitable for high-speed, automated production using low-cost materials. Simple design features, such as front surface texturing and BSF similar, to those developed for terrestrial crystalline-silicon PV cells in the early 1980s are still adopted in most current industrial crystalline cells. To improve cell efficiencies, many cell manufactures are systematically attempting to introduce high-efficiency features, such as finer gridlines, selective emitters or more shallowly doped n regions, into existing manufacturing processes. The BC-BJ cells and HIT cells have exceptionally high efficiencies for industrial monocrystalline PV cells, but have complex cell structures that require a much longer production process and more specialized equipment compared with the other industrial cells. As a result, it is difficult for these advanced cell types and modules to compete commercially in terms of production cost per output watt. There remains a dilemma in the balance between efficiency improvement and cost reduction for solar cells and modules using existing manufacturing technologies. Innovative and simple manufacturing technologies and equipment for the fabrication of high-efficiency solar cells are therefore needed in order to realize significant cost reductions for the production of crystalline silicon PV modules.
Another drawback of the monocrystalline cell technologies is that monocrystalline cells based on p-type CZ silicon substrates are susceptible to light-induced degradation (LID) caused by the recombination of reactive boron–oxygen complexes (B s–O 2i). Many studies have been undertaken in attempts to eliminate LID effects in monocrystalline-silicon PV cells, and permanent deactivation of the complex at high temperature ( 170 °C) has been reported 18. Boron-doped magnetic-field CZ wafers and gallium-doped CZ wafers also show promise for eliminating LID effects in monocrystalline solar cells, and CZ-silicon cells based on phosphorous-doped n-type CZ wafers are also free of LID effects. The high-efficiency PV cells of SunPower and Sanyo are made using n-type CZ-silicon wafers.
Polycrystalline solar cells
Polycrystalline silicon ingots and wafers were developed as a means of reducing the production costs for silicon ingots, and have been investigated since the mid-1970s [19,20]. Modern polycrystalline furnaces are designed for maximum productivity, casting ingots of around 450 kg. Polycrystalline cells are currently the most widely produced cells, making up about 48% of world solar cell production in 2008 1. Standard polycrystalline industrial cells offer efficiencies of 15–17%, roughly 1% lower than for monocrystalline cells fabricated on the same production lines. The efficiencies of polycrystalline cell modules, however, are almost the same as those for monocrystalline cells (14%) due to the higher packing factor of the square polycrystalline cells; monocrystalline cells are fabricated from pseudo-square CZ wafers and have relatively poor packing factors.
The efficiencies of both monocrystalline and polycrystalline PV cells will be improved in the future through the introduction of high-efficiency structures. The difference in efficiency between monocrystalline and polycrystalline cells is expected to become larger with the introduction of such high-efficiency structures due to the difference in crystal quality (i.e. minority carrier lifetimes). The best of the current research polycrystalline silicon cells, a PERL cell developed by Fraunhofer ISE 21. provides an energy conversion efficiency of 20.3%. This PERL cell has a laser-fired contact back structure that gives a Voc of as high as 664 mV. The efficiency of this polycrystalline cell, however, remains about 5% lower than that for the best of the research monocrystalline PERL cells, attributable mainly to the quality difference between mono- and polycrystalline substrates. Polycrystalline substrates are subject to higher rates of minority carrier recombination, both at active grain boundaries and within crystalline grains due to high dislocation and impurity densities in comparison with FZ or CZ monocrystalline substrates. A considerable amount of research and development has been conducted on improving the efficiencies of polycrystalline solar cells over many years, by both public and industrial laboratories, and recent high-efficiency polycrystalline silicon solar cells now have the features listed in Table 2. These features are generally the same as for recent monocrystalline solar cells.
The EWT cells have a larger number of close-spaced through-holes, which direct photogenerated electrons to the back surface solely through n-doped emitters. The EWT cells produce even higher photocurrents by eliminating the both busbar (main electrode) and gridline (sub-electrode) shading on the front surface. A high Jsc of 37.5 mA cm–2 and efficiency of 17.1% were reported recently for EWT cells by Q-Cells. The target for industrial polycrystalline PV cells is to realize average cell efficiencies of 17% in large-scale production 24.
Many methods have been investigated to improve the quality of polycrystalline substrates to match that of the more expensive CZ monocrystalline wafers. The dendritic casting method is one such approach that allows the grain orientation and size to be controlled, resulting in high-quality dendritic crystals with parallel twinning. Solar cells based on dendritic polycrystalline wafers show efficiencies of as high as 17%, comparable to the efficiencies provided by CZ monocrystalline cells using the same cell fabrication process 27.
