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Instruments / Technical Area	Publication Details	Published	Actions
	Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(II)/Sn(IV) doping Chantalle J. Krajewska , Seán R. Kavanagh , Lina Zhang , Dominik J. Kubicki , Krishanu Dey , Krzysztof Galkowski , Clare P. Grey , Samuel D. Stranks , Aron Walsh , David O. Scanlon , Robert G. Palgrave Chemical Science 12, 14686 - 14699 DOI: 10.1039/D1SC03775G Open Access Show Abstract ▼ Abstract: Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs3Bi2Br9, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr2 to the solution-phase synthesis of Cs3Bi2Br9 leads to substitution of up to 7% of the Bi(III) ions by equal quantities of Sn(II) and Sn(IV). The nature of the substitutional defects was studied by X-ray diffraction, 133Cs and 119Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(II) and Sn(IV) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(II) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(II) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure–property relationships of lead-free vacancy-ordered perovskite structures.	Nov 2021
	Hidden spontaneous polarisation in the chalcohalide photovoltaic absorber Sn2SbS2I3 Seán R. Kavanagh , Christopher N. Savory , David O. Scanlon , Aron Walsh Materials Horizons 8, 2709 - 2716 DOI: 10.1039/D1MH00764E Open Access Show Abstract ▼ Abstract: Perovskite-inspired materials aim to replicate the optoelectronic performance of lead-halide perovskites, while eliminating issues with stability and toxicity. Chalcohalides of group IV/V elements have attracted attention due to enhanced stability provided by stronger metal-chalcogen bonds, alongside compositional flexibility and ns2 lone pair cations – a performance-defining feature of halide perovskites. Following the experimental report of solution-grown tin-antimony sulfoiodide (Sn2SbS2I3) solar cells, with power conversion efficiencies above 4%, we assess the structural and electronic properties of this emerging photovoltaic material. We find that the reported centrosymmetric Cmcm crystal structure represents an average over multiple polar Cmc21 configurations. The instability is confirmed through a combination of lattice dynamics and molecular dynamics simulations. We predict a large spontaneous polarisation of 37 μC cm−2 that could be active for electron–hole separation in operating solar cells. We further assess the radiative efficiency limit of this material, calculating ηmax > 30% for film thicknesses t > 0.5 μm.	Oct 2021
	Solvent engineered synthesis of layered SnO for high-performance anodes Sonia Jaśkaniec , Seán R. Kavanagh , João Coelho , Seán Ryan , Christopher Hobbs , Aron Walsh , David O. Scanlon , Valeria Nicolosi Npj 2d Materials And Applications 5 DOI: 10.1038/s41699-021-00208-1 Open Access Show Abstract ▼ Abstract: Batteries are the most abundant form of electrochemical energy storage. Lithium and sodium ion batteries account for a significant portion of the battery market, but high-performance electrochemically active materials still need to be discovered and optimized for these technologies. Recently, tin(II) oxide (SnO) has emerged as a highly promising battery electrode. In this work, we present a facile synthesis method to produce SnO microparticles whose size and shape can be tailored by changing the solvent nature. We study the complex relationship between wet-chemistry synthesis conditions and resulting layered nanoparticle morphology. Furthermore, high-level electronic structure theory, including dispersion corrections to account for van der Waals forces, is employed to enhance our understanding of the underlying chemical mechanisms. The electronic vacuum alignment and surface energies are determined, allowing the prediction of the thermodynamically favoured crystal shape (Wulff construction) and surface-weighted work function. Finally, the synthesized nanomaterials were tested as Li-ion battery anodes, demonstrating significantly enhanced electrochemical performance for morphologies obtained from specific synthesis conditions.	Mar 2021
	Bandgap lowering in mixed alloys of Cs2Ag(SbxBi1−x)Br6 double perovskite thin films Zewei Li , Seán R. Kavanagh , Mari Napari , Robert G. Palgrave , Mojtaba Abdi-Jalebi , Zahra Andaji-Garmaroudi , Daniel W. Davies , Mikko Laitinen , Jaakko Julin , Mark A. Isaacs , Richard H. Friend , David O. Scanlon , Aron Walsh , Robert L. Z. Hoye Journal Of Materials Chemistry A 8, 21780 - 21788 DOI: 10.1039/D0TA07145E Open Access Show Abstract ▼ Abstract: Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide bandgaps, which limits photoconversion efficiencies. The bandgap can be reduced through alloying with Sb3+, but Sb-rich alloys are difficult to synthesize due to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We develop a solution-based route to synthesize phase-pure Cs2Ag(SbxBi1−x)Br6 thin films, with the mixing parameter x continuously varying over the entire composition range. We reveal that the mixed alloys (x between 0.5 and 0.9) demonstrate smaller bandgaps than the pure Sb- and Bi-based compounds. The reduction in the bandgap of Cs2AgBiBr6 achieved through alloying (170 meV) is larger than if the mixed alloys had obeyed Vegard's law (70 meV). Through in-depth computations, we propose that bandgap lowering arises from the type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch between the Bi and Sb s and p atomic orbitals, coupled with their non-linear mixing, results in the alloys adopting a smaller bandgap than the pure compounds. Our work demonstrates an approach to achieve bandgap reduction and highlights that bandgap bowing may be found in other double perovskite alloys by pairing together materials forming a type II band alignment.	Oct 2020

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