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Ingot Carving the Future

solar cells

Silicon is the most abundant solid element on earth; it makes up more than 25% of the earth’s crust. However, it rarely occurs in elemental form, virtually all of it is existing as compounds. Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or “boule” of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.

From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multiwire saw. (A diamond saw produces cuts that are as wide as the wafer—. 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell. The wafers are then cut, cleaned, and coated according to the specifications of the system manufacturers.

The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process. Recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. According to madehow this new process, however, has generally not been accepted by commercial manufacturers.

Latest research

In recent years there have been technological advances in photovoltaic cells’ efficiency, and increased production has brought down their cost. But they remain relatively expensive in large part because fully half of the costly silicon wafers at the heart of solar cells are destroyed during production. Now Researchers at the Fraunhofer Institute for Solar Energy Research (ISE) in 2015, developed a technology that cuts these losses in half and, at the same time, reduces fabrication energy costs by 80 percent. Manufacturing of these wafers is very time and energy consuming, and correspondingly expensive. Silicon costs about $17 per kilogram, which means that about $9 worth of silicon is wasted in making solar cells because the process ruins approximately half the silicon.

But the Fraunhofer researchers say they’ve solved both the waste and energy problems. “We are reducing material losses by 50 percent while using 80 percent less energy,” said Stefan Janz, the lead ISE researcher on the project.

Until now, making a solar cell starts with impure silicon that’s liquefied, then purified with the addition of chlorine, creating a material known as chlorosilane. With the addition of hydrogen, chlorosilane becomes highly pure chunks of polysilicon.

To turn polysilicon into the crystals needed for solar cells, the chunks are broken, melted at temperatures exceeding 2,600 degrees Fahrenheit, then allowed to grow randomly, sometimes into amorphous and therefore useless silicon, and sometimes into the crystalline form suitable for photovoltaic cells. The usable polysilicon is then molded into huge ingots, then cut with saws into small wafers.

Both the high heat used to melt the polysilicon, the random growing of the material and the silicon dust from the sawing cause great waste of an expensive resource. But the ISE method addresses all three causes of the waste, Janz says. The Fraunhofer method heats the chlorosilane under lower heat, about 1,800 degrees Fahrenheit, then mixes it with hydrogen. Lower heat translates into lower cost.

Next, the silicon is vaporized and allowed to flow past a crystalline silicon wafer, which acts as a kind of template that orients the silicon vapor to eventually become crystalline itself. “We don’t let the silicon just grow randomly,” Janz said.

This process drastically reduces silicon waste. It’s also economical because the “template” wafer can be reused dozens of times. It also gives the wafers their final shape, eliminating the need for wasteful sawing.

“In this way we get a very good monocrystal, which is the best type of crystal, and the wafers are of the same quality as those produced using conventional methods,” Janz said.

Material loss isn’t the only disadvantage of sawing. Franunhofer says sawn wafers can’t be thinner than 150 to 200 micrometers or they’ll shatter during cutting. Under the new manufacturing method, the wafers can be half as thin. This material savings alone reduces the cost of a solar cell by 20 percent.

High-quality mono-silicon crystal grown at low cost for solar cells

A joint research team in Japan has developed a “single-seed cast method,” a new casting method making it possible to grow highquality mono silicon at low cost. New casting method may facilitate the return of a marketcompetitive solar cell industry.

A research team led by Takashi Sekiguchi, a leader of the Nano Device Characterization Group, Nano-Electronic Materials Unit, International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Japan, and Koichi Kakimoto, a professor at the Research Institute for Applied Mechanics, Kyushu University, Japan, has developed a new method to grow high-quality mono silicon at low cost. The research resulted in the invention of a new casting method called a single-seed cast method. It dramatically improved the quality of crystals created compared to conventional casting methods, which potentially leads to the development of more efficient silicon solar cells.

As the current conversion efficiency of mainstream silicon-type solar cells has already reached 20%, it is required in future development to increase conversion efficiency even more to add higher value to the cell products. However, this goal is not achievable using conventionally cast polycrystalline silicon. In addition, there is demand for the development of a new silicon material to replace polycrystalline silicon and single-crystal silicon for semiconductors, because dislocation-free single crystal silicon for semiconductors is not adequately competitive pricewise.

