Breakthrough for plastic solar cells

Plastic solar cells have several important advantages compared to normal solar cells produced from silicon. They are cheap to make from simple raw materials, light and flexible, and can therefore be incorporated in clothing or rucksacks, for example. However, the yield is not that great. Whereas traditional solar cells convert about 15 to 20% of the sunlight into energy, plastic solar cells currently convert far less than 10%.

"Scientists who work on plastic solar cells suffer from a sort of inferiority complex," says professor Kees Hummelen, Professor of Organic Chemistry at the University of Groningen and one of the pioneers of the plastic solar cell. "They think that their solar cells will never be as good as those of silicon." Many experts believe that yields above 14% are not possible.

The result of all the calculations is an article that describes which conditions new materials for plastic solar cells must satisfy. The exact appearance of these materials is still not known. “We are of course working on that," says Hummelen. "However that is an enormous search. We have therefore published this article now so that as many other research groups as possible can join the search." The calculations showed that a plastic solar cell with a yield of more than 20% is quite possible. "Indeed it means that they can even be better than silicon solar cells," laughs Hummelen. He points to a shoulder bag with a plastic solar cell the size of a sheet of A4 paper on it. “With that you can currently just about charge up your iPod. With the ‘Next Generation’ plastic solar cells it should be possible to charge up your laptop."

Hummelen, leader of the FOM focus group that started in 2011 and that will spend the next 10 years working on the 'Next Generation' organic solar cell, had a flash of inspiration two years ago that led to an extensive theoretical investigation into the possibilities for solar cells made from organic molecules and/or polymers (plastics). The basis for this was laid in the project within the FOM Joint Solar Programme, in which Hummelen collaborates closely with Prof. Sean Saheen (University of Denver) and Dr Jan Anton Koster (University of Groningen). "Traditionally researchers in this field use non-polar compounds to produce their solar cells." These are oily substances in which charged ions dissolve poorly. “Such compounds also process electrical charges with difficulty." However, that is exactly what a solar cell must be good at: free charges produced by the sunlight falling onto the solar cell must be conducted properly to the outside.

Together with his colleagues, Hummelen examined what would happen if they captured the sunlight with polar or polarisable molecules. Just like water, for example, these are uncharged but they do have a plus and a minus pole or they can easily form these temporarily. Consequently, the material acquires a high dielectric constant. The research was carried out with the help of a computer model that could calculate the efficiency with which new materials can convert sunlight into electricity. The outcome of this was that the use of such materials considerably improves the efficiency of the solar cells.

The three researchers also examined several properties of the light-sensitive material, such as the colour and how the donor and acceptor work together. In a solar cell electricity is produced because a light particle (photon) gives an electron in the donor material extra energy. After that the electron can jump to an acceptor, which consequently receives a negative charge. "The jump made by such an electron costs energy. By optimising the interaction between donor and acceptor this energy loss can be minimised. In our article we show what the consequences of this are for the efficiency."

In addition to this, some molecules change shape if they absorb or emit an electron. "That also costs energy, which is lost in the form of heat. This is undesirable because you want a solar cell to produce as much electricity as possible," says Hummelen. The researchers therefore calculated how large the change in shape may be without losing too much efficiency.

Contact
Prof. J.C. (Kees) Hummelen, Stratingh Institute for Chemistry & Zernike Institute for Advanced Materials
University of Groningen
+31 (0)50 363 55 53 / +31 (0)6 12 53 27 79

Reference and link to publication
'Pathways to a new efficiency regime for organic solar cells', L.J.A. Koster, S.E. Shaheen and J.C. Hummelen, Advanced Energy Materials, DOI: 10.1002/aenm.201200103