Stereo‐Hindrance Engineering of A Cation toward ‐Oriented 2D Perovskite with Minimized Tilting and High‐Performance X‐Ray Detection

2D ‐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. ‐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 ‐oriented 2D perovskite EPZPbBr4 (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 [PbBr6]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, EPZPbBr4 simultaneously demonstrates high µτ product (1.8 × 10−3 cm2 V−1) and large resistivity (2.17 × 1010 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 × 108 µC Gy−1 cm−1 A−1/2) is the highest among all reported perovskite X‐ray detectors. A ‐oriented 2D EPZPbBr4 (EPZ = 1‐ethylpiperazine) perovskite with minimized tilting and interlayer distance is designed through A‐site stereo‐hindrance engineering. The piperazine functional group pushes Pb─Br─Pb closer to a straight line (maximum Pb─Br─Pb angle ≈180°), suppressing the tilting and electron–phonon coupling. The EPZPbBr4 X‐ray detector exhibits specific detectivity of 1.23 × 108 µC Gy−1 cm−1 A−1/2.