Novel zero-dimensional lead-free bismuth based perovskites: from synthesis to structural and optoelectronic characterization

Abstract

Despite high photo-conversion efficiency, the short long-term stability and toxicity issues have prevented lead-based perovskites from becoming the standard in high efficiency solar cells. Hence, lead-free perovskites with low inorganic framework dimensionality have attracted great attention because of their optoelectronic properties and superior stability. In this work, we present a combined experimental and theoretical investigation of the structural and optoelectronic properties of three novel highly stable zero-dimensional perovskites (discrete connectivity of the inorganic framework), where fused octahedra [Bi_xI_y]^z− are interspersed by large organic cations: H₂bpy²⁺ for 1, Hdmphen⁺ for 2 and the combination of H₂terpy⁺² and Hterpy⁺ for 3 (bpy = 4,4-bipyridine, dmphen = 2,9-dimethyl-1,10-phenanthroline, terpy = 2,2′;6′,2′′-terpyridine). In each of these compounds, the inorganic moieties display different morphologies, either edge-sharing bi-octahedra (Bi₂I₁₀⁴⁻, 1), face-sharing bi-octahedra (Bi₂I₉³⁻, 2) or isolated octahedra (BiI₆³⁻, 3). The consistent results of experimental and theoretical characterization reveal that distortions in the coordination octahedra are most prominent as the number of shared iodide species increases. The experimental optical response presents similar trends between all three materials, which is dominated by transitions inside the octahedron, while the transitions between cations and anionic bismuth-iodide clusters displayed very low intensity at a lower photon energy, an aspect that was described qualitatively well by theoretical characterization. Overall, these materials show different potential prospects, such as application in hybrid dimensional materials, acting as dielectrics for instance, or as a source of Bi for the synthesis of other lead-free perovskites.