Low Trap Density Para-F Substituted 2D PEA 2 PbX 4 (X = Cl, Br, I) Single Crystals with Tunable Optoelectrical Properties and High Sensitive X-Ray Detector Performance.

Exploring halogen engineering is of great significance for reducing the density of defect states in crystals of organic-inorganic hybrid perovskites and hence improving the crystal quality. Herein, high-quality single crystals of PEA 2 PbX 4 (X = Cl, Br, I) and their para-F ( p -F) substitution analogs are prepared using the facile solution method to study the effects of both p -F substitution and halogen anion engineering. After p -F substitution, the triclinic PEA 2 PbX 4 (X = Cl, Br) and cubic PEA 2 PbX 4 (X = I) crystals unifies to monoclinic crystal structure for p -F-PEA 2 PbX 4 (X = Cl, Br, I) crystals. The p -F substitution and halogen engineering, together with crystal structure variation, enable the tunability of optoelectrical properties. Experimentally, after the p -F substitution, the energy levels are lowered with increased Fermi levels, and the bandgaps of p -F-PEA 2 PbX 4 (X = Cl, Br, I) are slightly reduced. Benefitting from the enhancement of the charge transfer and the reduced trap density by p -F substitution and halogen anion engineering, the average carrier lifetime of the p -F-PEA 2 PbX 4 is obviously reduced. Compared with PEA 2 PbI 4 , the X-ray detector based on p -F-PEA 2 PbI 4 perovskite single-crystal has a higher sensitivity of 119.79 μ C Gy air -1 ·cm -2 . Moreover, the X-ray detector based on p -F-PEA 2 PbI 4 single crystals exhibits higher radiation stability under high-dose X-ray irradiation, implying long-term operando stability.