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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. [ABSTRACT FROM AUTHOR]

Halide double perovskites have gained significant attention, owing to their composition of low‐toxicity elements, stability in air, and recent demonstrations of long charge‐carrier lifetimes that can exceed 1 µs. In particular, Cs2AgBiBr6 is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs2AgBiBr6 thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through field‐effect transistor and temperature‐dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths >1 µm found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single‐crystal properties. [ABSTRACT FROM AUTHOR]