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Can dye-sensitised solar cells deliver low-cost PV?


George Marsh

Dye-sensitised cells (DSCS) have emerged alongside organic solar photovoltaics (PV) as a technology that might just transform solar energy prospects by delivering cheap "commodity" PV. George Marsh investigates.

Don't eat your spinach — consider it as a basis for solar PV. In fact, extracting chlorophyll from spinach and using it to convert sunlight into energy via a photo-electrochemical process akin to photosynthesis, may not be such a wild idea. As imitating plants is a time-honoured gambit, it came as little surprise to Eastman Kodak researchers in the early 1970s that small electrical currents could be generated using the agent that accounts for the green colouring in plants, sandwiched between electrode metals. While conversion efficiency was low, later researchers boosted performance with a combination of dyes.

As with Organic PV (OPV), the dye cells are ultra-thin, very low in weight, bend without breaking, avoid use of silicon and, best of all, should cost very little once their constituent materials are coated at low temperature over large areas of substrate. With their high performance/price ratio, dye-sensitised cells should be viewed as a competitor to organic PV. They are, in fact, ahead in the race for commercialisation.

The Graetzel Cell

The dye-sensitised solar cell in its modern form is also known as the Graetzel Cell, after its development by Prof. Michael Graetzel and Brian O'Regan at Ecoles Polytechniques Federales de Lausanne (EPFL) in Switzerland. This technology was launched publicly in 1991. Early cells were 7–8% efficient, though researchers have since boosted this figure to around 11%. As with organic PV, mainly low-cost materials are used. However, the energy conversion mechanism is different from organic PV.

A dye-sensitised solar cell can be likened to a conventional battery in which an external stimulus is needed — in this case, light. Light is used to initiate the electrochemical reaction that produces electron flow and hence current. Thus the overall action is photo-electrochemical. Such a cell requires a fourth ingredient over and above the well-known anode-cathode-electrolyte trio: a material that can release electrons when excited by photons of light. Some light-sensitive dyes have this power, as different dyes and colours are sensitive to different light wavelengths.

Semiconductor action is also involved, provided by a layer of nanostructured titanium dioxide, to which the dye is attached. The associated band gap dynamics mean that the various material layers can be extremely thin, in contrast to the “thick” crystalline silicon currently used in conventional solar cells

In the original form of the Graetzel Cell, ruthenium polypyridine was the dye, with its molecules held within the highly porous surface of the nanocrystalline titanium dioxide. This semiconductor material was then applied to the back of a transparent front plate which, by virtue of another thin transparent coating of an oxide of tin and fluorine, constituted a conductive anode. A back plate of platinum formed the cathode. Between the anode and cathode was a thin liquid layer of an iodide solution. The two plates were sealed to retain this liquid electrolyte.

While certain constituent materials — ruthenium, titanium, platinum, etc — are costly in bulk, savings can be made due to the extremely low volumes of material used in the thin coatings. Together with rapid wide-area application of the materials onto substrates in continuous roll-to-roll production processes, the cell costs can be much lower than those for conventional solar PV cells, especially based on crystalline silicon expensively produced in semiconductor foundries.

Other material combinations offer promise, such as “black dyes”. The titanium dioxide (titania) and dye combination used in the original Graetzel Cell was most active at high frequencies, towards the ultra-violet end of the light spectrum. However, the so-called “black dyes” that were subsequently developed convert over a broad light spectrum (hence their dark colour), thereby achieving higher efficiencies.

Organic dyes

While work on the ruthenium complex continues at Lausanne, other materials have been investigated, including copper-diselenium and various organic formulations. Researchers such as Wayne Campbell at Massey University, New Zealand, have experimented with organic dyes based on porphyrin, a basic building block of nature's hemoproteins, which include chlorophyll in plants. Campbell has reported efficiencies of about 7% for these low-cost dyes.

