Multistage Fluorination in a Metal-Free Perovskite Ferroelectric Enables Multidirectional Self-Powered X-Ray Detection.

Metal-free perovskites (MFPs) have emerged as eco-friendly candidates for next-generation x-ray detection. However, their development is severely hindered by poor material stability and the reliance on high driving voltages, which exacerbate device complexity and operational risks. Herein, we adopt a multistage fluorination strategy to develop a novel MFP ferroelectric, FMDABCO-NH4(PF6)3 (FMDABCO2+ = N-fluoromethyl-N'-diazabicyclo[2.2.2]octonium). X-site fluorination endows FMDABCO-NH4(PF6)3 with excellent stability, while A-site fluorination enhances molecular dipoles and rotational barriers, leading to multiaxial ferroelectricity, an elevated Curie temperature (TC) of 340 K, and a spontaneous polarization (Ps) of 16 µC cm-2, the highest value among reported fluorinated MFP ferroelectrics. As a result, FMDABCO-NH4(PF6)3 single crystals enable self-powered x-ray detection along three equivalent polar axes, representing the first multiaxial self-powered MFP x-ray detector. Furthermore, under a low electric field of 80 V mm-1, FMDABCO-NH4(PF6)3 delivers a record-high sensitivity of 2860 µC Gyair -1 cm-2 among MFP ferroelectrics, while retaining 93% of its initial sensitivity after 1 year of ambient exposure. This work establishes a viable pathway toward environmentally benign, high-performance, and self-powered x-ray detectors.