by Johanna Denecke

You conduct research on organic solar cells, among other things. What advantages can offset their lower efficiency compared to conventional solar cells? Will they really ever be cheaper?

If you really want to use organic solar cells just to generate electricity, it does not really make sense. There is also the question of who would invest in something when they do not know if they get a very good return on investment? That is why we looked into what other fields of application there are, apart from the exclusive generation of electricity. There are, for example, special applications in architecture, the integration into textiles, or in other objects that can benefit from a non-breakable solar cell. These are applications where very high mechanical stability is required, where you really do not want glass or silicon that can break. These are mostly smaller self-sufficient energy applications. At an exhibition in Milan, for example, a kind of tent was presented with a roofing that had special organic solar cells integrated in it. In this specific case, the main purpose was of course to generate electricity, but it also showed other advantages, such as the transparency of these solar cells. In addition, the organic solar cells, that are not bound to a specific shape, can be interconnected, which allows the creation of entirely new geometric shapes in contrast to the silicon technology, which is bound to be square, rectangular, good. The question of whether this will ever be cheaper depends on what kind of market will develop in the future. Will there be a mass application for organic solar cells? Only in a mass market will the cost-benefit ratio be worthwhile. How much are the manufacturing costs – this will still be challenging – and who will buy it then?

What do these properties and limitations mean for OPV applications?

Above all, organic solar cells can of course be flexible and some of them can also be transparent. These are the main advantages over the conventional thin film solar cells or silicon solar cells or other types of highly efficient solar cells, with the limitation of course of a lower efficiency. Although work is being done to increase these efficiencies, there are also limits. For example, in order to really achieve an efficiency that comes close to a silicon solar cell, you do not just have to build one cell to absorb the light, you have to stack two cells on top of each other in order to create so-called tandem solar cells. 

Do you see any possibility that future research could increase efficiency even further? 

I would say so, yes. I am following Heliatek’s developments for example. They are trying to make the production technology for tandem solar cells more efficient so that they really catch up with existing technologies. Only then will they be able to serve the mass market and only then will they be able to generate the corresponding revenues that would allow a production plant to be fully utilized. This would then also be welcomed by the investors, if they could prove that there is a market for it. That is the crux that makes it so difficult, which is why this technology has not yet been established – although there are many interesting advantages that cannot be achieved with conventional technologies, such as flexibility and transparency.

What role does the possibility of printing solar cells play? Surely many different printable functions can be combined in this way? Is the solar cell still a single product or should it be thought and developed in a more systemic way? How do different research areas cooperate here?

So far, solar cells have been manufactured in a rather energy-intensive process. You have to grow the silicon crystal, then you have to separate it, then you have to reassemble it, then they have to be connected to each other. The organic solar cells, on the other hand, have a very small payback time, which means that you can get the investment you put into the production back relatively quickly by generating energy. There are two technologies in the production of OPV – either to evaporate the individual layers or to print them. In the evaporation process you put the substrates into a large vacuum chamber, pump out the air and evaporate the materials that absorb the light. For the other technology, the printing technology, we distinguish two different processes. One is the so-called roll-to-roll process. This means that I have a roll with a film on it, place the individual process steps there and roll up the film again. The second technology is the sheet-to-sheet technology, which means that the coating of one larger or several small solar cells is actually produced at the same moment. So we put a substrate in, it gets coated, it comes back out, next substrate in, out, there is no continuous process. And then there are the different coating processes that can be applied both to roll-to-roll and to sheet-to-sheet and that is what we are working on – these different printing processes. So first the materials have to be made printable. Simply put, we make an ink out of it. In the inkjet process the material required for the solar cell is applied only at the position where it is really needed, which is very efficient in terms of material utilization and therefore also very cost-effective. Surely not all layers can be printed properly yet. There are still electrodes, metallic materials, which cannot yet be printed so easily. However, we basically reformulate a wide variety of materials into so-called inks in order to build solar cells. It is important that the printed properties (besser: characteristics) of these are just as good as those from the vapour deposition layers. This is a major developmental step that is often underestimated. The inks have to be formulated specifically according to the structure of the solar cell.

What role does sustainability play, with a particular focus on renewable resources and recycling? In principle, organic materials in particular can also be produced from renewable raw materials and thus also represent nature’s ability to recycle.

