11.08.22 „The perfect tandem partner“ Interview with Assistant professor Ulrich Paetzold from the Karlsruhe Institute of Technology • Reading time: 5 min.

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Assistant professor Ulrich Paetzold from the Karlsruhe Institute of Technology is carrying out research into innovative solar cells made from silicon and perovskite semi-conductors. In this interview, he gives details of the new approach and potential future efficiency levels. 

Ulrich Paetzold, what exactly does your research involve? 

We are working on perovskite-based tandem solar cells. The term “perovskite” describes materials with a crystal structure that were discovered in the 19th century. A few years ago, a new sub-category of perovskites was identified which have the characteristics of a semiconductor. These perovskite semi-conductors have amazing optoelectronic properties and we aim to use them in photovoltaic systems. In simple terms, we are applying a thin layer of these perovskite semiconductors onto conventional solar cells made of silicon to increase their efficiency by more than 25 percent. 

What brings about the improvement in efficiency? 

The silicon in normal solar panels can make particularly efficient use of only part of the energy from sunlight. The optical properties of the perovskite semiconductors can be tailored in such a way that they utilize the high-energy part of the solar spectrum much more effectively than the silicon solar cells. This gives us the amazing opportunity of creating the perfect tandem partner for existing silicon solar panels from this class of perovskite materials. Perovskite semiconductors can also be combined with other photovoltaic materials or with other perovskite semiconductors to make the best possible use of the solar energy. We have just presented a first module of this kind in a scientific paper. 

How is the work divided between the perovskites and the silicon in a tandem module? 

The perovskite semiconductor absorbs the high-energy, short-wavelength sunlight while the lower-energy, longer-wavelength light passes into the silicon solar cell underneath. In the “27plus6” project, we are working with the Institute for Solar Energy Research in Hamelin (ISFH) and centrotherm International AG with the aim of achieving an efficiency level of 33 percent: 27 percent in the silicon layer and an additional 6 percent with the help of the perovskites. 

What exactly is the structure of these solar cells? 

The “monolithic interconnection method” is particularly promising. This involves placing the perovskite layer directly onto the silicon module. The perovskite semiconductors in their liquid phase can be applied as a thin film on the surface of the silicon, for example using an inkjet printer, the slot-die process or co-evaporation. These are all relatively simple procedures, although the technical details can, of course, be highly complex. 

It sounds as if it will be relatively straightforward to take these tandem solar cells out of the lab and into mass production… 

The substrate for the tandem solar cells consists of conventional silicon solar cells. The second manufacturing step involves applying the perovskite top solar cell with the perovskite semiconductor layer and other functional layers, which naturally increases the complexity of the component. The first pilot production lines are already being operated by our industrial partners and we are working with German machinery manufacturers and technology companies to develop efficient manufacturing processes with high throughput rates for large modules. 

Are there other challenges involved? 

We need to improve the stability of the perovskites even further. I am optimistic that we will succeed in doing this, because a few years ago the perovskites were much less stable than they are today. At that time, we were talking about stability for only a few hours, but now they are stable for thousands of hours. But, of course, that is nowhere near 30 years, which is the lifetime of silicon solar cells. This means that over the next few years we will still have some challenges to overcome relating to technologies and materials. Long-term support for research and development remains crucial for the rapid transfer of the technology. 

So it is not worth waiting for the new, more efficient tandem solar cells. 

It will definitely be a few years before our new technology is widely available on the market, despite the fact that we have seen a huge increase in interest from industry along the entire supply chain over the last two years. For that reason, if you are able to install a photovoltaic system on your roof, you should do it now rather than waiting. 

At the moment, the security of supply of energy and raw materials is an important consideration. What is the situation with the perovskite semiconductors? What are they made from and are the raw materials readily available? 

Perovskite semiconductors are based on an organic cation consisting of lead and halogenides, so the raw materials are available. The problem is the use of lead, depending on the area, which means that recycling concepts for the solar modules are needed. However, it is used in very small quantities, as we only apply very thin layers of perovskite to the tandem solar cells. 

To finish, can we take a brief look into the future? How efficient will commercially available solar modules be in the long term? 

The theoretical limit for silicon is around 29 percent. In the case of commercial silicon modules, I think the threshold is between 25 and 26 percent. Tandem modules made from silicon with perovskite semiconductors could in theory achieve an efficiency level of 40 percent. I am certain that we will soon see the first panels with efficiency levels above 33 percent. In the medium term, the commercially available panels will be around 30 percent efficient. That will not happen tomorrow, but it will not take ten years either. 


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