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] However, the semiconductors explored thus far have gradually-increasing absorption onsets, and their charge-carrier transport is not yet well understood. The first half of this talk discusses our recent work on cation-disordered NaBiS₂ nanocrystals,^[3] which 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 nanocrystals. These unusual features arise because of the non-bonding S <i>p</i> character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes.<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.^[4]<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, Nodari, …, Gasparini,* Hoye,* arXiv: 2308.12250