NaBiS2 as a next-generation photovoltaic absorber

Yi-Teng Huang1,2, Seán R. Kavanagh3,4,5, Marcello Righetto6, Marin Rusu7, Szymon J. Zelewski1, Samuel D. Stranks1,8, Akshay Rao1, Laura M. Herz6, David O. Scanlon3,5, Aron Walsh4, Robert L. Z. Hoye2 1Cavendish Laboratory, University of Cambridge, Cambridge, --, United Kingdom /2Inorganic Chemistry Laboratory, University of Oxford, Oxford, --, United Kingdom /3Department of Chemistry, University College London, London, --, United Kingdom /4Department of Materials, Imperial College London, London, --, United Kingdom /5Thomas Young Centre, University College London, London, --, United Kingdom /6Department of Physics, University of Oxford, Oxford, --, United Kingdom /7Struktur und Dynamik von Energiematerialien, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, --, United Kingdom /8Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, --, United Kingdom

I-V-VI2 ternary chalcogenides were gaining more attention recently owing to their high air stability along with earth-abundant and nontoxic compounds. More interestingly, some of them have been predicted to be potential “perovskite-inspired materials” that can exhibit defect tolerance. In this work, we investigated the optoelectronic properties of cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients >105 cm−1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, an ultrafast photoconductivity decay and long-lived charge-carrier population almost independent of defect densities were simultaneously observed in NaBiS2 nanocrystals. Through the density functional theory calculations, these unusual features are found to be highly associated with the non-bonding S p orbitals above the upper valence band, which can be further enhanced by cation inhomogeneity. Our work hence reveals the critical role of cation disorder on both absorption characteristics and charge-carrier kinetics of I-V-VI2 ternary chalcogenides.