High-Performance Photodetector Enabled by Melamine Cation Based Lead Free Perovskitoid Single Crystals

Abstract

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)]. Benefiting from 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, enabling lower defect density (4.30 × 109 vs. 3.84 × 1010 cm−3) and higher carrier mobility (4.17 × 10−2 vs. 3.29 × 10−4 cm2 s−1 V−1) than 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 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.