MRS Meetings and Events

 

EN03.05.03 2022 MRS Spring Meeting

NaBiS2 as an Emerging Lead-Free Perovskite-Inspired Material—Defect Tolerance and PV Application

When and Where

May 10, 2022
9:00am - 9:15am

Hawai'i Convention Center, Level 3, 323B

Presenter

Co-Author(s)

Yi-Teng Huang1,Seán Kavanagh2,3,Marcello Righetto4,Igal Levine5,Alexander Sneyd1,Laura Herz4,Akshay Rao1,Robert Hoye3

University of Cambridge1,University College London2,Imperial College London3,University of Oxford4,Helmholtz-Zentrum Berlin5

Abstract

Yi-Teng Huang1,Seán Kavanagh2,3,Marcello Righetto4,Igal Levine5,Alexander Sneyd1,Laura Herz4,Akshay Rao1,Robert Hoye3

University of Cambridge1,University College London2,Imperial College London3,University of Oxford4,Helmholtz-Zentrum Berlin5
Perovskites have emerged as a new PV material exhibiting remarkable efficiency over the last decade. Apart from strong absorption coefficient and high mobility, their long minority carrier lifetime in the presence of numerous defect states has been considered as the most essential factor leading to the success of perovskite PVs. Owing to this "defect-tolerance" feature, efficient perovskite solar cells can be fabricated through low-temperature and facile solution-processing, which can hardly apply in silicon or gallium arsenide counterparts. Unfortunately, almost all the efficient perovskite solar cells contain lead component, and the lead toxicity as well as its ready solubility in water still impedes the industrialization of this potential material. This challenge has motivated researchers to investigate other lead-free alternatives, or perovskite-inspired materials (PIMs) that may preserve the features of perovskites.<br/>Obviously, one PIM will be expected to own the similar defect-tolerance to perovskites. The origin of this unusual feature has been claimed to be related to electronic band structure of materials. It has been shown that defect states tend to be within the bands or "shallow" relative to the band edges when the valence band is mainly composed of anti-bonding orbitals, which can be frequently found in compounds with partially oxidized cations such as Pb<sup>2+</sup> or Bi<sup>3+</sup>. The "ns<sup>2</sup>" electron configuration of cations can be thus regarded as a figure of merit for defect-tolerance. Taking into account of the electron configuration along with band gaps, energy level alignment, and crystal structures etc., several PIMs have been identified.<br/>NaBiS<sub>2</sub> is one of the potential PIMs with Bi<sup>3+</sup> as the partially oxidized cations. It is made from earth-abundant elements and can be synthesized based on solution processed method at low temperature. We have shown that NaBiS<sub>2</sub> has a band gap of 1.2-1.3 eV along with high absorption coefficients over 10<sup>5</sup> cm<sup>-1</sup> from NIR to UV region. NaBiS<sub>2</sub> is also rather stable in normal environment, which has been confirmed by its unchanged XRD peaks after storing in air for several months. Particularly, we also apply transient-absorption spectroscopy technique to show that NaBiS<sub>2</sub> has an extremely long carrier lifetime up to μs order, and this lifetime will not drop significantly when percent-level of Na/S vacancies are introduced via annealing. This result implies that NaBiS<sub>2</sub> may be defect-tolerant in some extent. However, from optical pump-THz-probe measurement, we also observe that free carrier population seems to decrease much faster within a few ps, which suggests that most carriers after excitation might be captured by a large amount of shallow defects in a short time, and these trapped carriers can somehow live longer due to the low recombination probability with other carriers. Recently, the DFT calculation results from our collaborators have provided some explanation for the experimental results. It has been shown that NaBiS<sub>2</sub> might be able to tolerate Na vacancies since Na<sup>+</sup> does not really contribute to the electronic state near the bandgap. But NaBiS<sub>2 </sub>will also tend to form localised S p states just above its band edge due to the lack of the orbital hybridization from Na<sup>+</sup>. These localised states will then serve as hole traps, which can lead to fast depopulation after photoexcitation.<br/>We also demonstrated that metal iodides are effective on the ligand exchange treatment on NaBiS<sub>2</sub> thin films while fabricating PV devices. Although a record high efficiency of ~0.7% has been achieved in our NaI-treated devices, the performance of devices might be still limited by numerous shallow defects and relatively short diffusion length of NaBiS<sub>2</sub>. Our work provides an in-depth investigation on the optoelectronic characteristics and defect tolerance of NaBiS<sub>2</sub>, which might provide some insights on the future application strategies with this material.

Keywords

defects | optical properties

Symposium Organizers

Sage Bauers, National Renewable Energy Laboratory
Jeffrey Neaton, University of California, Berkeley
Lydia Wong, Nanyang Technological University
Kazuhiko Maeda, Tokyo Inst of Technology

Symposium Support

Bronze
University of Pennsylvania’s Master of Chemical Sciences
MilliporeSigma
MRS-Singapore

Publishing Alliance

MRS publishes with Springer Nature