Making two out of one

FAU chemists and physicists aim to increase the efficiency of solar cells considerably.

Doubling the amount of electricity produced by solar cells is just a difficult as it sounds. Chemists and physicists at FAU are carrying out intensive research on this topic. They are investigating singlet fission, a process in which one photon excites two electrons. Their work has recently produced key results that contribute to a better understanding of the process as a whole – and provide the basic foundations for considerably more efficient solar cells.

The idea behind singlet fission is not new – the process was in fact discovered 50 years ago. However, research in this field has only begun to make real progress over the last ten years after researchers in the USA had first recognised the potential of singlet fission for significantly increasing the efficiency of organic solar cells. Since then, researchers across the globe have been working on gaining a more detailed understanding of the fundamental processes and mechanisms behind it.

What is a singlet exciton state?

When a photon meets and is absorbed by a molecule, the energy level of one of the electrons in the molecule is increased. This high-energy state is known as a singlet exciton state. On its way back to its original low-energy state, the electron can be discharged via an external circuit, creating an electrical current. In a small number of cases a molecule may use its excess energy to excite a second molecule. After this has occurred, one electron in each of the two molecules is in a high-energy state. These states are known as triplet exciton states. In total, one photon generates two excited electrons, which in turn can be used to produce electrical current – two are made out of one.

As singlet fission often occurs on a picosecond timescale (1 picosecond is 10^-12 seconds) – i.e. in a millionth of a millionth of a second – it is not yet fully understood and is very difficult to control. For this reason, a team of Erlangen-based researchers led by Prof. Dr. Dirk M. Guldi, Chair of Physical Chemistry I, Prof. Rik R. Tykwinski, Chair of Organic Chemistry I, and Prof. Dr. Michael Thoss, Chair of Theoretical Solid-State Physics, are researching this process in order to gain a better understanding of it which will allow them to control and influence it. Their work is being supported by FAU’s Emerging Fields Initiative.

In their study the researchers investigated pentacene dimers – hydrocarbons that are promising potential candidates for use for singlet fission in solar cells – in an organic solution. They exposed the liquid to light to see how the molecules reacted to the radiation, whether the process took place and, if it did, according to what mechanisms and to what extent. In their experiments they varied the different parameters, such as the proximity and position, and the electronic communication between the individual pentacene molecules. They recorded the molecules’ reactions to the light radiation with special laser spectrometers that are capable of measuring such ultra-fast processes which take place within just a few picoseconds.

The results:

The arrangement of the molecules and their proximity is a crucial parameter. Another factor which increases the success is sufficient electronic coupling, i.e. the communication between the pentacenes involved at the electron level. Depending on the arrangement of the molecules, this can be achieved through the chemical bond or directly through the space. In their experiments the researchers were ultimately able to increase the triplet yield – which is usually 16 percent for pentacene – to an impressive 156 percent.

Following on from the results of their research their next goal is to produce and study other molecules based on pentacene. In doing so they aim to improve the molecular structure step by step in order to design systems that are even more suitable for singlet fission and can therefore be used as a basis for highly efficient solar cells made from low-cost, environmentally friendly materials. In the long term this method could help increase the efficiency of solar cells to up to 44 percent. The theoretical limit for silicon-based solar cells is currently around 30 percent, although the efficiency achieved in practice is considerably lower at 20 percent.

The abstract is available in the online journal PNAS at www.pnas.org/cgi/doi/10.1073/pnas.1422436112.

Further information:

Prof. Dr. Dirk M. Guldi
Phone: +49 9131 8527340
dirk.guldi@fau.de

Prof. Rik R. Tykwinski
Phone: +49 9131 8522540
rik.tykwinski@fau.de

Prof. Dr. Michael Thoss
Phone: +49 9131 8528834
michael.thoss@physik.uni-erlangen.de