Materials and processing
The raw, high-purity polysilicon material used for the fabrication of crystalline silicon solar cells is generally made by the Siemens method. The market price for raw silicon is affected by the demand–supply balance for solar cell and semiconductor fabrication, and can fluctuate markedly. In 2006–2008, for example, the cost of raw silicon as a proportion of total solar cell module cost jumped from 20–30% to more than 50% due to a market shortage of silicon. Reducing the cost of silicon in a cell module by reducing the substrate thickness is therefore an important aspect of achieving overall cost reductions for solar cell modules. Wire-saw wafer slicing is one of the key production technologies for industrial crystalline silicon PV cells, and improvements in wafer slicing technology have resulted in a reduction in raw wafer thickness from 370 μm to 180 μm since 1997 for Sharp industrial polycrystalline-silicon cells (Figure 6). To introduce wafers thinner than 150 μm, sophisticated manufacturing processes suitable for ultrathin wafers will be needed, and the processes will need to provide high processing speed and high manufacturing yield in each of the process steps of wafer slicing, cell fabrication and module assembly.
Several alternative growth methods have been proposed over the past four decades for the production of polycrystalline substrates directly from molten silicon, including edge-defined film-fed growth (EFG), string ribbon growth (SRG), and ribbon growth on substrate (RGS) [28–30]. These methods potentially make it possible to reduce the amount of silicon used in PV cell fabrication. The EFG and SRG methods are used on industrial production scales by SCHOTT Solar and Evergreen Solar, respectively 31. These two methods have the advantages of low silicon consumption per Wp and high cell efficiencies in comparison with the RGS method. These methods afford inexpensive but slightly wavey polycrystalline substrates in comparison with the standard polycrystalline substrates. Recently manufactured cells based on direct-grown substrates have almost the same efficiencies as those of standard cast-silicon polycrystalline cells. However, the smaller EFG- and SRG-based cells, which are roughly half the size of standard industrial cells, incur higher cell and module processing costs. A crystallization on dipped substrate method, which can be used to produce standard-sized wafers (156 mm × 156 mm) directly from molten silicon in a crucible, was recently proposed by Sharp 32.
The front emitter layer of crystalline silicon PV cells is formed by phosphorus diffusion techniques in a quartz tube or belt furnace. Solid P2O5 or liquid POCl3 is used as the phosphorus diffusion source. Phosphorous diffusion techniques the exploit gettering effects to reduce impurity densities in a silicon wafer and thereby improve minority carrier lifetime have been demonstrated to be effective provided diffusion is conducted under phosphorus supersaturation conditions (doping level above the solid solubility in silicon) [33–35].
The BSF layers in industrial cells are formed by alloying of screen-printed aluminum paste in a belt furnace. This process provides high productivity and relatively low process cost for BSF formation. Aluminum-paste alloying has the additional advantage of inducing wafer gettering effects in both polycrystalline and monocrystalline silicon PV cells similar to the phosphorus diffusion technologies [36,37]. Metal impurities, such as iron or copper, can be eliminated from bulk silicon by aluminum gettering effects, which can improve the minority carrier diffusion length.
The screen printing and firing of silver paste to make contact with the bulk silicon surface by penetrating the ARC is a well-established, simple and fast process for forming front and rear electrodes. It is also the most widely used and lowest-cost method for forming electrodes in industrial crystalline silicon PV cells. The front gridlines are designed so as to optimize the trade-off between shadow loss and series resistance. As an alternative to the screen-printed silver paste approach, plated electrodes of layered nickel, copper and silver have been developed by researchers at the UNSW for use in buried-contact (BC) cells [38,39]. The BC cell fabricated by BP Solar is shown in Figure 4(e) 40. Crystalline silicon PV cells with plated electrodes have excellent electrical characteristics due to their low series resistance and fine gridlines, which result in a much smaller shadow area. However, plated electrodes, which are formed by a wet process, have not yet become as widely used as the screen-printed silver paste electrodes. The silver used as the electrode material in crystalline silicon cells will become a critical material resource when crystalline silicon solar cell production reaches the large volumes predicted in the future. Copper and aluminum have therefore been considered as substitutes for silver in silicon PV contacts.
Future views on crystalline silicon solar cells
Industrial solar cells module must reach a price level of 1/Wp with a total system price level of 2/Wp to reach grid parity, and to become competitive with coal or nuclear power generation will need to be mass produced at a total system cost of less than 1/Wp. Achieving even a module price of 1/Wp will require modules to be produced at a cost of less than 0.7/Wp. Although such low costs remain very challenging for modules based on crystalline silicon solar cells, cost reductions to such a level are considered to be possible based on the technologies presented in this review, and the cost reduction must be accomplished while public incentives for PV systems remain in effect. The annual production volume for all kinds of solar cells is expected to exceed 100 GWp/year by around 2020. Crystalline silicon cell modules have a long history of proven field operation and offer high efficiencies while presenting fewer resource issues than many competing technologies. As such, crystalline silicon PV cells are expected to be strongly represented in the future solar cell market.
