Cation Disorder in ABZ 2 Chalcogenide Photovoltaics (NaBiS 2 & AgBiS 2 ) Seán Kavanagh 1,2 , Yi-Teng Huang, Robert L. Z. Hoye, Aron Walsh 2 , David O. Scanlon 1 1 Thomas Young Centre and Department of Chemistry, University College London, London WC1H 0AJ, U.K. 2 Thomas Young Centre and Department of Materials, Imperial College London, London SW7 2AZ, U.K. I-V-VI2 ternary chalcogenides have recently attracted growing attention as earth-abundant, nontoxic, and air- stable absorbers for photovoltaic applications.1,2 In particular, our recent work on the NaBiS2 & AgBiS2 members of this family has revealed ultra-strong optical absorption for these compounds – the highest of all current PV materials.3,3 A key benefit of such intense light absorption is that it allows for ultrathin (<100 nm) solar cell devices, dramatically reducing material consumption, weight and manufacturing demand, directly lowering the cost and facilitating applications in space for example, in addition to benefitting quantum efficiency and photovoltaic (PV) performance. Our work on AgBiS23 showed the crucial importance of controlling cation distribution and disorder in these materials, yielding record-breaking efficiencies >9% – the highest of any Bi-based solar absorber.3 However, the impact of disorder on the charge-carrier properties in these materials is remains poorly understood. Herein, we investigate the key properties which dictate the relationship between disorder on the cation sublattice and carrier transport in these materials. We find the band-edge orbital character to be a crucial factor in the sensitivity of carrier localisation (and thus solar cell efficiency) to cation disorder, resulting in ultra-fast carrier trapping, despite slow carrier recombination, in NaBiS2.3 This work reveals the critical role of cation disorder in the photovoltaic performance of these systems, alongside key considerations for future research in this area. References 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. Schnepf, R. R.; Cordell, J. J.; Tellekamp, M. B.; Melamed, C. L.; Greenaway, A. L.; Mis, A.; Brennecka, G. L.; Christensen, S.; Tucker, G. J.; Toberer, E. S.; Lany, S.; Tamboli, A. C. Utilizing Site Disorder in the Development of New Energy-Relevant 3 Cavendish Laboratory, University of Cambridge, Cambridge, U.K. 4 Department of Chemistry, University of Oxford, Oxford, U.K. Semiconductors. ACS Energy Lett. 2020, 5 (6), 2027–2041. https://doi.org/10.1021/acsenergylett.0c00576. 3. Wang, Y.‡ & Kavanagh, S. R.‡; Burgués-Ceballos, I.; Walsh, A.; Scanlon, D. O.; Konstantatos, G. Cation Disorder Engineering Yields AgBiS2 Nanocrystals with Enhanced Optical Absorption for Efficient Ultrathin Solar Cells. Nature Photonics 2022, 16, 235–241. https://doi.org/10.1038/s41566-021-00950-4. 4. Huang, Y.-T.‡ & Kavanagh S. R. ‡; . R.; Scanlon, D. O.; Walsh, A.; Hoye, R. L. Z. et al. Strong Absorption and Ultrafast Localisation in NaBiS2 Nanocrystals with Slow Charge-Carrier Recombination. Nature Communications 2022, 13, 4960 https://doi.org/10.1038/s41467-022-32669-3
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