Perovskite solar cells: perfection not required!
Simplified cross-section of a perovskite solar cell: the perovskite layer does not cover the entire surface, but instead exhibits holes. The scientists could show that a protective layer is being built up which prevents short circuits. © HZB
Experiments at BESSY II reveal why even inhomogeneous perovskite films are highly functional
Metal-organic perovskite layers for solar cells are frequently fabricated using the spin coating technique. If you follow the simplest synthesis pathway and use industry-relevant compact substrates, the perovskite layers laid down by spin coating generally exhibit numerous holes, yet attain astonishingly high levels of efficiency. The reason that these holes do not lead to significant short circuits between the front and back contact and thus high-rate charge carrier recombination has now been discovered by a HZB team headed by Dr.-Ing. Marcus Bär in cooperation with the group headed by Prof. Henry Snaith (Oxford Univ.) at BESSY II.
The early metal-organic perovskites exhibited efficiency levels of only a few per cent (2.2 per cent in 2006). That changed quickly, however: the record level now lies considerably above 22 per cent. The equivalent efficiency increase in the current commercially dominant silicon solar cell technology took more than 50 years. The fact that thin films made of low cost metal-organic perovskites can be produced on a large scale for example by spin coating and subsequently baking (whereby the solvent evaporates and the material crystallizes), makes this technology additionally attractive.
Holes in the perovskite film
Nevertheless, the thin perovskite film that results from spin coating on compact substrates is generally not perfect, but instead exhibits many holes. The samples from the pioneering perovskite group headed by Prof. Henry Snaith exhibit these holes as well. The problem is that these holes could lead to short circuits in the solar cell by the adjacent layers of the solar cell coming into contact. This would reduce the efficiency level considerably, which is not observed.
Thin layer is built up
Now Marcus Bär and his group, together with the Spectro-Microscopy group of the Fritz Haber Institute have carefully examined samples from Henry Snaith. Using scanning electron microscopy, they mapped the surface morphology. They subsequently analysed the sample areas exhibiting holes for their chemical composition using spectromicrographic methods at BESSY II. “We were able to show that the substrate was not really exposed even in the holes, but instead a thin layer is being built up essentially as a result of the deposition and crystallization processes there that apparently prevents short circuits”, explains doctoral student Claudia Hartmann.
.. and prevents short circuits
The scientists were able to ascertain at the same time that the energy barrier the charge carriers had to overcome in order to recombine with one another in the event of a direct encounter of the contact layers is relatively high. “The electron transport layer (TiO2) and the transport material for positive charge carriers (Spiro MeOTAD) do not actually come into direct contact. In addition, the recombination barrier between the contact layers is sufficiently high that the losses in these solar cells is minute despite the many holes in the perovskite thin-film”, says Bär.
The study is published in Advanced Materials Interfaces (2018): Spatially-resolved insight into the chemical and electronic structure of solution processed perovskites – why to (not) worry about pin-holes, C. Hartmann, G. Sadoughi, R. Félix, E. Handick, H. W. Klemm, G. Peschel, E. Madej, A. B. Fuhrich, X. Liao, S. Raoux, D. Abou-Ras, D. Wargulski, Th. Schmidt, R.G. Wilks,, H. Snaith, and M. Bär
DOI: 10.1002/admi.201701420
- The future of corals – what X-rays can tell usThis summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO2 is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?
Prof. Dr. Tali Mass, a marine biologist from the University of Haifa, Israel, is an expert on stony corals. Together with Prof. Dr. Paul Zaslansky, X-ray imaging expert from Charité Berlin, she investigated at BESSY II the skeleton formation in baby corals, raised under different pH conditions. Antonia Rötger spoke online with the two experts about the results of their recent study and the future of coral reefs.
- Susanne Nies appointed to EU advisory group on Green DealDr. Susanne Nies heads the Green Deal Ukraina project at HZB, which aims to support the development of a sustainable energy system in Ukraine. The energy expert has now also been appointed to the European Commission's scientific advisory group to comment on regulatory burdens in connection with the net-zero target (DG GROW).
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.