Power Polymers Fuel Solar Cell Advances

It sure isn't easy being green. America launched major initiatives to ease dependence on oil in the 1970s after two oil price spikes orchestrated by Mideast oil powers. The big push for more fuel-efficient cars and alternative sources of energy quickly stalled, however, as oil prices dropped.

One of the big players on this roller coaster has been solar power, which still accounts for less than 1 percent of world energy production, mostly because its cost is often seven or eight times higher than coal-generated power. There is renewed interest now, with states such as California leading the push. Solar is a new darling of venture capital and major polymers companies, such as DuPont and Bayer MaterialScience, see significant potential.

Another big driver now is the U.S. military, which wants to improve soldier mobility. Packs carried by soldiers weigh almost 100 lb, including a three-day supply of batteries to power their gear, weighing 20 lb. The Defense Research Projects Agency (DARPA) recently awarded $12.2 million to the Very High Efficiency Solar Cell Consortium that includes DuPont and the University of Delaware. DARPA wants researchers to develop solar rechargers that could be integrated into battlefield gear such as radios, GPS navigation systems and night-vision goggles.

DuPont is developing materials for conducting the electricity produced by solar cells and for the encapsulation of cell assemblies into environmentally stable panels that are protected from moisture, UV rays and impact. One of DuPont's contributions is a material called Solamet, which is a system of conductive metallization materials provided as a thick film. Its purpose is to enhance efficiency levels. The University of Delaware, in fact, recently announced its new technology achieved an efficiency level of 42.8 percent, up from the previous record of 40.7 percent. DARPA wants a minimum of 50 percent efficiency.

Silicon wafer-based solar cells, such as those used by the Delaware consortium, account for more than 86 percent of the commercial solar cell market. And therein lies the rub. Supply constraints and rising prices for silicon have slowed commercial development since 2004. Next generation technologies explore use of thin films made of materials such as amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride and copper indium selenide/sulfide. These materials are typically less efficient than silicon, but can be manufactured at much lower costs.

Printable Polymers

One new technology with promise uses a class of semi-conductive polymers based on regioregular poly (3-substituted) thiophenes that can strongly absorb sunlight and separate electric charges, the key functions required in a photovoltaic unit. The approach is under study by a Pittsburgh-based company called Plextronics that received a $3 million grant from the U.S. Dept. of Energy Initiative to develop thin film organic photovoltaic technology for the federal Solar America Initiative. Plextronics describes itself as the world leader in developing active layer technology for printed electronics devices, which can be used for polymer solar cells, plastic circuitry or organic light-emitting displays (OLED).

"We expect to see early organic photovoltaic (OPV) device products in the market as early as 2008 through our customers," says James Dietz, vice president of business development for Plextronics. "The key issues relate to scaling cell and module efficiencies, optimizing the key production processes and providing the lifetime of the modules for early applications."

Plextronics will not reveal much information about its technical building blocks. "We do not divulge our manufacturing recipes," says Dietz. He will say Plexcore PV is a proprietary ink system that incorporates p+ and n+ type polymer semiconductors that are unique to Plextronics. Plexcore PV by itself is a p-type semiconductor. When mixed with materials that conduct negative charges (such as carbon fullerenes, titanium dioxide or cadmium selenide) a complete solar cell can be made.

The company is working with the University of Denver and Arizona State University to boost solar efficiencies by broadening the spectral absorbance and aligning it more fully with the higher flux portion of the solar spectrum.

Plextronics' goal is to drive costs toward what it considers a commercially viable $1 per watt. The printed systems can also produce flexible cells, tremendously expanding their design range. It's hoped, for example, the rolled-up panels could be used for tents. The power polymers could also be fabricated into roof shingles or used to coat windows. The chief scientist at Plextronics is Richard D. McCullough, who is the dean of the College of Science at Carnegie Mellon University and a leading expert in the synthesis of regioregular polythiophenes.

One company that has met with Plextronics is Bayer MaterialScience. "Roll-to-roll manufacturing has the potential to drive costs way down," says Kevin Elsken, a project manager for future business at Bayer MaterialScience in Pittsburgh. "Long-term, flexible cells will replace conventional silicon cells," says Elsken. "Maybe in 10 years. Certainly in 20 years."

Molding Approach

Bayer has patented a process for producing solar panels with a front side made of transparent thermoplastic polyurethane, which replaces glass that is 3 to 4 mm thick with a transmittance in the spectral region of 90 to 92 percent. The adhesive typically used for the structure is ethylene vinyl acetate (EVA), which is melted during a vacuum lamination process at 150C. It then flows into the gaps between the soldered solar cells and becomes thermally crosslinked.

The vacuum is necessary to prevent formation of air bubbles. The rear side of the panel is often covered with a film (polyvinyl fluoride-polyethylene terephthalate) to provide environmental and mechanical protection. The materials must protect the cells to allow an economical lifetime of 20 to 30 years. The module materials account for about 30 percent of a panel's total cost and also slow production. Manual assembly plus the vacuum lamination process often requires production cycle times of 30 minutes.

The primary goal of the Bayer invention is to accelerate manufacturing times. The key material is transparent polyurethane, which at a layer thickness of 1 mm exhibits a transmission of >85 percent at wavelengths in the range between 400 and 1,150 nm. Transparent polyurethanes are produced from aliphatic polyisocyanates. Adding copper to the recipe boosts the material's thermal conductivity, boosting cell efficiency.

In thin film systems, such as those proposed by Plextronics, conductive polymeric material is printed on substrates and then embedded completely in transparent polyurethane. The modules can be made through reaction injection molding, with the optically active side facing the mold cavity. Reactive urethane materials are then injected into the cavity under high pressure, encapsulating the solar cell.

Bayer has studied many variations of the process and materials technology. In another approach, active materials are deposited on polyimide film. The front of the structure consists of thermoplastic urethane and fluoropolymer films, which are applied in a roll-to-roll process.

In one system, tested on a one-person ocean-going kayak, electricity was provided using silicon solar cells encapsulated in thermoplastic polyurethane film made by Etimex Primary Packaging of Dietenheim, Germany. The solar panels were manufactured by SunWare Solartechnik. Production cycle times are reduced to 10 and 20 minutes, according to Dr. Gunther Stollwerck, an expert on plastic encapsulation for solar modules at Bayer MaterialScience.

Many companies are pursuing alternative approaches to solar cells. Nanosolar of Palo Alto, CA, for example has raised $100 million in venture capital to develop a process for printing solar cells on rolls of film using ink filled with nano particles of copper indium gallium selenide. As with Plextronics, the ink is the secret sauce. A company called Luz II built several commercial power plants using solar thermal parabolic trough technology, which focuses sunlight on a pipe carrying synthetic oil.

Several things seem likely about solar. Several technology approaches will be used to solve different problems. Support for the technology will vary widely. Germany, for example, has significantly spurred development even though sunlight can be hard to find near the Baltic Sea. Costs for photovoltaic cells will improve. But how competitive will solar be versus hydrocarbons? Today it seems hard to believe how little progress has been made since the 1970s.