FAU researchers develop innovative dye-sensitized solar cell

The picture shows solar cells.
Solar cells (Image: pixabay | blickpixel)

More power from sunlight

Chemists at FAU have developed a process which can be used to significantly boost the performance of solar cells. Thanks to a process called singlet fission, researchers were able to double the number of electrons excited by incoming photons. The results were published in the renowned scientific journal ‘Angewandte Chemie’ (doi: 10.1002/anie.201801041 – ‘Singlet Fission for Photovoltaics with 130 % Injection Efficiency’, ‘Angewandte Chemie’).

If Germany wants to reach the climate protection targets it has set, it will have no choice but to tap into renewable energy sources. One of the most important ‘green’ energy sources available in abundance is the sun, whose rays can be converted into electricity. Modern solar cells, which consist of layers of silicon, are technically advanced and relatively cost-effective thanks to mass industrial production, but they have one disadvantage: they can reach a maximum efficiency of just 33 percent.

The so-called Shockley-Queisser limit assumes 100 percent internal fluorescence yield, as one photon of sunlight can in theory excite one electron in the silicon grid. In practice, however, external factors lead to considerable energy losses. These include reflections on surfaces and interfaces, electrical resistance and the fact that it is not possible to optimally exploit the entire spectrum of visible sunlight. ‘The most powerful silicon solar cells have now reached 30 percent efficiency, and the available technology has just about reached its limits,’ explains Dr. Andreas Kunzmann from the Chair of Physical Chemistry I. ‘New impulses can only be reached by taking an alternative technological approach.’

Singlet fission doubles number of charge carriers

This is exactly what Head of Department Prof. Dr. Dirk Guldi and his team are doing. As the external energy loss of a solar cell can only be reduced to a certain degree, the researchers are taking an alternative approach and have turned their focus to primary energy yield. They use the principle of singlet fission, in which one incoming photon excites not one, but two electrons. The principle itself is not new, but until now the process has only been able to be proven in solutions or highly crystalline materials, making it not particularly well-suited for use in solar modules.

Together with chemists at the University of Alberta, Canada, the scientists from Erlangen have now succeeding in making a more practical structure. They synthesised a novel pentacene molecule, a hydrocarbon compound in a solid state that acts as an organic dye and absorbs sunlight. The pentacene is combined with a photoelectrode consisting of indium-zinc oxide and an electrolyte made of lithium and iodine. According to Dr. Andreas Kunzmann, ‘this structure allows singlet fission to move forward from the realms of basic research to practical application.’

Internal quantum yield of 130 percent

Singlet fission in dye-sensitized solar cells could in theory lead to twice as much internal energy yield compared to conventional silicon cells. At present, the chemists from Erlangen currently reach an average of 130 percent. ‘At the moment we are keeping the charge very low, as exciting electrons in neighbouring molecules at the same time still leads to an unacceptably high loss of energy,’ explains Dr. Kunzmann. ‘We can increase efficiency once we are able to better control these loss processes.’ At the same time, the researchers are working to make their invention more durable and suited to the requirements of industrial production.

Further information

Dr. Andreas Kunzmann
Phone: +49 9131 8567468

Prof. Dr. Dirk M. Guldi
Phone: +49 9131 8527340