At present, dye-sensitive solar cells are not yet efficient enough and cheap enough to be implemented on a wide scale. “But step by step we are improving them. Within ten years they will be good enough to supply electricity that will be able to complete with mains electricity”, ECN researcher Jan Kroon says.
Together with colleague Paul Sommeling he recently told a solar cell conference in Hamburg that ECN had managed to make the so-called dye-sensitive solar cells much more stable. As these cells are relatively inexpensive, they offer perspective for wide-scale use in the long term.
In the field of solar cells ECN is currently active on three fronts at once: improving the current solar cells, 80% of which are made from crystalline silicon, developing thin-film-silicon solar cells, and organically based solar cells including dye-sensitised and polymer solar cells. In fact ECN recently set a new world record with a panel of crystalline silicon solar cells with DSM coating on the glass: this panel converted 16.4% of the sunlight into electricity.
Time and patience
“The challenge is to produce ever better solar cells at ever lower cost”, Mr Kroon explains. “It’s all about the euros-per-watt capacity in full sunlight; the watt-peak. This is calculated by dividing the euros per square metre by the number of watts per square metre. At ECN we particularly examine how we could further reduce the cost per square metre and also whether we can improve the efficiency of the solar cells and thus increase the capacity per square meter.”
Once the price of solar energy has been reduced by half, wide-scale implementation is within reach. But this is not to be expected for some years yet. “These kinds of development are very gradual and take time and patience”, he warns. He is referring, among other things, to the development of dye-sensitised solar cells, invented by the Swiss Michael Grätzel in 1991. “They’re easy to make. The Dutch company Mansolar, for instance, supplies kits to schools where children make dye-sensitised solar cells in next to no time, using blackberry juice as colouring. Initially, the best dye-sensitised solar cells at lab scale offered an efficiency of approx. 7%. Hardly surprising therefore that expectations were high in the 1990s. But it is only now that the first commercial applications are emerging. Dye-sensitised solar cells currently provide 12% efficiency. The advantage is that no expensive technology is required to produce them. It can be done in small factories, in developing countries for instance. The problem with this kind of solar cell is the durability. “For a roof panel, for example, you need a solar cell that will last twenty years. This type of solar cell won’t last that long yet”, Mr Kroon explains.
Nanoparticles of titanium dioxide and colouring
The dye-sensitised solar cell consists of a sandwich of a flat anode and cathode with a layer of porous titanium dioxide in between. This is 10-20 nanometre particles of titanium dioxide which are partly fused to form a porous material with a large internal surface area. Colouring is then applied to this surface. This absorbs the light, releasing electrons that are absorbed by the titanium dioxide. This then ‘discharges’ them to the cathode. The electrolyte contains iodide ions (I-), which discharge the electrons to the colouring, creating tri-iodide ions (I3-). These diffuse through the cell to the anode where they absorb the electrons coming from the cathode through the electric circuit. And so we come full circle.
Complex process
“It is a complex electrochemical process in several stages”, Mr Kroon explains. “The slowest stage determines the efficiency of the solar cell. If the colouring does not discharge the electrons quickly enough it might generate a nice fluorescent or luminescent effect, but no electricity. If the layer is too thin most of the light will go through it and only heat the cathode. Too thick a layer of colouring doesn’t work either.”
If the colouring has been treated with titanium chloride the efficiency remains higher than if this is not the case.
Endurance tests in climatic chamber
In ECN’s climatic chamber Kroon and Sommeling subjected the dye-sensitised solar cells to high levels of heat and light to see how quickly they aged. During one test, for instance, the cells were exposed to a temperature of 80 °C and full sunlight for 1000 hours. Cells where the titanium dioxide had been treated with titanium tetrachloride (TiCl4) proved to be resistant to this, even if there was a little (5%) water in the electrolyte. Kroon: “This takes the stability to such a level that commercial implementation is within sight. But we will continue with our research. We don’t quite understand yet why the cell is sometimes stable and sometimes less stable. We have not made much progress with trial and error methods. We first have to acquire a fundamental understanding of the ageing process. Only then we will be able to focus on improving the cell.”
Kroon and his colleagues want to get a handle on the process that takes place in the solar cell. “We also want to get the dye-sensitised solar cell ready for industrial production. For that we have to demonstrate that the cell has high efficiency and that we can make the cells reproducible i.e. with a low failure rate. We are working on this together with the Fraunhofer ISE institute in Freiburg, Germany. We are considering setting up a joint spin-out company to take up the commercial development.”
Mobile telephone
As a matter of fact, the British company G24i already produces dye-sensitised solar cells on flexible film as chargers for mobile telephones, among other things. The fact that the efficiency is only 2-3% and the durability about three years does not make much difference for this particular application. “Within ten years dye-sensitised solar cells could cost around 100 euros per square metre with efficiencies of between 5% and 12%. The cell would then be cheap enough to integrate into external walls of buildings or window frames, because the cell can also be made partially transparent. Various colours are possible, offering designers additional freedom”, Kroon predicts. The ECN researcher remains realistic, however. “We haven’t resolved every problem yet but developments are promising, considering that we already have a dye-sensitised solar cell that remains stable for long periods of time at 80 degrees Celsius and full sunlight. And we can learn a lot from the early applications such as the mobile phone. Step by step we are progressing towards wide-scale application”, Kroon concludes.
Contact
Jan Kroon
ECN Solar energy
Phone: +31 (0)22 456 4734
E-mail: Jan Kroon
Text: Erik te Roller
This ECN Newsletter article may be published without permission provided reference is made to the source: www.ecn.nl/nl/nieuws/newsletter-en/
