These Plastics Create Electricity

DN Staff

December 16, 2010

9 Min Read
These Plastics Create Electricity

The race ison to commercialize photovoltaic cells made with conductive plastics.

A Californiastartup announced last July that it had achieved a new world record for organicphotovoltaic (OPV) cell efficiency: 8.13 percent. On Nov. 29,Massachusetts-based Konarka said it achieved an efficiency of 8.3 percent,breaking the record.

The new OPVrecords compare to 11 to 15 percent efficiency for traditional inorganicsilicon photovoltaic cells in converting sunlight to electricity. The maximum recorded cellefficiency for crystalline silicon is 24.7 percent.

Improvementsin efficiency in organic photovoltaics based on conductive plastics have beenslow in coming. Although organic photovoltaics have been around for 20 years,they still represent close to zero percent of global energy production.

But, conductiveplastics are considered an important part of the world's energy future becausethe cost of electricity from silicon photovoltaics is very high-roughly 10times higher than fossil fuels such as coal and oil.

There probablywill be plenty of room for growth in several new photovoltaic technologies. In2010, global cumulative installed photovoltaic (PV) capacity grew by 40percent, with compounded annual growth forecast at 20 percent.

Conductiveplastics offer these advantages:

Better Designs. Conductive plastics are flexible andare easy to bond to flexible substrates such as plastic and metal foils, creatingopportunity for integrated, flexible applications. For example, mobile phones couldgenerate their own power.

Costs. Relatively inexpensive plastic isused as the active material to convert solar energy into electricity. Only afew tenths of a micrometer of conductive plastics are needed to generateelectricity.

Printing capability. Conductive solar cells can be printedin processes similar to printing on newspaper. The fabrication process is describedas low temperature and environmentally friendly.

Photovoltaiccells contain a light-sensitive semiconducting material that starts the processof producing electricity. Today that material typically is silicon. OPVsgenerally use two materials, one acting as a donor that captures electrons fromlight (even indoor light) and the other acting as an electron acceptor. Acharge transfer process then begins, creating electricity.

Climate change
Thediscovery of pure conductive organic polymers (such as oxidizediodine-doped polyacetylene) in the late twentieth century coupled with high petroleum pricesaccelerated interest in OPVs. Concerns about climate change boosted research againin OPVs in the last ten years.

Solarmer Energy of El Monte, CA announcedthe efficiency record of 8.13 percent and in June 2010 the National RenewableEnergy Laboratory (NREL) and Solarmer signed a cooperative research anddevelopment agreement to improve the lifetime of plastic solar cells.

Tests willbe conducted with NREL's combinatorial degradation system, which compares theperformance of up to 360 OPV devices in parallel under simulated solarconditions that accelerate the test results.

"Throughthis collaboration we hope to improve plastic solar cell performance and extendthe cells' lifetime through the development of stable device architectures andpackaging systems," said NREL senior scientist Dana C. Olson, who is theprincipal investigator on the collaboration project.

Solarmer'stechnology, which includes making electrically active plastics
in multiplecolors, is representative of an explosion of research that is leading tooptimism about organic photovoltaics.

To date,organic photovoltaics have been largely limited to charging consumerelectronics because of their short lifetimes, lasting only a couple years, andlow efficiencies (3 to 8 percent). One early applicationis to provide power at poolside umbrellas for laptop recharging that cost$10,000.

Research isfocusing on increasing the lifetime and efficiencies for organic photovoltaics.Several companies hope to break the 10 percent efficiency barrier this year. Improvingstability may be farther off.

Konica invests
A leadingglobal player is Konarka Technologies, aspinoff of the University of Massachusetts Lowell. Last year Konica Minoltainvested $20 million in Konarka to further develop organic thin-filmphotovoltaics.

"KonicaMinolta positions the organic thin-film photovoltaic business as one of themost promising in the environment and energy field,next to the organic light-emittingdiode (OLED) business, where our photographic film manufacturing technology isleading mass production," says Masatoshi Matsuzaki, CEO at Konica Minolta.