To address this issue, the research team developed a single-seed cast method, a new silicon casting method using a seed crystal, and succeeded in growing a highquality single-crystal silicon (mono silicon) ingot with low impurity. In the new casting method, silicon is melted in a crucible, and a single crystal is grown from a small seed crystal. This method is less expensive than the method to create single crystal silicon for semiconductors due to reduced raw material use and manufacturing costs. Moreover, the conversion efficiency of a solar cell prototype created using the crystal grown by this method was as high as 18.7%. This efficiency was very close to the efficiency of dislocation-free single-crystal silicon (Czochralski (Cz) silicon) wafers for semiconductors (18.9%), which were evaluated concurrently. In future studies, the conversion efficiency of mono silicon may exceed that of Cz silicon by further reducing crystal defects and the impact of impurities.

It is also feasible to grow an ingot as large as a 50-cm cube using the current facility. As such, the facility is compatible with and can be integrated into the existing production line. In the future, it may be possible to make the solar cell industry market competitive again by transferring this new technology and other technologies derived from it to solar cell manufacturers in Japan.

Advanced silicon solar cells

MIT mechanical engineers are leading an effort to detect defects that reduce efficiency. As the world transitions to a low-carbon energy future, near-term, largescale deployment of solar power will be critical to mitigating climate change by midcentury. Climate scientists estimate that the world will need 10 terawatts (TW) or more of solar power by 2030 — at least 50 times the level deployed today. At the MIT Photovoltaics Research Laboratory (PVLab), teams are working both to define what’s needed to get there and to help make it happen. “Our job is to figure out how to reach a minimum of 10 TW in an economically and environmentally sustainable way through technology innovation,” says Tonio Buonassisi, associate professor of mechanical engineering and lab director.

Their analyses outline a daunting challenge. First they calculated the growth rate of solar required to achieve 10 TW by 2030 and the minimum sustainable price that would elicit that growth without help from subsidies. Current technology is clearly not up to the task. “It would take between $1 trillion and $4 trillion of additional debt to just push current technology into the marketplace to do the job, and that’d be hard,” says Buonassisi. So what needs to change?

Using models that combine technological and economic variables, the researchers determined that three changes are required: reduce the cost of modules by 50 percent, increase the conversion efficiency of modules (the fraction of solar energy they convert into electricity) by 50 percent, and decrease the cost of building new factories by 70 percent. Getting all of that to happen quickly enough — within five years — will require near-term policies to incentivize deployment plus a major push on technological innovation to reduce costs so that government support can decrease over time.

Making strides on efficiency

Making strides on efficiency

Major gains are already being made on the conversion efficiency front — both at the MIT PVLab and around the world. One especially promising technology is the passivated emitter and rear cell (PERC), which is based on low-cost crystalline silicon but has a special “architecture” that captures more of the sun’s energy than conventional silicon cells do. While costs must be brought down, the technology promises to bring a 7 percent increase in efficiency, and many experts predict its widespread adoption.

But there’s been a problem. In field tests, some modules containing PERC solar cells have degraded in the sun, with conversion efficiency dropping by fully 10 percent in the first three months. “These modules are supposed to last 25 years, and within just weeks to months they’re generating only 90 percent as much electricity as they’re designed for,” says Ashley Morishige, postdoc in mechanical engineering. That behavior is perplexing because manufacturers thoroughly test the efficiency of their products before releasing them. In addition, not all modules exhibit the problem, and not all companies encounter it. Interestingly, it took up to a few years before individual companies realized that other companies were having the same problem. Manufacturers came up with a variety of engineering solutions to deal with it, but its exact cause remained unknown, prompting concern that it could recur at any time and could affect next-generation cell architectures.

To Buonassisi, it seemed like an opportunity. His lab generally focuses on basic materials problems at the wafer and cell level, but the researchers could equally well apply their equipment and expertise to modules and systems. By defining the problem, they could support the adoption of this energyefficient technology, helping to bring down materials and labor costs for each watt of power generated.

Working closely with an industrial solar cell manufacturer, the MIT team undertook a “root-cause analysis” to define the source of the problem. The company had come to Figure 1 them for help with the unexpected degrad-ation of their PERC modules and reported some odd trends. PERC modules stored in sunlight for 60 days with their wires connected into a closed loop lost no more efficiency than conventional solar cells typically do during their break-in period. But modules stored in sunlight with open circuits degraded significantly more. In addition, modules made from different silicon ingots displayed different powerloss behavior. And, as shown in Figure 1, the drop in efficiency was markedly higher in modules made with cells that had been fabricated at a peak temperature of 960 degrees Celsius than in those containing cells fired at 860 C.

