Jamie Novak, Applied Nanotech, Inc.
January 27, 2012 | 6 Comments
One dollar per watt: that's the mark the solar industry is hoping to hit in order to become cost-competitive with conventional energies. In a bid to reach this magic number in the next two years, various Asian manufacturers are striving to reduce costs by increasing volume production. An alternative direction that will benefit the industry in the long term is to target increased efficiencies primarily through new manufacturing processes, new higher-quality materials for metallization and changing the structure of the cell. Nanotechnology companies have developed these new materials and processes that enable manufacturers to both increase quality and lower-cost production, bringing the era of solar grid parity closer than ever before.
Grid parity: Quantity vs. quality
China is already winning the race in the solar manufacturing capacity. China and the rest of the world’s producers are starting to fall into two camps in a bid to maintain supremacy.
The first camp assumes that costs come down only by increasing production rates. Quantity is the key to this approach by achieving economies of scale. Manufacturing capacity is good for the industry but there is no advancement of technology; this approach simply maintains solar cell manufacturing technology that has essentially been used for decades.
The second operates on the principle that costs will come down as solar cell efficiencies improve. In this case, quality coupled with technological innovation are the key drivers. These manufacturers are developing technologies that change the structure of the cell through the use of higher quality materials, stringent manufacturing processes and the use of advanced cell architectures.
As cell efficiencies increase, the cost per watt (and therefore per cell) decreases significantly. One company, for instance, is demonstrating a 23% cell efficiency through interdigitated backside contact cell architectures and superior, albeit expensive, manufacturing methods.
But the false dichotomy created between quantity and quality doesn’t have to exist. Not with new solar nanotechnology inks and pastes and the new manufacturing methods that take advantage of their unique properties. These new materials promise a convergence of reduced manufacturing cost with advanced technology solar cells and represents the future of the industry.
Solar cell breakage challenges
Currently, industry standard solar cell manufacturing processes use screen printing equipment that directly contacts the wafer, requiring the solar cell wafers to be thick enough to survive the pressure generated through this direct contact metallization process. This process can exert enough force to cause breakage of thin wafers, in turn increasing costs. If high-throughput metallization could be done without touching the wafer, it would allow for silicon solar cell wafers to be much thinner than they are today, which could reduce the overall cost of the module.
There are non-contact printing methods currently on the market, such as vacuum-based physical vapor deposition (PVD) coatings, however equipment can be extremely expensive to maintain and temperamental, making it an unviable long-term solution for low-cost production.
Non-contact print techniques like inkjet and aerosolized jet technologies provide a route to realize the opportunity for thin-silicon wafer technology in high-throughput production environments.
Benefits of thin silicon wafers
Wafer thickness is likely the largest factor that impacts solar cell costs; nano-inkjet offers several important advantages in this regard. As non-contact printing technologies allow for a thinner wafer, the modules use significantly less silicon than can be processed using conventional screen printing. Thin silicon wafers mean lower silicon materials costs. Even as silicon prices drop, silicon still accounts for roughly 50−60% of the overall cost of the solar cell, the largest cost in conventional solar cell production.
Market trends show that silicon wafer thickness could decrease from ~180µm to less than 100µm, through non-contact printing. Of course, the availability of ink materials is the key to making this transition to thinner wafers.
Higher conductivity with non-contact printing
Besides the reduced materials cost, what are the other benefits of using thin silicon wafers? Generally, it is accepted that thinner wafers can increase efficiency. Separated charges from the photovoltaic effect have less chance for recombination if their travel distance is reduced. This allows for more efficient extraction of electricity from the solar cell. However, there is more to the story of reduced cost than just wafer thickness.
Non-contact printing provides processing flexibility, allowing multiple metal layers of different materials to be printed simultaneously without having to dry or fire wafers between steps. This increases production efficiency, reducing manufacturing timelines and costs.
If you tried to print multiple layers simultaneously using today’s screen-printing methods, the printed patterns would be smeared or corrupted therefore requiring one metal to be patterned, dried, and in some cases fired, before the next metal can be printed.
By utilizing non-contact printing materials, wafers allow more sunlight to filter through because of the ability to print narrower lines. Screen printers are not able to achieve the fine resolution of inkjet and other non-contact printing technologies, which means nano-inkjet achieves higher sunlight capture levels (Fig. 1). The primary differences in resolution are related to the metallic particles used in the metallization materials. Nanoparticle-based inks can easily be deposited at high resolution using inkjet printing without limits of screen clogging from the micron-sized particles uses in metallization pastes.
Conventional printing methods also result in greater ink waste. For example, screen printing methods leave a lot of paste on the screens, squeegees, and other printer components. In addition, there is loss from dying and open air contact during the print process. However, because nano-inkjet and aerosol jet printing delivers “drop on demand” printing techniques, it places inks only where required and when required, drastically reducing waste. These non-contact inks are also contained within effectively sealed environments preventing solvent loss due to evaporation.
Because of the finite size of the non-contact print nozzles, large particles are precluded from passing. This eliminates the possibility of using glass frit in formulations, yet opens up exciting new ink chemistries that can also burn-through anti-reflective coatings (ARC). Properly formulated, non-contact nanoparticle-based metallic inks can burn-through nitride ARC coatings with improved contact resistivity compared to conventional frit-based Ag pastes.
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