Combined expertise for the energy transition

Mr. Albrecht, Mr. Jošt, what is the TAPAS project about?

Steve Albrecht: TAPAS is a European Partnering project between Germany and Slovenia. On the one hand, we have two working groups from Helmholtz-Zentrum Berlin and, on the other, Marko Jošt's working group at the Faculty of Electrical Engineering at the University of Ljubljana. Together we are researching tandem solar cells. For the energy transition and for the fight against climate change. We are developing these cells and we are trying to simulate and understand the performance under real operating conditions. By combining experimental measurements and computer simulations, we can learn a lot about these solar cells.

Marko Jošt: The good thing about TAPAS is that we combine the strengths of all the teams involved. For example, Steve's group is very experienced in the fabrication of tandem solar cells while my group in Slovenia specializes in studying the stability of just such solar cells under operating conditions.

Tell us more about the know-how that your working groups bring to the project.

Steve Albrecht: At Helmholtz-Zentrum Berlin, my working group is concerned with materials and cell development, and about increasing efficiency. In short, we build and optimize tandem solar cells.

Marko Jošt: My working group is in the department of electrical engineering. We are engineers and we have a lot of experience with optical simulations and with electrical and electronic development. So we bring a lot of expertise in optical modeling and in energy yield modeling to TAPAS. We predict how much energy the solar cells can produce over their lifetime. That allows us to compare different technologies, and we develop the electronics, both for stability tests and for solar simulators.

In the case of tandem solar cells, you also use perovskites in addition to conventional silicon. What makes this group of materials so special?

Steve Albrecht: One outstanding property of perovskites is that we can tailor them to our applications. The solar cells that you buy today for the most part are made of silicon. That is a chemical element. And its properties cannot be changed. But the characteristic feature of perovskites is their special structure, to which different chemical elements are combined. And depending on which elements we use in which mixing ratio, we change the properties of the finished perovskite. For example, we can define the colors of the material and thus determine what proportion of sunlight it absorbs particularly well. Unlike silicon, perovskites can be processed at low temperatures. This saves energy during production. Only extremely thin layers of it are needed. This saves starting material and weight and it makes it possible to apply them to flexible substrates such as films. These can adapt to any curvature and be used, for example, for tents, textiles, smart watches and in building integration. This means there is a wide variety of applications.

Marko Jošt: When it comes to energy, everything is also about price. This is especially evident in the current energy crisis. For a long time, photovoltaics were not competitive compared to conventional energy sources. With the development of solar cells made of crystalline silicon, they have now become a competitive energy source. Of course, we want to become even cheaper and more efficient, and this is where perovskite solar cells come into play as we can produce them very cost-effectively. The efficiency of a single layer is already comparable to that of silicon solar cells. But the great potential lies in combining several layers on top of each other, also in combination with silicon. This allows the yield to be increased significantly, and at a very low price increase per additional layer. On the one hand, this makes solar power cheaper. On the other hand, it also means that we need less space for the same amount of energy. Especially in Slovenia, there are always discussions about where we should put solar modules. I think this is also a very important point.

What are the goals you want to achieve with the TAPAS project?

Steve Albrecht: Of course, we want to further increase efficiency. We have also succeeded in doing that, and we recently set a new world record for tandem solar cells with over 32 percent. However, the performance of the solar cells under real conditions, i.e., outside on the roof or in the field, is at least as important. That's something else entirely compared to all the performance tests we can perform in the lab. To measure our solar cells, we have developed an infrastructure and special hardware. The University of Ljubljana is very strong in the development of hardware, of software and in the field of simulation. It is a very important goal of the project to understand how solar cells work indoors and how they do it outdoors.

Marko Jošt: There is one thing that is very interesting about this: If you test the stability of silicon solar cells indoors, you can infer the stability outdoors. With perovskite, that's not always the case. So what we measure indoors under simulated light and continuous conditions sometimes does not translate to outdoor operation. It’s therefore very important that we do stability tests outside and see how the cells behave in real life, because outdoors we have changing lighting conditions like day-night cycles, and there are temperature fluctuations. So we are now also in Ljubljana adapting our systems for different conditions and different device architectures. Whatever the current research trends reveal, we have to implement it in electronics.

What are the biggest challenges you are facing in this project?

Steve Albrecht: The biggest challenge in the last two years has been materials and hardware development and, of course, strengthening the post-Covid collaboration. We started in the middle of the pandemic so there was a little bit of a delay in hiring people and in meetings, especially traveling between Germany and Slovenia. But I think we made it a success. And for the next step, let's say for the next two years in TAPAS, we would like to make comparisons between the indoor and outdoor performance tests and make predictions. I mean, if we're going to sell these cells, we need stability guarantees on the order of a couple of decades. And nobody can wait 30 years until we have long-term experience under real conditions.

Marko Jošt: For that, we need a lot of statistical data, not just one or five solar cells. We need more like 500 to see that the processes are reproducible. So far, there are no reliable stability statistics. That's why we're developing electronics that can test a few hundred solar cells at a time over longer periods of time. And on this basis, we can then draw conclusions about what needs to be optimized. Because sometimes it turns out that the most powerful cell is not necessarily the most stable.