Zero-Dimensional Bismuth Halide Thin Films with High Orientation and Broadband Emission

Abstract

Lead-free low-dimensional metal halides are emerging as highly tunable materials for optoelectronic applications. Here, we report a new class of zero-dimensional bismuth halides incorporating phenethylammonium (PEA) cations, namely PEA4BiI7 and PEA4BiBr7, and investigate their structural, thermal, and optoelectronic properties. Single-crystal X-ray diffraction reveals that both compounds consist of isolated [BiX6] octahedra separated by bulky organic cations, although they crystallize in different crystal systems and exhibit high stability under ambient conditions. In contrast, the chloride analogue adopts a distinct stoichiometry, PEA4Bi2Cl10, featuring edge-sharing octahedral dimers, highlighting the strong impact of halide substitution on the structural connectivity. Optical measurements show a systematic bandgap evolution following the trend Cl > Br > I, with pronounced excitonic absorption features in all compounds; density functional theory calculations indicate that the conduction band edge is dominated by the inorganic Bi-X framework, while the valence band edge progressively changes from the organic cation to the inorganic sublattice. Thin films were fabricated via a simple one-step spin-coating procedure. Particularly, PEA4BiI7 forms highly oriented, phase-pure thin films and exhibits broadband visible photoluminescence with excitation-dependent behavior. These results establish phenethylammonium bismuth halides as a versatile platform for stable, lead-free zero-dimensional semiconductors with promising potential for solution-processed optoelectronic devices.