A perfect match: perovskite meets perovskite

With your team, you have already been involved in several world records with tandem solar cells. Why are tandem solar cells such an exciting research topic?

Steve Albrecht: Already today, photovoltaics is one of the cheapest methods to sustainably generate electricity. Most solar modules are based on silicon and achieve efficiencies of around 21 %. However, this technology is already approaching its physical limits and only minor improvements are still possible. This is completely different with tandem solar cells: The first publication on perovskite-based tandem solar cells appeared in 2015, and since then we have experienced a rapid development towards efficiencies that are significantly higher than those of silicon.

How does a tandem solar cell manage to convert a larger proportion of sunlight into electrical energy?

A tandem solar cell usually consists of two different semiconductor materials with different band gaps. The semiconductor with a lower band gap efficiently converts red light into electrical energy, while the semiconductor with a larger band gap efficiently converts blue sunlight. This significantly increases the efficiency. We can imagine to not just stack two or three, but many different semiconductors in the future. Such devices have a much higher efficiency potential than the tandems that we currently consider.

What is the special charm of a tandem solar cell made of two different perovskite materials?

The hybrid organic/inorganic semiconductor materials from the perovskite family have very intriguing properties: Their band gap can be precisely tuned by changes in their chemical composition. We combine a perovskite material with a low band gap with a second one that has a high band gap. To do this, for example, we partially replace the element lead with tin, which reduces the band gap to about 1.25 eV. We then combine this material with a perovskite absorber layer with a particularly large band gap of about 1.8 eV, which in turn contains a lot of bromine. These perovskites have excellent optical and electrical properties, which allow to achieve high efficiencies. Theoretically, even tandem efficiencies well above 40 % are possible.

Why are perovskite materials so suitable?

Not only can the properties of perovskite semiconductors be entirely adapted to the respective requirements, there are already various cost-effective and large-scale manufacturing processes available. This allows to produce PV cells of arbitrary shapes or solar modules on flexible substrates. There are hardly any limits to the imagination. In addition, the production of pure perovskite tandem PV modules costs very little energy, so such modules already have a lower CO₂ footprint than, for example, silicon-based solar modules currently available on the market.

How did the work on all-perovskite tandem solar cells start?

The starting signal was the HIPSTER project, which is funded by the German Research Foundation (DFG). The name is of course a great fit for our young team here in Berlin; we constructed it of letters in this research project (see box). In the first phase of HIPSTER, we already showed that we can develop highly efficient full perovskite tandem solar cells. For this, we also used the self-assembled monolayers (so-called SAMs) that we developed. In the second phase of the project, we want to further improve the stability of the full perovskite tandem solar cells. To do this, we need to locate electrical losses within the entire tandem stack, for example at interfaces and contact materials. At the end of this fundamental research project, we would like to significantly increase the stability of the full perovskite tandem solar cells through better understanding of degradation mechanisms. Furthermore, we expect these tandem cells to achieve efficiency values similar to those of tandem cells made of silicon and perovskite.

You and your team are also involved in a large EU project on full perovskite tandem cells. What is that about?

In the SuPerTandem project, we want to develop a low-cost photovoltaic technology with a low CO₂ footprint together with partners from eight countries. Full perovskite tandem cells, which can be produced on flexible substrates using printing processes, are perfectly suited for this. Specifically, we want to contribute to the development of a perovskite tandem module on 10 × 10 cm² with an efficiency of over 30 %.

So far, perovskite solar cells are still considered quite fragile. How long will they work and what should happen to such modules when they have reached the end of their useful life?

There has already been a lot of improvement in long-term stability. We are aiming for stabilities that are on par with those of established silicon modules. Several project partners are also working on recycling concepts. The goal is a closed-loop recycling economy for full perovskite tandem modules.

How good are full perovskite tandem solar cells currently?

The highest efficiency reported so far for tandem solar cells consisting exclusively of perovskites is 28.0 %. However, no scientific details have been published on this yet. We recently succeeded in achieving a certified efficiency of 27.2 % with an all-perovskite tandem solar cell entirely developed and manufactured at HZB. This value is very close to the best mark and we already have ideas for further improvements. We assume that through further optimisations, efficiencies of 33 % are realistic with this technology.

Thank you very much for the interview.