A‐site Cation Engineering for Highly Efficient MAPbI 3 Single‐Crystal X‐ray Detector

Metal halide perovskites have emerged as a new generation of X‐ray detector materials. However, large‐sized MAPbI 3 single crystals (SCs) still exhibit lower performance than MAPbBr 3 SCs in X‐ray detection. DFT (density functional theory) simulations suggest the problem could be overcome by alloying large‐sized cations at the A site. The alloyed process could notably decrease the electron–phonon coupling strength and increase the material defect formation energy. Accordingly, centimeter‐sized alloyed DMAMAPbI 3 (DMA=dimethylammonium) and GAMAPbI 3 (GA=guanidinium) SCs are obtained. Electrical characterizations confirm the GAMAPbI 3 SCs display improved charge collection efficiency. It also exhibits a remarkable reduction of dark current, an important figure of merit for X‐ray detectors. With a judiciously designed device architecture, the overall detector performance confirms GAMAPbI 3 SCs as one of the most sensitive perovskite X‐ray detectors to date.