Jamie Novak, Applied Nanotech, Inc.
January 27, 2012 | 6 Comments
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.
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 email@example.com
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