Publication date: 11th March 2026
Over the last years, metal chalcogenide-based nanocrystalline compounds were shown to be promising candidates for various optoelectronic application and solar cells in particular. They provide the unique opportunity to tune the band gap and other properties of a specific material through changes in the particle size or through surface treatment and passivation. This adaptability can result in making compounds available or further optimizing them for certain applications they would otherwise be less suitable for.[1] Two prominent examples of this material class that have been successfully utilised as solar cell absorber layers are PbS and AgBiS2, resulting in device efficiencies exceeding 10% and 8%, respectively.[2,3]
Nevertheless, developing new material platforms around non-toxic and earth-abundant materials is still crucial for optimal resource management. With that in mind, NaBiS2 nanocrystals have been identified as an interesting and promising candidate due to its high absorption coefficient, stability and a suitable band gap.[4] However, its implementation into solar cell devices up to this point has proven to be challenging, but at the same time still remains a largely unexplored topic.[5,6]
Therefore, the main focus herein is to investigate potential reasons preventing NaBiS2 nanocrystals from being applied more successfully in solar cells thus far, as well as to explore strategies to overcome these issues. We have found that the conductivity of the nanocrystal film and its passivation remains a limiting factor, properties that are closely tied to the ligands employed in the nanocrystalline system. Results from X-ray photoelectron spectroscopy indicate that ligand exchange with commonly employed passivation agents, such as TBAI, does not occur as readily, compared to similar systems like AgBiS2. Further testing revealed that for NaBiS2 nanocrystals, it is rather the presence of a suitable inorganic cationic passivation component that is crucial in determining the resulting surface and overall chemistry of this system after the ligand exchange. Consequently, functional devices could be fabricated, while the new found insights regarding passivation and chemical modification in this material system will hopefully provide a starting point for further optimisation and improvement.
This work was financially supported by the Swedish Research Council (VR), the Wallenberg Initiative Materials Science for Sustainability (WISE), and ÅForsk.
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