When cells age

Working in their laboratory at the Helmholtz-Zentrum Berlin in Adlershof, Dr. Alexander Schnegg and Dr. Klaus Lips are tracking down aging processes. However, the cells they are investigating in-between the enormous magnetic coils have nothing to do with biology, even if the Sun is their elixir of life. Rather, these are solar cells, or to be more precise, thin-film solar cells made of amorphous silicon. The Sun releases electrons in these cells so that the power can flow. Unfortunately, the Sun also impairs their function. In the first 100 operating hours their efficiency drops by between ten and twenty per cent, and only then stabilises. This aging process was already observed in 1976 and is called Staebler-Wronski Effect in recognition of the scientists who discovered it. However, what exactly happens is still not clear. Previously, there were simply no analysis procedures that were sensitive enough to study the damage caused in the ultra-thin layers.

The solar researchers headed by Lips and Schnegg are now using tailor-made electron paramagnetic resonance spectroscopy methods (EPR spectroscopy in short) to gain new insights into the material. In the EPR Solar project, physicists from Berlin coordinate the collaboration with solar experts at the Forschungszentrum Jülich, with EPR specialists at the TU München and the FU Berlin, as well as with simulation professionals at the Max Planck Institute for Iron Research in Düsseldorf. The project has been funded by the Federal Ministry of Education and Research since 2008 for a five-year term in total. By the end of this period, the researchers not only want to understand the processes taking place in the material itself, but also want to advance the development of EPR analysis.

After all, thin-layer solar cells are seen as a cost-efficient alternative to the expensive solar cells made of single crystalline silicon, many of which already shimmer blue on roofs practically everywhere. Although they only achieve half their efficiency at rates of up to nine per cent, the thin layers of amorphous silicon, which are only a few nanometres thick, use much less material and can easily be vapour deposited on substrates like glass or plastic. “This means they already pay back their energy consumption in less than a year, rather than only after two to four years, and that is the decisive advantage,” says Schnegg.

The aging processes in amorphous silicon are, as far as we know today, generated by defects in the nuclear structure, caused, in turn, by the sunlight. Defects like these impair the power flow and so reduce the efficiency levels. Hydrogen atoms are thought to play an important role both in the formation and repair of the defects, which also exist in the material itself. However, it is still unclear where these hydrogen atoms reside and exactly how they affect the defects.

To explain these questions, the researchers use a particular physical characteristic of the defects. Because that is exactly where the unpaired electrons are found that possess a magnetic moment. This so-called spin is influenced by the atoms in the surrounding area like a compass needle, a process that can be observed with the help of EPR spectroscopy. The physicists do this by attaching an external magnetic field and additionally beam microwaves into the samples. Knowing at which energies the spins change direction allows conclusions to be drawn as to the type of defects and how they are distributed. So it is possible, for example, to recognise whether a hydrogen atom really is directly nearby.

Meanwhile, the researchers have modified their method in such a way that they can gain this information millions of times more precisely than previously possible directly from the solar cell’s photocurrent. Hence, they automatically only take defects like these into consideration. “Since the project began, we have been able to substantially improve the sensitivity and resolution of the EPR spectrometer,” says Lips. And so the scientists were able to acquire new insights into where the hydrogen atom actually sits in the amorphous silicon.

The physicists expect to make further progress with their new Terahertz EPR spectrometer which they have set up at the Synchrotron Radiation Source BESSY at the Helmholtz-Zentrum Berlin in Adlershof. "It’s really been worth the effort," says Schnegg, "for if we can prevent the solar cell from aging, the cost of energy generation would fall by up to 30 per cent, and thin layer solar technology would already be competitive today."