Bandgap Lowering in Lead-Free Cs2Ag(SbxBi1-x)Br6 Double Perovskite Alloys
Sean R. Kavanagh a b c, Zewei Li d, David O. Scanlon b c e g, Aron Walsh a b f g
a Department of Materials, Imperial College London, United Kingdom, Prince’s Consort Road, South Kensington Campus, London, United Kingdom
b Thomas Young Centre, University College London, UK, United Kingdom
c Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom
d Cavendish Laboratory, Department of Physics, University of Cambridge, UK, JJ Thomson Avenue, Cambridge, United Kingdom
e Diamond House, Harwell Science and Innovation Campus, Diamond Light Source, Didcot OX11 0DE, United Kingdom
f Department of Materials Science and Engineering, Yonsei University, Seoul, KR, Korea, Republic of
g The Faraday Institution, Quad One, Harwell Science and Innovation Campus, Didcot, UK, United Kingdom
Online Conference
Proceedings of Internet Conference on Theory and Computation of Halide Perovskites (ComPer)
Online, Spain, 2020 September 8th - 9th
Organizers: Giacomo Giorgi and Linn Leppert
Oral, Sean R. Kavanagh, presentation 022
Publication date: 4th September 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. Using relativistic hybrid Density Functional Theory, we probe the electronic structure of two promising members of this novel material class and their alloys, Cs2AgSbBr6 and Cs2AgBiBr6, which have only recently been synthesised.[2,3] The energetic alignment of electron states within these materials is accurately calculated, including consideration of deformation potentials to adequately account for the spurious supercell effects associated with such calculations. The electrostatic potential alignment method of Butler et al.[4] and the deformation potential method of Wei et al.[5] are implemented, though at a higher level of theory than previously performed (hybrid DFT). These computational investigations are combined with in-depth experimental measurements of composition, phase and electronic structure to yield detailed understanding of the underlying physical mechanism of bandgap lowering.

Our work reveals pathways to bandgap engineering in double perovskite alloys, such that they may be better suited to photovoltaic (indoor PV – Eg, ideal = ~2 eV[6] or tandem top-cells - Eg, ideal = 1.7-1.9 eV[7]) or photocatalytic applications.

Z.L. would like to thank Cambridge Trust and Chinese Scholarship Council for financial support. S.R.K. acknowledges funding from the EPSRC Centre for Doctoral Training in Advanced Characterisation of Materials (CDT-ACM)( EP/S023259/1), the use of the UCL Grace High Performance Computing Facility (Grace@UCL), the Imperial College Research Computing Service (doi.org/10.14469/hpc/2232), and associated support services in the completion of this work. Via membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work also used the UK Materials and Molecular Modelling (MMM) Hub for computational resources, which is partially funded by the EPSRC (EP/P020194). R.L.Z.H. acknowledges support from the Royal Academy of Engineering under the Research Fellowship programme (No. RF\201718\1701), the Isaac Newton Trust (Minute 19.07(d)), and the Kim and Juliana Silverman Research Fellowship at Downing College, Cambridge. 

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