Stereo-Hindrance Engineering of A Cation toward <110>-Oriented 2D Perovskite with Minimized Tilting and High-Performance X-Ray Detection.

2D <100>-oriented Dion-Jacobson or Ruddlesden-Popper perovskites are widely recognized as promising candidates for optoelectronic applications. However, the large interlayer spacing significantly hinders the carrier transport. <110>-oriented 2D perovskites naturally exhibit reduced interlayer spacings, but the tilting of metal halide octahedra is typically serious and leads to poor charge transport. Herein, a <110>-oriented 2D perovskite EPZPbBr 4 (EPZ = 1-ethylpiperazine) with minimized tilting is designed through A-site stereo-hindrance engineering. The piperazine functional group enters the space enclosed by the three [PbBr 6 ] 4- octahedra, pushing Pb─Br─Pb closer to a straight line (maximum Pb─Br─Pb angle ≈180°), suppressing the tilting as well as electron-phonon coupling. Meanwhile, the ethyl group is located between layers and contributes an extremely reduced effective interlayer distance (2.22 Å), further facilitating the carrier transport. As a result, EPZPbBr 4 simultaneously demonstrates high µτ product (1.8 × 10 -3 cm 2 V -1 ) and large resistivity (2.17 × 10 10 Ω cm). The assembled X-ray detector achieves low dark current of 1.02 × 10 -10 A cm -2 and high sensitivity of 1240 µC Gy -1 cm -2 under the same bias voltage. The realized specific detectivity (ratio of sensitivity to noise current density, 1.23 × 10 8 µC Gy -1 cm -1 A -1/2 ) is the highest among all reported perovskite X-ray detectors.