A high-performance photodetector enabled by melamine cation-based lead-free perovskitoid single crystals

Low-dimensional halide perovskitoids exhibit emerging potential in photodetection owing to their exceptional environmental stability and ultralow dark current. However, their extended inorganic interlayer spacing and weak inter-ligand interactions fundamentally limit their carrier transport efficiency and photodetector performance. In this study, we address the above challenges through melamine (MLA) cation engineering, constructing two isostructural Bi-/Pb-based single crystals [(MLA)₂Bi₂I₁₀·8H₂O (Bi-SC) and (MLA)₂PbI₆·2H₂O (Pb-SC)]. The extensive hydrogen bonds formed by the abundant -NH₂/-NH- terminal groups of MLA²⁺ and water molecules in Bi-SC, as well as the shorter octahedral I⋯I distances (3.83 Å vs. 4.30 Å, a 11% reduction) from Bi³⁺ substitution, enable lower defect density (4.30 × 10⁹vs. 3.84 × 10¹⁰ cm^-3) and higher carrier mobility (4.17 × 10^-2vs. 3.29 × 10^-4 cm² s^-1 V^-1) than those of Pb-SC. As a result, the Bi-SC-based photodetector demonstrates an on/off ratio 15 times higher than that of Pb-SC. Mechanism analysis reveals that the water-bridged hydrogen bonding network simultaneously compresses the inorganic lattice, enhances the stacking arrangement of MLA²⁺ ligands and thus strengthens charge transfer pathways. The hydrogen-bond engineering overcomes the inherent dimensionality limits in hybrid materials, establishing a general design principle for high-performance low-dimensional optoelectronic applications.