By D.A. Barber
Due to expansion in past 2 years of capacity, the PV market has changed dramatically and competition as far as technology and cost has become the main focus within the solar industry. This is a big challenge for the entire industry, particularly those involved in crystalline silicon (c-Si) PV manufacturing.
At the 6th PV Fab Managers Forum March 25-27, 2012, in Berlin, the third edition of the International Technology Roadmap for Photovoltaics (ITRPV) was presented designed to help make PVs cheaper and more efficient.
The Roadmap was originally created by European c-Si PV manufacturers in September 2009 and published the first roadmap document in March 2010 at an earlier PV Fab Managers Forum in Berlin. Since then, the SEMI PV Group has released a yearly update to inform PV suppliers and customers about technology trends in all aspects of crystalline silicon photovoltaics and sets a baseline for dialog on needed improvements. The ITRPV has now grown to include the entire supply-chain in identifying technology barriers for advancement specifically of c-Si solar cell manufacturing worldwide from crystallization, wafer production and PV cell production to PV module manufacturing. The current edition was jointly prepared by major PV manufacturers, module producers, and wafer suppliers as well as experts from various research institutes and equipment suppliers.
Production Processes Trends
According to the Roadmap, the “specific cost per Watt peak (WP) of PV modules will continue to decrease,” which corresponds with the significant reduction in cost of PV cells. This cost reduction will happen because “current mainstream technology will be optimized, new production technologies will be rolled out, and not yet known techniques have to be implemented in production around 2015.” But in the meantime, new technologies will need to be implemented particularly in production to enable this future growth and significant cost reduction.
“To reduce production costs, new technologies, materials and highly productive equipment including Statistical Process Control (SPC) are needed. By providing information on important key figures of the production, as well as details about the process to increase the cell efficiency and finally the power output of the modules, the roadmap will be the guideline to support these developments,” states the ITRPV.
Chart showing the relative investment per MWp for a c-Si cell production line.
The Roadmap notes that all along the c-Si manufacturing chain there will need to be a combination of more effective use of materials, more efficient use of manufacturing equipment and advancements in the c-Si process. One example given is the actual improvements in the Si-cast technology to enable crystallization of ingots with large fractions of mono-crystalline domains, or quasi-mono material. One way of increasing the crystallization process is by changing the common formats of the ingots through an increase in ingot mass for multi-crystalline silicon (mc-Si) and for mono-crystalline silicon (mono-Si).
Other trends noted that can contribute to lower costs per cell is a move to thinner wafers, improved wafer sawing methods, better material handling during the cell process and further development of more efficient interconnect technologies.
According to the ITRPV,material loss during cell production is expected to decrease to below 1 percent by 2020 as thinner wafers are introduced.
A significant improvement in reducing manufacturing costs will be during the wafering process as wafers continue to get thinner. This is expected to happen with the coming introduction of diamond wire sawing, a technology that will require “synchronization with the cell process development.”
Focus on Metallization Pastes
The ITRPValso took aim at the technology trends in the metallization process to enable finer line printing, a trend expected to be available by 2015. Part of this trend is the replacement of silver printing by copper plating which is expected to take place in order to reduce the rising costs of using silver. Other than the wafers, such metallization pastes and inks are currently the most expensive materials in cell production. The industry is also being encouraged to replace pastes and inks containing lead with lead-free products. The problem is that this can result in loss in performance of the PVs. But the ITRPV notes that production of lead-free PVs and modules also have a value in the market place among consumers who have environmental concerns.
But before any alternative metallization techniques can be introduced, the technical barriers pertaining to conductivity and adhesion as well as development of the appropriate processing equipment has to be solved, according to the ITRPV.
The International Technology Roadmap for Photovoltaics does not make any detailed recommendations for technical solutions, but rather it is a document designed to motivate the PV industry to come up with their own comprehensive answers to the identified areas that are open to improvement. But it is clear that the document favors, as it has in past years, crystalline silicon over other technologies as the dominant type of PVs during the near future. Keep in mind the Roadmap is published by SEMI’s PV Group in conjunction with the Crystalline Silicon PV Technology and Manufacturing Group. The difference is that this year’s Roadmap is specifically focusing on crystalline silicon materials and processes that are expected to come online by 2015.
The technological challenges the Roadmap believes the global PV makers face in the next few years include finding ways to continue to reduce the costs per watt and how to make the entire manufacturing process more efficient. In fact, discussed the most is the current state of the c-Si processes involving casting technology and the need for ingots that contain larger fractions of mono-crystalline that can be fabricated or “casted” in existing furnaces. This material has been dubbed quasi-mono, meaning that its emergence into the mainstream could bring a third c-Si material into the mix. The report warns that any development of alternative technology development for production needs to happen without sacrificing the quality of the silicon.
Besides such silicon improvements, the economy of scale was another crucial focus of the Roadmap, which the report noted doing this alone will not be sufficient to reduce costs to compete in the global PV market. But as for PV module production, the Roadmap did emphasize cost reduction over time will be achieved by using copper-plating to replace silver pastes. Unlike the environmental reasons for going lead-free in PV manufacturing, replacing silver is cited due to its volatile price changes in recent years, a price change that is expected to continue to trend upward.
Overall, the Roadmap makes a point of noting that it will be a combination of increased energy conversion efficiency and considerably reduced costs of manufacturing that are the key to the PV industry to reach the right cost balance to be viable. This will be done through standardized, coordinated efforts across the entire production chain to keep everyone in the loop with the latest technology trends.
The concept of this Roadmap has taken some criticism in the past for being strictly a Eurocentric group of mainly German-based PV makers concerned about competition in the face of low-cost Chinese c-Si PV cells and modules without much international participation from Asian companies in compiling the report. But that changed this year when the International Technology Roadmap for Photovoltaics was simultaneously released at SEMICON China 2012 in Shanghai on March 20, 2012 with an entire morning of presentations and discussion groups.