Konarka wasan attractive partner because of its commercial product, Power Plastic, whichis now being produced in a roll-to-roll process in New Bedford, MA. PowerPlastic captures indoor and outdoor light and converts it into direct current(DC) electrical energy.

PowerPlastic captures indoor and outdoor light and converts it into direct current(DC) electrical energy. The technology traces its roots to development of a wayto process photovoltaic materials at relatively low temperatures, enabling theuse of low-costpolymers as the top and bottom surfaces of the photovoltaiccell. The technology, invented by a team at UMASS Lowell, also enabled thephotovoltaic cells to be manufactured at high speed using coating and printingtechnologies.

At the heart of Konarka's technology is aphoto-reactive polymer invented by Konarka co-founder and Nobel Prize winnerAlanHeeger. This material can be printed or coated inexpensively onto flexiblesubstrates.

The newerapplications for Power Plastic are coming from partners focused onbuilding-integrated photovoltaics (BIPV) and other shade and structureapplications. In one example, Arch Aluminum & Glass Co. is integratingPower Plastic into a wall structure at Arch's office building in Tamarac, FL.The solar panels will 1.5 kW of power to the facility.

"The purposeof this project is to test the performance and robustness of our solar panelsolution for a curtain wall application with a variety of glass and windowconfigurations under a wide range of environmental and insolation conditions,"says Terri Jordan, vice president of business development at Konarka. "Theyielded data and information will guide our development of the first-of-its-kindvision application, a transparent, colored solar glass panel."

The currentlifetime for Power Plastic is three-to-five years when exposed. If theconductive layers are protected with glass or plastics, its lifetimes arecomparable to traditional solar cells.

Thecompanies are expected to establish a joint venture company
in Japan to produceorganic thin-film photovoltaic panels.
Ink systems
Plextronics'key product for the solar market is Plexcore PV, a ready-to-use ink system.

In printedsolar cells, sunlight is collected in the photoactive layer to create chargecarriers. When voltage is applied to the device, these carriers are separatedinto positive and negative charges and then directed to the conductiveelectrodes (cathode and transparent anode) to create power. A hole transportlayer improves extraction of positive charges from the photoactive ink bymatching of energy levels to photo-active ink.

"Our currentefficiency is just shy of 7 percent," says Mary Boone, director of ink businessdevelopment for Plextronics. "From a goal perspective, we are targeting greaterthan 8 percent efficiency in the next year." The technology is pre-commercialand Plextronics provides inks in 50-liter batch sizes or less.

Onepotential indoor application for the ink is point-of-purchase displays thatreceive energy from fluorescent lighting.

"Working with our partners and customers,we've talked with major retailers and consumer goods companies in the U.S. whowant to replace the traditional paper signage used in stores today withlow-cost, animated point-of-purchase displays that are self-powered by OPV,"says Boone. "Retailers don't want the ongoing cost and disruption of having topurchase and replace conventional batteries in these displays, so there istremendous interest in using printable OPV as the power source for these newdisplays."

In anotherrecent development, Plextronics says it has improved low-temperatureprocessability of OPV. While previous industry standard techniques required aglass substrate to be annealed at temperatures at or above 110C, theproprietary method developed at Plextronics enables annealing at less than 65C.This new method allows use of less-expensive substrates, especially once theprocess is transferred to flexible substrates such as conventional plastic.

Plextronicsexpect to introduce ink-jet printable inks for organic light-emitting diodes (OLED)displays soon.

Thenon-aqueous-based Hole Injection Layer (HIL) ink augments the company'sexisting aqueous-based HIL, and is geared specifically for solution processablephosphorescent OLED emitters.

Conductiveplastic photovoltaic technology is close, but not yet part of the designengineering toolbox on a practical level. But they may change in the next twoto five years.

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