Cation Disorder in Emerging Ternary Chalcogenides Absorbers (I-III-S₂; NaBiS₂ & AgBiS₂)

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 – <i>the highest of all current PV materials</i>.³ A key benefit of such intense light absorption is that it allows for ultrathin (&lt;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.<br/><br/>Our collaborative work on AgBiS₂³ showed the crucial importance of controlling cation distribution and disorder in these materials, yielding record-breaking efficiencies &gt;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 NaBiS₂.⁴ We extend this analysis by explicitly calculating the phononic properties of these compounds, alongside Inelastic Neutron Scattering (INS) measurements, to probe the carrier de-population process in these compounds.<br/><br/>This work reveals the critical role of cation disorder in the photovoltaic performance of these disordered inorganic PV compounds, alongside key considerations for future research in this area.<br/> <br/>(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. <i>Nanotechnology</i> <b>2021</b>, <i>32</i> (13), 132004. https://doi.org/10.1088/1361-6528/abcf6d.<br/>(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 Semiconductors. <i>ACS Energy Lett.</i> <b>2020</b>, <i>5</i> (6), 2027–2041. https://doi.org/10.1021/acsenergylett.0c00576.<br/>(3) Wang, Y.^‡ & Kavanagh, S. R.^‡; Burgués-Ceballos, I.; Walsh, A.; Scanlon, D.; Konstantatos, G. Cation Disorder Engineering Yields AgBiS2 Nanocrystals with Enhanced Optical Absorption for Efficient Ultrathin Solar Cells. <i>Nat. Photon.</i> <b>2022</b>, <i>16</i> (3), 235–241. https://doi.org/10.1038/s41566-021-00950-4.<br/>(4) Huang, Y.-T.^‡ & Kavanagh, S. R.^‡; Righetto, M.; Rusu, M.; Levine, I.; Unold, T.; Zelewski, S. J.; Sneyd, A. J.; Zhang, K.; Dai, L.; Britton, A. J.; Ye, J.; Julin, J.; Napari, M.; Zhang, Z.; Xiao, J.; Laitinen, M.; Torrente-Murciano, L.; Stranks, S. D.; Rao, A.; Herz, L. M.; Scanlon, D. O.; Walsh, A.; Hoye, R. L. Z. Strong Absorption and Ultrafast Localisation in NaBiS2 Nanocrystals with Slow Charge-Carrier Recombination. <i>Nat Commun</i> <b>2022</b>, <i>13</i> (1), 4960. https://doi.org/10.1038/s41467-022-32669-3.