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

Abstract

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.

Tuning the optoelectronic properties of emerging solar absorbers through cation disorder engineering

“…Valakh et al have shown that when a Cu 2 ZnSnS 4 particle was excited by a high-power 514.5 nm wavelength laser, its Raman peaks shifted and broadened, indicating an increased degree of cation disorder. 64 In terms of the impact of an applied electric field on cation disorder, although a direct investigation has not yet been reported, significant ion migration has been observed in AgBiS 2 and NaBiS 2 devices under bias, 65,66 suggesting that the cations may be mobile. Nevertheless, more quantitative investigations will be needed to verify the effectiveness and reproducibility of these light- or electric field-induced changes in cation disorder.…”