Subatomic Misbehavior

Understanding how defects can affect conversion efficiency requires understanding how solar cells work at a fundamental level. Within a photoreactive material such as silicon, electrons exist at two distinct energy levels. At the lower level, they’re in the “valence band” and can’t flow; at the higher level, they’re in the “conduction band” and are free to move. When solar radiation shines onto the material, electrons can absorb enough energy to jump from the valance band to the conduction band, leaving behind vacancies called holes. If all is well, before the electrons lose that extra energy and drop back to the valence band, they travel through an external circuit as electric current. Generally, an electron or hole has to gain or lose a set amount of energy to move from one band to the other. (Although holes are defined as the absence of electrons, physicists view both electrons and holes as “moving” within semiconductors.) But sometimes a metal impurity or a structural flaw in the silicon provides an energy “state” between the valence and conduction bands, enabling electrons and holes to jump to that intermediate energy level — a move achieved with less energy gain or loss. If an electron and hole both make the move, they can recombine, and the electron is no longer available to pass through the external circuit. Power output is lost.

The PVLab researchers quantify that behavior using a measure called lifetime — the average time an electron remains in an excited state before it recombines with a hole. Lifetime critically affects the energy conversion efficiency of a solar cell, and it is “exquisitely sensitive to the presence of defects,” says Buonassisi.

To measure lifetime, the team — led by Morishige and mechanical engineering graduate student Mallory Jensen — uses a technique called lifetime spectroscopy. It involves shining light on a sample or heating it up and monitoring electrical conductivity during and immediately afterward. When current flow goes up, electrons excited by the added energy have jumped into the conduction band. When current drops, they’ve lost that extra energy and fallen back into the valence band. Changes in conductivity over time thus indicate the average lifetime of electrons in the sample.

Locating and characterizing the defect

Locating and characterizing the defect

To address the performance problems with PERC solar cells, the researchers first needed to figure out where in the modules the primary defects were located. Possibilities included the silicon surface, the aluminum backing, and various interfaces between materials. But the MIT team thought it was likely to be in the bulk silicon itself.

To test that assumption, they used partially fabricated solar cells that had been fired at 750 C or at 950 C and — in each category — one that had been exposed to Future Of Manufacturing Industry light and one that had been kept in the dark. They chemically removed the top and bottom layers from each cell, leaving only the bare silicon wafer. They then measured the electron lifetime of all the samples. As shown in Figure 2, with the low-temperature pair, lifetime is about the same in the lightexposed and unexposed samples. But with the high-temperature pair, lifetime in the exposed sample is significantly lower than that in the unexposed sample.

Those findings confirm that the observed degradation is largely attributable to defects that are present in the bulk silicon and — when exposed to light — affect lifetime, thus conversion efficiency, in cells that have been fired at higher temperatures. In follow-up tests, the researchers found that by reheating the degraded samples at 200 C for just an hour, they could bring the lifetime back up — but it dropped back down with re-exposure to light.

So how do those defects interfere with conversion efficiency, and what types of contaminants might be involved in their formation? Two characteristics of the defects would help the researchers answer those questions. First is the energy level of the defect — where it falls between the valence and conduction bands. Second is the “capture cross section,” that is, the area over which a defect at a particular location can capture electrons and holes. (The area might be different for electrons than for holes.)

While those characteristics can’t easily be measured directly in the samples, the researchers could use a standard set of equations to infer them based on lifetime measurements taken at different illumination intensities and test temperatures. Using samples that had been fired at 950 C and then exposed to light, they ran lifetime spectroscopy experiments under varying test conditions. With the gathered data, they calculated the energy level and capture cross section of the primary defect causing recombination in their samples. They then consulted the literature to see what elements are known to exhibit those characteristics, making them likely candidates for causing the drop in conversion efficiency observed in their samples.

According to Morishige, the team has narrowed down the list of candidates to a handful of possibilities. “And at least one of them is consistent with much of what we’ve observed,” she says. In this case, a metal contaminant creates defects in the crystal lattice of the silicon during fabrication. Hydrogen atoms that are present combine with those metal atoms, making them electrically neutral so they don’t serve as sites for electron-hole recombination. But under some conditions — notably, when the density of electrons is high — the hydrogen atoms dissociate from the metal, and the defects become very recombination-active.

