Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.004
Publication date: 18th July 2023
Bismuth-based semiconductors have gained increasing attention as potential nontoxic alternatives to lead-halide perovksites [1]. Whilst early works focussed on the role of defects (particularly defect tolerance) in these materials, recent work has emphasized the important role of electron-phonon coupling [2]. This talk examines electron-phonon coupling in two emerging bismuth-based perovskite-inspired materials: NaBiS2 and BiOI.
NaBiS2 is part of a growing family of ternary chalcogenides. We show NaBiS2 to be phase-stable in ambient air for 11 months, with high absorption coefficients >105 cm-1 reached from its optical bandgap of 1.4 eV. As a result, a 30 nm thick film has a spectroscopic limited maximum efficiency of 26%, higher than lead-halide perovksites or established thin film solar absorbers. However, we show through ultrafast spectroscopy that the photogenerated charge-carriers in NaBiS2 slowly decay on a microsecond timescale, and yet the photoconductivity decays within 1 ps. This arises due to carrier localization, and we rationalize this as due to inhomogeneous cation disorder, leading to the formation of S 3p states just above the valence band maximum that facilitate the formation of small hole polarons [3].
The second part of the talk examines BiOI. We show through detailed spectroscopic measurements and state-of-the-art computations that this material is an exception to recent Bi-based perovskite-inspired materials and avoid carrier localization. As a result, mobilities exceeding 80 cm2 V-1 s-1 are achieved, along with high mobility-lifetime products exceeding 10-2 cm2 V-1 in single crystals. We show that BiOI photoconductors are highly promising radiation detectors, capable of resolving dose rates down to 22 nGyair s-1, which is well below the current medical standard of 5500 nGyair s-1.
R. L. Z. H. acknowledges funding from the Royal Academy of Engineering through the Research Fellowships scheme (no. RF \201718\1701) and the Engineering and Physical Sciences Research Council (no. EP/V014498/1).
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