Understanding the superpowers of perovskites

For the first time, researchers from Oxford’s Departments of Physics and Materials have managed to image hybrid metal halide perovskites with atomic-scale resolution providing new insights into these wonder-materials. A paper published in Science shares the groups’ findings about the materials’ remarkable self-healing powers; the findings further our critical understanding of how such perovskites work and are an essential step closer to the commercial production of perovskite solar cells.

Perovskites hold the promise of a new dawn for photovoltaic and optoelectronic applications – solar cells and light-emitting devices

Professor Laura Herz from Oxford’s Department of Physics and corresponding author, says: ‘Perovskites hold the promise of a new dawn for photovoltaic and optoelectronic applications – solar cells and light-emitting devices – but there is still much we need to know about them. We need to fundamentally understand these materials in order to fully harness their power. Up until now, researchers essentially had to guess how processing affected the material and so, in turn, the solar cell performance; our work will allow the field to directly determine how different fabrication routes affect the structure.’

Remarkably rapid recovery

The materials are very soft and therefore electron-beam sensitive however, using specialist electron microscopy imaging, the team were able to observe the perovskite structure at atomic resolution. They found that prolonged electron irradiation degraded the structure as expected but that an intermediate structure formed that allowed for the material’s rapid recovery. Further observations of the atomic arrangement within the hybrid perovskite films showed that small inclusions of lead iodide (which is used as an ingredient) form a surprisingly coherent transition boundary that nearly perfectly follows the surrounding perovskite structure and orientation. The observation suggests that lead iodide may seed perovskite growth and explains why an excess of lead iodide tends not to impede solar cell performance. Studying the material at this atomic level revealed essential information about the nature of the structure’s boundaries, defects and decomposition pathways.

Dr Mathias Rothmann, from Oxford’s Department of Physics and lead author, comments: ‘We have been able to study photoactive perovskite thin films with atomic resolution in their native thin-film state, which is what you find in a solar cell device. We were able to observe a range of different boundaries between the crystal domains in the film, which are very important for solar cell performance but not very well understood.

'We observed different types of defects that haven't been described for this material before, as well as the inclusion of a secondary material – all of which can potentially have a significant impact on the overall performance of the solar cells, but which are impossible to study without the level of resolution we have managed to get. This detailed level of understanding has been essential for the rise of silicon-based solar cells, and we can therefore only begin to imagine the impact it will have on the future of perovskite solar cells.’

Perovskites for commercial solar cells

Professor Herz concludes: ‘Our findings have revealed several, possibly unique, mechanisms that underpin the remarkable performance of this technologically important class of hybrid lead halide perovskites. The highly adaptive nature of the perovskite structure accounts for its exceptional regenerative properties and our atomic-resolution observations of interfaces within the material will contribute significantly to being able to eliminate defects and optimise interfaces. This is exactly the exciting kind of information we need to make perovskites ready for commercial solar cells.’

Read ‘Atomic-scale microstructure of metal halide perovskite’, in Science, Vol 370 Iss 6516.