Crystal and Band-Gap Engineering of One-Dimensional Antimony/Bismuth-Based Organic-Inorganic Hybrids

Hybrid halide perovskites are emerging semiconducting materials with a diverse set of remarkable optoelectronic properties. Besides the widely studied lead halide perovskites, Pb-free metal halides such as Bi- A nd Sb-containing hybrid organic-inorganic materials have shown potential as semiconductors and have been deemed candidates for optoelectronic devices. Here, we report a series of 1D Sb/Bi-based organic-inorganic hybrid alloys: [4ApyH]Sb_xBi_1-xI_yBr_4-y, where 4ApyH stands for the 4-aminopyridine cations. These compounds are assembled by edge-sharing octahedral [MX₆] units stabilizing 1D chains with organic cations filled in between. The crystallographic data of eight selected complexes show that [4ApyH]Sb_xBi_1-xI_yBr_4-y has at least five phases (space group) with the difference metal and halogen content: Pbca ([4ApyH]BiI₄), Pca2₁ ([4ApyH]Sb_0.5Bi_0.5I₄), P2₁/c ([4ApyH]SbI₄ (100 K), [4ApyH]BiI₂Br₂, [4ApyH]BiBr₄, and [4ApyH]SbBr₄ (100 K)), I2/a ([4ApyH]Sb_0.5Bi_0.5I₂Br₂and [4ApyH]SbI₂Br₂), and C2/c ([4ApyH]SbI₄ (298 K) and [4ApyH]SbBr₄ (298 K)). Powder X-ray diffraction shows that the phase of the sample changes with a change of the metal and halogen ratios, and the change law accords with Vegard's law. The optical band gaps are heavily affected by the metal and halide contents, ranging from 1.94 eV for [4ApyH]BiI₄ to 2.73 eV for [4ApyH]SbBr₄. When Sb substitutes for Bi to form an alloy, the band gap increases from 1.94 for [4ApyH]BiI₄ to 1.67 eV for [4ApyH]SbI₄, from 2.13 eV for [4ApyH]BiI₂Br₂ to 2.41 eV for [4ApyH]SbI₂Br₂, and from 2.55 eV for [4ApyH]BiBr₄ to 2.73 eV for [4ApyH]SbBr₄. The conductivity of [4ApyH]Sb_xBi_1-xI₄ increased from ∼1.00 × 10^-15 to 2.14 × 10^-8 S cm^-1 with an increase of the Sb content. Solution-deposited thin films of the nine complexes show the same (110) orientation, displaying a parallel growth orientation with respect to the substrate. The devices of [4ApyH]Sb_0.8Bi_0.2I₄ and [4ApyH]SbI₄ demonstrated stable open-circuit photovoltages of 0.55 and 0.44 V, steady-state short-circuit photocurrent densities of 1.52 and 1.81 mA cm^-2, and light-to-electrical energy conversion efficiencies of 0.29% and 0.30%, respectively.