Based on that possible mechanism, the That explanation fits with the company’s initial reports on their modules. Cells fired at higher temperatures would be more susceptible to light-induced damage because the silicon in them typically contains more impurities and less hydrogen. And performance would vary from ingot to ingot because different batches of silicon contain different concentrations of contaminants as well as hydrogen. Finally, baking the silicon at 200 C — as the researchers did — could cause the hydrogen atoms to recombine with the metal, neutralizing the defects.

Unintended consequences Based on that possible mechanism, the researchers offer manufacturers two recommendations. First, try to adjust their manufacturing processes so that they can perform the firing step at a lower temperature. And second, make sure that their silicon has sufficiently low concentrations of certain metals that the researchers have pinpointed as likely sources of the problem.

Unintended consequences

The bottom line, observes Buonassisi, is that the very feature that makes the PERC technology efficient — the special architecture designed to capture solar energy efficiently — is what reveals a problem inherent in the fabricated material. “The cell people did everything right,” he says. “It’s the quintessential law of unintended consequences.” And if the problem is the higher density of excited electrons interacting with defects in the silicon wafer, then developing effective strategies for dealing with it will only get more important because next-generation device designs and decreasing wafer thicknesses will bring even higher electron densities.

To Buonassisi, this work demonstrates the importance of talking across boundaries. He advocates communication among all participants in the solar community — both private companies and research organizations — as well as collaboration among experts in every area — from feedstock materials to wafers, cells, and modules to system integration and module installation. “Our laboratory is taking active steps to bring together a community of stakeholders and create a vertically integrated R&D platform that I hope will enable us to more quickly address the technical challenges and help lead to 10 TW of PV by 2030,” he says.

Future Of Manufacturing Industry

Future Of Manufacturing Industry

With a massive solar power installed capacity target of 100 GW by 2022, the Indian Government is promoting and expanding local manufacturing capacity for polysilicon ingots and wafers. Manufacturing has emerged as one of the high growth sectors in India after the Prime Minister Modi’s launch of ‘Make in India’ program. The program was launched to place India on the world map as a manufacturing hub and give global recognition to the Indian economy. However, there is no facility in the country that manufactures polysilicon ingots and wafers.

India’s ranking among the world’s 10 largest manufacturing countries has improved by three places to sixth position in 2015.

The Government of India has set an ambitious target of increasing the contribution of manufacturing output to 25 per cent of Gross Domestic Product (GDP) by 2025, from 16 per cent currently.

According to IBEF India’s manufacturing sector has the potential to touch US$ 1 trillion by 2025. There is potential for the sector to account for 25-30 per cent of the country’s GDP and create up to 90 million domestic jobs by 2025. Business conditions in the Indian manufacturing sector continue to remain positive.

In September 2016, Foreign Direct Investment (FDI) in electronic manufacturing has reached an all-time high of Rs 123,000 crore (US$ 18.36 billion) in 2016, from Rs 11,000 crore (US$ 1.65 billion) in 2014; on the back of enabling policies of the government and its Make in India initiative. Undoubtedly, India has become one of the most attractive destinations for investments in the manufacturing sector.

The Government of India has an ambitious plan to locally manufacture as many as 181 products. The move could help infrastructure sectors such as power, oil and gas, and automobile manufacturing that require large capital expenditure and revive the Rs 1,85,000 crore (US$ 27.42 billion) Indian capital goods business. The ingot, solar cells and panel manufacturers will befit from government’s Make in India program and may witness growth in the output.

Lanco Solar is setting up a fully-integrated PV panels manufacturing project for manufacture of high-quality polysilicon, silicon ingots/ wafers and modules in a 250 acre SEZ at Chhattisgarh, India. The state-ofthe-art plant boasts of many ‘firsts’ in India – crack-free modules, being one among them. The project, which is being built with latest technology and engineered by leading global players, has a production capacity of 1800 TPA of Polysilicon, 300MWp/ year of ingots and wafers, high efficiency solar cell panels and modules. The first phase of this project is being implemented with a total cost of US$ 300 million.

