New insights into the structure of organic-inorganic hybrid perovskites
Movie showing the 3D crystal structure as a function of the cell modulation phase. (grey: Pb, brown: Br, black: C, blue: N; white: H) © HZB
10.00 sIn photovoltaics, organic-inorganic hybrid perovskites have made a rapid career. But many questions about the crystalline structure of this surprisingly complex class of materials remain unanswered. Now, a team at HZB has used four-dimensional modelling to interpret structural data of methylammonium lead bromide (MAPbBr3), identifying incommensurable superstructures and modulations of the predominant structure. The study is published in the ACS Journal of Physical Chemistry Letters and was selected by the editors as an Editor's Choice.
Organic-inorganic hybrid perovskites have been intensively investigated for use in solar cells for about ten years. Thin films of such perovskites are inexpensive and already achieve high efficiencies. In addition, they can be perfectly combined with common solar cell materials such as silicon to form tandem cells. At the beginning of 2020, an HZB team was able to achieve a world record efficiency of 29.15 % with a tandem cell made of perovskite and silicon.
But despite the most intensive research, it has not yet been possible to precisely elucidate the crystal structures with their diverse modulations and superstructures as a function of temperature, even for the best-known perovskite compounds such as methylammonium and formamidinium lead halide.
Now, a team at HZB has analysed structural data of methylammonium lead bromide (MAPbBr3) with a novel model. Postdoc Dr. Dennis Wiedemann used a model that takes a fourth dimension into account in addition to the three spatial dimensions. The structural data were measured at a temperature of 150 Kelvin at the University of Columbia.
"The problem in these hybrid perovskites is the fact that the different modifications do not differ significantly in energy, so that even small temperature differences are sufficient to trigger phase transitions," explains Dr. Joachim Breternitz, co-author of the study. The data on the crystal structure therefore show an average value over many elementary cells, so that modulations and superstructures are not always recognisable. The new model explains the incommensurable superstructures observed in MAPbBr3 in a small temperature window around 150 K, which do not have the same periodicity as the crystal lattice. This complex structure comes from tilts and shifts in the crystal structure. "The new model will also provide more detailed insights into the modulated structures of other perovskite compounds," says Breternitz.
- 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.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.