**5. Challenges and open issues**

Hybrid perovskite single crystals have shown great potential in high-performance optoelectronic devices; however, several challenges and issues still remain in terms of their practical applications. They mainly include (1) the effects of surface defects, (2) the large-area fabrication, as well as (3) the stability of the perovskite single crystal devices and (4) the health and environmental concerns.