Solid electrolytes

Other work has focused on developing a solid electrolyte to supersede the iodide liquid which can escape if not effectively sealed within the cell. It can also freeze in a cold environment. Michael Graetzel himself, working with colleagues at the Chinese Academy of Sciences, has based a solid-state electrolyte on a melt of three salts and has used this in a cell to achieve a demonstrated efficiency of 8.2%. While somewhat less than the 11% efficiency of existing iodide solutions, the research team believes that this efficiency can be improved upon.

Low cost concentrator technology

Concentrating the light before it enters the cell is a non-materials strategy that shows promise. Recently a team at the Massachusetts Institute of Technology (MIT) made waves by announcing that they can increase the power produced by a solar cell by a factor of 40 using low-cost concentrator technology — that requires no optical sun-tracking or cooling.

According to team leader Marc A. Baldo, this is done by harvesting energy over a large area, such as a window, and concentrating it at the pane edges where solar cells are located. A mix of dyes spread over the glass, or plastic substrate absorbs the light over a range of wavelengths and re-emits it to the solar cells.

While this “luminescent solar concentrator” (LSC) concept was first tried in the 1970s, it failed to make headway at that time due to dye instability and loss of collected light before it reached the pane edges. The MIT team has since addressed these drawbacks by using a new class of molecular phosphor dyes in a “four level” dye mix. The resulting system both minimises the losses and promises future thermal stability.

Early devices currently lose some 8% of their initial efficiency over three months, but the researchers are confident that importing technology developed for television organic light emitting diodes (OLEDs) will solve this problem. If they are right and if manufacturing and commercialisation issues can be addressed satisfactorily, the MIT work, which is supported by the US National Science Foundation, could prove revolutionary.

Durability

The MIT team's difficulty with dyes exemplifies a major current downside of dye-sensitised solar cells — the rate at which the photo-active dye materials can be degraded by moisture, heat and, in some cases, strong light. Unprotected, dyes can break down in weeks or months. At present, researchers are targeting five to 10 years of useful life, compared with 20 or 30 years demonstrated by “mainstream” silicon-based solar PV.

However, progress is being made. In an experiment two years ago, a dark dye lost just 10% of its initial PV efficiency, after an accelerated ageing test that involved heating it at 80ºC in the dark for 1000 hours, and then light-soaking at 60ºC for a further 1000 hours. This level of thermal stability offered a significant improvement over previous results.

And a black dye system has been subjected to 60 million cycles, equivalent to 10 years of sun exposure. This dye system proved thermally stable, though strong incident light remained a problem.

Encapsulation

Germany's Fraunhofer Institute for Solar Energy Systems (ISE) has shown good accelerated ageing results for a DSC in which the screen-printed active dye and other cell layers were hermetically sealed within the cell. Fraunhofer ISE believes that a combination of effective encapsulation techniques with improved printing processes will result in durable 5% efficient cells of around 60 by 100 cm2 area by next year.

At the EU PVSEC event in Milan last year, the organisation showed several 3.5% efficient dye-sensitised solar cell modules (30 by 30 cm2). Each module contained 6, series-connected solar PV cells, together delivering 0.9A at 4.7V. Fraunhofer ISE and other dye-sensitised solar cell developers suggest that, by 2015, they will attain 10% efficient modules, able to pass accelerated ageing certification tests similar to that outlined above. By then, experts also expect that dye-sensitised solar cells will be demonstrating 20% efficiency in laboratories.

While dye-sensitised solar cells may be challenged on the thermal durability front, on the positive side an increase in the operating temperature has little effect on their performance. This contrasts with conventional silicon cells in which performance falls as the temperature rises. Dye-sensitised solar cells also compensate for their lower efficiency by being able to work indoors, and in other subdued light conditions, and by an ability to harness light impinging at angles that silicon devices would reject.

Commercialisation

Successful commercialisation of dye-sensitised solar cell technology requires efficient and durable material combinations that can be coated inexpensively in thin layers on rigid and flexible substrates using continuous roll-to-roll production processes. Several companies are actively driving commercialisation forward, including Konarka Technologies, Dyesol, G24 Innovations, Toppan Forms, Leonhard Kurz, Mitsubishi and Peccell.

