Plastic solar tape offers a flexible power source

23 Aug 2004

Konarka's thin-film photovoltaics could lead to a whole range of innovative solar-powered products. James Tyrrell speaks to the US company to find out more about the technology.

From Opto & Laser Europe September 2004

Imagine charging your ipod or mobile phone using "solar" fibres woven into your jacket or t-shirt. What if solar panels were available in different colours and patterns to blend in with their surroundings? These are just some of the possibilities that could become a reality if US renewable-energy firm Konarka is successful in developing its lightweight and flexible photovoltaics (PVs).

Unlike silicon-based PVs, first developed by US-based Bell Labs in the 1950s, Konarka's products use photoactive dye to convert light into electricity. Resembling photographic film, Konarka's flexible plastic PVs contain a layer of dye-activated titanium dioxide (TiO₂) particles and can be mass produced on a roll-to-roll process.

"What we are bringing forth is not a semiconductor-fab-type technology," Daniel McGahn, Konarka's executive vice-president and chief marketing officer, told Opto & Laser Europe. "It can be done in the ambient environment, it can be done in a warehouse, it doesn't have to be done in a class 100 cleanroom."

The theory Light falling on the PV film passes through a clear plastic barrier layer and a conductive electrode layer to reach a dye-impregnated composite TiO₂ particle region (see figure, p26). The dye reacts with incoming photons, releasing electrons into the semiconducting TiO₂ network, which is in turn connected to a counter electrode.

As well as being thin and flexible, Konarka says that its PVs work well in low-light conditions and under artificial light. The dye can also be tuned to absorb specific wavelength bands and Konarka has achieved efficiencies that make commercial devices feasible.

"At the lab level we're near 12% [photon to electron conversion]. The manufacturing level is nearer 8%, but what we're going to market with is a 5% film because we think that's what the initial threshold for the market needs," commented McGahn.

To put this into context, the conversion efficiency of commercial mono/polycrystalline silicon-based PVs is about 15%, with values approaching 30% for triple-junction cells containing a mix of semiconductor materials and using concentrators. Konarka plans to use its film's flexibility, appearance and low manufacturing cost to win market share from amorphous silicon devices that offer similar conversion efficiencies of 5-7%.

A big attraction of Konarka's dye-based PVs is that they can be coloured and even patterned to resemble normal roofing material or military camouflage. "In photovoltaics this is in itself a revolutionary concept. Forget the [ease of] manufacturing, forget the [low] cost, just the fact that you can make it look like a consumer electronic device, house or soldier - that's a big deal," said McGahn. The US military appears to agree, having already placed orders for PV material as part of Konarka's on-going development programmes with army, navy, air-force and marine corps.

The breakthrough that led to Konarka's first prototypes came when founding scientist Sukant Tripathy (see box, p28) and his researchers were able to reduce the temperature of the TiO₂ sintering process. The team found a way to convert single TiO₂ particles into a conductive network at 150ºC instead of the previous temperature of 450ºC. At 450ºC the choice of substrates was limited to rigid and often delicate glass materials. The new "cold sintering" technology transformed the number of possible applications and the manufacturing process itself by allowing the use of flexible plastic substrates.

"There's a little art to exactly how they [TiO₂ particles] are pre-conditioned," McGahn told Opto & Laser Europe, careful to protect the firm's proprietary knowledge. "The real trick is that there's a uniformity to the [particle] size. These are all nanoscopic of the order of tens of nanometres in diameter."

With several key R&D and production staff coming from Polaroid, it is no secret that Konarka's fabrication methods are rooted in the photographic-film industry. "What we are starting with is a very simplified version of a roll-to-roll coating process," McGahn pointed out.

Just as the photographic industry manufactures a whole range of film products, Konarka says that its PV films can be easily customized by varying the coating chemistry. McGahn says that the firm is now optimizing its recipe. "What we're trying to do is simplify this dye and TiO₂ interchange to be able to get a higher level of activity."

Exciting developments There are some interesting technical developments taking place behind the doors of Konarka's labs. "What we get very excited about, but it's at the research level, is the idea of being able to do the same coating process on a fibre," McGahn explained. "Instead of coating multiple layers on a flat surface, you could dip a fibre through different solutions to create a PV fibre."

Being able to weave "solar" fibres into textiles could open up a huge market for PVs, essentially the opportunity to power anything that needs a battery. "It could be an ipod, it could be the rifle sight for an infantryman," suggested McGahn.

Although it may be a few years before these applications are realized, Konarka's business plan is now in full flight. Despite having its headquarters in Massachusetts, US, the firm's business activities have a strong European focus.

"Germany is the second largest photovoltaic market today, Japan being number one," said McGahn. "Europe as a total, from a continent standpoint, is the largest PV market." The US firm's European presence extends to research operations in Linz, Austria, and Zug, Switzerland, with another facility due to be announced in Germany.

Boosting renewable energy Europe's political climate is also seen as receptive to "green" power such as solar energy. The European Commission aims to double renewable energy's share of gross domestic energy consumption from 6 to 12% by 2010.

Forecasts suggest that by then, 21% of France's electricity generation will involve renewable energies. "We have a partnership with Electricité de France (EDF), who would like to be a consumer of these products," said McGahn. With 41.6 million customers worldwide and 35.6 million in Europe, EDF is a big hitter in the energy business.

However, it is not just European energy firms that are interested in Konarka's technology. Europe's venture capitalists have also bought into the company's vision. Players such as Partech International, Prime New Energy (who backed Konarka on the advice of Credit Suisse) and Good Energies made significant investments in the company as part of its $18 m (€15 m) series C financing round in July. This cash will help fund the equipment and strategic investments needed to scale up manufacturing to commercial production levels.

"We won't necessarily go to the market as a traditional photovoltaic [supplier]," McGahn told Opto & Laser Europe. Instead, Konarka's plans involve growing to a size where it can join forces with global companies to focus on a specific use. For example, say a car manufacturer wanted to launch a new model featuring PVs, Konarka's goal would be to work with the firm to marry its material to the final product.

The low cost of fabrication equipment places Konarka in a position to offer application-specific production to the point where manufacturing could even take place in-line at a customer's premises.

In a US radio interview on THE Business Station, Konarka's chairman and chief executive officer Howard Berke told listeners that the firm plans to generate revenues in 2005, becoming profitable in 2007.

Company background The founding of Konarka was inspired by the work of Sukant Tripathy. During his 15 years at the University of Massachusetts, US, Tripathy established himself internationally in the field of material science. He was a major force in creating the University's Institute for NanoScience Engineering and Technology, raising the university's profile in the emerging nanotechnology sector - a cornerstone of Konarka's technology.

Born in India in 1952, but spending most of his professional life in the US, Tripathy's vision was to produce low-cost power that could be deployed in the developing world, specifically India. Tragically it was a dream that he never achieved as he drowned while swimming at the Pololu valley, Hawaii, in 2000. Konarka takes its name from one of Tripathy's favourite places - the magnificent temple dedicated to the sun god Surya in Orissa, India.

Along with Tripathy, Konarka's intellectual contributors include Nobel laureate (chemistry, 2000) Alan Heeger and dye-based photoelectric cell pioneer Michael Grätzel. Building on this knowledge and expertise, the company has amassed a strong intellectual-property base, featuring more than 75 patents, developing technology both in-house and licensing-in from EPFL Lausanne, Switzerland, and the University of California Santa Barbara and Dupont in the US.

Konarka - key facts

Founded: 2001

Headquarters: Lowell, Massachusetts, US

Funding: $32 m

Employees: 32