To reach these future price levels, new technologies as listed in Table 3 will be needed for crystalline silicon solar cells and modules. New technologies to break through the efficiency barrier of 25% for crystalline silicon PV cells are being studied by many researchers and institutes around the world, but there have yet to be any practical improvements in cell efficiency. The peak theoretical efficiency in a crystalline silicon solar cell based on a single homojunction and a bulk silicon energy bandgap of 1.1 eV is 30% under 1 sun AM1.5 illumination. To break through this ideal efficiency limit based on existing Schockley and Queisser solar cell theory, novel technologies based on quantum dot (QD) and quantum well structures have been proposed and studied by many researchers. Multi-junction designs have been attempted in many forms for improving solar cell efficiency beyond that of a single-junction cell. For example, a triple-junction solar cell with a silicon bottom cell is expected to give efficiencies of more than 40%. Researchers at the UNSW have also proposed a silicon-based tandem junction solar cell incorporating silicon QD technology 41. An effective bandgap of up to 1.7 eV has been demonstrated for 2 nm-diameter silicon QDs embedded in SiO2 42. Photon management, such as up- and down-conversion and plasmonic effects, are other potential approaches that could add extra efficiency based on existing high-efficiency silicon cells [43–45]. These technologies aim at shifting the photon energy of sunlight to match the sensitivity of the solar cell by adding special optical features (e.g. a fluorescent coating layer including rare-earth elements for up-and down-conversion) to the front and/or rear surface of the cells without modifying the structure of the solar cell itself.
These high-efficiency technologies, however, generally incur higher production costs compared to standard silicon cells. Cell and module manufacturing technologies that satisfy both high efficiency and low cost will be essential for industrial production in the near future. The impacts of novel technologies such as QDs and photon management will be interesting to watch as research and development on crystalline silicon solar cells continues.
Author information
China’s solar cell production capacity may reach 600 GW by year-end
The Asia Europe Clean Energy (Solar) Advisory revealed that most of the planned new solar cell production capacity relates to high-efficiency n-type cell technologies such as TOPCon and HJT.
Share
China’s total annual solar cell and module production capacity may increase from 361 GW at the end of last year to up to 600 GW at the end of 2022, according to the Asia Europe Clean Energy (Solar) Advisory (AECEA).
“ Since January, 20 companies disclosed to expand module production totaling 380 GW, planned to be executed within the next few months or up to 1,5 years,” the analyst firm said, noting that most of this capacity relates to n-type modules produced with tunnel oxide passivated contacts (TOPCon) solar cell s or panels based on cells with a heterojunction (HJT) design. “Reportedly, TOPCon related expansion plans exceed 220 GW, whereas HJT is nearing the 150 GW mark. As an example, recently one HJT company conducted an online pitching and according to them almost 800 people joined that call,” it added.
So far this year, the output of polysilicon, wafers, cells and modules has already beaten the achievements of the Chinese PV industry in 2021 by some 50%. “By June, module shipments of the TOP 10 manufacturer crossed the 100 GW and by the end of September may have reached 140-150 GW (2021: 133 GW),” said the AECEA. “Just five of these top 10 have set shipment targets of between 183-205 GW.”
Popular content
Furthermore, the AECEA revealed that the country’s polysilicon capacity should grow from around 530,000 MT at the end of 2021 to up to 1.2 million MT in 2022, jumping to 2.5 million MT in 2023, and up to 4 million MT in 2024.
“In the near term, the overall industrial landscape won’t fundamentally change. Incumbent companies are further consolidating their market positions through backward/forward integration,” the AECEA stated. “By and large, vertical integration remains their favored business model, which in times of external supply constraints or external supply dependencies has gained ever more weight.”
This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: editors@pv-magazine.com.
Emiliano Bellini
Emiliano joined pv magazine in March 2017. He has been reporting on solar and renewable energy since 2009.
From sand to solar panels: Unveiling the journey of solar panel manufacturing
By subscribing, you agree to our Terms of Use and Policies You may unsubscribe at any time.
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.
The role of sand in the solar panel manufacturing process
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.
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.
First step: Extraction and refinement of silica
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.
Second step: Polysilicon production
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.
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.
