A small Californiacompany called BioSolar isintroducing plastic components for photovoltaic (PV) cells based on cotton andcastor oil that are described as less costly than plastics based on petroleum.

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

The substrate of BioBackSheet-C is a cellulosic plasticcoated with a proprietary material to reduce the water vapor transmission rateand to increase abrasion resistance. The two-component system aimed atcrystalline silicon photovoltaic applications is said to possess highdielectric strength.

BioSolar's products attack one of the conundrums of theemerging solar cell industry - they use millions of square feet of materialsthat 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 of20-25 percent versus the prevailing material in the market," says Lee. "With our first product, we are achievingthat goal."

Most biobased plastics target packaging markets and cost apremium of 30 to 500 percent above the materials they replace. In the solarcell 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 Tedlarpolyvinyl fluoride (PVF), a thermoplastic fluoropolymer that is structurallyvery similar to polyvinyl chloride. It has excellent vapor permeability and isvery resistant to weathering, key requirements for the backsheet of aphotovoltaic cell. Tedlar constructions often replace glass.

Costly Structures

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

Tedlar is solvent-cast using an industrial solvent called dimethylacetamide (DMAC), which can produce systemic injury when inhaled or absorbedthrough the skin in sufficient quantities over a prolonged period of time. IfTedlar is burned, corrosive hydrogen fluoride fumes can be released. Anotherimportant issue: Tedlar has been in veryshort supply for the past two years because of its growing use in photovoltaiccells and aircraft interiors.

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

"Our capacity expansions are critical steps in growing theTedlar business and maintaining our market leadership in backsheets for solarpanels that deliver the long-term, reliable power supply that our customershave come to expect from their investment in renewable power generation," says DavidB. Miller, group vice president - DuPont Electronic & CommunicationTechnologies.

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

In other markets, such as automotive, PLA is alloyedwith a petroleum-based engineering plastic to boost durability. The soleadvantage is reduction of the carbon footprint. Such a solution could also beintroduced for the PV market.

Two Employees

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

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

Significant new technology may be required to process thebiomaterials for photovoltaic applications. RowlandTechnologies, a leading extruder of high performance films, is working onthin nylon 11 films. Lee would not disclose the name of BioSolar's supplier ofcellulosic materials.

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

"Special types of cellulosic films have been used indielectric applications for more than 80 years and meet or exceed the ULspecifications for use in PV modules," says Levy. They are inexpensive, buthave a high water vapor transmission rate (WVTR). BioSolar plans to laminatenylon 11 to the cellulosic film to reduce its WVTR and to improve wearproperties.

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

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