Screen printing technique

Konarka in particular, working with the benefit of a licence from EPFL (Lausanne), has developed a screen printing technique for a cell technology that has attained 10.4% efficiency. In 2006 Konarka licenced US investment company Renewable Capital to undertake large-scale dye-sensitised solar cell production, something that has now been achieved through G24 Innovations Ltd based in Cardiff, Wales.

G24i's UK chairman and co-founder, Robert Hertzberg, also founded Renewable Capital.

Hertzberg said, “this technology raises tremendous opportunities. It's a source of power that does not need direct sunlight and works just as well in the rain at Cardiff as it does in the sun in the south of France. You get more energy in a 24 hour real-life cycle than with glass/silicon technology, which is highly efficient in bright sunlight but cuts off in subdued light. DSC light cells even work indoors. We have attracted major funding so that we can reach the market fast and we are ramping up production.”

G24i operates a 187,000 ft2 manufacturing facility on a 23-acre site in Wales.

G24i claims to be first in the world to manufacture commercial-grade dye-sensitised thin-film cells, regarded as the third generation of solar cells, on an industrial scale. It is now supplying material for solar chargers used with phones and other mobile devices, especially in off-grid parts of the world. One early deal, secured last year, was with Master IT Ltd in Kenya, which is producing solar chargers for mobile phones used in Africa.

Meanwhile, the company continues to develop its technology. Last October, G24i signed an agreement with BASF to jointly develop ionic liquids and formulations with which to further improve solar cell performance.

Translucent tiles

Dyesol in Australia is also well ahead of the game with a proprietary dye-sensitised solar cell manufacturing technology described as simple, low energy, easy to operate, affordable and based on relatively benign materials. Dyesol, which like G24i boasts links with EPFL, produces tiles that can be connected to form solar panels suitable for building integrated PV (BIPV) applications. These translucent tiles, in which active material is sandwiched between glass panes and encapsulated in a UV-resistant transparent polymer, are offered in ochre, grey, green and blue tints depending on the dye combinations used. Solar wall panels are designed for exterior mounting and can be structural, avoiding duplication of existing façade.

The company is working with steel maker Corus to develop a PV-active building material based on stainless steel substrate coated with dye-sensitised solar cell materials. This is in line with Dyesol's policy to promote widespread use of its technology through joint ventures.

Meanwhile, under contract to the Australian Defence Science and Technology Organisation, Dyesol has developed a third-generation solar module for military applications. Light, flexible, foldable and camouflaged, this and similar products can generate current for military equipment in low light conditions, such as dappled light under trees.

According to managing director Gavin Tulloch, Dyesol also produces turnkey manufacturing solutions. Processing stations developed by the company can be reprogrammed to suit a wide range of dye-sensitised solar cell formulations, making the equipment ideal for evaluations, prototyping and validation of third-generation solar PV designs. Tulloch hints that there are plans to establish a presence in Europe “where the markets are” and where legislation will require future buildings to be carbon neutral.

A DSC powered Swatch watch?

Innovative applications will help drive demand for dye-sensitised solar cell technology. Watchmaker Swatch plans to launch a dye-sensitised solar cell powered watch using cells laid directly onto metal substrates. Konarka has worked with Textronics on weaving dye-sensitised solar cell fibres into fabrics usable in handbags, clothing, curtains, blinds, tents, vehicle covers, etc.

And in a recent report, analyst NanoMarkets LLC cites good performance in dim or variable light as probably the leading dye-sensitised solar cell advantage at present. This explains why solar chargers are likely to be the first products to market. NanoMarkets suggests that the flexibility and cost advantages will become predominant later in the technology's evolution, to be realised in products ranging from smart curtains and wearable fabrics, to tiles for roofs or walls.

Costs per watt generated could, if early promise is borne out, eventually fall well below those for conventional power generation, a breakthrough that still seems well in the future for silicon-based PV. Proponents even suggest that dye-enabled solar PV could become ubiquitous, attain commodity status and prove transformational for renewable energy prospects.

We can only hope that enough spinach is left over for dinner.

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