Researchers around the world are competing to develop highly efficient solar cells that can generate electricity in an environmentally friendly manner. The widespread use of solar modules made of silicon are not the sole solution. University of Cologne (UoC) research teams rely on less common materials with very special properties.

By Mathias Martin

A laboratory room with several biological safety cabinets, so-called glove boxes. In a box, a pair of rubber gloves protrude from the glass front into the air, inflated by overpressure of inert gas. Inside the box, a rotating turntable slowly comes to a standstill, and a square, glass substrate is recognizable on the plate. It contains eight small dots arranged in a circle: ›Seven round organic solar cells and a connection contact,‹ explains Klaus Meerholz, slowly pulling his arms out of the glove box.

Professor Dr Klaus Meerholz is a group leader at the Institute of Physical Chemistry. The chemist came to the University of Cologne in 2002 to build up basic research on organic electronics at the institute. His research focuses on organic light-emitting diodes, which have become known as OLED and are mainly used in displays. Half of all smartphones now have such an OLED display. ›There is not nearly as good technology for displays as the OLEDs,‹ says Meerholz, who has applied for about 40 patents in the field of organic light-emitting diodes.

Substrate – the (mechanical) support on which the components are mounted, a square glass plate with 25 millimetres edge length, on which the various layers of material and the electrodes (anode and cathode) of the solar cells are applied. Alternatively, flexible plastic foils, for example, could be considered.

Organic electronics - Organic electronics uses semiconducting hydrocarbon compounds (small molecules and polymers). The electronic circuits are usually manufactured using thin-film technology on flexible plastic films.

In 2008, based on his numerous patents, he achieved extraordinary success in his research – the development of an OLED micro display of just one centimetre diagonally, but with the resolution of a Full HD TV.

Silicon or plastic?

Success in basic research, however, has a downside. As soon as an application such as the OLED is on the market, basic research becomes less relevant. Meerholz is therefore also researching organic solar cells, which in practice are very niche even though there is great potential for new fields of application for them. According to Meerholz ›Organic solar cells and OLEDs are closely related. Their production process is identical which is why the same tools are used in the laboratory.‹

Organic solar cells consist of hydrocarbon compounds, more precisely semiconducting polymers. These are specially processed plastics that can be used as a conductor material for electronic components. On Germany's roofs, and more recently on balconies, on the other hand, you can see almost exclusively solar modules made of semi-metal silicon. Solar cells made of silicon achieve an efficiency of about 25 percent. ›The efficiency indicates how much percentage of the solar energy incident on the solar cell is converted into electrical energy, i.e. how efficiently the solar cell works,‹ explains Dr Selina Olthof, who, like Meerholz, works as a group leader at the Institute of Physical Chemistry, researching solar cell materials. Solar cells made of silicon are now considered to be optimized and their efficiency can hardly be improved.

Meanwhile, in the laboratory, a turntable rotates again in a glove box, extremely fast; the substrate with the seven solar cells in the construction cannot be identified at all. The scientists call this process ‘spin coating‘, or rotational coating, which is carried out in the box under inert gas in order to protect the sensitive materials from oxygen and humidity. Due to the strong centrifugal forces generated, the substrate is evenly coated with a wafer-thin organic absorption layer, which later absorbs the solar radiation and converts it into electrical current. After the spin coating, a connection contact is attached and the substrate is then encapsulated. The protective layer has to be extremely dense, the organic material is very sensitive – otherwise the seven cells will be quickly destroyed.

All this effort is for the sole purpose of the experiment. The seven solar cells on the substrate are part of a series of tests with a total of fifteen different substrates. Using special equipment, the scientists carry out complex measurements on the solar cells in order to learn more about how different materials affect the efficiency but also the other properties of the cells.

A solar cell from the printer

It is precisely the other properties that make organic solar cells particularly interesting for researchers. In terms of efficiency, they are not as effective as the silicon cells. Organic cells made from polymers achieve an efficiency of approximately 18 percent in the laboratory. This is not bad but less than the 25 percent of silicon cells. Compared to silicon cells, however, solar cells made of organic materials can be produced in a more environmentally friendly manner. In addition, they can be produced very thinly and semi-transparently. Solar films on three-dimensional roof surfaces, on window panes or integrated into sun protection slats are thus possible. In addition, the organic absorber materials can be dissolved in special liquids, so that they can be produced cost-effectively with a specially designed inkjet printer. However, the shelf life of organic cells is worse than that of silicon cells, and so they have not yet established themselves on the market for the construction of larger solar modules.

Klaus Meerholz sees the great potential of organic solar cells primarily in indoor photovoltaics. Indoor devices that require little electrical energy, such as sensors or measuring devices are often operated. Energy from room lighting or daylight could be used to power these devices. ›Organic cells can use the light spectrum indoors ten times better than cells made of silicon, which is why they are particularly suitable for such indoor applications,‹ explains Meerholz.

A twin pack with more performance

In 2009, research discovered another material for solar cells. ›It was a bit of like going mad in science. Everyone wanted to participate, everyone wanted to set a new record. It was a bit of a gold rush mood,‹ says physicist Olthof. We are talking about the material class of perovskites. Through intensive research on solar cells made of perovskite, their efficiency was increased from 3 percent at the beginning to 22 percent. Perovskite cells are still not suitable for practical use, as their shelf life is very limited. However, they have a significant advantage over other types of solar cells, which suggests that further research is being carried out on them. Perovskite solar cells can absorb the sunlight spectrum particularly well.

On the basis of measurements in the laboratory, the perovskite material can be specifically ‘assembled’ and the researchers determine which wavelength range of the light spectrum is to be absorbed. The perovskite cell can thus make very good use of the available solar energy. In addition, perovskite can be dissolved in liquids, so that solar cells can be produced from perovskite as pure organic cells by printing on flexible films. Such a process requires significantly less energy than the production of silicon cells.

Perovskite – The mineral has a specific crystal structure with a mostly cubic lattice. For the production of perovskite cells, semiconducting, lead- and iodine-containing perovskite crystals are stacked with wafer-thin organic layers.

Even if a single perovskite cell already has a considerable efficiency, its efficiency can be increased even further. In a ‘twin pack’, these solar cells become even more powerful. For this purpose, two or more solar cells, for example two perovskite cells, are ‘stacked’ into a so-called tandem solar cell. By optimizing the individual cells for different areas of the sunlight spectrum, a tandem solar cell can make even better use of solar energy. ›The challenge is to adapt the material so that the upper cell absorbs as much sunlight as possible, allowing sufficient light and electrical energy to pass through to the lower cell,‹ explains Selina Olthof.

The solution is in the institutes's cellar: A futuristic looking, room-filling apparatus made of stainless steel, with numerous pipes, levers, hoses and hatches made of glass. ›This is our photoelectron spectrometer,‹ explains Olthof. ›This shows whether the electrons move between the two cells without a barrier. This allows us to precisely adapt the 1.5 nanometer thin layer of indium oxide, which is supposed to connect both cells in the best possible way, both optically and electrically, the so-called interconnect.‹

Last year, the two Cologne working groups, together with scientists from the University of Wuppertal and other universities as well as research institutions, succeeded in developing a tandem solar cell made of perovskite and organic absorber layers with an efficiency of 24 percent – a world record for this material combination. The measurements by Olthof and her team with the photoelectron spectrometer have contributed significantly to the optimization of the layer that connects the two individual cells in the tandem component.

›There are not many locations worldwide that combine as many different tools and the necessary know-how in one place as we do here at our institute. Physicists and chemists work closely together with us. This means that we can evaluate very quickly what and how something can be realized successfully,‹ says Meerholz.