Perovskite materials: Neutrons show twinning in halide perovskites
Dr. Michael Tovar working at FALCON at the neutron Source BER II. © HZB
The Laue camera captured the diffraction pattern. © HZB
Solar cells based on hybrid halide perovskites achieve high efficiencies. These mixed organic-inorganic semiconductors are usually produced as thin films of microcrystals. An investigation with the Laue camera at the neutron source BER II could now clarify that twinning occurs during crystallisation even at room temperature. This insight is helpful for optimising production processes of halide perovskites.
A good ten years ago, research teams discovered the class of semi-organic halide perovskites, which are now making a rapid career as new materials for solar cells. The mixed organic-inorganic semiconductors achieved efficiencies of over 25 percent within a few years. They take their name from their basic structure, which is very similar to that of the mineral perovskite (CaTiO3), but contains other components: halide anions, lead cations and organic molecular cations.
MAPI examined
In the case of the most important compound of the class, methylammonium lead iodide CH3NH3PbI3 (usually abbreviated as MAPI), which was also studied here, the molecular cations are methylammonium cations and the anions are iodide anions. Although more than 4000 publications on halide perovskites have appeared in 2019 alone, it has not yet been possible to fully understand their structure. In the case of MAPI this was attributed, among other things, to the fact that they are produced as polycrystalline films at elevated temperature and it was assumed that twinning occurs when they are cooled to room temperature.
Close examination with neutrons
The formation of twins is complex and can significantly change the material properties. It is therefore exciting to investigate this process more closely. "We have now crystallised MAPI at room temperature and analysed the crystals thus formed with the Laue camera Falcon on BER II," says Dr. Joachim Breternitz, HZB. Together with his colleagues Prof. Susan Schorr and Dr. Michael Tovar, he was able to determine from the data that crystals grown at room temperature also form twins. This gives a new insight into the crystallization and growth process of MAPI. "Our results indicate that the crystallisation nuclei have a higher symmetry than the bulk crystals," explains Breternitz.
With these insights, the synthesis of the technologically important thin films can be specifically optimised.
The neutron source BER II has provided neutrons for research until its scheduled shutdown in December 2019. "This was one of our last experiments at FALCON on BER II and I hope that we were able to make useful contributions right up to the end," says Breternitz.
- Thin nickel films boost green hydrogen productionUltra-thin nickel oxide coatings can significantly improve the performance of anion exchange membrane (AEM) water electrolysers, a promising technology for producing green hydrogen. This was demonstrated by a team led by HZB scientist Dr. Michelle Browne. The results show that a nickel oxide film only 10 nanometres thick outperformed conventional nickel oxide powder electrodes while using less than 1% of the catalyst material. The study is published in the Journal of Materials Chemistry A,
- Detailed insights at BESSY II into the oxidation processes of copperBefore pure copper oxide builds up, complex superstructures such as ‘29’CuxO, are formed. The new results on the '29' CuxO superstructure have relevance for catalyst research and corrosion protection in the development of safe copper containers for nuclear waste repositories.
- BESSY II: Evaporated perovskites in tandem solar cells improvedPerovskite-silicon tandem solar cells achieve significantly higher efficiencies than silicon solar cells on their own. One particularly attractive method is co-evaporation of the perovskite precursor molecules on top of the silicon subcell. Scientists at HZB have analysed film growth on the nanoscale at BESSY II and found a new way to improve the quality of the perovskite layer: adding a thin seed layer of caesium chloride between the two sub-cells promotes uniform perovskite growth and suppresses the formation of undesired lead iodide at the interface.