Limitations and Prospects of NaBiS₂ Nanocrystals in Solar Cell Applications

The material class of metal chalcogenides has yielded a lot of promising compounds for optoelectronic devices and solar cells in particular. Besides bulk-based thin film technologies, there are also prominent examples of metal chalcogenide nanocrystals employed as solar cell absorber layers such as PbS and AgBiS₂.^1–3 They provide the unique opportunity to tune the materials band gap and other optoelectronic properties through changes in the particle size or through surface treatment and passivation, making compounds available or optimizing them for certain applications they would otherwise be less suitable for.⁴ But while the aforementioned two examples have shown great potential with solar cell efficiencies exceeding 10% and 8%, respectively,^2,5 developing new material platforms around non-toxic and earth-abundant materials is still crucial for optimal resource management. In that context, NaBiS₂ nanocrystals have been identified as an interesting and promising candidate with a high absorption coefficient, stability and a suitable band gap.⁶ However, its implementation into solar cell devices so far has proven to be challenging, but at the same time remains a largely unexplored topic.^7,8<br/>Therefore, the main focus herein is to investigate what has been preventing NaBiS₂ nanocrystals from being applied more successfully in solar cells so 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 does not occur as readily compared to similar systems such as AgBiS₂ for commonly used passivation agents like TBAI or EDT. Thus, other compounds and strategies are explored and tested to further a better understanding of the surface chemistry of this system to provide new selection guidelines for improved ligand exchange as a basis for successful device implementation.<br/> <br/><u>References</u><br/>1. Yuan, M., Liu, M. & Sargent, E. H. Colloidal quantum dot solids for solution-processed solar cells. <i>Nat. Energy</i> <b>1</b>, 16016 (2016).<br/>2. Liu, M. <i>et al.</i> Hybrid organic-inorganic inks flatten the energy landscape in colloidal quantum dot solids. <i>Nat. Mater.</i> <b>16</b>, 258–263 (2017).<br/>3. Bernechea, M. <i>et al.</i> Solution-processed solar cells based on environmentally friendly AgBiS2 nanocrystals. <i>Nat. Photonics</i> <b>10</b>, 521–525 (2016).<br/>4. Carey, G. H. <i>et al.</i> Colloidal Quantum Dot Solar Cells. <i>Chem. Rev.</i> <b>115</b>, 12732–12763 (2015).<br/>5. Wang, Y. <i>et al.</i> Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells. <i>Nat. Photonics</i> <b>16</b>, 235–241 (2022).<br/>6. Huang, Y. T. <i>et al.</i> Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination. <i>Nat. Commun.</i> <b>13</b>, 1–13 (2022).<br/>7. Medina-Gonzalez, A. M., Rosales, B. A., Hamdeh, U. H., Panthani, M. G. & Vela, J. Surface Chemistry of Ternary Nanocrystals: Engineering the Deposition of Conductive NaBiS2Films. <i>Chem. Mater.</i> <b>32</b>, 6085–6096 (2020).<br/>8. Huang, Y. T. <i>et al.</i> Elucidating the Role of Ligand Engineering on Local and Macroscopic Charge-Carrier Transport in NaBiS2 Nanocrystal Thin Films. <i>Adv. Funct. Mater.</i> <b>2310283</b>, (2024).