Ultrathin Chalcogenide Light Harvesters—Charge-Carrier Transport and Fast NIR Photodetectors

Bismuth-based semiconductors have gained increasing attention as potential nontoxic alternatives to lead-halide perovskites [1]. Whilst most attention has been on bismuth-halide-based compounds, there is growing interest in broader families of materials, including chalcogenides, such as ABZ₂ materials (A = monovalent cation; B = Bi³⁺ or Sb³⁺; Z = chalcogen) [2]. This talk discusses our work on two such compounds: NaBiS₂ and AgBiS₂.<br/><br/>We show NaBiS₂ nanocrystals to have a steep absorption onset, with absorption coefficients reaching &gt;10⁵ cm^-1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-timescale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS₂. These unusual features arise due to cation disorder, with inhomogeneous disorder leading to localised S p states forming that contribute to the formation of small hole polarons [3]. Whilst this severely reduces charge-carrier mobilities, we find that it is still possible to extract charge-carriers in photovoltaic devices, with external quantum efficiencies &gt;50% reached at 450 nm wavelength [4].<br/><br/>The second half of this talk covers our recent work on AgBiS₂, which also has high absorption strength, such that films only 50 nm thick are required to achieve adequate light absorption. Given the small bandgap of 1.2 eV, we demonstrate the utility of this material in near-infrared photodetectors. We achieve high cut-off frequencies reaching 0.5 MHz at 940 nm wavelength, along with &gt;1 MHz cut-off frequencies in the visible wavelength range. Through detailed characterisation, we reveal the electronic-ionic transport properties of this material, and how these properties can be controlled to achieve fast NIR photodetectors. Finally, we demonstrate the practical application of these devices for heart beat monitoring [5].<br/><br/>Overall, in this talk, the critical role of cation disorder in these ternary chalcogenide systems is revealed, especially how they influence optical absorption and charge-carrier kinetics.<br/><br/><br/>[1] Ganose, Scanlon, Walsh, Hoye,* <i>Nat. Commun.</i>, 2022, <i>13</i>, 4715.<br/>[2] <i>Nat. Photon.</i>, 2022, <i>16</i>, 235.<br/>[3] Huang, Kavanagh, … Hoye,* <i>Nat. Commun.</i>, 2022, <i>13</i>, 4960.<br/>[4] Huang, … Hoye,* <i>Adv. Funct. Mater.</i>, 2024, 2310283. DOI: 10.1002/adfm.202310283<br/>[5] Huang, Nodari, …, Gasparini,* Hoye,* <i>Small</i>, 2024, <i>20</i>, 2310199