1:30 PM - EQ05.02.01
Enhanced Visible Light Absorption in Layered Cs3Bi2Br9 Through Mixed-Valence Sn(II) / Sn(IV) Doping
Seán Kavanagh1,2,3,Chantalle Krajewska4,Lina Zhang1,Dominik Kubicki5,Krishanu Dey5,Krzysztof Galkowski5,Clare Grey5,Samuel Stranks5,Robert Palgrave1,Aron Walsh2,David Scanlon1
University College London1,Imperial College London2,CDT-ACM3,Massachusetts Institute of Technology4,University of Cambridge5
‘Perovskite-inspired’ materials have been placed under the scientific spotlight as promising candidates for next-generation photovoltaic and optoelectronic technologies.1–4 Amongst these, vacancy-ordered triple perovskites were initially proposed due to their A3B2X9 stoichiometry which facilitates the replacement of the toxic Pb2+ cation with a benign isoelectronic B3+ cation (e.g. Bi3+ or Sb3+) while preserving the perovskite crystal structure. Unfortunately, however, these materials tend to exhibit large bandgaps (> 2 eV), impeding their application in many photo-catalytic/voltaic devices.5,6
In this work,7 we demonstrate a drastic shift of over 1 eV in the optical absorption onset of Cs3Bi2Br9 (from 2.58 eV to 1.39 eV), upon doping with tin. The origin of this intense visible and near-infrared absorption is identified through a combination of theoretical and experimental characterisation (XRD, solid-state NMR, XPS). Sn atoms are found to disproportionate in the doped material, forming a mixed-valence species and inducing a strong intervalence charge transfer (IVCT) transition as well as electronic transitions to and from localised Sn-based states within the band gap, whilst preserving the structural integrity of the perovskite framework.
Sn(II) and Sn(IV) ions 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. Moreover, using hybrid Density Functional Theory, including spin-orbit coupling effects, alongside the Marcus-Hush theory of IVCT behaviour, we comprehensively elucidate the origin of unusual concentration dependence of absorption in the doped perovskite.
These results expand upon research on inorganic mixed-valent halides and perovskite-inspired materials, to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.
1 Huang, Y.-T.; Kavanagh, S. R.; Scanlon, D. O.; Walsh, A.; Hoye, R. L. Z. Perovskite-Inspired Materials for Photovoltaics and beyond—from Design to Devices. Nanotechnology 2021, 32 (13), 132004.https://doi.org/10.1088/1361-6528/abcf6d.
2 Buchanan, M. Perovskite Fever. Nature Physics 2020, 16 (10), 996–996. https://doi.org/10.1038/s41567-020-01060-8.
3 Li, Z.; Kavanagh, S. R.; Napari, M.; Palgrave, R. G.; Abdi-Jalebi, M.; Andaji-Garmaroudi, Z.; Davies, D. W.; Laitinen, M.; Julin, J.; Isaacs, M. A.; Friend, R. H.; Scanlon, D. O.; Walsh, A.; Hoye, R. L. Z. Bandgap Lowering in Mixed Alloys of Cs2Ag(SbxBi1-x)Br6Double Perovskite Thin Films. J. Mater. Chem. A 2020, 8 (41), 21780–21788. https://doi.org/10.1039/D0TA07145E.
4 Kavanagh, S. R.; Savory, C. N.; Scanlon, D. O.; Walsh, A. Hidden Spontaneous Polarisation in the Chalcohalide Photovoltaic Absorber Sn2SbS2I3. Mater. Horiz. 2021, 8 (10), 2709–2716. https://doi.org/10.1039/D1MH00764E.
5 Bass, K. K.; Estergreen, L.; Savory, C. N.; Buckeridge, J.; Scanlon, D. O.; Djurovich, P. I.; Bradforth, S. E.; Thompson, M. E.; Melot, B. C. Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9. Inorg. Chem. 2017, 56 (1), 42–45. https://doi.org/10.1021/acs.inorgchem.6b01571.
6 Nie, R.; Sumukam, R. R.; Reddy, S. H.; Banavoth, M.; Seok, S. I. Lead-Free Perovskite Solar Cells Enabled by Hetero-Valent Substitutes. Energy Environ. Sci. 2020, 13 (8), 2363–2385. https://doi.org/10.1039/D0EE01153C.
7 Krajewska, C.; Kavanagh, S.; Zhang, L.; Kubicki, D.; Dey, K.; Galkowski, K.; Grey, C.; Stranks, S.; Walsh, A.; Scanlon, D.; Palgrave, R. Enhanced Visible Light Absorption in Layered Cs3Bi2Br9 through Mixed-Valence Sn(II) / Sn(IV) Doping. Chem. Sci., 2021, https://doi.org/10.1039/D1SC03775G.