MRS Meetings and Events

 

EN02.06.04 2023 MRS Spring Meeting

Role of Ligand Exchange on the Optoelectronic Properties of NaBiS2 Nanocrystals

When and Where

Apr 12, 2023
11:00am - 11:15am

Moscone West, Level 2, Room 2002

Presenter

Co-Author(s)

Yi-Teng Huang1,Igal Levine2,Hannes Hempel2,Markus Schleuning2,Marin Rusu2,Artem Musiienko2,Szymon Zelewski1,Akshay Rao1,Robert Hoye3

University of Cambridge1,Helmholtz-Zentrum Berlin2,University of Oxford3

Abstract

Yi-Teng Huang1,Igal Levine2,Hannes Hempel2,Markus Schleuning2,Marin Rusu2,Artem Musiienko2,Szymon Zelewski1,Akshay Rao1,Robert Hoye3

University of Cambridge1,Helmholtz-Zentrum Berlin2,University of Oxford3
Ternary chalcogenides are gaining increasing attention for photovoltaics, and we recently showed NaBiS<sub>2</sub> to have intriguing properties, including &gt;10<sup>5 </sup>cm<sup>-1</sup> absorption coefficient at its pseudo-direct bandgap of 1.4 eV<sup>1</sup>. As a result, the spectroscopic-limited maximum efficiency (SLME) of this material can reach 26% with an absorber thickness of only 30 nm. This demonstration was made with nanocrystals (NCs) coordinated with long-chain organic ligands, which allowed us to probe the bulk properties, but severely limited intra-NC transport. Furthermore, we found that the inhomogeneous distribution of Na<sup>+</sup> and Bi<sup>3+</sup> cations, which both occupy the same crystallographic lattice site, results in the formation of S <i>p</i> states within the bandgap of NaBiS<sub>2</sub> and facilitates the formation of small hole polarons that intrinsically limit the bulk mobility of this material. <br/> <br/>In this work, we examined whether the intra-NC and inter-NC charge-carrier transport could be improved through ligand engineering. We showed through FTIR measurements that the long-chain organic ligands used during synthesis were completely substituted by short LiI, NaI, KI, or TMAI (tetramethylammonium iodide) ligands dissolved in methanol. NaBiS<sub>2</sub> NC films treated by these iodides all showed surface photovoltage (SPV) under illumination, with enhanced sum mobility from OPTP (optical-pump terahertz probe) and TRMC (time-resolved microwave conductivity) measurements. Together, these measurements suggest an enhancement in intra-NC mobility following ligand exchange. However, OPTP measurements showed that the enhanced mobility in iodide-treated films still decayed by ~80% within 1 ps, suggesting that carrier localisation cannot be suppressed through ligand exchange treatment. Nevertheless, from TRMC measurements, we noticed that the residual mobility after localisation was higher than the non-treated samples, meaning that more carriers could be extracted. Solar cells based on these iodide-treated NaBiS<sub>2</sub> NC films have reached external quantum efficiencies exceeding 50%, but their power conversion efficiencies were still below 1%. We showed that, apart from the low photocurrents limited by carrier localisation, ion migration is another hidden limitation for NaBiS<sub>2</sub> devices. We finish with a discussion of possible routes forward to overcome these limiting factors.<br/> <br/>1. Huang, Y. T. <i>et al.</i> Strong absorption and ultrafast localisation in NaBiS<sub>2</sub> nanocrystals with slow charge-carrier recombination. <i>Nat. Commun.</i> <b>13</b>, (2022).

Symposium Organizers

Eric Colegrove, National Renewable Energy Laboratory
Jessica de Wild, imec
Byungha Shin, Korea Advanced Institute of Science and Technology
Colin Wolden, Colorado School of Mines

Publishing Alliance

MRS publishes with Springer Nature