Progress in research into organic solar cells has been rapid in recent years. These cells are environmentally sensitive, can be printed onto large surfaces, and open completely new possibilities for husbanding the energy of the sun.
Photo credit: Thor Balkhed
What about printing organic solar cells onto windows not only to give shade but also to generate electricity, or built into a handbag to charge a telephone?
Two of the most highly cited researchers in the world in this field are active at Linköping University: Fengling Zhang and Olle Inganäs, both of them professors of biomolecular and organic electronics. They are not working on their own: the group around them includes Feng Gao, senior lecturer in the field, together with Mats Fahlman, professor in surface physics and chemistry, and Martijn Kemerink, professor of complex materials and devices, to name just three. At the Department of Physics, Chemistry and Biology alone, for example, around 20 doctoral students were working on organic solar cells in the autumn of 2016.
The most prestigious journals
The results of the hard work of these scientists can be seen in the most prestigious journals throughout the year: Nature Energy, Science, Advanced Materials, and several others. The research is advancing along many fronts and in collaboration with researchers in China, Germany and the US.
A recently concluded EU project, Sunflower, in which Mats Fahlman and his group took part, has shown that the environmental impact of the organic solar cells is very small. In addition, they discovered new combinations of materials for the next generation of organic solar cells.
The material is, namely, the key in the quest for evermore efficient organic solar cells. The researchers test new polymers and are seeking more efficient acceptor materials, which help to release the charge carriers in the solar cells and initiate electricity production.
Combinations of materials containing polymers and a stable acceptor have given the best solar cells currently available, where Fang Gao, Fengling Zhang and Olle Inganäs are holders of the world efficiency record: 11%.
This means that 11% of the energy in sunlight can be converted to electricity with the aid of plastic solar cells. However, the researchers expect better results.
This summer saw a further breakthrough in the field: combinations of materials in which not only the speed of transport of charge carriers to the electrodes is higher, but also the parameter known as the driving force - which is the energy that must be used to release electrons in the polymer - is lower. The driving force in the organic solar cells is approaching the driving force of silicon-based solar cells, which makes them increasingly interesting for commercial manufacture.
Photo credit: GORAN BILLESON
Fengling Zhang is working intensely to make solar cells easy to manufacture. New materials in the electrodes and a very thin layer next to them that allows only positively charged particles to pass through on one side and only negatively charged on the other, are two of her many contributions that increase the efficiency of the organic solar cells.
She is also working with new methods to achieve an even quality across the active layer when the solar cells are to be produced, printed, onto large areas.
The printed, flexible organic solar cells are on their way out from the laboratory, in work being done in collaboration with Tekniska verken in Linköping. Solar cell modules have been mounted onto windows at Tekniska verken and at Linköping University. This is part of the work to evaluate the manufacturing methods for semitransparent solar cell modules across large areas.
“It’s only a couple of years ago that we started to raise the efficiency of organic solar cells from a few percent to today’s level of 11%, so you can see that development is rapid,” says Fengling Zhang. She shows the most recent Chinese call for applications in which large grants are available for research into optical energy conversion and flexible organic solar cells.
Chinese PhD students
Many of the doctoral students who work with solar cells at LiU are from China. The US and the EU are also investing heavily in the field, while Swedish efforts are small.
“We would never be able to conduct the research with Swedish money alone. This technology can solve the world’s energy problems in an environmentally sensitive manner, but this is something that few Swedish funding agents have realised,” Olle Inganäs concludes.
Fengling Zhang agrees: “We would dearly like to have more Swedish doctoral students.”
The theoretical limit for the fraction of the sun’s energy that can be obtained in solar cells is around 33%. Laboratory experiments with silicon-based solar cells have achieved 25% at best. The limit for organic solar cells is lower.
“Most people say 15%, but I’m convinced that 20% can be achieved,” says Fengling Zhang.
If you instead measure energy, rather than power, the organic solar cells will very soon be on a par with silicon-based cells.
“Organic solar cells give 30% more energy than silicon-based cells. If we can manage to gather 15% of the energy from organic solar cells, this would correspond to the current efficiency of silicon-based cells, 18-20%,” says Olle Inganäs.
We can hope that the day is not so far away, not with all of the work that is being carried out around the world in which LiU, despite a low level of Swedish financing, is well to the fore. These cells may very well be on a window or handbag close to you in only a few years’ time.