Exciton Dynamics in Hybrid Halide 2D Perovskites—The Role of Organic Spacers in Low-Dose X-Ray Detection

Two-dimensional (2D) layered halide perovskites are unique due to their integration of organic spacer molecules, affecting properties like interlayer spacing, electronic band structure, ion migration suppression, and charge carrier transport. These aromatic spacers relax dielectric confinement in π-conjugated molecules, lowering dark current in X-ray detectors and enhancing operational stability. However, reducing the limit of detection for low-dose X-ray applications requires compositional engineering of 2D perovskites. In this study, we explored how tuning organic spacers influences X-ray detection performance, using femtosecond-transient absorption spectroscopy (fs-TAS) to elucidate the mechanisms behind high-sensitivity, low-dose detection. Specifically, we synthesized 2D perovskite single crystals with organic spacers N,N,N′,N′-tetramethyl-1,4-phenylenediammonium (TMPDA) and N,N-dimethylphenylene-p-diammonium (DPDA). Our findings show that DPDA-based perovskites exhibit reduced interplanar spacing, leading to tighter lattice packing. Density functional theory (DFT) analysis indicates that (DPDA)PbBr₄ has a lower effective mass and reduced lattice distortion, minimizing self-trapped excitons (STEs) and electron-phonon coupling, while promoting carrier delocalization. X-ray detection results reveal that (DPDA)PbBr₄ outperforms (TMPDA)PbBr₄ with superior sensitivity due to enhanced carrier dynamics and a higher mobility-lifetime product, achieving an LoD of 18 nGy/s—far below both commercial and advanced perovskite-based detectors. Additionally, fs-TAS data shows that (DPDA)PbBr₄ has extended hot STE cooling and decay lifetimes, correlating with its higher sensitivity. This research highlights the crucial role of organic spacers in modulating interlayer spacing, lattice structure, and electron-phonon interactions, directly influencing exciton dynamics and charge transport efficiency. These insights pave the way for designing ultra-sensitive X-ray detectors with ultra-low LoD, critical for health and security applications.