Third step: Silicon ingots and wafer production
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.)
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.
The Process of Solar Panel Manufacturing
A solar panel contains a set of solar cells whose function is to convert the sun’s light into electric energy. The primary material in solar panel manufacturing is silicon. Many of the solar panels that you see on roofs are monocrystalline or polycrystalline. Solar panel manufacturing process determines its efficiency.
As people switch to solar energy, it is good to know a few things about how solar panels are made. In this article, we will look at a few things you need to know about solar panel manufacturing. Let’s get started.
Solar Energy Background
Due to research in this field, solar panels have changed in effectiveness since invention. Before the inception of the first silicon photovoltaic cells in 1954, several scientists contributed to solar growth as we know it today. The silicon cell produced in 1954 had 4% efficiency. As research grew, solar panel’s efficiency continued to increase.
Currently, the solar cells being used can meet commercial and homes electricity demand. Solar panels are also being installed to generate electricity to power business and manufacturing operations. As further research continues on solar energy, it could emerge as the preferred source to run economies.
Solar Panel Raw Materials
The first crucial component required to make solar cells is pure silicon. However, silicon is not pure in its natural state. It is derived from quartz sand in a furnace requiring very high temperatures. Natural beach sand is the main component in making pure silicon. Though it is an abundant resource in the world, the process of getting pure silicon comes at a cost and requires a lot of energy.
Solar Panel Manufacturing Process
The first process in solar panel manufacturing is purifying the silicon from quartz sand. Once silicon is purified, it is collected into solid rocks. These rocks are then molten together, forming cylindrical ingots. A steel and cylindrical furnace is utilized to achieve the desired shape. When manufacturing is underway, there is a keen attention to have all atoms align in desired orientation and structure.
To give the silicon positive electrical polarity, boron is included in the process. To make monocrystalline cells, the manufacturer only uses one silicon crystal. As a result, such solar panels have high efficiency. However, they come at a higher cost.
For polycrystalline cells, the manufacturers melt several silicon crystals together. These panels have a shattering glass appearance derived from the various silicon crystals. Once the formed ingot cools постільна білизна купити, it is shinned and polished to leave flat sides.
Making Wafers
The next step in solar panel manufacturing after making ingots. To make wafers, the cylindrical ingot is thinly sliced into thin disks. It is done so one at a time using a cylindrical saw. Manufacturers can also use a multiwire saw to cut many at a time.
Thin silicon is shiny, which reflects light. A thin anti-reflective coating is put on the disks reducing the amount of sunlight lost. The anti-reflective coating is commonly made from titanium dioxide and silicon oxide, but other materials can also be used.
This coating material can be heated until the molecules boil, or it can go through spattering. Under the spattering process, the manufacturers use a high-voltage to know the materials’ дропшипінг molecules and depositing them on the silicon.
The wafers can be further polished to remove saw marks. However, some manufacturers are choosing to skip these steps as the saw marks help increase efficiency.
Making Solar Cells
Manufacturers follow several steps to convert the silicon wafers into usable solar cells. They treat each wafers and add metal conductors on the surface. The added conductors result in a grid-like matrix appearance on the surface. They ensure sunlight is converted into electricity.
The coating on the silicon wafers reduces sunlight reflection, ensuring the sun is absorbed, leading to more production. In oven-like chambers, the manufacturer’s phosphorous is spread in a thin layer over the wafers’ surface. The phosphorous charges покривало на ліжко the wafers with a negative electrical orientation.
Solar Cells to Solar Panels
Once the solar cells are made, the manufacturers connect them using metal connectors. Solar panels are a combination of solar cells in a matrix-like structure. The market standard of solar panels are:
- 48 cell panels – ideal for small residential roofs.
- 60-cell panels – the standard size.
- 72-cell panels – suitable for large-scale installations
After the manufacturers combine the solar cells, thin glass casing is placed on the side to face the sun. They also use highly durable polymer-based material to make the back sheet. This prevents things such as water, soil, and others from getting to the solar cells.
To allow connections to the modules, a junction box is added. Once this step is complete, the manufacturer adds the frame, providing more protection to the cells. Ethylene-vinyl acetate or EVA ( Plaid) is used to bind everything together.
Testing the Solar Panels
Once the solar module manufacturing is completed, testing is done to ensure it meets the expected performance. Normally, STC (Standard Test Conditions) is used. After testing, solar panels are now cleaned and inspected, and the model is shipped to homeowners.
Solar panels’ efficiency continues to increase. As many home and business owners choose clean energy, the solar manufacturing industry is expected to grow. There is hope that solar manufacturing costs will continue to reduce as research постільна білизна and development continue.