Aligning Out-of-Plane Polarization in 2D Ferroelectric Perovskite Wafer Boosts Interlayer Carrier Transport for Efficient X-Ray Detection.

2D metal halide perovskites (MHPs) featuring suppressed ion migration and enhanced structural stability are highly promising for direct X-ray detection but suffer from poor interlayer carrier transport and difficulty in scaling single crystals. Here, we overcome these limitations by fabricating high-quality, centimeter-sized polycrystalline wafers of a 2D ferroelectric MHP with out-of-plane spontaneous polarization, (Mpda)PbBr4 (Mpda = N-methyl-1,3-propanediamine), via a facile hot-pressing strategy. This process induces a horizontal orientation of microcrystals, resulting in vertical alignment of their polar axes and thus macroscopic polarization in the wafer. Such an intriguing polarization generates a switchable built-in electric field that can promote separation and interlayer transport of photogenerated carriers, exhibiting a bulk photovoltage of up to 2 V. Consequently, the vertical X-ray detector can not only be operated in self-powered mode but also achieve a high sensitivity of 2285 μC Gy-1 cm-2 and an ultralow detection limit of 7.2 nGy s-1 under a small bias (16 V). The device also exhibits remarkable operational stability and high-resolution X-ray imaging capability. This work demonstrates that aligning the intrinsic ferroelectric polarization in polycrystalline wafers is a viable strategy to enhance vertical carrier transport, paving the way toward high-performance, flat-panel X-ray detectors based on 2D MHPs.