Vacancy-Ordered Hybrid Two-Dimensional Bi(III) Iodides with (100)-Oriented Dion-Jacobson Perovskite-Related Structures

Two-dimensional (2D) hybrid iodide perovskites, (R-NH₃)₂MI₄ and (H₃N-R-NH₃)MI₄ (R = alkyl group; M = divalent metal ion), are promising materials for optoelectronics. Traditionally, these compounds contain Pb²⁺ and Sn²⁺ ions in the M-site; however, concerns over the toxicity of Pb²⁺ and the instability of Sn²⁺ ions have driven interest in Bi³⁺ halide-based alternatives. This study reports two Dion-Jacobson type, vacancy-ordered 2D Bi-I perovskites: (H₂DAC)Bi_2/3□_1/3I₄, with vacancy in every third metal site and (H₂DAP)BiBi_1/2□_1/2I₃·(I₃)_1/2, with vacancy in every second metal site (H₂DAC = trans-1,4-diammoniumcyclohexane, H₂DAP = 1,5-diammoniumpentane, and □ = vacancy). The band gaps of (H₂DAC)Bi_2/3□_1/3I₄ and (H₂DAP)Bi_1/2□_1/2I₃·(I₃)_1/2 are 2.11 and 1.97 eV, respectively─both narrower than that of Pb²⁺-based analogue (H₂DAC)PbI₄ (2.36 eV). These compounds show a positive photoresponse under light exposure, with the highest response observed in the case of (H₂DAP)Bi_1/2□_1/2I₃·(I₃)_1/2. This enhancement is attributed to the presence of I₃^- ions, which not only cross-link the perovskite layers and stabilize the H₂DAP cation in its zigzag conformation but also contribute to the frontier orbitals. DFT calculations corroborate these experimental results. Overall, this study introduces an approach for synthesizing hybrid Bi(III)-I perovskites, which may be further investigated as lead-free optoelectronic materials, including in perovskite photovoltaics.