Metformin Spacer Conformation Is a Structure-Directing Element in a 2D Perovskite with Strong X-ray Response

Direct X-ray detection in two-dimensional (2D) hybrid metal-halide perovskites is dictated by the interplay between organic spacer chemistry, crystal structure, and charge transport. Among these materials, the metformin (MF2+)-based perovskite MFPbI4 has recently emerged as a promising X-ray detector, exhibiting high X-ray detection sensitivity. Here, we present a structure-to-performance investigation of this material, revealing that the conformational asymmetry of the doubly protonated MF2+ spacer is associated with anisotropic octahedral tilting and an anomalous pseudo-Ruddlesden-Popper interlayer registry despite its diammonium classification, influencing the electronic structure and transport anisotropy of the material. High-quality single crystals up to 8 mm in length are grown, and calculations show that protonation-induced twisting of the metformin spacer contributes to the observed organization of the inorganic framework. Optical spectroscopy, Kelvin probe measurements, and trap density analysis then relate this structure to the band gap, carrier dynamics, and charge transport behavior. Millimeter-scale in-plane and out-of-plane X-ray detection measurements, enabled by electrode geometries validated through electric field simulations, yield an in-plane sensitivity of 4020 μC Gyair-1 cm-2 at a 3 mm electrode separation and nearly balanced electron and hole out-of-plane sensitivities of 2379 and 2517 μC Gyair-1 cm-2 across a 1 mm crystal thickness, with a detection limit of 65 nGyair s-1 and less than 7% photocurrent drift under continuous irradiation. The in-plane sensitivity is comparable to the highest previously reported value for this material while employing a 60-fold larger electrode separation, demonstrating efficient charge collection across relevant transport distances. Together, these results indicate that spacer conformation propagates from molecular geometry into framework topology, electronic anisotropy, and detection performance.