Ion engineering enabled lattice-matching in germanium perovskite heterojunctions for high-performance X-ray detectors.

Developing high-performance, lead-free X-ray detectors urgently requires environmentally friendly new material systems. Among various candidate materials, germanium (Ge) perovskites have attracted significant attention due to their superior structural stability and tunability. Constructing two-dimensional/three-dimensional (2D/3D) Ge perovskite heterojunctions is an effective way to enhance detector sensitivity and stability synergistically, but their performance is fundamentally limited by the core challenge of interfacial lattice mismatch. Here, we report a lattice matching strategy via doping at the A- and B-sites in lead-free Ge perovskites. By doping the 4-fluorophenethylammonium (FPEA⁺) organic cations at the A-site and Bismuth (Bi3+) ions at the B-site of the 2D bis(4-fluorophenylmethylammonium) germanium iodide (FPMA2GeI4) perovskite, we precisely adjust its lattice constant to match that of the 3D perovskite methylammonium germanium iodide (MAGeI3). This approach reduces the lattice mismatch to an ultralow value of approximately 1%. Consequently, the optimized heterojunction detector demonstrates a high sensitivity of 21,237 μC Gy-1 cm-2, a low detection limit of 43 nGy s-1, and excellent operational stability under continuous X-ray irradiation. This work not only presents a straightforward method for fine-tuning lattice matching in perovskite heterojunctions through A- and B-site ion-radius engineering, but also offers new insights into designing more stable and efficient lead-free perovskite X-ray detectors.