Mixed-Cation Vacancy-Ordered Perovskites (Cs2Ti1–xSnxX6; X = I or Br): Low-Temperature Miscibility, Additivity, and Tunable Stability

Shanti M. Liga, Seán R. Kavanagh, Aron Walsh, David O. Scanlon, Gerasimos Konstantatos

Research output: Contribution to journalArticlepeer-review

Abstract

Lead toxicity and poor stability under operating conditions are major drawbacks that impede the widespread commercialization of metal–halide perovskite solar cells. Ti(IV) has been considered as an alternative species to replace Pb(II) because it is relatively nontoxic and abundant and its perovskite-like compounds have demonstrated promising performance when applied in solar cells (η > 3%), photocatalysts, and nonlinear optical applications. Yet, Ti(IV) perovskites show instability in air, hindering their use. On the other hand, Sn(IV) has a similar cationic radius to Ti(IV), adopting the same vacancy-ordered double perovskite (VODP) structure and showing good stability in ambient conditions. We report here a combined experimental and computational study on mixed titanium–tin bromide and iodide VODPs, motivated by the hypothesis that these mixtures may show a stability higher than that of the pure titanium compositions. Thermodynamic analysis shows that the cations are highly miscible in these vacancy-ordered structures. Experimentally, we synthesized mixed titanium–tin VODPs as nanocrystals across the entire mixing range x (Cs2Ti1–xSnxX6; X = I or Br), using a colloidal synthetic approach. Analysis of the experimental and computed absorption spectra reveals weak hybridization and interactions between Sn and Ti octahedra with the alloy absorption being essentially a linear combination of the pure Sn and Ti compositions. These compounds are stabilized at high percentages of Sn (x of ∼60%), as expected, with bromide compositions demonstrating greater stability compared to the iodides. Overall, we find that these materials behave akin to molecular aggregates, with the thermodynamic and optoelectronic properties governed by the intraoctahedral interactions.

Original languageEnglish
Pages (from-to)21399–21409
Number of pages11
JournalThe Journal of Physical Chemistry C
Volume127
Issue number43
Early online date19 Oct 2023
DOIs
Publication statusPublished - 2 Nov 2023

Bibliographical note

Acknowledgments:
S.R.K. acknowledges the EPSRC Centre for Doctoral Training in the Advanced Characterisation of Materials (CDT-ACM) (EP/S023259/1) for funding a Ph.D. studentship. D.O.S. acknowledges support from the EPSRC (EP/N01572X/1) and from the European Research Council, ERC (Grant 758345). The authors acknowledge the use of the UCL Kathleen High-Performance Computing Facility (Kathleen@UCL), the Imperial College Research Computing Service, and associated support services in the completion of this work. Via membership of the UK’s HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/L000202, EP/R029431, and EP/T022213), this work used the ARCHER2 UK National Supercomputing Service (www.archer2.ac.uk) and the UK Materials and Molecular Modelling (MMM) Hub (Thomas EP/P020194 and Young EP/T022213). The authors acknowledge financial support from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (Grant Agreement 725165) as well as from the European Union Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement 713729. This project has received funding also from the Spanish State Research Agency, through the Severo Ochoa Center of Excellence (CEX2019-000910-S), the CERCA Programme/Generalitat de Catalunya, and Fundacio Mir-Puig. The authors also acknowledge funding by the Fundacio Joan Ribas Araquistain (FJRA). This project was funded also by EQC2019-005797-P (AEI/FEDER UE).

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