Of course, the question of sustainability always comes up with us as well. Organic materials – what does that actually mean? Carbon, hydrogen and some other atoms, but there are no heavy metals in them. The layers of our OPVs are very thin, a hundred times smaller than human hair. This means that there is very little material in it, which means that the waste actually only consists of the substrates the solar cells are built upon and perhaps the foil with which the solar cell is closed against water and oxygen. So most of it are the films and you would have to ask the film manufacturers how to recycle it. We know that plastic is not particularly hip at the moment. It is important to take care of the recycling of the films, just as it is to recycle the silicon solar cells or the thin-film solar cells. Some examples of thin-film solar cells do not contain environmentally friendly metals, but the manufacturers say that nothing happens, because they are all collected and recycled. So let us believe this. But organic solar cells have the advantage that they can easily be burned, which is not as easy with silicon solar cells, at least not with so little energy. 

You said that the inks for the solar cells are specially tailored. Is it possible, hypothetically, to separate and recycle these mixtures?

No, not really, because the layers are so thin that they could not be seperated. There are always at least three layers and nobody would do that. I do not think there is any procedure for that.

The Application Centre for Innovative Polymer Technologies at Fraunhofer IAP offers the opportunity to work closely with the industry. Are there also (experimental) collaborations with representatives from the field of design? If so, are they fruitful?

If you take a look around right now – how far away is your next power outlet? In Europe that is not a problem at all, not even in America or Asia, but of course there are other countries in the world that do not have such an easy access to electricity everywhere. Among other subjects we work on realizing a self-sufficient energy supply, in combination with rechargeable batteries. The organic solar cells are very robust, you could step on them, you cannot cut into them, but there are a whole series of advantages that a polymer film has compared to a glass that can break. I am thinking, for example, of the integration into tents. We had a discussion about a developmental project in Africa and of course something like that would play a major role there, but as I said, we cannot find any manufacturers or a suitable business model.

In the field of OLEDs, for example, we did have collaborations with designers. We worked with Nike as a user who wanted to develop a jacket or with Puma and Hexonia. Also in the field of photovoltaics we have a regular communication and we ask time and time again questions like what else a solar cell could look like. We are dependent on the interaction between ourselves and the design players. Another example is our collaborative work with Mareike Gast in the field of flexible electronics. How can we design lighting surfaces today? How can we design solar surfaces? I do not think it makes sense economically to install solar cells on new roofs. Unfortunately, so far there are only a few in-roof solutions in this area. They are usually also very expensive. It does not really make sense to cover a roof and then screw solar cells on top of it. In my opinion that is a total waste of resources. That is what this project was supposed to inspire, a fertile exchange between design and research. 

I have another question about the Africa project you mentioned. Have you ever tried to work with designers, away from the design of products, but in the social field?

Yes, this particular project was actually such a collaboration. Morethanshelters GmbH was the name of our partner. Of course, he also needed a producer who could provide him with something like this. You know how difficult it is to acquire money in the humanitarian sector. Who gives the money for the investments, so that such things can be produced on a larger scale. That is how it usually fails. We can see for ourselves that the solar industry has virtually collapsed in Germany or even in Europe and that solar modules basically all come from China in the end. This competition has ruined the industry here in Germany, although there used to be a lot of investment in the recent past. Now you would not even need to apply for funding anymore related to organic photovoltaics, that is all over.

On the other hand, there are many really great approaches, such as the integration of solar cells into glasses or contact lenses. The KIT in Karlsruhe is doing this for example, as is the University of Ghent. This kind of thing can also be an application, not only on a large scale, but very small, in order to provide electricity for a small electronic system.

Where do you see the future of organic photovoltaics – the obstacles and potentials?

I see the potential mainly in two areas. Firstly, of course, to generate energy for energy self-sufficient applications on a scale where efficiency does not necessarily play a role, for example in areas where the sun shines for a long time and where the duration of the charging time of a battery does not play a role. There are regions in the world where this is quite conceivable. And then, of course, in special applications, as I mentioned earlier, when you need small energy sources to produce a self-sufficient power supply, to charge wearables or other electronic devices attached to the body.

I had already formulated the obstacles. It was mainly the lack of sufficient producers and investors, because nobody knows yet in which direction it will really go and how to make money with it.

Photo: Fraunhofer IAP

Armin Wedel studied physics at the University of Rostock and received his doctorate at the University of Potsdam. Since 1992 he has been working at the Fraunhofer IAP (Institute for Applied Polymer Research) in different functions and is engaged in the development of OLEDs, organic solar cells and quantum dots for display applications.