Lead-free double perovskite halides are emerging optoelectronic materials that are alternatives to lead-based perovskite halides. Recently, single-crystalline double perovskite halides were synthesized, and their intriguing functional properties were demonstrated. Despite such pioneering works, lead-free double perovskite halides with better crystallinity are still in demand for applications to novel optoelectronic devices. Here, we realized highly crystalline Cs2AgBiBr6 single crystals with a well-defined atomic ordering on the microscopic scale. We avoided the formation of Ag vacancies and the subsequent secondary Cs3Bi2Br9 by manipulating the initial chemical environments in hydrothermal synthesis. The suppression of Ag vacancies allows us to reduce the trap density in the as-grown crystals and to enhance the carrier mobility further. Our design strategy is applicable for fabricating other lead-free halide materials with high crystallinity.
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