Solar tariffs are expected to see a rise of 10 percent once the Goods and Services Tax (GST) is rolled out, according to a study by the Council on Energy, Environment and Water (CEEW). The Council also found that the GST will give a boost to the government’s “Make in India” initiative, improving competitiveness of Indian manufacturers of solar cells, panels and modules, eliminate the cascading effect of the existing tax structure and introduce an input tax credit. Increased competitiveness of domestic solar manufacturers could create an additional 37,000 new jobs in the solar manufacturing sector by 2022, according to the report. The government is trying to promote domestic manufacturing with a 20-25 percent capital subsidy and incentives such as interest free loans and tax breaks says Mercom capital in its report. In addition to focusing on solar manufacturing through this program, the government is also considering subsidizing solar manufacturing through Viability Gap Funding (VGF).

The Goods and Services Tax (GST) is further expected to help solar manufacturers once it is ratified by all states. “Currently, manufacturers have to pay a countervailing duty of 12 percent and 5 percent Value Added Tax. The GST will do away with these and help indigenous manufacturers compete globally,” said official at Ministry of Commerce to the research firm .

Challenges

According to Mercom Capital competing with low-cost imports, low profit margins and lack of scale are all hurting India’s solar manufacturing sector.

“The major problems plaguing the sector are a lack of scale, insufficient government support and an underdeveloped supply chain,” said an official at Waaree Energies to the research firm.

Access to financing is also a challenge. Manufacturers are citing lack of funding available to build manufacturing units. “Even if private banks are willing to lend, it is at exorbitant rates ranging from 16 to 17 percent,” stated Mr. Thakur of Shukra Solar, a solar manufacturer.

Financing challenges struck a chord with another solar manufacturer: “You won’t find banks financing manufacturing units because it is considered risky.” Any financing that has happened so far is because of foreign direct investment, voiced another manufacturer.

Most manufacturers agree that the DCR ruling by the WTO has hurt the indigenous manufacturing sector. “The government can still make DCR a pre-requisite for government tenders for projects installed on government land or buildings,” stated another manufacturer.

Manufacturers would also like to see more investment in research and development to support new innovations that can bring down costs over the long run. “The investment in research in the sector is almost negligent compared to other countries like China,” commented another manufacturer.

The Ministry of New and Renewable Energy (MNRE) called a meeting with manufacturers that have a capacity of 500 MW or more to discuss these issues in June last year.

polysilicon

The MNRE asked the manufacturers to build polysilicon manufacturing facilities of about 500 MW each, either in partnership with a foreign company or joining forces with Indian companies like Waaree, Vikram and Goldi Green. In return, these companies would get independent power producer rights to develop a 1,500 MW solar project at a fixed tariff by MNRE. “While the offer was made, we have not heard back from MNRE on this topic,” said a source at Vikram Solar to Mercom Capital

Manufacturers were hoping for some kind of subsidy or incentive from the government to scale up production but were disappointed that the current budget did not provide any. Manufacturers also want more clarity around state-sponsored incentives so they can determine which states are better and more profitable for building manufacturing units.

Budget Impact

The basic customs duty on solar tempered glass and components used for solar cell, panel, module manufacturing has been reduced by 5-6%. However , according to Industry experts this will have very marginal impact on the cost of solar panels as it is a low value component and further a lot of the solar tempered glass used by domestic manufacturers is locally procured.

Driver of overall PV costs

Most solar companies today manufacture solar panels using large portions of silicon, called ingots, and cut it into small rectangular shapes. These silicon components account for approximately 40% of the cost of production for solar panels. While some companies have been finding ways to manufacture panels for cheaper using the same materials, the expectations are now somewhat different. According to a report by Fortune, “today as the industry matures, much more of the expected lowered production costs will come from new components that plug into traditional silicon solar panels, new ways to manage the electrons from panels, or new ways to finance and sell the panels.” In addition, some innovative companies are coming up with entire new techniques to salvage the sun’s energy. Around 85% of polysilicon being produced today can be used for mono ingot pulling and wafer production, according to industry it is less of an issue than some reason. Solar industry often works under assumption that mono ingot pulling is much more expensive than it is for multicrystalline, but this is changing said Chinese monocrystalline ingot and wafer producer LONGi

Years ago mono ingot costs were high, the non-silicon costs for mono ingot pulling was as around $30/kg, today it is below $10/ kg. LONGi predicts that in 2018 it will be around $5/kg . The company also forecasts that sawing costs, for wafer production in 2018 will be less than $0.15/piece than what it is today.

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