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Think Ink for Increased Efficiency

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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Reducing the cost of solar

The use of non-contact printing methods can demonstrate reduced manufacturing costs. It has been generically discussed in previous articles, but let’s put some details to the idea. Screen printing is most commonly a serial process – one wafer at a time. In comparison, inkjet printing can be serial. Large-scale print heads can print a wafer in a single pass creating the same throughput as a screen printer. The display industry is already using inkjet heads with greater than 1m-wide print heads. Now the possibility exists, today, to print six 156mm solar cells in a single pass. Just by increasing the throughput using a parallel process you have cut the price of the metallization print step up to 18% of its original cost.

It is not just the print process that saves manufacturing cost. Nanoparticle inks can be processed at lower temperatures, reducing the tendency for over-firing or p-n junction damage such as internal, diffusion-based shunts.

Collectively, these changes in processes translate into dramatic material cost savings close to 44%, or overall solar cell cost savings potentially exceeding 27%. These numbers do not account for reduced energy consumption during manufacturing that can increase the cost savings nor the technological advancements that translate into higher cell performance.

We have developed metallic nanoparticle-based non-contact print compatible inks for the metallization of solar cells that offer several key technological advantages compared with currently available materials. The inks are targeted at next-generation solar cell manufacturing and design. Such non-contact print-compatible inks enable the industry to transition to thinner silicon wafers and promise higher conductivity, improved metal-silicon contact resistivity and overall improved cell efficiency. The inks can offer ease of use, reduced manufacturing cost and higher cell efficiency that targets both the cost and performance benefits to achieve grid parity. However, the widespread acceptance of non-contact printing will take additional time because of the lack of integrated and turn-key process equipment that can reliably shuttle thin silicon wafers and print at high speed. Such equipment does exist in other industries, yet has not infiltrated the solar manufacturing and PV industry.

Despite the delay in complementary manufacturing equipment reaching the fab line, there are other ways to take advantage of the cell enhancements that are enabled by nanotechnology. The inks can be reformulated into pastes. Pastes coming out of these high-conductivity inks show promise to utilize up to 30% less material when compared to competing screen-printed pastes, resulting in materials cost savings. The reduction in material coverage also benefits module assembly yield by demonstrating reduced wafer bowing.

Aluminum is entrenched in the silicon solar cell market, especially for back-side and p-type contacts. We are producing screen printable aluminum pastes that have similar properties as the non-contact printable ink formulations. This gives solar cell manufacturers the ability to try the material in paste form on their existing solar cell lines, before fully transitioning to a non-contact printable approach.

Similarly with silver, high-conductivity pastes derived from ink formulations can replace traditional silver pastes, which is currently the industry standard for front-side metal contacts. We have proprietary chemistry available that can burn through the ARC layer without glass frit. The removal of the glass frit increases finger line conductivity allowing higher cell performance.

Aluminum and silver aren’t the only metals being used. Aluminum is easily available for low cost and is positioned to remain as the metal of choice for the backside contact. However, silver is expensive and has experienced highly volatile market pricing, clearly driving the search for alterative metallization materials. Though not as popular, nickel and copper are now being used by companies looking at next-generation architectures of solar cells that could potentially make a significant impact in the future. Nickel can provide a low-resistance contact with silicon and can be an excellent barrier layer for copper, where copper’s high silicon diffusion constants threaten to shunt solar cells during the firing process. The ability to print nickel using, for example, inkjet, can provide narrow finger contacts compared with screen printing (shown to the right). When combined with specialized chemistry to burn-through the ARC coating, direct print nickel ink can offer significant cost savings compared to the laser etch followed by electroplating of the base contact. Copper plating, electroplated using an industrial light-induced plating (LIP) method allows for high-aspect ratio, high-conductivity contacts not possible with printed inks or pastes.

We have developed commercially available inks to enable next-generation wafers. The material list includes non-contact print compatible inks comprised of aluminum, nickel and copper providing the necessary solutions for incorporation of non-contact printing in the solar cell industry. Paste formulations are also available allowing advanced metallization schemes on existing process equipment.

Material availability and the ability to deliver them in quantity has long been one of the concerns echoed by CTOs and chief scientists in the industry. To address this need, the DOE recently funded Applied Nanotech’s pilot manufacturing facility in Austin Texas, which can produce 25 tons of inks and pastes per year. This is sufficient quantity to supply material for validation and pilot scale integration. Licensing of the inks for mass manufacturing to address the need for future product delivery has already begun.

Conclusion

There are many cost factors that influence the final price per watt for solar technologies. The cost of base materials and how much they’re required, processing technologies, and the time required to complete a manufacturing cycle are all important for companies looking to reach the magical $1/watt target. Nano-materials are having a significant impact in lowering the costs of solar cell production, making the timeline to solar grid parity shorter than could have been previously imagined. However, to fully realize industry goals, manufacturers must be fully aware of the possibilities of these new technologies, their potential cost savings and ease of introduction with current process lines.

Acknowledgment: Aerosol Jet is a registered trademark of Optomec

Jamie Novak received a PhD in chemistry from N.C. State U. studying the optical and electronic properties of nanomaterials and is a senior scientist at Applied Nanotech, Inc., 3006 Longhorn Blvd., Suite 107, Austin, TX 78758 USA; ph.: 512-339-5020; email jnovak@appliednanotech.net

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