In the photovoltaics sector, the use of silver is predominant in the production of metallization pastes. Within a solar cell, there are approximately 250 milligrams of silver present. Used as a key element in the formation of contact grids, PV manufacturers consume approximately 11% of the worlds silver supply according to Bloomberg. The price fluctuation of silver as a result has a direct influence on production costs. As Andy London from Heraeus Photovoltaics states, silver is a significant cost component of metallization pastes, representing about 80% of the pastes price. With the solar installation boom in the last few years, the amount of silver needed by the industry has also increased steadily. The prices of PV panels have been drastically falling but with silver as one of the components in thick film PV, these panels tend to be prone to price fluctuations that affect silver too.
Casey Research states that since the year 2000, silver used in solar panel production has increased by an average of 50% per year. In 2011, the demand from the PV sector accounted for 11% of total industrial demand for silver (excluding jewelry) as the consultancy states. Silver prices are expected to remain bullish in the months to come with prices already having gone up in the beginning of this year. The unpredictable nature of silver prices has however led to the search for alternatives. Lux Research analyst Fatima Toor tells pv magazine that the cost of silver pastes are going to influence the development of new technologies and it is good to have numerous options. Copper, nickel and tin have been making ripples in the PV crystalline scene as potential replacements. And manufacturers have introduced several production tools that have made headlines with regard to their elimination or decreased use of silver.
Alternatives to silver
Toor brings attention to Meyer Burger Technologys group member Roth & Rau AG developing a new tool which uses nickel in busbar metallization for electrical contacting of solar cells. The new system, named Helia, does not completely eliminate silver but promises that silver savings between 50% and 70% can be achieved, thereby reducing overall production costs of solar cells. The Helia system coats the solar cells with nickel in a shortened system configuration to form the front and rear busbars via a sputtering process.
Copper has also been touted as a material that holds great potential both cost-wise and from a sustainability standpoint. Lux Research in its report last year, Key Issues and Innovations in Photovoltaic Metallization stated that innovative new technologies, including copper metallization, nickel phosphide and non-contact printing techniques, which can replace silver, could reduce costs by up to 50%. However with the replacement with copper the PV panel production steps get more complex. The standard metallization and co-firing steps cannot be utilized with copper. Hence extra process steps may be required. The decision to invest in new research and development and eventually new equipment to accommodate copper as the substitute material may involve heavy investment costs. Still the manufacturer might save tremendously after such investments are made with the lower price of the metal copper itself.
Research work at imec indicates that copper can have positive impacts on cell efficiencies. In 2011, imec achieved a 19.4% conversion efficiency on large area Cz-Si PERC type solar cells with copper-based front metallization. Last year, imec stated that it developed a new process for creating front contacts on silicon solar cells by single-sequence plating of nickel, copper and silver followed by thermal annealing. Imecs new plating process is also said to minimize the use of silver in comparison to standard screen-printed metallization with silver paste thereby leading to an improved aspect ratio. Shadow losses are thus reduced and conversion efficiency is increased. Jef Poortmans, Director Photovoltaics Research at imec stated last year that this metallization process is an optimization to imecs PERC process, optimizing the cost-of-ownership and conversion efficiency of cells through structure and materials optimizations. Additionally he predicted, The next steps in our R&D should boost efficiencies of PERC solar cells to well over 21%. Another material, tin, really came on to the crystalline PV scene with Schmid and Schott Solar winning the Intersolar Award 2012 for the TinPad. The TinPad system was recognized by the jury in Munich for its ability to apply tin contacts to six inch solar cells. The Schott-patented technology allows the production of completely silver-free backside contacts. What the TinPad does is deposit tin busbars instead of silver on the backside of the solar cell. The system developed by the two companies can be integrated into an already existing cell production line after firing or sorting or into a module production line before the stringing process. This is an important point to note as extra costs may be incurred should standard metallization and co-firing steps not be suitable.
Team Leader of the Business Unit Cell of the Schmid Group Martin Magrian explains, We provide the possibility to integrate into a cell or module line. We can install it after the firing, as tin has a melting point of about 230 degrees and the firing process reaches 800-900 degrees, according to customer profiles. We can also install it before the stringer on a module line. With an old line, the customer can shut off one of the printing steps: the busbar printing step. So after the firing process, the next step would be the movement into the TinPad, and then into the cell soldering step.
An element with a lower resistivity will allow more movement of electric charge, something vital in PV. Fatima Toor explains that if contact resistance is high, then the fill factor will be affected. The fill factor (FF) is a parameter, which in conjunction with open circuit voltage (Voc) and short circuit current (Isc), determines the maximum power of a solar cell. The fill factor decreases with higher contact resistance. And amongst all metals known to humankind, silver actually has the lowest resistivity, closely followed by the cheaper and more abundant copper. Toor adds that many other substitute materials have had difficulty competing against silver because of this resistivity advantage.
Silver has a resistivity of 1.59×10?8 ?m (ohm meters) at 20°C. At a similar temperature, copper has a resistivity of 1.68×10?8 ?m. Tin, interestingly, has a resistivity of 1.09×10?7 ?m and Toor sees this factor as something that equipment manufacturers who look to replace silver need to take into consideration.
The topic of adhesion strength has been garnering increasing attention as module manufacturers want their 25 year warranties to stand. Paste adhesion is a significant factor in determining lifetime of cells. Many silver paste manufacturers have also been working on adhesion improvements to give the good old silver use a boost.
DuPont Microcircuit Materials introduced their Solamet PV51G last year which claims to improve adhesion when used in conjunction with backside aluminum compositions. With silver paste manufacturers working hard to reduce materials usage and improve components like adhesion, newer materials like tin have to be on par.
It has been difficult to substitute silver with tin due to the insufficient adhesion problems with aluminum. Schmids Magrian tells us how this issue was rectified with the TinPad, The interface between aluminum and tin is ultrasonically done. We use heat as well. The heat is needed to melt the tin and with ultrasonic we generate the space between the aluminum for the tin to enter. We get a 10-15 microns interface between aluminum and tin. The main discussion with customers right now, as Magrian highlights, is the pull-off force which is mainly used to see if the connections between string and cell is strong enough. It is not the connection between the aluminum and tin anymore. It is not the pull-off force caused by the aluminum. Before, we had the silver completely on the silicon, but the tin is now on the aluminum so there is no connection to the silicon, he elaborates.
Schmid claims that it is now possible to produce completely silver-free backside contacts with a peel-off strength or pull-off force which is twice as high as required.
The researchers at imec also believe in their metallization mainly with copper to be on par. Adherence of the plated contacts is apparently good, as imec announced last year, with solder tab adhesion pull tests showing a pull strength in excess of 2 Newtons.
Cost-wise, being completely silver-free results in savings of about US$0.06 per wafer as Schmid claims with regards to its TinPad. Magrian explains that there has been a standard in the last one to two years of about 90 mg of silver and manufacturers are even reaching 40 to 50 mg of silver with newer printing options.
The Helia system with its 50-70% silver savings with the use of nickel can also lead to massive cost reductions in terms of materials price. Overall it is clear that with the reduction of silver and the introduction of other metals like nickel, copper and tin, materials costs can be reduced.
Fatima Toor believes that silver-free solutions will be introduced in crystalline technologies more than thin film and this can possibly be a trend that takes off. Whether tin will be the leading replacement material however remains to be seen.
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