**5.1 Surface defects**

The absence of grain boundaries makes perovskite crystals acquire better optical and charge transport properties than their polycrystalline counterparts. However, the surface of crystals usually possesses lots of chemical impurities, dangling bonds, surface dislocations, and under-coordinated atoms, and becomes disordered owing to hydration, thus decreasing the carrier mobility and carrier diffusion length and promote the recombination of carriers near the crystal surface [76, 104–106]. Thus, the further decrease of defects, especially the surface defects, is required, aiming to gain high-quality perovskite crystals. To realize high-performance optoelectronic devices based on perovskite crystals with low-level surface defects, more research should be carried out on the surface passivation.
