46 47 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 Nanoscale impurities seed degradation in novel solarmaterials Surfaces - Earth Sciences & Environment - Sustainable Energy Systems - Energy - Physics - Climate Change - Physical Chemistry - Energy Materials - Chemistry - Materials Science - Interfaces and Thin Films - Perovskites - Metallurgy Perovskite materials offer a cheaper alternative to silicon for producing solar cells and also show great potential for other optoelectronic applications, including energy-efficient LEDs and X-ray detectors. Metal halide salts - abundant andmuch cheaper to process than crystalline silicon - can be prepared in a liquid ink used to print a thin film of the material. In the past decade, improvements in the design and fabrication of metal halide perovskite (MHP) based solar cells have seen their efficiencies rise to compete with incumbent technologies and have laid the pathway to commercialisation. However, MHP stability, and thus the longevity of these light-harvesting devices, remains deficient. A multidisciplinary team of researchers used Diamond’s Hard X-ray Nanoprobe beamline (I14) and the electron Physical Science Imaging Centre (ePSIC) to gain new insight into the perovskite materials that hold so much potential in the field of optoelectronics. Low-dose Scanning Electron Diffraction (SED) measurements performed at ePSIC allowed the team to map the crystallography of their MHP samples with 5 nm resolution at various stages of ageing under light exposure, without triggering additional electron beam-induced degradation. They also used complementary experiments at I14 to survey the various crystallographic structures present. Their results showed that photochemical degradation of the MHP samples (manifesting as a change in crystal structure and eventual amorphisation) initially occurs in very localised sample regions. Crucially, these sample regions are crystalline grains or boundaries associated with unwanted material phases such as hexagonal polytypes and lead iodide - the same nanoscale structures that compromise light harvesting efficiency. The team concludes that degradation seeds at phase impurities due to their high density of defects, which act as both non-radiative recombination sites for charge carriers and fuel for fatal redox photochemistry. They uncovered one method of mitigating the formation of sinister hexagonal polytypes: controlled octahedral tilting of the perovskite lattice. Their findings suggest that the localised presence of phase impurities are direct indicators of failure points in the absorber layer. The detection of such species through nanoscopic screening (e.g. high resolution electron microscopy) offers a means of predicting sites of instability during film optimisation and manufacturing for application in solar cells. There are several strategies for inducing beneficial octahedral tilt, including tuning the perovskite A-site cation or adding passivating organic molecules. New approaches should be developed to realise scalable, uniformly tilted and, thus, photo-stable MHP films on the manufacturing line. This research could significantly accelerate the development of long- lasting, commercially available perovskite photovoltaics. Related publication: Macpherson, S. et al . Local nanoscale phase impurities are degradation sites in halide perovskites. Nature 607 , 294–300 (2022). DOI: 10.1038/s41586- 022-04872-1 Funding acknowledgement: Royal Society and Tata Group (UF150033). Horizon 2020 programme (HYPERION - grant agreement number 756962) EPSRC (EP/R023980/1, EP/V012932/1, EP/T02030X/1 and EP/S030638/1) Corresponding authors: Prof Samuel D. Stranks, University of Cambridge,
[email protected] Dr Stuart Macpherson, Lumai LTD,
[email protected] Imaging andMicroscopy Group ePSIC and Beamline I14 Scanning Electron Diffraction (SED) maps of nanoscale photodegradation on ametal halide perovskite thin film. (a) Diffraction contrast image of fresh sample region before light exposure showing polycrystalline grain structure. Diffraction patterns identifying (b) a “pristine” perovskite grain and (c) hexagonal polytype perovskite phase impurities (highlighted yellow in (a)). (d) Diffraction contrast image of the same region after 1 hour of solar-equivalent light exposure. Light contrast regions are indicative of local material loss. (e, f ) SED patterns from the same locations showing minimal evolution in the pristine grain, and loss of crystallinity in the polytype impurities. Nanoprobe and ePSIC analysis techniques for sample return missions Earth Sciences & Environment – Geology – Geochemistry – Planetary Geology Space weathering is a collective process that causes a gradual alteration in the composition, structure and optical properties of asteroids and other bodies that move through the solar system without a protective atmosphere. The Hayabusa2 sample-return mission to near-Earth asteroid (162173) Ryuguprovides the first opportunity for laboratory studies of space-weathering signatures on a C-type asteroid, the most abundant type of inner solar system body. C-type asteroids are composed of materials that have remained largely unchanged since the solar system formed. The Hayabusa2 mission was designed to bring back samples from two different types of site on Ryugu. One sample collection was made at the surface, and the other was in a small crater excavated by firing a copper projectile at the asteroid from the spacecraft. The aim of comparing excavated samples from the crater to surface samples was to identify the processes that control the spectral properties – how we classify asteroid types from Earth – and the surface mineralogy of asteroids. The Hayabusa2 Earth Return Capsule landed in the Nullabor desert of Western Australia in 2020. Curated at the Japanese Space Agency Sagamihara Facility near Tokyo, samples were prepared for the international analytical team, including the University of Leicester, as a result of the preparatory work done at Diamond and ePSIC. The research team performed scanning and high-resolution Transmission Electron Microscopy on the Ryugu grains at ePSIC. They also used Diamond’s I14 beamline for nanoscale X-ray Absorption Spectroscopy and Fluorescence mapping on the same samples. Their results gave the most accurate determinations of the valency state of the iron in the outer rind of the Ryugu grains that had been exposed to space for millions of years. From the textural and spectroscopy results at I14 and ePSIC, combined with work from Hayabusa2 mission colleagues in Japan and worldwide, it has now been established that the surface of Ryugu has undergone intense bombardment by micrometeorite grains, the solar wind and galactic cosmic rays. Together this is what we now recognise as ‘space weathering’. New sample return missions to asteroids, Phobos (Mars’ largest moon and perhaps a captured asteroid), the Earth’s moon and, most ambitiously of all, Mars, over the coming decade, are designed to provide new insights about the evolution of the Solar System that can’t be gained from in situ analyses by landers and orbiters. However, the mass of returned samples is constrained, perhaps 40 drill tubes totalling 500 g from Mars. Synchrotron analyses will be essential to perform 3D spectroscopy on complex mineral assemblages at a microscopic scale for upcoming sample return missions. In addition to wanting a better understanding of the processes that have formed and altered the surfaces of the Solar System’s asteroids, including targets for new sample return missions such as Phobos, new concepts in space exploration of in situ resource utilisation - and even asteroid mining – require a better understanding of how remote spectra of asteroid surfaces relate to the composition of their interiors. Related publication: Noguchi, T. et al. A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu. Nature Astronomy 7 , 170–181 (2023). DOI: 10.1038/ s41550-022-01841-6 Funding acknowledgement: UK Space Agency Corresponding authors: Prof John Bridges, University of Leicester,
[email protected] Dr Leon Hicks, University of Leicester,
[email protected] Imaging andMicroscopy Group Beamline I14 and ePSIC Figure 1: TEM image taken at E01 ePSIC showing Ryugu serpentine and Fe oxide minerals. Figure 2: nano Fe XANES spectra from I14 indicating the Fe 3+ to Fe 2+ reduction due to space weathering, credits: 10.1038/s41550-022-01841-6.