Band Gap Engineering and Metastable Phase Discovery in Cu2BaSnS4–xSex Nanocrystals via Topotactic Anion Exchange

Sun, Zhaohong; Chen, Yizhen; Brutchey, Richard L.

doi:10.1021/jacs.5c07324

Band Gap Engineering and Metastable Phase Discovery in Cu₂BaSnS_4–xSe_x Nanocrystals via Topotactic Anion Exchange

Journal Article · Mon Jun 09 00:00:00 EDT 2025 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/jacs.5c07324· OSTI ID:3015113

^[1]; Chen, Yizhen ^[1]; ^[1]

University of Southern California, Los Angeles, CA (United States)

Earth-abundant multinary inorganic chalcogenides, such as Cu₂BaSnS_4–xSe_x (CBTSSe), have emerged as promising absorbers for next-generation solar cells. However, accessing colloidal CBTSSe nanocrystals, including metastable polymorphs with more favorable optoelectronic properties, remains a critical synthetic challenge. Herein, we report the first use of topotactic anion exchange to access metastable crystal structures in soft chemistry syntheses. By varying the stoichiometry of selenium precursors, we directly synthesize Cu₂BaSnS_4–xSe_x (0 ≤ x ≤ 4) nanocrystals with a broad band gap tunability of 1.51–2.04 eV. At x = 4, a new metastable trigonal phase of Cu₂BaSnSe₄ is isolated. This phase arises via an in-situ topotactic anion exchange from sulfur-rich Cu₂BaSnS_4–xSe_x intermediates, while retaining both the cation sublattice and overall nanocrystal morphology. The trigonal Cu₂BaSnSe₄ exhibits a nearly direct band gap that is 280 meV lower than its thermodynamically stable orthorhombic counterpart, aligning more closely with the optimal band gap for single-junction solar cells. In contrast, a post-synthetic anion exchange route leads to the thermodynamically preferred orthorhombic polymorph, pointing to the critical nature of the in-situ transformation. Our findings open a versatile pathway for both polymorphic and morphological control in colloidal nanocrystals, expanding the synthetic design space for new optoelectronic materials.

View Accepted Manuscript (DOE)

Research Organization:: Univ. of Southern California, Los Angeles, CA (United States); University of Southern California, Los Angeles, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

Grant/Contract Number:: SC0006812

OSTI ID:: 3015113

Alternate ID(s):: OSTI ID: 2570063
OSTI ID: 3013923

Journal Information:: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 24 Vol. 147; ISSN 0002-7863; ISSN 1520-5126

Publisher:: American Chemical Society (ACS)Copyright Statement

Country of Publication:: United States

Language:: English

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Band Gap Engineering and Metastable Phase Discovery in Cu2BaSnS4–xSex Nanocrystals via Topotactic Anion Exchange

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