Elucidating the Role of Ligand Engineering on Local and Macroscopic Charge-Carrier Transport in NaBiS₂ Nanocrystal Thin Films

Ternary chalcogenides have emerged as potential candidates for ultrathin photovoltaics, and NaBiS₂ nanocrystals (NCs) have gained appeal because of their months-long phase-stability in air, high absorption coefficients >10⁵ cm⁻¹, and a pseudo-direct bandgap of 1.4 eV. However, previous investigations into NaBiS₂ NCs used long-chain organic ligands separating individual NCs during synthesis, which severely limits macroscopic charge-carrier transport. In this work, these long-chain ligands are exchanged for short iodide-based ligands, allowing to understand the macroscopic charge-carrier transport properties of NaBiS₂ and evaluate its photovoltaic potential in more depth. It is found that ligand exchange results in simultaneous improvements in intra-NC (microscopic) and inter-NC (macroscopic) mobilities, while charge-carrier localization still takes place, which places a fundamental limit on the transport lengths achievable. Despite this limitation, the high absorption coefficients enable ultrathin (55 nm thick) solar absorbers to be used in photovoltaic devices, which have peak external quantum efficiencies > 50%. In addition, temperature-dependent transient current measurements uncover a small activation energy barrier of 88 meV for ion migration, which accounts for the strongly hysteretic behavior of NaBiS₂ photovoltaic devices. This work not only reveals how the charge-carrier transport properties of NaBiS₂ NCs over several length and time scales are influenced by ligand engineering, but also unveils the facile ionic transport in this material, which limits the potential of NaBiS₂ in photovoltaics. On the other hand, the discovery shows that there are opportunities to use this material in memristors, electrolytes, and other applications requiring ionic conduction.