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Copper in the power-generation world is typically thought of as a conduit for carrying electricity from one place to another. It is in power plant components, such as heat exchangers. Current research is attempting to make copper valuable in photovoltaic systems.

In June, the New York State Energy Research and Development Authority (NYSERDA) awarded $887,000 to Intrinsiq Materials Inc., a developer of nanotechnology printable electronic ink, to help the company research and commercialize a method of using copper in solar power cells.

Intrinsiq Materials - with facilities in Farnborough, U.K., and Rochester, N.Y. - is developing nanoparticle-based copper ink formulations for printed electronics applications, including photovoltaic modules. The company’s copper inkjet ink and copper screen print paste are formulations designed for photonic curing at room temperature in air, by laser or broadband flash techniques. Intrinsiq claims the conductivities of its copper-based materials are comparable to commercial silver inks on a range of substrates. The company has additional inks under development, including nanoparticle-based nickel and silicon.

The company grew out of a research effort of the U.K. Ministry of Defense to develop printable copper ink for circuit boards and other applications. Intrinsiq has established an independent headquarters in Rochester to focus on opportunities to develop the technology for semiconductor and PV industries in the U.S., among other applications.

The U.S. company currently has eight employees but expects to create up to 25 new positions as a result of this research.

Under the terms of the NYSERDA award, Intrinsiq will repay the grant if the product is successful. Intrinsiq is also matching the funding amount with help from private investors. In September, Intrinsiq Materials closed $4.1 million in funding from the Cayuga Venture Fund to continue its research, development and commercialization of technology for manufacturing conductive nanoparticle inks.

Rochester is also getting the Photovoltaic Manufacturing and Technology Development Facility at the State University of New York’s College of Nanoscale Science and Engineering with the assistance of state and federal funds. The goal is to develop the hub of a PV manufacturing industry in New York in which copper-based metallization could play an important, differentiating role.

 

Finding an alternative to silver

The most common conductive material used in the manufacture of solar cells is silver, which is about 70 times more expensive than copper and much less abundant. According to the NYSERDA, Intrinsiq’s technology has the potential to significantly reduce the cost of PV manufacturing, lowering the costs of solar power for consumers.

“When we started talking to various solar companies at industry conferences, it seemed that most people were using silver or silver paste, which is a very expensive solution,” says Sujatha Ramanujan, vice president and chief operating officer of Intrinsiq Materials. “Copper is very conductive and is much more affordable. So we looked at what has forced people to use silver, and we realized that the primary issue - if you look at the PV industry - is that copper is a tough solution. In a standard formulation, the copper tends to oxidize, which reduces its conductivity. The novelty of our ink is that it is highly conductive and can be deposited. In that process, we take away the greatest risk to somebody trying PV metallization.”

Deposition is not a new process for PV manufacturing. According to Lux Research, deposition tools are capable of reducing silver usage by up to 30% - relative to conventional screen printing - and improving absolute cell efficiencies by 0.3-0.5%. Nevertheless, over the past decade, silver prices have risen six-fold, motivating PV manufacturers to try to reduce the amount they use. Lux Research says copper is the leading contender to displace silver due to its significantly lower cost and widespread use in the manufacture of semiconductors for the electronics industry. However, for performance and durability reasons, limitations in formulations based on copper are not yet ready for prime time.

 

Creating a formulation

Intrinsiq’s technique uses a form of printable copper that is air-stable and highly conductive and doesn’t need to be baked in a traditional oven. Intrinsiq’s material design process creates coated particles customized to control factors such as substrate compatibility, particle chemistry and protection from corrosion by external substances. A proprietary coating protects pure nanometal particles from atmospheric corrosion. The company says coating enables effective particle dispersion in inks, making printing a relatively straightforward process. Screen pastes are mixtures of micron and nanocopper. Binders are optimized for rheology and adhesion.

Inkjet ink and screen paste can be effectively printed on a variety of substrates. Photonic curing can give resistivity values close to five times that of bulk copper for inkjet. Laser curing of screen paste gives four times the resistivity of bulk copper - although the company admits that the broadband curing performance needs to be improved. The company reports achieving such results at its Farnborough lab in the U.K. by laser sintering screen-printed images on glass.

The resistivity of bulk copper is calculated as follows:

• Bars are printed on glass and sintered
• Contact profilometry is used to determine width and height of sintered bars
• Resistivity = Resistance × Cross-sectional area / Length
• Bulk copper resistivity = 1.68 x 10-6 Ω-cm
• Bulk copper factor = Resistivity / Bulk copper resistivity

The resistance performance of Intrinsiq’s copper-based ink is calculated as follows:

Inkjet calculation

• Dimensions: 1050 μ wide x 1.25 μ high x 95 mm long
• Resistance on PEL inkjet paper = 10 Ω
• Resistivity = 1.38 x 10-5 Ω-cm
• Bulk copper factor = 8.2 times bulk copper resistivity

Screen print calculation

• Dimensions: 958 μ wide x 34.2 μ high x 48 mm long
• Resistance on HP Brochure paper = 0.8 Ω
• Resistivity = 5.46 x 10-5 Ω-cm
• Bulk copper factor = 32.5 times bulk copper resistivity

 

Putting copper to work

Ramanujan says Intrinsiq’s copper ink and paste formulation, deposition, and curing process is well-developed, having been used for other applications. The focus of most of the company’s current research and development work is to integrate it into existing PV manufacturing processes.

“The trick here is to make the ink work in the process that the PV cell manufacturer has already adopted,” she says. “You don’t want to flip the whole process; you want to integrate into it. We want to make an ink and a process that is compatible.”

The company is currently working with a number of unnamed PV manufacturers to integrate its copper formulation and deposition system into existing lines. Ramanujan says the NYSERDA grant allows Intrinsiq to work with additional clients in the PV industry and that the company is looking for additional industry partners.

Ramanujan says that in terms of process integration, a deposition and curing process for metallization has the added advantage of being much easier to manage from the standpoint of factory environmental management.

The more typical subtractive process commonly employed in the PV industry uses large amounts of toxic chemicals that have to be stored, applied and then disposed of. In a subtractive process, the manufacturer coats the entire surface of a substrate with metal and then chemically removes the unwanted portions.

“Those are very harsh chemicals, and they are very damaging to our environment,” she says. “It is also very wasteful. We avoid chemical etching through deposition. We try put the material where you need it in the amount that you need it.” S