A small number of companies are developing plastics from sustainable resources, such as plants not used as food sources, for photovoltaic cells, which are on the verge of an explosive growth burst.

Conventional silicon devices have been manufactured on a glass or silicon substrate, but there is widespread interest in thinner, lighter, flexible plastic substrates. The Holy Grail may be a roll-to-roll solar cell that can be integrated into building structures, possibly as a roof.

David Lee, CEO of a start-up company called BioSolar, says, "Most of the plastic materials that have been used for solar cells have worked pretty well, but most of the material being used is based on petroleum." It makes no sense to drive up demand for a material that solar energy is intended to replace, say companies like BioSolar.

On the surface, bioplastics seem like a long shot for solar cell applications. "The potential use of bioplastics in solar applications is very remote because of the temperature required, and the technical requirements often demanding up to 30 years guarantee," says Frederic Scheer, CEO of Cereplast, a leading developer of proprietary starch-based plastics.

But whole new technologies are emerging that are changing the game.

One of the most intriguing combines electronic printing with bioplastics. Due to their poor thermal characteristics, the most widely used plastic substrates, such as polyester film, cannot withstand the conventional silicon sintering process, which has a high temperature of 500 to 600C.

NanoGram, a company in Milpitas, CA, developed a laser pyrolysis-based Nanoparticle Manufacturing (NPM(TM)) process for high-volume production of crystalline silicon nanoparticles. Intrinsic and doped silicon nanoparticles are collected and dispersed into a variety of ink formulations that meet specific printing specifications. A year ago, NanoGram joined forces with Teijin to develop silicon-on-plastic integration technology. Late last year, they developed the first technology for sintering silicon nanoparticles onto a polycarbonate substrate at low temperatures.

Silicon on Plastic

The companies are now in phase two of the technology development agreement to establish silicon-on-plastic integration technology comparable to that of amorphous silicon or organic semiconductors. Efforts will focus on areas such as the development of solar cells and thin film transistor liquid crystal displays. The markets for these products is expected to expand to as much as $11 billion by 2018.

The potential use of bioplastics makes this technology even more fascinating.

Teijin has been developing a heat-resistant bioplastic called Biofront since 2007 and late last year, brought on-line a production plant in Matsuyama, Japan with a capacity of 1,000 metric tons. "Teijin plans to increase the annual capacity of Biofront to 5,000 tons by 2011, and eventually to tens of thousands of tons," says Rie Mashiba, a communications officer for Teijin.

The plant was originally built by Toyota Motor Corp. to produce polylactic acid that would be converted to polymers for use in its green prototype cars. Teijin acquired the facility in 2008 and added proprietary processes required for producing a more heat-resistant biopolymer.

The former Toyota plant increases capacity for Teijin and, more importantly, advances the technology.

Teijin had been making its bioplastics from lactide, a compound produced from lactic acid by fermenting starch extracted from plant matter. The new facility can start from an earlier lactic acid stage, affording more flexibility in the production of varied polymer feedstocks.

Another potential technology player is conductive plastics coupled with plastic substrates that create structure.

Conductive Plastics

The electricity in flexible solar panels from Konarka of Lowell, MA comes from Power Plastic(R), a conductive material. A review of Konarka patents shows that much of its work has focused on conjugated polymers that behave as metallic conductors and semiconductors. The polymers include at least the following: polythiophenes, polyalkylthiophene, polydihexylterthiophene (PDHTT), polythienylene vinylenes and polyfluorene derivatives. A Pittsburgh-based company called Plextronics opened a small-scale manufacturing facility last year to produce solar demonstration modules with printable solar inks made from conductive polymers, such as those used by Konarka.

So far, there are no plans to use bio-based plastics as part of the conductive plastic cells, but the idea is intriguing.
Also underway is the development of cellulosic and castor-oil-based plastics for use as backsheets on conventional silicon solar panels.

BioSolar is commercializing backsheets for solar panels that are made with cotton-based cellulosics and castor-oil-derived nylon 11. Renewable 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, Rhodia and BASF, for example, are now offering the material.

Significant new technology may be required to process the nylon material for photovoltaic applications. Rowland Technologies, a leading extruder of high-performance films, is working on thin nylon 11 films.

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. They were the first successful thermoplastic polymer, predating plastics such as polyethylene by more than 80 years.

Nylon Laminates

"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 Stan Levy, the chief technical officer for BioSolar. 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.

These new materials are not a sure bet, even if their economies are as favorable as promised by BioSolar. OEMs may require several years of field testing to determine the worthiness of the new biomaterials for applications expected to last 15 years or longer.

Other types of bioplastics are strong contenders for structural components of solar panels. A strong player could be DuPont's new Sorona family, which contains 37 percent corn-based feedstock and is offered in 15 and 30 percent glass-filled grades. Another candidate is a corn-based polymer called Mirel, which is said to have mechanical properties comparable to ABS, a crossover resin between commodity and engineering thermoplastics.

For more information:

- Editor Doug Smock answers questions about bioplastics asked by engineers.
- Read about the latest developments in flexible solar cell technology.

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