Bandgap Lowering in Lead-Free Cs2Ag(SbxBi1-x)Br6 Double Perovskite Alloys

Seán R. Kavanagh^a^,^b^,^c, Zewei Li^d, Robert Palgrave^a, Robert Hoye^c, Aron Walsh^b^,^c^,^e, David O. Scanlon^a^,^b^,^f

^aDepartment of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom

^bThomas Young Centre, University College London, UK, United Kingdom

^cDepartment of Materials, Imperial College London, United Kingdom, Prince’s Consort Road, South Kensington Campus, London, United Kingdom

^dCavendish Laboratory, Department of Physics, University of Cambridge, UK, JJ Thomson Avenue, Cambridge, United Kingdom

^eDepartment of Materials Science and Engineering, Yonsei University, Seoul, KR, Korea, Republic of

^fDiamond House, Harwell Science and Innovation Campus, Diamond Light Source, Didcot OX11 0DE, United Kingdom

Materials for Sustainable Development Conference (MATSUS)
Proceedings of Online nanoGe Fall Meeting 20 (OnlineNFM20)

#PerEmer20. Perovskite III: Emerging Metal Halide Semiconductors

Online, Spain, 2020 October 20th - 23rd

Organizers: Dmitry Dirin, Jacky Even and Constantinos Stoumpos

Contributed talk, Seán R. Kavanagh, presentation 170
Publication date: 4th October 2020

Double perovskites have emerged as promising candidate materials for high-performance next-generation optoelectronic technologies, owing to the ability to replace the toxic Pb²⁺ cation with a pair of more benign cations (e.g. Ag⁺ and Bi³⁺), while preserving the perovskite crystal structure.[1] Although double perovskites are air-stable and have demonstrated long charge-carrier lifetimes,[2] most double perovskites, including the prototypical Cs₂AgBiBr₆, have prohibitively wide bandgaps, limiting photoconversion and photocatalytic efficiencies.[2]

In this work, we demonstrate a novel route to lowering the bandgap of these materials through non-linear mixing of metal-cation orbitals. We develop a solution-based route to synthesize phase-pure Cs₂Ag(Sb_xBi_1-x)Br₆ thin films, with the mixing parameter x tunable over the entire composition range. In doing so, we observe this system to disobey Vegard’s law, exhibiting significant bandgap bowing, such that mixed alloys demonstrate significantly reduced bandgaps, relative to the pure materials. We investigate the possible mechanisms for this nonlinear bandgap variation through relativistic hybrid Density Functional Theory (DFT) calculations, combined with in-depth measurements of the composition, phase and grain structure to yield detailed understanding of the underlying physical mechanisms of bandgap lowering. A type II staggered alignment of electronic states in these materials is found to facilitate non-linear orbital mixing at the band extrema, thus narrowing the alloy bandgaps.

Our work reveals pathways to bandgap engineering in double perovskite alloys, such that they may be better suited to photovoltaic (indoor PV – E_{g, ideal} = ~2 eV or tandem top-cells - E_{g, ideal} = 1.7-1.9 eV) or photocatalytic applications.

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Preprint available here: https://arxiv.org/abs/2007.00388

(Link to my contributed talk for the NanoGe ComPer (Conference on Theory and Computation of Halide Perovskites) focused solely on the theory aspect of this research work: https://www.youtube.com/watch?v=txaTYU9Pq1I)

References:

[1] Savory, C. N., Walsh, A. & Scanlon, D. O. Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells? ACS Energy Lett. 1, 949–955 (2016).

[2] Slavney, A. H., Hu, T., Lindenberg, A. M. & Karunadasa, H. I. A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications. J. Am. Chem. Soc. 138, 2138–2141 (2016).

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