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

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jats:titleAbstract</jats:title>jats:pTernary chalcogenides have emerged as potential candidates for ultrathin photovoltaics, and NaBiSjats:sub2</jats:sub> nanocrystals (NCs) have gained appeal because of their months‐long phase‐stability in air, high absorption coefficients >10jats:sup5</jats:sup> cmjats:sup−1</jats:sup>, and a pseudo‐direct bandgap of 1.4 eV. However, previous investigations into NaBiSjats:sub2</jats:sub> 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 NaBiSjats:sub2</jats:sub> 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 NaBiSjats:sub2</jats:sub> photovoltaic devices. This work not only reveals how the charge‐carrier transport properties of NaBiSjats:sub2</jats:sub> 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 NaBiSjats:sub2</jats:sub> 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.</jats:p>


Publication status: Published

Funder: Helmholtz International Research School “Hybrid Integrated Systems for Conversion of Solar Energy”

Funder: Agency for Science, Technology and Research, Singapore

Funder: Institute of Materials Research and Engineering; doi:

Funder: Ministry of Education, Taiwan


ion migration, ternary chalcogenides, perovskite‐inspired materials, charge‐carrier dynamics, cation disorder, ligand exchange, nanocrystals

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Advanced Functional Materials

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European Union's Framework Programme for Research and Innovation HORIZON EUROPE (2021‐2027)
EPSRC (EP/R00661X/1, EP/P022464/1, EP/V014498/2, EP/W017091/1)
Royal Academy of Engineering (RF ∖201718∖17101)
AiF Projekt (ZIM‐KK5085302DF0)
Narodowa Agencja Wymiany Akademickiej (PPN/BEK/2020/1/00264/U/00001)
HORIZON EUROPE Marie Sklodowska‐Curie Actions (101061809)