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Low-Cost Bioplastics Emerge for Solar Cells

Low-Cost Bioplastics Emerge for Solar Cells

A small California company called BioSolar is introducing plastic components for photovoltaic (PV) cells based on cotton and castor oil that are described as less costly than plastics based on petroleum.

"Designed specifically for cost-sensitive, economical PV solar cell modules, we expect the BioBacksheet-C to be instrumental in driving down the cost per watt of solar power," says David Lee, chief executive officer of BioSolar. "An initial full production run was recently completed at one of our manufacturing partners and samples will be available for all interested PV module manufacturers shortly."

The substrate of BioBackSheet-C is a cellulosic plastic coated with a proprietary material to reduce the water vapor transmission rate and to increase abrasion resistance. The two-component system aimed at crystalline silicon photovoltaic applications is said to possess high dielectric strength.

BioSolar's products attack one of the conundrums of the emerging solar cell industry - they use millions of square feet of materials that are based on petroleum or require use of toxic chemicals to produce. BioSolar's approach is also unusual in the green world because it saves money - at least according to the company's business plan.

"Our goal is to sell any given configuration at a savings of 20-25 percent versus the prevailing material in the market," says Lee. "With our first product, we are achieving that goal."

Most biobased plastics target packaging markets and cost a premium of 30 to 500 percent above the materials they replace. In the solar cell market, materials are generally much more expensive, and in some cases, have been in short supply.

One material in BioSolar's gunsite is DuPont's Tedlar polyvinyl fluoride (PVF), a thermoplastic fluoropolymer that is structurally very similar to polyvinyl chloride. It has excellent vapor permeability and is very resistant to weathering, key requirements for the backsheet of a photovoltaic cell. Tedlar constructions often replace glass.

Costly Structures

Tedlar is typically used as the outside layer of a sheet that includes polyester to reduce costs and improve mechanical strength and dielectric properties. Price of the sheet is around $1.60 a square foot.

Tedlar is solvent-cast using an industrial solvent called dimethyl acetamide (DMAC), which can produce systemic injury when inhaled or absorbed through the skin in sufficient quantities over a prolonged period of time. If Tedlar is burned, corrosive hydrogen fluoride fumes can be released. Another important issue: Tedlar has been in very short supply for the past two years because of its growing use in photovoltaic cells and aircraft interiors.

In August, DuPont announced plans to boost Tedlar production by 50 percent.

"Our capacity expansions are critical steps in growing the Tedlar business and maintaining our market leadership in backsheets for solar panels that deliver the long-term, reliable power supply that our customers have come to expect from their investment in renewable power generation," says David B. Miller, group vice president - DuPont Electronic & Communication Technologies.

There are at least 10 companies that make backsheet materials for solar cells. Some of them are testing other biobased plastics such as polylactic acid. Stan Levy, a former DuPont official who is now the chief technical officer at BiSolar, says PLA falls short as a solar cell material. "It is designed as a biodegradable product, and is almost destroyed during the damp heat test (used to test materials)." In addition, extruded PLA film can be brittle. Levy spent 27 years at DuPont working on many premiere films, including Teflon, Mylar and Kapton.

In other markets, such as automotive, PLA is alloyed with a petroleum-based engineering plastic to boost durability. The sole advantage is reduction of the carbon footprint. Such a solution could also be introduced for the PV market.

Two Employees

A fascinating aspect of BioSolar is that the company has only two employees, yet is developing and introducing a broad slate of bio-based back sheets that offer different properties and economics.

The two bioplastics deployed by BioSolar are available from well-known suppliers. The castor oil-derived nylon 11 was developed in Europe in the 1940s and has been used for industrial applications for more than 50 years. BioSolar will initially buy nylon 11 from the primary supplier, Arkema, but other major companies are developing capacity as demand grows for the unique bioplastic. DuPont and BASF, for example, are now offering the material.

Significant new technology may be required to process the biomaterials for photovoltaic applications. Rowland Technologies, a leading extruder of high performance films, is working on thin nylon 11 films. Lee would not disclose the name of BioSolar's supplier of cellulosic materials.

The films using cellulose are manufactured much the same way that electrical insulation papers are made, using Fourdrinier machines. Cellulosic polymers go back to the 19th Century when they used to replace ivory in billiard balls. In fact, they were the first successful thermoplastic polymer, predating plastics such as polyethylene by more than 80 years.

"Special types of cellulosic films have been used in dielectric applications for more than 80 years and meet or exceed the UL specifications for use in PV modules," says Levy. They are inexpensive, but have a high water vapor transmission rate (WVTR). BioSolar plans to laminate nylon 11 to the cellulosic film to reduce its WVTR and to improve wear properties.

BioSolar isn't the only game in town in the growing photovoltaic market. Several manufacturers are developing new materials' configurations to improve economics and performance. And DuPont obviously doesn't agree with BioSolar's assessment of Tedlar. DuPont is investing more than $120 million in the Tedlar expansion.

In addition, some manufacturers may require several years of field testing to determine the worthiness of the new biomaterials for applications expected to last 15 years or longer.

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