The future of energy is renewable. The composition of the mix of solar, wind, and hydroelectric power is still uncertain today. But one thing is sure: it must be possible to convert these forms of energy into each other. However, high losses occur in the process – the interface between materials is critical. This is precisely where e-conversion scientists from various disciplines come in and investigate the conversion processes so that they can run more efficiently in the future. We spoke to one of them about his research: Prof. Peter Müller-Buschbaum and his team at the TU Munich are focusing on energy materials for solar cells, among other things, and are convinced that with suitable materials, the sun could easily make households energy self-sufficient.
Prof. Peter Müller-Buschbaum is convinced that the right materials can significantly increase the yield of solar energy. Together with his team at the Technical University of Munich, he is looking for promising materials for a wide variety of solar cell types. (Photo: Andreas Heddergott/TUM)
What research topics does your research group focus on?
We research functional materials, particularly in the energy sector, and focus on batteries and thermoelectrics – the latter can convert heat into electricity – but above all, we deal with solar cells. I have long been enthusiastic about the latter, especially the further development of organic solar cells, perovskite, and quantum dot solar cells. I see great potential for these new types and a wide range of applications in the future.
Why can’t silicon solar cells keep up in this respect?
It is often misunderstood that the aim is not to replace many people’s classic solar cells on their roofs. Instead, we are interested in opening up further solar cell applications that are impossible with rigid silicon modules. These new devices can be semi-transparent, for example, so that windows can be fitted with them. This gives glass facades an additional function by allowing them to produce electricity, thus making high-rise buildings energy self-sufficient. The new solar cell types also score points for their high flexibility and low weight, making them attractive as photoactive fibers for clothing or photoactive paints for cars. There are still so many possibilities for harvesting solar energy. So far, we are only using a fraction of them.
How does the light yield of the new solar cell types compare with those based on silicon?
Silicon solar cells need a blue sky and full sunlight. If it is cloudy, they do not provide any electricity. The new types can compensate for this because they still work very well, especially when the light intensity is low. In addition, they absorb solar energy even better than silicon due to their semiconducting properties. Very thin layers of just 100 nanometers are often sufficient for these high-tech materials.
A thinner layer thickness is, of course, economically attractive. Can the new energy materials keep up economically regarding their manufacturing processes?
They do indeed have the potential to become really cheap. That’s why we are also working with our research to ensure that they are widely used in the future. The materials needed for organic solar cells, perovskite, and quantum dot solar cells can be produced using wet chemical processes. The starting materials are often available in large quantities and are inexpensive. In addition, smaller amounts of substances are required and processing usually does not require complex clean rooms. Another advantage is that the materials can be printed in layers on carrier substances. This makes the new solar cells very interesting for industrial production.
Off into space: The team led by TUM researcher Prof. Dr. Peter Mueller-Buschbaum (right) has sent solar cells into space for material tests. Together with Dr. Lennart K. Reb (left), he shows the payload module equipped with organic and hybrid solar cells0 that was installed in a rocket and tested under space conditions in a project with the German Aerospace Center. (Photo: Wei Chen / TUM)
Is printing solar cells comparable to printing paper?
In part, yes. Basic printing techniques such as inkjet and screen printing can also be used for solar cells. The roll-to-roll printing process is particularly interesting because it allows very large areas to be printed in a very short time. However, you need photovoltaic inks and the printing process itself is very complex from a physical point of view. I am looking at this in detail with my team. The active layers consist of complex structures built up through the self-organization of the molecular building blocks. The optimal nano-architecture is needed to efficiently generate, separate, and transport charge carriers. This is exactly where our research comes in: We assemble the solar cells ourselves and use special scattering experiments to observe live, so to speak, how the layers and structures form during the printing process. This, in turn, allows us to conclude how different components, additives, or solvents influence film formation and self-organization. And, of course, we also measure the structures themselves and the performance of the finished solar cell.
Can organic solar cells or perovskite solar cells keep up in terms of efficiency and service life?
Absolutely. You only need to take a look at the diagram from the National Renewable Energy Laboratory, which is well-known among energy researchers: Perovskite solar cells have almost caught up with silicon solar cells in terms of cell efficiency, for example, and organic solar cells are on the cusp of reaching 20 percent. One weak point of the new solar cell types is still their relatively short lifetime. That is why we want to use our research to better understand the aging process and determine why the materials become unstable at a molecular level over time. Unfortunately, the record-breaking solar cells often do not last as long. That’s why our findings are very helpful in designing a more stable molecule or improving the film formation process. Temperature profiles, drying times, and the selection of solvents play a decisive role here.
Are there other aspects that play a role for you?
We are also trying to establish more environmentally friendly components for the solar cell manufacturing process, for example “green solvents”. This brings its own challenges because surface tensions and evaporation rates change. However, we have already managed to adapt other process parameters so that more environmentally friendly solvents also work. This is a crucial aspect for industrial production. What is important for me and my team and what motivates us is that we have a big goal – the broad application of solar cells. Compared to mountain climbing, it is the summit we want to reach. No one would stop ten meters short of it and say: others can go the last ten meters. We deliberately have the entire value chain in mind.
The solar cell stars of tomorrow: in the glass tubes, quantum dots made of perovskite nanocrystals glow in all the colors of the rainbow under ultraviolet radiation. (Photo: Adobe Stock)
The fact that you care about the final steps and the overall context of the research is also evident from your overarching commitment. How are you networked when it comes to energy?
I founded the TUM Solar Keylab for solar energy research more than ten years ago, which I also head. It is integrated into the so-called Soltech network, which includes four other key labs at LMU Munich and the universities in Erlangen, Würzburg, and Bayreuth. The good idea here is that research is not carried out in competition but in collaboration. In the third funding period, the focus is now on solar water splitting. I am also very involved in the Renewable Energy Network (NRG) at the Munich Institute of Integrated Materials, Energy and Process Engineering (MEP) at the TU Munich. This brings together experts from a much broader context – engineers, architects, and electrical engineers. These interdisciplinary meetings broaden the perspective enormously. This particularly inspires young researchers and offers exciting starting points for joint projects. The circle is even larger on the Sustainability Board of the TU Munich, where I represent the natural sciences and energy aspects. Medical scientists, social scientists, and political scientists are also involved here, for example, and the range of issues and projects is even broader. The sustainable transformation of our society must be considered holistically.
Back to the solar cell: Where do you see perovskite, organic and quantum dot solar cells in the future?
The latter is something of a rising star. Quantum dots are a new class of fluorescent nanocrystals that can efficiently produce brilliant colors. The particle size is used to adjust the absorption properties in a unique way and almost the entire spectral range is accessible. For example, we have combined them with triboelectric components, which convert kinetic energy into electrical energy. The idea is that the solar cell harvests solar energy during the day and wind energy can be used at night when the component is bent by the wind, for example. In this way, two energy generation modes could be combined. As already indicated at the beginning, the sun could make every household energy self-sufficient if we capture it everywhere: Windows could be designed as solar cells, photovoltaic wall paints or indoor photovoltaics will be possible in the future, as will photovoltaic clothing. However, all of these applications require new materials, which we are researching at full speed.